Chapter 3

SECTION I

THE SYNTHESIS OF VEGETABLE TANNINS

1. TANNIN

The first investigations of gall-tannin date from the year 1770, at which time, however, no exact differentiation between tannin and gallic acid was made. The first step in this direction was made when Scheele,[Footnote: Grell's _Chem. Ann._, 1787, 3, I.] in 1787, discovered gallic acid in fermented gall extract, and in the same year Kunzemuller [Footnote:_Ibid._, 1787,3,413.] separated gallic acid (or pyrogallol) as a crystalline body from oak galls. Dize [Footnote: _Jour. Chim. et Phys._, 1791, 399.] continued the investigations, which were brought to a conclusion with Deyeux' work [Footnote: _Ann. Chim._, 1793, 17, I.]; both recognised that the substance isolated was not a single substance, but was a mixture of gallic acid, a green colouring matter, a rosin (tannin?), and extraneous matter. Proust [Footnote: _Ibid._, 1799, 25, 225.] was the first to differentiate the crystalline gallic acid from the amorphous, astringent substance, which latter he named "Tannin."

Amongst the numerous subsequent investigations of tannin must be especially noted the one by Berzelius [Footnote: Pogg,_Ann._, 1827, 10, 257.], who purified the potash salt and decomposed this with sulphuric acid. Pelouze [Footnote: Liebig's _Ann._, 1843, 47, 358.], later on, observed the formation of the crystalline gallic acid from tannin, when the latter is boiled with sulphuric acid; this had already been observed by J. Liebig.[Footnote: _Ibid._1843, 39, 100.] Both had noticed the absence of nitrogen. In addition to the methods of preparation of tannin then in vogue neutral solvents were mainly employed by subsequent investigators; Pelouze [Footnote: _Jour. Prakt. Chem._, 1834, 2, 301, and 328.] treated powdered galls with ether containing alcohol and water, and considered the upper layer to be a solution of gallic acid and impurities, the bottom layer to contain the pure tannin.

The EMPIRICAL FORMULA of tannin has also been the subject of much speculation by the different investigators, the difficulty here being that of obtaining a pure specimen of the substance free from sugars, and which could be submitted to elementary analysis. Whereas these early purified substances were thought to correspond to the formula of digallic acid (galloylgallic acid), C_14H_10O_9, Fischer and Freudenberg [Footnote: _Ber._, 1912, 915 and 2709.] were able to show, with approximate certainty, that the constitution of tannin is that of a pentadigalloyl glucose.

Early attempts at _hydrolysing tannin_ gave varying results, some investigators claiming the presence, and others the absence of sugars. Here, again, E. Fischer and Freudenberg [Footnote: _Ibid._] were able to conclusively prove that on hydrolysing tannin with dilute acids, 7.9 per cent. glucose is dissociated, and that hence glucose forms part of the tannin molecule. Fischer and Freudenberg also determined the optical activity of pure tannin in water: [Greek: a]_D was found to lie between +58° and +70°.

Graham found [Footnote: _Phil. Transact._, 1861, 183.] that the _tannin molecule_ is of considerable size, since its diffusion velocity is 200 times less than that of common salt. Paternò [Footnote: _Zeits. phys. Chem._, 1890, iv. 457.] was the first to determine the molecular weight of tannin, employing Raoult's method; he found that tannin in aqueous solution behaves like a colloid and that hence Raoult's method is not applicable. When, on the other hand, he dissolved tannin in acetic acid, results concordant with the formula of C_14H_10O_9, corresponding to a molecular weight of 322, were obtained. Sabanajew [Footnote: _Ibid._, 1890, v. 192.] later determined the molecular weight of tannin in aqueous solution as 1104, in acetic acid solution as 1113-1322, Krafft [Footnote: _Ber._, 1899, 32, 1613.] as 1587-1626 in aqueous solution. Walden [Footnote: _Ibid._, 1898, 3167.] determined the molecular weight of tannin-schuchardt as 1350-1560, tannin-merck as 753-763, digallic acid as 307-316 (calculated 322). Feist [Footnote: _Chem. Ztg._, 1908, 918.] determined the molecular weight of tannin as 615 and one of his own preparation as 746, Turkish tannin as 521 and Chinese tannin as 899. In this connection it should be noted that the calculated molecular weight of pentagalloyl glucose, which in E. Fischer's opinion forms a substantial part of the tannin molecule, is 940, but Fischer also thinks that this compound possesses a much higher molecular weight.

STRUCTURE OF TANNIN--The oldest structural formula of tannin is Schiff's digallic acid formula:--[Footnote 1: _Ber_., 1871, 4, 231.]

---------CO.O.---------- ^ ^ OH | | | | HO | | OH HOOC | | OH V V OH

A drawback to the acceptance of this formula is the absence of an asymmetrical C-atom; the formula, therefore, does not explain the optical activity exhibited by tannin. Schiff attempted to overcome this difficulty by adopting a diagonal structural formula, but even when adopting Clauss' diagonal formula for benzene the optical activity of a number of other compounds depends upon the existence of the asymmetrical C-atom. Biginelli [Footnote 2: _Gazz chim. Ital_., 1909, 39, 268.] also opposed the digallic acid formula, and supported his view by referring to the arsenic compounds obtained by him on heating arsenic acid and gallic acid, instead of obtaining digallic acid. Walden, [Footnote 3: _Ber_., 1898, 31, 3168.] on the other hand, found, on analysing the digallic acid thus prepared, only slight traces of arsenic and, by the elementary analysis, obtained figures closely corresponding to those of digallic acid.

Bottinger [Footnote 4: _Ibid_., 1884, 17, 1476.] prepared the so-called _[Greek: b]_-digallic acid by heating ethyl gallate with pyroracemic acid and sulphuric acid and proposed the so-called ketone-tannin formula:--

HO_____OH ______OH HO{_____}--------CO--------{______}OH COOH OH

Schiff completed this formula by a diagonal, so as to explain the optical activity observed--

HO OH ______OH HO{_____}--------CO--------{______}OH COOH OH [Diagonal bond between HO and COOH on left.]

The ketone formula was corroborated by Nierenstein, [Footnote: _Ber._ 1905, 38, 3641.] who distilled tannin with zinc dust and obtained diphenylmethane (smell of benzene) and a crystalline product, M.P. 7O°-71° C. (M.P. of diphenyl = 71° C.). König and Kostanecki [Footnote: _Ibid._, 1906, 39, 4027.] sought to find the constitution of the tannins in the leuco-compounds of the oxyketones, to which catechin belongs. Nierenstein (see above), however, emphasises that the high molecular weight and the optical activity speak against the digallic acid formula, but in favour of this are the following points: (1) the decomposition of tannin with the formation of gallic acid; (2) the decomposition of methylotannin with the formation of di- and trimethyl esters of gallic acid; and (3) the production of diphenylmethane on distillation with zinc dust. The latter reaction especially illustrates the analogous formation of fluorene from compounds of the type--

--CO.O ^ ______ ^ | | | | | | | | V V

Nierenstein gave the name "Tannophor" to the mother-substance of tannin, phenylbenzoate, C_6H_5-COO-C_6H_5.

Dekker [Footnote: "De Looistoffen," vol. ii, p. 30 (1908).] was, however, unable to detect diphenylmethane on distilling with zinc dust, and did, therefore, not accept Nierenstein's views. In proposing the formula--

O || HO ^ _ __C | | | | | }O | | | __OH | |____|_C_/ \OH HO V \__/ OH OH OH

Dekker [Footnote: _Ber._, 1906, 34, 2497.] was enabled to account for most of the details in the behaviour of tannin, viz.: (1) the empirical constitution, C_14H_10O_9; (2) the almost complete hydrolysis into gallic acid (the dotted line indicates the decomposition of the molecule into 2 molecules gallic acid by taking up water); (3) the formation of diphenylmethane as a result of distillation with zinc dust; and (4) the electrical non-conductivity. Since tannin on acetylating yields a considerable amount of triacetylgallic acid, it should, according to Dekker, contain at least six acetylisable hydroxyls.

Nierenstein [Footnote: _Chem. Ztg._, 1906, 31, 880.] objected to this formula on account of its containing seven hydroxyl groups, whereas Dekker found six, Nierenstein five, and Herzig still fewer hydroxyl groups. The formula would also favour the conception of tinctorial properties which could hardly be ascribed to tannin. Lloyd [Footnote: _Chemical News_, 1908, 97, 133.] proposed a very intricate formula containing three digallic acid groups joined into one six-ring system, which would then explain the optical activity; it would, on the other hand, also require an inactive cis-form.

Iljin [Footnote: _Jour. of the Russian phys. chem. Soc._, 1908, 39, 470.] prepared two phenylhydrazine derivatives of tannin (C_74 H_58 N_8 O_30 and C_98 H_82 N_14 O_96) and proposed the formula, C_58 H_40 O_33, the constitution of which would be--

R_1 R_1 | | }C--O--O--C{ | | | R_2 | R_2 O R_1 | R_1 | | | }C--O--O--C{ | | R_2 R_2

where R1= CO C_6 H_2 (OH)_3 and R2= C_6 H_2 (OH)_2

Nierenstein [Footnote: _Ber._, 1905, 38, 3841; 1907, 40, 917; 1908, 41, 77 and 3015; 1909, 42, 1122 and 3552; _Chem. Ztg._, 1907, 31, 72; 1909, 34, 15.] considers tannin to be a mixture of digallic acid and leucotannin, the latter possessing the formula--

^-------CH.OH--O----^ OH | | | | HO V OH HOOC V OH OH

The optical activity of tannin is expressed in this formula and its probability is corroborated by Nierenstein, who was able to resolve the acetylated tannin by fractional precipitation into pentacetyl tannin (M.P. 203°-208° C.) and pentacetyl leucotannin (M.P. 166° C.). By oxidation, the former is converted into ellagic acid, and on hydrolysis with dilute sulphuric acid readily yielded gallic acid. Hydrolysis of the pentacetyl leucotannin, however, yielded gallic aldehyde, and oxidation yielded purpurotannin (a naphthalene derivative) in addition to ellagic acid.

Nierenstein [Footnote: _Ber._, 1910, 43, 628.] also succeeded in converting tannin into carboethoxytannin, the latter on saponification yielding crystalline, inactive digallic acid. On acetylating pentacetyl leucotannin with acetyl chloride a hexacetyl derivative (M.P. 159° C.) is obtained, the strychnine salt of which is resolved into both of the active components. This proves the presence of digallic acid and leucotannin in tannin lev. pur. Schering investigated by Nierenstein. The latter author [Footnote: Liebig's _Ann._, 1912, 386, 318; 388, 223.] later considered tannin to be polydigalloylleucodigallic acid anhydride and the simplest tannin to be a digalloylleucodigallic acid anhydride. This view, however, would not stand subsequent criticisms, being in disagreement with the earlier observations of molecular weight and acidic properties of tannin. Manning [Footnote: _Ibid._, 1912, 34, 918.] believed to have isolated a pentethylester of the pentagalloyl glucoside from tannin, but this was shown to be the ethyl ester of gallic acid.

Feist [Footnote: _Ber._, 1912, 45, 1493.] had arrived at the conclusion that tannin was a glucose compound, and maintained that tannin from Turkish galls was a compound of glucogallic acid combined as an ester with 2 molecules gallic acid. But Fischer and Strauss [Footnote: _Ibid._, 1912, 45, 3773.] synthetically prepared a glucoside of gallic acid exhibiting differences from Feist's preparation which were so great that the latter no longer could be considered a single glucoside of gallic acid.

Fischer and Freudenberg [Footnote: _Ibid._, 1912, 45, 2717; 1913, 46, 1127.] subsequently elaborated a method of purifying tannin, and on investigating the purified substance, arrived at the conclusion that no other hydroxybenzoic acid than gallic acid was present in tannin. On repeating Strecker's hydrolysis they obtained 7-8 per cent, sugar, and hence concluded that 1 molecule of glucose was combined with about 10 molecules of gallic acid. Owing to the difficulty of isolating the intermediary hydrolysis products, and the subsequent impossibility of drawing any conclusions as to the constitution of tannin, the latter investigators decided to adopt the methods offered by synthesis. Their basic idea was the absence of carboxylic groups in tannin, and that hence the total gallic acid must be present in ester form. These conditions are fulfilled if one views tannin as being an ester compound of 1 molecule of glucose and 5 molecules of digallic acid, of similar construction as, for example, pentacetyl glucose. Fischer and Freudenberg succeeded in preparing the former by shaking a mixture of finely powdered glucose, chloroform, and quinoline with an excess of tricarbomethoxygalloyl chloride for twenty-four hours and precipitating the resulting product with methyl alcohol; suitably purified, a light amorphous colourless substance was obtained which proved to be penta-(tricarbomethoxygalloyl) glucose. Careful saponification with excess alkali in acetone-aqueous solution at room temperature yielded a tannin very closely resembling tannin, identified as pentagalloyl glucose. It is doubtful, however, whether this substance is homogeneous, and it is probably a mixture of two stereoisomers.

Fischer and Freudenberg, therefore, further concluded that tannin is mainly an ester compound of glucose and 5 molecules _m_-digallic acid. Elucidation on this point offered itself advantageously in Herzwig's methylotannin, [Footnote: _Ber._, 1905, 38, 989.] which is obtained by the interaction of diazomethane and tannin. The first step was then to prepare pentamethyl-_m_-digallic acid

CH_3.O_______ ______COOH CH_3.O{_______}--CO.O--{______} CH_3.O CH_3.O O.CH_3

from trimethylgalloyl chloride and the _m-p_-dimethyl ether of gallic acid; the chloride of this substance, coupled with [Greek: a]- and [Greek: b]-glucose, yields--

_CH.OR | | | CH.OR H_______O.CH_3 | | R=CO{_______}O.CH_3 O{ CH.OR H O | | H_____O.CH_3 | CH CO{_____}O.CH_3 | | H O.CH_3 |_CH.OR

CH_2.OR

The [Greek: a]- and [Greek: b]-derivatives thus obtained differ in their behaviour towards polarised light, and are, again, probably mixtures of two stereoisomers, _i.e._, mixtures of derivatives of [Greek: a]- and [Greek: b]-glucose. Compared to methylotannin, these preparations exhibit very close resemblance to the former, from which it may be concluded that they are closely related to this substance, and probably possess the same or a very similar structure; the result of the above experiments has, therefore, brought us at least in close proximity to the structure of tannin. It must, however, be borne in mind that the analysis and hydrolysis of tannin does not afford an explanation of the question as to whether tannin is a compound of glucose and 10, 9, or 11 molecules of gallic acid; it is also possible, though not probable, that tannin would contain a polysaccharide instead of glucose itself. Similarly to sugar, the true glucosides can be coupled with hydroxybenzoic acids, which is proved by the preparation of tetra-galloyl-[Greek: a]-methyl glucoside; this substance, also, exhibits tannoid character.

2. DIGALLIC ACID

Whereas, until recently, tannin had been considered to be gallic acid anhydride, or digallic acid, closer investigations have revealed that neither is tannin digallic acid nor is the synthetically prepared digallic acid identical with tannin. Schiff [Footnote: _Ber._, 1871, 231 and 967.] prepared digallic acid by the interaction of phosphorus oxychloride and gallic acid, and believed the product obtained to be identical with tannin; to this latter he first ascribed an ether formula (I.), later an ester formula (II.)--

(OH)_2 (OH)_2 ¦¦ ¦¦ C_6H_2---0---C_6H_2 ¦ ¦ COOH COOH (I.)

(OH)_2 ¦¦ C_6H_2(OH)_3--C--O.C_6H_2 ¦¦ ¦ O COOH (II.)

Froda [Footnote: _Gasz. chim._, 1878, 9.] held that Schiff's condensation product contained phosphorus or arsenic acid and ascribed its tanning properties to the latter; according to this investigator, digallic acid, when completely freed from arsenic acid, does not react with gelatine or quinine. Biginelli [Footnote: _Ibid._, 1909, 39, ii. 268 and 283.] did not consider the action of arsenic acid that of a catalyst, but held that it entered into reaction; according to his investigations products containing arsenic (C_7H_7O_8As and C_14H_11O_12As) are obtained when gallic acid is heated with arsenic acid.

In his preparation of digallic acid, Iljin [Footnote: _Jour. f. prakt. Chem._, 1911, 82, 451.] could only obtain gallic acid, and the ethyl ether of gallic acid showing no characteristics of the tannins; when, however, he heated gallic acid with arsenic pentoxide, he obtained bodies exhibiting the reactions given by tannins.

Bottinger [Foonote: _Ber._, 1884, 1503.] made the first attempt at synthesising tannin; he heated gallic acid or its ethyl ester with glyoxylic acid or pyroracemic acid, and obtained a substance of the composition C_14H_10O_9.2H_2O, which certainly showed some of the characteristics exhibited by tannin, but which by no means was identical with the latter. Bottinger's preparation is probably identical with [Greek: b]-digallic acid, one of two dibasic isomers having the composition--

C_6H_2(OH)_2COOH | C_6H(OH)_3COOH

the other possible isomer having the composition

C_6H(OH)_3COOH CO | C_6H_2(OH)_3

Fischer [Footnote: _Ber_., 1908, 41, 2875.] obtained a digallic acid (M.P. 275°-280° C) by coupling tricarbomethoxygalloyl chloride with dicarbomethoxygallic acid.

Nierenstein [Footnote: _Ibid_., 1910, 43, 628.] obtained, from the carbethoxy compound of tannin, a crystalline, optically active digallic acid, M.P. 268°-270° C. The pentacetate of this substance, obtained by reduction and acetylisation, yielded hexacetylleucotannin. A pentamethyldigallic acid methyl ester of the composition

((O.CH_3)_3)C_6H_2----COO-----C_6H_2((OCH_3)_2)COO.CH_3

was obtained by Mauthner [Footnote: _Jour. f. prakt. Chem_., 1911, 84, 140.] from the chloride of trimethylgallic acid and the methyl ester of the acid from the glucoside of syringin; on saponification with caustic potash the former compound yielded trimethylgallic acid and syringic acid.

Fischer [Footnote: _Ber_., 1913, 46, 1116.] synthesised the so-called _m_-digallic acid by coupling tricarbomethoxygalloyl chloride with carbonylgallic acid and subsequent splitting off of CO_2. The _m_-digallic acid appears as rather thick, colourless, microscopic needles containing about 16 per cent. water of crystallisation, M.P. 271° C. They are slightly soluble in cold, soluble in hot water, and very soluble in methyl and ethyl alcohols. Their aqueous solution gives dark blue coloration with ferric chloride, and precipitates gelatine and quinine.

Fischer and his students [Footnote 5: _Ibid_., 1912, 45, 915, 2709; 1913, 46, 1116.] prepared quite a number of digallic acid derivatives, amongst which are the following:--

Pentamethyl-_m_-digallic acid methyl ester, C_20H_22O_9. Pentacetyl-_m_-digallic acid, C_24H_20O_14. Pentamethyl-_m_-digallic acid, C_19H_20O_9. Pentamethyl-_m_-digalloyl chloride, C_19H_19O_8Cl. Pentamethyl-_p_-digallic acid, C_19H_20O_9. Pentamethyl-_p_-digallic acid methyl ester, C_20H_22O_9.

Hydrolysis of digallic acid yields gallic acid; oxidation, on the other hand, ellagic acid and luteic acid (Luteo Säure), which can be separated by shaking with pyridine. The reduction of digallic acid yields, by different methods, the same reduction compound, [Footnote: Nierenstein, Abderhalden's "Handb. d. biochem. Arbeitsm.," vi. 154.] viz., the racemic leucodigallic acid, which differs from digallic acid by being devoid of any tannoid properties; the latter distinction may be ascribed to the transformation of the tannophor group--CO.O--, to the tannoid-inactive group CH(OH)--O--.

The successful resolving of racemic leucodigallic acid into both of its optically active components can only be brought about through the _d_- or _l_-hexacarbethoxyleucodigallic acid on introducing the latter into a 1 per cent. pyridine solution and heating to 45°-50° C., whereby the _d_- or _l_-acid is formed accompanied by a strong evolution of carbon dioxide.

Hydrolysis of leucogallic acid yields gallic acid and gallic aldehyde; oxidation by means of hydrogen peroxide yields ellagic acid and luteic acid, and oxidation with potassium persulphate and sulphuric acid, in acetic acid solution, yields purpurotannin (see below) [Footnote: Liebig's _Ann_., 1912, 386, 318.].

Another distinct difference between digallic acid and leucodigallic acid is the fact that the formaldehyde condensation product of the former resembles gallic acid, whereas that of the latter resembles tannin; it is therefore probable that the leucodigallic acid part of the tannin molecule imparts this characteristic property to tannin.

---CO.O--- ^ ^ | | | | HO V OH COOH V OH OH OH [Illustration: Digallic Acid becomes...]

---CO.O--- ^ ^ OH | | | | HO V OH COOH V OH OH OH [Illustration: Luteic Acid becomes...]

---CO.O--- ^ ^ OH | | | | HO V --O.CO-- V OH OH OH [Illustration: Ellagic Acid becomes...]

COOH COOH ^ _______ ^ | | | | HO V ---O--- V OH OH OH [Illustration: Purpuro Tannin.]

3. Ellagic Acid

Ellagic acid was discovered in 1831 by Braconnot, who named it "acide ellagique." Its presence in the vegetable kingdom was not quite comprehended for some time, and Nierenstein [Footnote: _Chem. Ztg._, 1909, 87.] was the first to prepare this substance from algarobilla, dividivi, oak bark, pomegranate, myrabolarms, and valonea. The acid is obtained by precipitating it with water from a hot alcoholic extraction of the plants referred to, and recrystallising the precipitate from hot alcohol. Another method of preparation consists in boiling the disintegrated plants with dilute hydrochloric acid, washing the residue, and extracting with hot alcohol, from which the acid will then crystallise. According to Lowe, [Footnote: _Zeits. f. analyt. Chem._, 1875, 35.] it may be obtained from dividivi, an aqueous extract of which is heated to 110° C. in a tube closed at both ends, when crystalline ellagic acid is deposited. Heinemann [Footnote: Ger. Pat., 137,033 and 137,934.] obtained ellagic acid by simply boiling repeatedly aqueous tannin solutions.

Lowe [Footnote: _Jour. f. prakt. Chem._, 1868, 103, 464.] first synthesised ellagic acid by heating gallic acid with arsenic acid or silver oxide. Herzig [Footnote: _Monatshefte fur Chemie_, 1908, 29, 263.] states that ellagic acid is deposited when air is conducted through a mixture of the ethyl or methyl ester of gallic acid and ammonia. Perkin [Footnote: _Proc. Chem. Soc._, 1905, 21, 212.] obtained a substance very similar to ellagic acid by electrolysis of gallic acid in sulphuric acid solution; on oxidising gallic acid in concentrated sulphuric acid solution, Perkin and Nierenstein [Footnote: _Ibid._, 1905, 21, 185.] obtained flavellagic acid. Ellagic acid is also obtained by heating luteic acid in a 10 per cent. soda solution.

Ellagic acid thus prepared crystallises with 2 molecules of water as yellow micro-crystalline rhombic prisms or prismatic needles. The crystals lose this water when heated to 100° C., and it is possible that it is water of constitution, in which case the substance would be hexoxydiphenylcarboxylic acid, and the substance left after drying at 100° C., the dilactone.[Footnote: _Arch. d. Pharm_., 1907, 244, 575.] Ellagic acid is slightly soluble in water, alcohol, and ether, but is easily soluble in caustic potash. With concentrated nitric acid the product assumes a red colour, which appears to be due to the presence of impurities; ellagic acid is commercially known as "alizarin yellow."

The constitution of ellagic acid was uncertain for a long time, and different structural formulae were proposed which more or less corresponded to its properties. The most satisfactory structural formula was proposed by Graebe--[Footnote: _Chem. Ztg_., 1903, 129.]

---CO.O--- ^ -------- ^ OH | | | | HO V --O.CO-- V OH OH

This would represent a tetroxydiphenylmethylolide.

The probability of the correctness of this formula is supported by the possibility of the following derivatives: monomethylellagic acid, C'14H'6O'7(O.CH'3); dimethylellagic acid, C'14H'4O'6(O.CH'3)'2; tetramethylellagic acid, C'14H'2O'4(O.CH'3)'4; phenylhydrazinellagic acid, C'14H'6O'8.N'2H'3C'6H'5.

By the electrolytic reduction of ellagic acid, hexoxydiphenyl, (OH)'3C'6H'2-C'6H'2(OH)'3, is obtained; the ordinary methods of reduction yield leucoellagic acid, C'14H'10O'8, which crystallises in small sharp needles, melting with decomposition at 294°-295° C. Leucoellagic acid is soluble in ethyl and methyl alcohols, and in glacial acetic acid, insoluble in chloroform, benzene, toluene, carbon tetrachloride, and petrol ether; it gives a bluish-green colour with ferric chloride which quickly turns black. Leucoellagic acid is soluble in alkalies, the solution assuming a deep-red coloration; it reduces silver nitrate in the cold, but is not adsorbed by mordanted cotton cloth, in which respect it differs from ellagic acid.[Footnote: Liebig's _Ann_., 1912, 394, 249.

ELLAGITANNIC ACID, C'26H'28'O'10-3H'2O, is closely related to ellagic acid; the former consists of faintly yellow needles, M.P. 329°-336°C. It is soluble in water, precipitates gelatine, and is adsorbed by hide powder. It occurs with gallic acid, tannin, and ellagic acid in dividivi, myrabolams, algarobilla, and chestnut wood extracts.

Other bodies of this class include:--

METELLAGIC ACID, Cl_4H_6O_5, derived from methoxybenzoic acid, and recrystallised from acetic acid, forms small crystalline needles, M.P. 273°-276° C., and yields fluorene on distillation with zinc dust.

----CO.O---- ^ ---------- ^ | | | | V ---O.CO--- V OH

FLAVELLAGIC ACID, C_14H_6O_9, is obtained by the oxidation of gallic acid with concentrated sulphuric acid and potassium persulphate. It crystallises from pyridine in prismatic needles melting above 360° C. Distillation with zinc dust yields fluorene (see above)--

----CO.O---- ^ ---------- ^ OH | | | | HO V ---O.CO--- V OH OH OH

By heating ellagic acid for three-quarters of an hour at 185° C. with concentrated sulphuric acid, ceruleo-ellagic acid (dioxyellagic acid), C_14H_6O_10, is formed as yellowish needles, M.P. 360° C., which are but little soluble in the usual solvents. The acid is slightly soluble in strong caustic soda solution, the colour of the solution, on diluting, changing to green and blue.

LUTEIC ACID (Luteo Saure, pentoxybiphenylmethylolide carboxylic acid),C_14H_8O_9, occurs, in addition to ellagic acid, in myrabolams-- [Footnote: _Ber_., 1909, 42, 353.]

----CO.O---- ^ ---------- ^ OH | | | | HO V OH HOOC V OH OH OH

It is obtained by extracting myrabolams for one hour and a half, under reflux condenser, with pyridine, filtering and adding twice the volume of water to the filtrate and boiling till complete solution is obtained. After about thirty hours a reddish powder deposits, from which ellagic acid may be extracted with pyridine; the mother-liquor on being concentrated yields luteic acid. It is also obtained by oxidising tannin with hydrogen peroxide, the other oxidation product being ellagic acid, and the two may then be separated as indicated above. Luteic acid forms reddish needles which are decomposed, with evolution of gas, at 338°-341° C. Heated with 10 per cent. caustic soda solution it yields ellagic acid. In pyridine solution the carboxyl group maybe eliminated by hydrogen iodide, whereby pentoxybiphenylmethylolide is formed as long silky needles, which do not melt below 300° C. The same substance may also be obtained when ellagic acid is boiled with concentrated caustic potash solution. When luteic acid is treated with diazomethane, it yields the methyl ester of pentamethoxybiphenylmethylolidcarboxylic acid.

4. DEPSIDES

The most common decomposition products of the natural tannoids are hydroxybenzoic acids, notably gallic and proto-catechuic acids; furthermore, other aromatic and aliphatic hydroxy compounds frequently occur. So far, however, attempts at explaining the constitution of the complex decomposition products obtained by hydrolysing high molecular tannoids have not been successful. On the other hand, the constitution of the simpler natural tannoids is known to a greater or less extent; of these, lecanoric acid (Lecanorsäure) is the best known, being an ester anhydride of orsellic acid (a dihydroxytoluylic acid). It combines with erythrite, forming another tannoid, erythrine. The fact that hydroxybenzoic acids are constantly encountered together with the products obtained on hydrolysis of the tannins, seems to point toward the conclusion that anhydrides of hydroxybenzoic acids are frequent constituents of the natural tannoid molecules.

The assumption that, for instance, in tannin at least part of the gallic acid radicals are combined with one another is highly probable, and is supported by the formation of tri- and dimethylgallic acid from methylotannin, [Footnote: Herzig, _Monatshefte f. Chemie_, 1909, 30, 343.] and by the formation of ellagic acid when tannin is oxidised. [Footnote: Nierenstein, _Ber_., 1908, 41, 3015.] Further proof is brought forward by the existence of the pentacetyl-tannin, [Footnote: Schiff, _Ann. d. Chem_., 1873, 170, 73.] and by the results of hydrolysis which has yielded up to 104 per cent. anhydrous gallic acid fiom tannin [Footnote: Sisley, _Bull. Soc. Chim_. 1909, 5, 727.]

Of the three classes of isomeric anhydrides which can be formed from hydroxybenzoic acids, the chemistry of the natural tannins is only concerned with the class comprising the ester anhydrides. If the carboxyl of the first molecule combines with a hydroxyl of the second molecule (ester formation), then a substance possessing character similar to that of a hydroxybenzoic acid is formed, which is capable of combining up with a further molecule in the same way. It is natural to assume that this ester form is much more prevalent in Nature than a combination of two carboxyls by the elimination of water. From the point of view of the chemistry of the tannins, therefore, the starting-point would naturally be that of synthesising the ester anhydrides of hydroxybenzoic acids. Amongst the small number of synthetically prepared ester anhydrides of hydroxybenzoic acids, a few occur exhibiting the properties of the natural tannoids.

In order to simplify the terminology of these substances, Fischer [Footnote: Liebig's _Ann_., 1910, 372, 35.] proposed the name "Depsides" from [Greek: depheiv] = to tan. In analogy with peptides and saccharides, the names di-, tri-, and polydepsides of hydroxybenzoic acids would be suitable for these substances.

The principles underlying the synthesis of depsides are the following:--If the chlorides of carbomethoxy (or carbethoxy) hydroxybenzoic acids are coupled with the sodium salts of hydroxybenzoic acids, esters are formed, _e.g._,

CH_3CO O.O.C_6H_4.CO.Cl + NaO.C_6H_4.COO.Na = NaCl + CH_3.COO.O.C_6H_4.CO.O.C_6H_4.COO.Na

On gently saponifying the esters, these are converted into the corresponding hydroxy derivatives--

OH.C_6H_4.CO.O.C_6H_4.COOH

According to Fischer and Freudenberg, [Footnote: Liebig's _Ann._, 1909, 372, 32.] this method possesses the following advantages:--

1. The synthesis takes place at low temperatures, so that any intramolecular rearrangements are improbable.

2. The composition of the substances is controlled by the intermediary compounds, the carboalkyloxy derivatives.

3. The synthesis permits of more definite evidence as regards the structure of the resulting compounds.

4. The substances obtained are easily purified.

Depsides produced in this manner are by no means new, and were obtained by Klepl by simply heating _p_-hydroxy-benzoic acid (_cf._ Introduction, p. 4). This simple procedure, however, is not applicable to most other hydroxybenzoic acids which are decomposed at the high temperature necessary to induce reaction. Lowe and Schiff (_loc. cit._) have obtained products similar to tannins, the latter investigator by removing the elements of water from gallic acid, protocatechuic acid, salicylic acid, _m_-hydroxybenzoic acid, cresotinic acid, phloretinic acid, and pyrogallolcarboxylic acid. These depsides, however, are amorphous substances, and it is hence difficult to substantiate their homogeneity.

Carbomethoxylation of Hydroxybenzoic Acids

Amongst other compounds chlorphydroxybenzoic acid is used in the preparation of the materials employed in the synthesis of depsides; the free phenolic group, however, exerts a disturbing influence when aromatic acids are acted upon by phosphorus chloride, and another group, which can subsequently be easily removed, must therefore be introduced to cover the disturbing influence referred to. For this purpose, Fischer [Footnote: _Ber_., 1908, 41, 2860.] chose the carbomethoxy group, and this investigator succeeded, by the action of chlorocarbonic alkyl ester and alkali upon hydroxybenzoic acid in cold aqueous solution, in obtaining substances with the properties required. [Footnote: _Ber._, 1908, 41, 2875.] In such substances (_e.g._, salicylic acid) where the hydroxyl occupies the ortho-position to the carboxyl, complete carbomethoxylation does not take place, whereas the _m_- or _p_- positions offer no hindrance. In the case of the _o_-position, however, the action of chlorocarbonic alkyl ester is successfully assisted by the presence of dimethylaniline in an inert solvent, _e.g._, benzene.[Footnote: U.S. Pat, 1,639,174, 12, xii., 1899.] The difficulty encountered by the _o_-position is eliminated when the carboxyl is not directly linked to the benzene nucleus, _e.g._, _o_-cumaric acid. Many hydroxybenzoic acids require an excess of chlorocarbonic methyl ester, which then also, to some extent, attacks the carboxyl group; but on dissolving the product in acetone and treating it with bicarbonate the carboxyl group as such is again restored without splitting off the carbomethoxy group.[Footnote: _Ber._, 1913, 46, 2400.] In this way all hydroxybenzoic acids may be carbomethoxylated. [Footnote: _Ibid._, 1908, 41, 2877, 2881, 2882; 1909, 42, 226, 218, 223, 225; Liebig's _Ann._, 1912, 391, 357, 366; _Ber._, 1913, 46, 1145, 2390, 2400.] The carbomethoxy group is easily removed by excess of aqueous alkali in the cold, and is also partially removed when insufficient alkali is present; the latter fact is of importance in the synthesis of didepsides.

Chlorides of Carbomethoxyhydroxybenzoic Acids

The chlorides of these compounds are obtained when phosphorus pentachloride is allowed to act upon the acids, and are as a rule crystalline. For the purpose of synthesis they may be employed as follows:

1. They readily form esters with alcohols, which on subsequent saponification with alkali are converted into the esters of the free hydroxybenzoic acids.

2. The chlorides interact energetically with esters of amino-acids, and may be coupled with amino-acids in aqueous alkaline solution. On subsequently removing the carbo-methoxy group derivatives of hydroxybenzoic acids are obtained, _e.g._,

CH_3.CO_2.O.C_6H_4.CO.Cl + 2NH_2CH_2.CO.C_2H_5 = NH_2.CH_2.CO_2.C_2H_5 + HCl + CH_3.CO_2.O.C_6H_4 CO.NH.CH_2CO_2C_2H_5. CH_3.CO_2.O.C_6H_4.CO.NH.CH_2.CO_2.C_2H_5 + 3NaOH = Na_2CO_3 + C_2H_5OH + CH_3OH + HO.C_6H_4.CO.NH.CH_2.COONa.

3. In the presence of AlCl_3 the chlorides easily combine with benzene, and on removing the carbomethoxy group unsymmetrical hydroxy derivatives of benzophenone are formed:--

CH_3.CO_2.O.C_6H_4.CO.Cl + C_6H_6 = CH_3.CO_2.O.C_6H_4.CO.C_6H_5 + HCl CH_3.CO_2.O.C_6H_4.CO.C_6H_5 + 3NaOH = NaO.C_6H_4.CO.C_6H_5 + Na_3CO_3 + CH_3OH + H_2O

4. The chlorides may be coupled with free hydroxybenzoic acids, and on removing the carbomethoxy group didepsides are obtained. Repetition of these operations yields tri- and tetradepsides.

Preparation of Didepsides

A simple application of these syntheses is offered by _p_-hydroxybenzoic acid. When the chloride of its carbomethoxy derivative is allowed to interact with _p_-hydroxybenzoic acid in aqueous alkaline solution, in the cold, the alkali salt of carbomethoxy-_p_-hydroxybenzoic acid is formed:--[Footnote 1: _Ber._, 1909, 42, 216.]

CH_3.CO_2.O.C_6H_4.CO.Cl + NaO.C_6H_4.COONa = CH_3.CO_2.O.C_6H_4.CO_2.C_6H_4.CO_2.Na + NaCl.

Being sparingly soluble, the salt in this case is readily deposited as crystals, but is readily converted into the free acid by hydrochloric acid. In most other cases, however, the alkali salts are easily soluble and the aqueous solution is then directly acidified with a mineral acid. The chlorides, being for the most part solids, the mode of procedure is as follows:--the hydroxybenzoic acid required for coupling is dissolved in normal or double-normal alkali (the volume calculated per molecule acid), a little acetone added, and the mixture well cooled; a further molecule of 2N caustic soda and the chloride (I molecule) dissolved in dry acetone are added in small portions, whilst stirring, to the mixture. In spite of the low temperature the coupling proceeds quickly and the sparingly soluble product can in most cases be precipitated from the solution by acidifying and diluting with water. In case of more easily soluble coupling products the acetone is driven off under reduced pressure or the liquid acidified and diluted, and the substance extracted with ether. Instead of alkali, dimethylaniline may be employed, with the exclusion of water as a solvent for the purpose of coupling.

Another suitable method of obtaining _o_-didepsides is that of treating _o_-hydroxybenzoic acids with phosphorus trichloride and dimethylaniline (_e.g_., synthesis of disalicylic acid, Boehringer & Sons).[Footnote: Ger. Pat., 211,403.]

The carbomethoxy derivatives of the depsides are as a rule crystalline substances of distinct acidic character, and decompose alkaline carbonates.

The elimination of the carbomethoxy group may be brought about by dilute alkaline solutions in the cold, or by aqueous ammonia. If the depside formed is so stable as to resist the action of alkali for several hours, the use of the latter is very convenient for the purpose required. The substance is dissolved directly in sufficient normal alkali to neutralise the carboxyl group and a further 2 molecules of caustic soda for each carbomethoxy group to be eliminated are added. The temperature should be about 20° C., when the reaction as a rule is completed after one-half to three-quarters of an hour. It is usual, however, to use an aqueous ammonia solution in considerable excess, whereby the temperature should again be about 20° C., and the solution of ammonia normal or half normal.

The didepsides so far investigated are crystalline bodies, sparingly soluble in cold water; they--as a rule--decompose when fused, possess acid reaction, and are dissolved by bicarbonates. On account of the presence of a free phenolic group they give a coloration with ferric chloride; if the phenolic group occupies the _o_-position to carboxyl, the coloration with ferric chloride is red or bluish-violet Excess of dilute alkali resolved all didepsides into their components at ordinary temperatures. The didepsides of gallic, proto-catechuic, gentisinic, and [Greek: b]-resorcylic acids precipitate gelatine and quinine acetate, and in this respect approach the natural tannins.

The following summary gives an account of depsides which have been prepared synthetically or which occur naturally:--[Footnote 1: _Ber._, 1908, 41, 2888; 1909, 42, 217; 1913, 45, 2718; 1913, 46, 1130, 2396, 1141, 1143; Liebig's _Ann._, 384, 230, 233, 238; 391, 356, 362.]

Di-_p_-hydroxybenzoic acid. Di-_m_-hydroxybenzoic acid. Disalicylic acid. Diprotocatechuic acid. Digentisinic acid. Di-[Greek: b]-resorcylic acid. _p_-Diorsellic acid. _o_-Diorsellic acid. _m_-Digallic acid. Disyringic acid. Di-_o_-cumaric acid. Diferulic acid. Di-[Greek: b]-hydroxynaphthoic acid. _p_-Hydroxybenzoyl-_m_-hydroxybenzoic acid. _m_-Hydroxybenzoyl-_p_-hydroxybenzoic acid. Salicyl-_p_-hydroxybenzoic acid, Vanilloyl-_p_-hydroxybenzoic acid. Feruloyl-_p_-hydroxybenzoic acid. [Greek: a]-Hydroxynaphthoyl-_p_-hydroxybenzoic acid. Orsellinoyl-_p_-hydroxybenzoic acid. Protocatechuyl-_p_-hydroxybenzoic acid. Galloyl-_p_-hydroxybenzoic acid. Pyrogallolcarboy _p_-hydroxybenzoic acid. Syringoyl-_p_-hydroxybenzoic acid. _p_-Hydroxybenzoyl-syringic acid. Pentamethyl-_m_-digallic acid. Pentamethyl-_p_-digallic acid. Vanilloyl vanillin.

Preparation of Tridepsides

Monohydroxybenzoic acids allow theoretically of tri-depsides of the type HO.C_6H_4COO.C_6H_4.COO.C_6H_4.COOH only; if, on the other hand, di- or trihydroxybenzoic acids are dealt with, two formulae are possible, viz.:--

HO.C_6H_4.COO } C_6H_3.COOH HO.C_6H_4.COO

Of the former type, two compounds are known, _i.e._, di-_p_-hydroxybenzoyl-_p_-hydroxybenzoic acid and vanilloyl-_p_-hydroxybenzoyl-_p_-hydroxybenzoic acid--

HO } C_6H_3.COO.C_6H_4.COO.C_6H_4.COOH CH_3O

The first named of these two compounds was obtained by Klepl, in addition to the didepside, by heating _p_-hydroxybenzoic acid. Fischer and Freudenberg obtained a beautifully crystalline form in the following way: carbethoxyhydroxy-benzoyl chloride was coupled with _p_-hydroxybenzoyl-_p_-hydroxybenzoic acid in alkaline solution, the compound dissolved in a mixture of pyridine and acetone, and ammonia added for the purpose of removing the carbethoxy group. The tridepside was then obtained as long needles by re-dissolving in acetone.

Both tridepsides melt well above 200° C., are practically insoluble in water, and are but sparingly soluble in practically all organic solvents. In alcoholic solution they give colour reaction with ferric chloride similar to those given by _p_-hydroxybenzoic acids.

Preparation of Tetradepsides [Footnote: Fischer and Freudenberg, Liebig's _Ann._, 1910, 372, 32.]

Here, again, two forms are known, _e.g._, tri-_p_-hydroxybenzoyl-_p_-hydroxybenzoic acid--

HO.C_5H_4.COO.C_6H_4.COO.C_6H_4COO.C_6H_4 COOH

and vanilloyl-di-_p_-hydroxybenzoyl-_p_-hydroxybenzoic acid--

HO } C_6H_3.COO.C_6H_4.COO.C_6H_4.COO.C_6H_4.COOH CH_3O

The former has been prepared from carbethoxyhydroxy-benzoyl-_p_-hydroxybenzoyl chloride and _p_-hydroxybenzoyl-_p_-hydroxybenzoic acid in alkaline solution; the second tetradepside was prepared from carbomethoxyvanilloyl-_p_-hydroxybenzoyl chloride and _p_-hydroxybenzoyl-_p_-hydroxy-benzoic acid.

The preparation of these compounds is rendered difficult by the slight solubility of the substances and their slight affinities for entering into reaction. Both tetradepsides were obtained in crystalline form, and are but very little soluble in most organic solvents. They decompose on being fused.

Tannoid Substances of the Tannin Type

The preparation of pentagalloyl glucose has proved this compound to be nearly identical with tannin obtained from galls (_tannin_); a few other natural tannins belong to this type which Fischer terms acyl compounds of sugar with hydroxybenzoic acids. The method of preparation employed in the synthesis of pentagalloyl glucose may be easily applied to other hydroxybenzoic acids, _e.g._ penta[_p_-hydroxybenzoyl] glucose [Footnote: Fischer and Freudenberg, _Ber._, 1912, 45, 933.] was prepared in this way. Similar characteristics are exhibited by pentasalicylo glucose. Mention must also be made of the corresponding derivative of pyruvic acid and the compound with pyrogallolcarboxylic acid, penta-[pyrogallolcarboyl]glucose. [Footnote: Fischer and Rapoport, _Ber._, 1913, 46, 2397.] The latter is isomeric with pentagalloyl glucose and possesses similar properties; there is, however, a vast difference in the solubility of the two. Whereas the galloyl compound is easily soluble in cold water, its isomer is hardly soluble in hot, and completely insoluble in cold water. Considering the very similar structure of these two tannins, such differences appear surprising, but an analogy may be readily found in the existence of colloidal solutions of tannin and the (nearly) identical pentagalloyl glucose. These properties clearly show how dependent is the colloidal state on small differences in the structure of two substances. On the other hand, the formation of hydrosols is of the greatest importance relatively to the part played by these substances in Nature as well as relating to their chemical characteristics; thus it is extremely difficult to make a solution of penta-[pyrogallolcarboyl]-glucose, at the same time ascertaining its astringent taste and its property of precipitating gelatine.

The experience gained by the methyl glucosides makes it exceedingly probable that the simpler polyhydric alcohols also are suitable substances to employ in these syntheses; as a matter of fact, glycerol has been condensed with gallic acid. [Footnote: Fischer and Freudenberg, _Ber., 1912, 45, 935.]

One of the chief characteristics of synthetic tannins is their high molecular weight; for instance, the molecular weight of penta-[tricarbomethoxygalloyl]-glucose is 1,810, that of penta-[pentamethyl-_m_-digalloyl]-glucose 2,051. Employing gallic acid derivatives, especially the tribenzoyl compounds, coupled with glucose, _e.g._, mannite, yielded a neutral ester of molecular weight 2,967.

The determination of the elementary composition of compounds of high molecular weight is greatly facilitated by employing their halogen derivatives; so, for instance, is _p_ iodophenyl maltosazone very suitable. Coupling the latter with tribenzoylgalloyl chloride yielded hepta-[tribenzoyl-galloyl]-_p_-iodophenyl maltosazone, the structure of which is represented by--

CH:N_2H.C_6H_4I | C:N_2H.C_6H_4I | CH.O.R R = CO.C_6H_2(O.CO.C_6H_6)_2 | CH.O.R | CH.O.R R R R R | O O O O | | | | | CH_2.O.CH.CH.CH.CH.CH.CH_2 | | ---O---

The molecular weight of this substance is 4,021, and probably represents the highest molecular organic body obtained in any chemical synthesis.

From a physiological standpoint the recognition of tannins as esters of glucose and hydroxybenzoic acids, possessing characteristics similar to those of tannin, is of great importance. Especially interesting appears the fact of plants utilising sugars for the esterification of acids, just as glycerol or monohydric alcohols may be employed for the same purpose. Free acids, as a rule, are only tolerated in certain parts of the organism, the latter usually striving to neutralise acidic groups which may be brought about by salt formation; formation of amino compounds (proteins) or esterification (fats); and, lastly, esterformation by means of sugars.

Why Nature should always build up substances of very complex constitution can only be explained by biochemical investigations, but it may, at any rate, be assumed that by this means any substance poisonous to the living organism is rendered inactive. The function of the tannins present in plants may thus be explained; if, for instance, phenols are formed by the oxidation of corresponding sugars, [Footnote: Mielke, "Ueber die Stellung der Gerbstoffe im Stoffwechsel der Pflanzen" (Hamburg, 1893).] the poisonous character of the former would be lessened by the introduction of the carbonic acid esters and subsequent coupling of the substances (depside formation). The depsides thus formed would serve as vehicle of the sugars and transport the migrating tannins, [Footnote: Kraus, "Grundlinien zu einer Physiologie der Gerbstoffe" (1889).] and, after subsequent deposition of the sugars, would then be eliminated from the plant organism, either by oxidation into ellagic acid and phlobaphenes or by condensation with the formation of cork.

Diagrammatically, the following would represent the physiology of the tannins:--[Footnote: Nierenstein, "Chemie der Gerbstoffe" (Stuttgart, 1910).]

Sugar-->Phenol-->Hydroxybenzoic Acid-->Depside-->

|Phlobaphene -->Migrating Depside-->Glucoside-->Free Depside-->-{Ellagic Acid |Cork.

SECTION II

SYNTHESIS OF TANNING MATTERS

1. AROMATIC SULPHONIC ACIDS

In organic chemistry distinction is made between sulphonic acids of the aliphatic and the aromatic series, the characteristic group of these acids being the so-called _sulphonic acid group_, HSO_3.

When sulphides or mercaptans in glacial acetic acid solution are heated with permanganate, the resulting sulphonic acid compounds exhibit great similarity to compounds containing free carboxyl groups. The sulphonic acid group may also be directly introduced either by concentrated, or by fuming sulphuric acid, or by elimination of halogen by the action of sodium or silver sulphite on the halogen derivatives of the aliphatic compounds. Saturated hydrocarbons do not react with sulphur trioxide, but unsaturated hydrocarbons are readily attacked by SO_3. Similarly, halogenated compounds and alcohols react with concentrated or fuming sulphuric acid forming sulphonic and hydrosulphonic acids respectively. The aromatic compounds form, as a rule, sulphonic acids with much greater facility. Benzene, for instance, is easily converted into the _m_-disulphonic acid by gently heating with fuming sulphuric acid; stronger heating converts the _m_- into the _p_-disulphonic acid, and at 190° C. the trisulphonic acid is formed. Toluene treated with fuming sulphuric acid first yields _o_- and _p_-sulphonic acids, finally _o_- and _p_-disulphonic acids, ethylbenzene at the boiling point _p_-ethylbenzene-sulphonic acid. Of the three isomeric xylenes _o_- and _m_-xylene dissolve in concentrated, _p_-xylene in fuming sulphuric acid only.

The action of sulphuric acid on naphthalene is stronger even than on benzene. Equal parts of naphthalene and sulphuric acid heated to 100° C. yield 80 per cent. [Greek: a] and 20 per cent. [Greek: b]-monosulphonic acid. At 160°-170°C. 25 per cent [Greek: a]- and 75 per cent. [Greek: b]-sulphonic acid is formed, and at higher temperatures [Greek: b]-monosulphonic acid only. If, on the other hand, 8 parts of naphthalene are heated with 3 parts of concentrated sulphuric acid to 180° C., two different naphthyldisulphonic acids are obtained.

Complete solution of the substance in sulphuric acid is, generally speaking, a criterion of complete sulphonation. A completely sulphonated compound should remain clear on dilution with water, or, in case precipitation occurs, the precipitate should be completely soluble in alkali or ammonia. It is necessary to submit the product to this test, since many organic substances are soluble in concentrated sulphuric acid without undergoing any alteration in composition.

Phosphoruspentoxide or potassium sulphate considerably increase the sulphonating property exhibited by fuming sulphuric acid.

The separation of the sulphonic acids from sulphuric acid is effected by salting out the former with common salt, or by removing the sulphuric acid with calcium, barium, or lead salts, provided that the sulphonic acid salts of these metals are soluble in water.

The sulphonic acid, in its chemically pure state, is best obtained from its crystalline barium salts, which are decomposed with the equivalent of sulphuric acid; another way is to decompose the calcium salts of the sulphonic acids with oxalic acid. The sulphonic acids are frequently hygroscopic and are easily soluble in water; the majority of their barium and lead salts are also soluble in water. The sulphonic acids are insoluble in ether. The halogens do not easily react with sulphonic acids, but when they do they usually replace the sulphonic acid group. In order to prepare the halogen substitution products, therefore, use is made of sulphonic chlorides. The latter are obtained by the action of chlorosulphonic acid on aromatic hydrocarbons; a simpler method, however, is to treat the dry alkali sulphonates with phosphorus pentachloride--

C_6H_5SO_3Na + PCl_5 = C_6H_5SO_2.Cl + NaCl + POCl_3

Derivatives of sulphonic chlorides are sulphonamides, which are easily prepared from the former by grinding with ammonium carbonate--

C_6H_5SO_2.Cl + (NH_4)_2CO_3 = C_6H_5.SO_2.NH_2 + NH_4Cl + CO_2 + H_2O

Sulphonic chlorides react with alkaline sulphides to form thiosulphonic acids--

C_6H_5SO_2.Cl + K_2S = C_6H_5SO_2.SK + KCl

Sulphonic chlorides, dissolved in ether, yield sulphinic acids on reduction with zinc dust or metallic sodium--

C_6H_5SO_2.Cl + H_2 = C_6H_5SO_2.H + HCl

In the sulphonic acid compounds it is assumed that the sulphur is hexavalent, and it is hence possible to consider the sulphones to be esters of sulphinic acid.

==O R--S==O --H

The sulphones are mostly solid bodies, which soften prior to melting when heated. They are very stable towards chemical reagents; for instance, saponification of a mono-sulphone very rarely yields sulphinic acid.

If a hydroxyl is substituted for a hydrogen atom in the aromatic hydrocarbons, the action of sulphuric acid is greatly facilitated; thus, by merely mixing phenol with sulphuric acid, the sulphonic acid is at once formed, whereby, in the cold, _o_-phenolsulphonic acid prevails which on heating for some time to 100°-110° C. is completely converted into _p_-phenolsulphonic acid. In the absence of free sulphuric acid the conversion of _o_- into _p_-phenolsulphonic acid is brought about by heating the aqueous solution. Phenol-2,4-disulphonic acid is prepared from _o_- or _p_-phenolsulphonic acid, whereas phenol-2,4,6-trisulphonic acid is prepared directly from phenol by heating with concentrated sulphuric acid in presence of phosphorus pentoxide. Phenolsulphonic acids are also obtained by fusing benzenedisulphonic acid with alkali.

Cresol is not so easily sulphonated as is phenol; _o_-cresol when heated eight to ten hours at 90° C. with one and one-half times its weight of concentrated sulphuric acid, yields _o_-cresol-_p_-sulphonic acid.

The phenolsulphonic acids are strong, rather stable acids; their alcoholic hydroxyl-hydrogen atom may, similarly to that of the phenols, be substituted by a metal or an alkyl radical.

From [Greek: a]- and [Greek: b]-naphthol a number of sulphonic acids may easily be prepared; viz., mono-, di-, and trisulphonic acids. Nearly all these acids are important as basic materials in the dyestuff industry, especially 2,6-[Greek: b]-naphtholmonosulphonic acid (S-acid), 2,3,6-[Greek: b]-naphtholdisulphonic acid (R-acid) and 2,6,8-[Greek: b]-naphtholdisulphonic acid (G-acid).

2. Condensation of Phenols

Phenolsulphonic acids exhibit pronounced tendencies to condensation, for which purpose A. v. Baeyer (1872) employed aldehydes. The reaction is rather violent, and yields, in addition to well-defined crystalline substances, amorphous bodies resembling rosins. In addition to formaldehyde, paraformaldehyde, trioxymethylene, methylal, hexamethylene-tetramine, and other substances containing a reactive methylene group, as well as acetaldehyde, benzaldehyde and other aldehydes may be employed to induce reaction.

A number of these condensation products are derivatives of diphenylamine or hydroxybenzyl alcohols. When the latter are heated, either by themselves or in presence of acids, anhydrides and polymerisation products are formed producing hard, brittle, fusible substances, insoluble in water but fairly soluble in organic solvents. The same substances are formed when phenols are condensed with formaldehyde, especially in the presence of acid contact substances and excess of phenol by sufficiently long heating at certain temperatures. The substances referred to are termed "Novolak": similar to these are the so-called "Resols," insoluble and non-fusible substances, very resistant to chemical and physical action. Another member of the series is the so-called "Bakelite" or "Resitol," which does not fuse but softens when heated and swells in organic solvents. The ultimate product of this class of substances is "Resit" which is obtained when concentrated hydrochloric acid is allowed to act upon a mixture of phenol and formaldehyde; the temperature rises spontaneously, and a hard, porous, insoluble mass of great resistance is obtained. By heating resols, resitols are formed which, on further heating, are finally converted into resits. [Footnote: _Ber.,_ 1892, 25, 3213.]

Of all these products, bakelite (resitol) has found the greatest industrial application; in its purest form, this substance is a nearly colourless or light yellow body of sp. gr. 1.25 and, being a poor conductor of heat and electricity, constitutes an excellent insulating material; it is exceedingly resistant towards most chemical reagents even in concentrated forms of the latter. Its pronounced refractivity, and the ease with which it may be worked, makes bakelite a favourite substitute for amber (Ger. Pat, 286, 568). Similarly, the resols which can be easily moulded are used either as such or mixed with sand, pulverised cork, asbestos or wood, and the moulded substances then converted into the more highly resistant bakelite by heating.

The constitution of these bodies no doubt depends largely on their method of preparation; Baekeland [Footnote: _Chem. Ztg.,_ 1913, 73, 733.] considers resit a polymerised hydroxybenzylmethylene glycol anhydride; Raschig, a diphenylmethane derivative (e.g., dihydroxydiphenylmethane alcohol); Wohl [Footnote: _Ber.,_ 1912, 45, 2046.] considers them polymerisation products of methylene derivatives of tautomeric phenol.

CH===CH H_2C:C{ }CO CH===CH [Note: Lower Right CH has double bond to CO]

This group possesses the characteristic property of being capable of converting animal hide into leather when suitably dissolved. The author has dissolved a number of these water-insoluble condensation products in alkali and alcohol and was able to demonstrate their tanning effects on pelt; bakelite is easily soluble in alkali; a faintly alkaline solution partially precipitates gelatine, and completely so when the alkali is neutralised. This latter solution gives a dirty brown precipitate with iron salts.

These condensation products gained extraordinary importance for the tanning trade when Stiasny [Footnote: Ger. Pat, 262,558; Austr. Pat, 58,405.] succeeded in preparing them in water-soluble form when they are enabled to directly exert their tannoid properties. This may be done by acting upon two molecules of concentrated phenolsulphonic acid with one molecule of formaldehyde, the temperature thereby not exceeding 35°C. By condensation, however, considerable heat is liberated, and hence the rise in temperature can only be limited by adding the diluted formaldehyde drop by drop, whilst stirring and cooling, to the phenolsulphonic acid. The original letters patent is worded as follows: 10 kilos each of crude phenol and sulphuric acid (66° Bé.) are heated with stirring for two hours at 105°-106°C., cooled to about 35°C., and 463 kilos 30 per cent. formaldehyde added during three hours, the temperature thereby not exceeding 35°C.; the stirring is continued for a couple of hours after the final addition of formaldehyde. This yields about 24 kilos of the crude condensation product. On a commercial scale, however, cresol (cresylic acid) is substituted for phenol. There are three isomers of cresol, viz., _o_-, _m_-, and _p_-cresol, and it was naturally of interest to investigate whether one or the other of the isomers exerted any particular influence on the properties of the final product. It was found, however, that condensation products from the three isomers were distinguishable from one another neither in physical nor in tannoid properties. It is hence possible to employ crude cresol, which contains varying quantities of the _o_-, _m_-, and _p_-compounds, in the manufacture of these tanning matters. [Footnote: Gen Pat, 291,457.]

The tar obtained from the Rochling coal-gas generator contains considerable quantities of phenols (B.P.=200°-250°C.), and the author has protected the use of these for the production of synthetic tannins by Ger. Pat, 262,558. A deep brown viscous mass is obtained which, when partly neutralised, yields similar results to those given by the product above referred to.

It may be anticipated that by analogy from the chemical reactions taking place in the condensation of phenols on the one hand and cresolsulphonic acid on the other, that all other homologues of phenol, its polyvalent derivatives, substitution products and acids, would yield similar condensation products.

The particular position occupied by the aromatic hydroxy compounds in the chemistry of substance possessing tannoid character is not only evidenced by the natural classification of the tannins, tannin derivatives, and decomposition products so far isolated and investigated, but also by other chemical behaviour shown by these substances. Meunier and Seyewetz [Footnote:_Collegium_, 1908, 315, 195.], for example, were able to show that phenol, _p_-aminophenol, chlorophenol, trinitrophenol, catechol, resorcinol, hydroquinone, monochlorohydroquinone, orcinol, pyrogallol, and gallotannic acid precipitate gelatine from its aqueous solution, that is, to a certain extent possess tanning properties.

The author has extended this series somewhat and obtained the following results:--

Relative Behaviour Towards Substances Gelatine. Hide Powder. Pelt. Tribromophenol Slight ppte. Tans Surface tannage [Footnote: In alcoholic solution] _o_-Nitrophenol No ppte. " " Br-_o_-Nitrophenol Slight ppte. " " Tribromopyrogallic Ppte. " " acid Bromophloroglucinol " " No tannage Galloflavine Slight ppte. " " Bromosalicylic acid " " " Bromo-[Greek: b] " " Tans -naphthol [Footnote: In alcoholic solution] Rosolic acid " " " [Footnote: In alcoholic solution] Gallic acid No ppte. No tannage No tannage

By the condensation of their sulphonic acids, it may be demonstrated experimentally how the tannoid properties of nearly every member of the series are intensified. Investigattion in this direction, however, has not been systematically undertaken, for which reason the author determined to examine this subject; but the enormous number of samples required, obtainable only with great difficulty during the war, made it impossible to conclude completely the researches in this field. What little has so far been done relatively to this subject should, when collected, indicate the way to be pursued in this wide field of investigation. What follows will hence comprise the conversion of a few of the most important members of this series of substances into their methylene-condensation products with a brief discussion of the qualitative and tannoid reactions of the latter.

The didepside of phenolsulphonic acid is obtained by condensing carbomethoxyphenolsulphonic chloride with sodium phenolsulphonate in the presence of the calculated amount of caustic soda. A product of the composition

CH_3.0.COO.C_6H_4SO_2.0.C_6H_4.SO_3Na

is first obtained, which on saponification with soda yields the pure didepside--

HO.C_6H_4.SO_2.C_6H_4.SO_3.Na

By acidifying the concentrated solution the didepside is obtained as a white crystalline substance; a solution of which precipitates gelatine without, however, exhibiting any tanning effect upon animal hide. If, on the other hand, the above ester is converted into the chloride

CH_3O.COO.C_4H_4SO_2.O.C_6H_4.SO_2Cl

by treatment with PCl_5, and the chloride thus obtained further condensed with sodium phenolsulphonate, saponified, and the solution acidified, the pure tridepside

HO.C_6H_4.SO_2.O.C_6H_4.SO_2.O.C_6H_4.SO_3Na

is precipitated as white crystalline needles which not only precipitate gelatine, but are capable of converting animal hide into leather.[Footnote: _Chem. Ztg._, 1919, 43, 318.]

Of the class of hydroxy-cymenes _thymol_,

C_6H_3.CH_3.C_3H_7OH,

was converted into the water-soluble sulphonic acid by warming with concentrated sulphuric acid at 50° C., the sulphonic acid being subsequently easily condensed with formaldehyde by slightly heating the mixture. The condensation product thus obtained is a viscous brown mass which is easily soluble in water, precipitates gelatine completely, gives a bluish-black coloration with iron salts, and gives a precipitate with aniline hydrochloride. To investigate its tannoid properties, the mixture was brought to the acidity 1 gm = 10 c.c. N/10 NaOH and a piece of bated calf skin was then introduced into a solution measuring about 2° Bé. After eighteen hours the pelt was nearly tanned through, and a further twenty-four hours completed the tanning process, after which a light fat-liquor was given. The dried leather was brownish-grey in colour, possessed soft and full feel and good tensile strength.

On account of their importance, the three dihydroxybenzenes were examined with a view to test their suitability for conversion into tannoid substances.

_o_-Dihydroxybenzene, catechol, yields a sulphonic acid easily soluble in water, which on the careful addition of formaldehyde assumes a blue colour. The compound thus obtained may be heated to 100° C., without depositing insolubles. A further addition of formaldehyde, however, results in the formation of a considerable quantity of insolubles whilst the liquid assumes a brown coloration. If, on the other hand, the sulphonic acid is diluted with twice its volume of water, formaldehyde added and the mixture heated on the water bath, the liquid immediately turns brown, the formaldehyde is completely fixed, and a condensation product soluble in water results. The latter gives a brownish-black coloration with ferric chloride, completely precipitates gelatine, but gives no opalescence with aniline hydrochloride. Tanning experiments with the partly neutralised (1 gm.= 10 c.c. N/10 NaOH) substance resulted in both grain and flesh being tanned with a black colour, whereas the interior of the pelt was pickled (white colour). After a further forty-eight hours, however, the black colour penetrated the pelt, and tannage was complete. The washed and lightly fat-liquored leather was soft, of full feel and good tensile strength, and was greyish coloured throughout.

With regard to the black colour possessed by leathers tanned with synthetic tannins the following should be noted. When sulphonating and especially when condensing substances, black dyestuffs or very finely divided carbon in the colloidal state are often formed. Such a substance does not deposit the black particles, even when filtered through kaolin, and hence convert pelt into leather possessing black colour on the surface. The hide in this case acts as a perfect filtration medium, whereby the surface layers retaining the coloured particles assume their colour; thus only the pure tanning matter enters into the interior, which then, according to the composition of the former, imparts a colour varying from white to light brown to the inner layers.

_m_-Dihydroxybenzene, resorcinol, is also easily sulphonated by concentrated sulphuric acid, the brownish-coloured sulphonic acid being easily soluble in water. If the sulphonic acid is diluted with three times its volume of water, cooled down, a few drops of formaldehyde added and the mixture heated on the water bath to completely fix the formaldehyde, and this process repeated till no more formaldehyde is taken up, a brown water-soluble condensation product results, the aqueous solution of which precipitates gelatine completely, aniline hydrochloride only partly and which gives a deep blue colour with ferric chloride.

A piece of calf skin immersed in a solution of the partly neutralised (as above) product was tanned through in twenty-four hours; when lightly fat-liquored, the resulting leather possessed a yellowish-green colour and good tensile strength, and was soft and full.

_p_-Dihydroxybenzene, hydroquinone, was converted into the water-soluble sulphonic acid by heating it with concentrated sulphuric acid at 100° C.; the sulphonic acid, mixed with formaldehyde at ordinary temperature, immediately solidifies to a white mass, which is soluble in water and which had completely fixed the formaldehyde. If, however, this mass is heated for some time to 100°C, it assumes a light brown coloration and its solubility in water is diminished. A slight excess of formaldehyde and the application of heat result in dark violet insoluble condensation products. The aqueous solution precipitates gelatine, gives a deep blue colour with ferric chloride, but gives no precipitate with aniline hydrochloride; on the other hand, addition of potassium nitrite produces the yellow colour characteristic of hydroquinone.

The product effects a slower tannage (seven days) than the former product, when a brown, soft, but rather empty leather of good tensile strength is obtained.

Of the _trihydroxybenzenes_ pyrogallol and phloroglucinol only were included in these investigations.

When pyrogallol is sulphonated with concentrated sulphuric acid a violet-coloured sulphonic acid, soluble in water, is obtained, which, when treated with formaldehyde first in the cold and then when heated, yields a solid deep red-coloured mass, which precipitates gelatine but not aniline hydrochloride, and gives a blackish-brown colour with ferric chloride. The partly neutralised substance in aqueous solution tans pelt in twenty-four hours with black colour on the surface only, the intermediary layer being pickled (white colour) only, but the black-coloured tanning matter ultimately penetrates the pelt, which tanned through in seven days. The resultant leather is coloured black throughout, is full, soft, and possesses good tensile strength.

Sulphonation of phloroglucinol succeeds at higher temperatures only, the sulphonic acid being a solid which is scarcely soluble in water, the latter then assuming a wine-red colour. The condensation product--prepared as described for resorcinol, but requiring higher temperature--is a brick-red powder, insoluble in water.

The same end-product also seems to be obtained by simply heating the sulphonic acid at a higher temperature; this also induces condensation with the formation of a reddish-brown mass insoluble in water. It is, of course, impossible to attempt any tanning experiments with this product in aqueous solution; attempts at dissolving the condensation product in alcohol proved barren of result, since only traces of impurities accompanying the substance dissolved, imparting a light reddish-brown colour to the solution. In highly concentrated alcohol, however, the condensation product is somewhat soluble, yielding a reddish-brown solution. A piece of pelt introduced into the alcoholic solution was surface tanned only after forty-eight hours, leaving the remainder of the pelt pickled; extending the experiment over a further four days produced no change in the pelt. The latter was therefore rinsed with water, lightly fat-liquored and dried, when a soft but empty leather of grey colour and good tensile strength was obtained. It appears, therefore, to be a case of pseudo-tannage, where an infinitesimal amount of synthetic tannin produces a tanning effect without, however, a true tannage being effected.

The Elberfelder Farbenfabriken have protected the use of the condensation products of di- and polyhydroxybenzenes by Ger. Pat., 282,313; owing to the high cost of the latter substances, however, it is doubtful whether synthetic tannins prepared from these materials would not be too expensive for any other than pharmaceutical purposes.

Before leaving the phenols, mention must be made of the quinones, the use of which for tanning purposes was first protected by Ger. Pat., 206,957 (30th April 1907). According to this patent, only 400 gm. of quinone are required for the conversion into leather of 400 kilos pelt, drum tannage being preferable. During the process the leather first assumes a reddish colour, changing through violet to brown; its resistance to water, acids, and alkalies is said to be considerably greater than that exhibited by all other kinds of leather.

The chemistry of the quinone tannage has been investigated, and an explanation given by Thuau [Footnote: _Collegium_, 1909, 363, 211.] assumes a reaction between the quinone and the amino groups of the hide protein with the formation of hydroquinone--

+-O OH | | | 2R.NH_2 + 2C_8H_4 | = C_6H_4 + C_6H_4(O.NH.R)_2 | | | +-O OH (Pelt.) (Leather.)

Fahrion has shown that, during the tanning process, the quinone loses its active oxygen, and this can only be brought about by the amino group of the hide protein, the amino group only being capable of effecting reduction of the quinone. An analogy is here offered by dianilinoquinone. A spent quinone liquor contains considerable amounts of hydroquinone. The tannage may also be effected by exposing pelt saturated with hydroquinone to oxidation by the air. The pelt, which is unaltered by the hydroquinone bath, on being removed from the latter, and in the presence of alkali, assumes a red colour at first, which changes into violet, blue, and finally brown, the pelt being thereby converted into a quinone-tanned leather.

It may be noted that quinone only effects pseudo-tannage; quinone mixed with water deposits, in time, a black amorphous substance practically insoluble in water. This substance is easily adsorbed by hide powder, but is not capable of converting the latter into that insoluble form into which it is converted by the natural tannins.

Amongst polyhydric alcohols, the behaviour of the methyl ester of catechol, _guaiacol_ was investigated. The sulphonic acid was prepared by heating guaiacol with concentrated sulphuric acid, the resulting water-soluble product possessing a light, brownish-green colour. On condensing the sulphonic acid with formaldehyde, the same precautions were observed as in the case of resorcinol, but complete fixation of the formaldehyde could only be obtained by finally heating the product for a short time over a free flame, at about 105° C. Condensation was indicated by the brownish appearance of the liquid. No insoluble products were formed. The condensation product easily dissolves in water, the solution assuming a rich brown colour and exhibiting the following reactions: gelatine is completely precipitated, aniline hydrochloride produces opalescence, and ferric chloride a deep brown coloration.

Tannage, with the partly neutralised product, was rapid, the pelt being nearly tanned through in twenty-four hours, excepting a small white streak in the middle; after a further twenty-four hours this streak had vanished, and the completely tanned, dark grey-coloured leather, after washing, fat-liquoring, and drying, was soft, full, and of good tensile strength, very similar to the leather yielded by the catechol-condensation product.

Of the nitro-compounds, trinitrophenol, C_6H_2(NO_2)_3OH (picric acid), was investigated. If a concentrated solution of picric acid is brought into contact with pelt it will penetrate the latter completely in a few days; it is, however, difficult to fat-liquor the resultant leather, since the fat is absorbed only with difficulty. If a pelt treated in this way be dried, a soft but rather flat leather results, the colour of which easily rubs off, the leather also tasting intensely bitter. These disagreeable qualities prevent a general use of this material for tanning purposes; in spite of them, however, picric acid, in admixture with boracic acid, salicylic acid, and glycerol, is used in the production of the so-called transparent leather. The latter is very flexible and possesses great tensile strength, but loses the latter quality when exposed to heat, and, when stored, also loses its flexibility. By simply washing with water, the leather is reconverted into pelt.

When picric acid is treated with hot sulphuric acid and formaldehyde gradually added, a dark coloured water-soluble condensation product is formed which strongly precipitates gelatine. Exposed to the action of bromine, the condensation product yields a mass which is insoluble in water.

Experience has taught that the amino bodies--the basic N-derivatives of the phenols--do not yield substances possessing tannoid properties on condensation. On account of their importance, however, a few have been included in this series of investigations.

Aminobenzene, C_6H_5NH_2, aniline, treated with sulphuric acid, yields the water-soluble aniline sulphate, which, on cautious addition of formaldehyde, yields a reddish-coloured gel, insoluble in water, in addition to a small volume of a reddish-yellow liquid. The latter precipitates gelatine, but is not capable of converting pelt into leather. The insoluble gel is likewise insoluble in alcohol, so that tanning experiments with this substance are excluded.

Dimethylaniline, C_6H_5N(CH_3)_2, when treated with sulphuric acid, yields a product soluble in water which neither reacts with nor fixes formaldehyde. Hence the substance does not precipitate gelatine.

If, on the other hand, nitrosodimethylaniline,

NO | C_6H_4 | (CH_3)_2

is sulphonated, and the water-soluble sulphonation product heated with formaldehyde for some time, the product remains soluble in water and precipitates gelatine. No tanning effect could, however, be detected.

Arylsulphaminoarylsulphonic acids and arylsulphoxyarylsulphonic acids precipitate gelatine but are devoid of tannoid character. The latter is acquired by compounds belonging to this class containing two or more sulphamino groups, or when they, in addition to one sulphamino group, contain a sulphoxy group and another sulphonic group. According to Ger. Pat., 297,187 (Society oc Chemical Industry, Basle), such compounds are obtained when, for instance, sodium sulphanilide in alkaline solution acts upon nitrotoluenesulphochloride, and the resulting nitrotoluenesulphamino compound is subsequently reduced with acetic acid and iron. The resulting aminotoluenesulphaminobenzenesulphonic acid is finally treated with p-toluenesulphonic chloride till the latter disappears. A compound of the composition

-----NH-----SO_2----- ^ ^ ^ | | | | | | | | | |---NH---| | V V V SO_2Na CH_2

is thereby obtained, which, when acidified, is readily capable of being used for tanning purposes.

The intermediary product of the aminotoluenesulphaminobenzenesulphonic acid obtained by this process may again be employed for the purpose of reacting with one-half molecule soda and 1 molecule nitrotoluenesulphonic chloride. The following compound is obtained--

---NH---SO_2--- ---NH---SO_2--- ^ ^ ^ CH_3 ^ | | | | | | | | | | | | | | | | v v ---NH---SO_2--- v v SO_3Na CH_3 CH_3

If _p_-toluenesulphaminobenzenesulphonic chloride is condensed with sodium sulphanilide, a compound,

---SO_2---NH--- NaSO_3 ^ ^ ^ | | | | | | | | | | | | v v ---SO_2---NH--- v SO_3Na

is obtained which, when acidified, exhibits tannoid properties.

On condensing sodium phenolsulphonate with nitrotoluenesulphonic chloride, reducing the condensation product and condensing the latter with _p_-toluenesulphonic chloride, a compound similar to the above is obtained--

---O---SO_2--- ^ ^ ^ CH_3 | | | | | | | | | | | | v v ---NH---SO_2--- v NaSO_3 CH_3

Again, a similar product is obtained when _p_-toluenesulphaminobenzenesulphonic chloride or its homologues or isomers are condensed with sodium-_o_-cresylsulphonate--

---SO_2---NH--- SO_3Na ^ ^ ^ | | | | | | | | | | CH_3| | v v ---SO_2---O--- v CH_3

The chloride of this compound may again be condensed, for instance, with sodium aminotoluenesulphaminobenzene-sulphonate, and yields the compound--

---NH---SO_2--- ^ ^ ^ ---NH---SO_2--- ^ | | | | | | | | | | | | | | | | v v ---NH---SO_2--- v v CH NaSO_3 CH_3

The three latter compounds, when dissolved in water and the solution acidified, exert tanning action.

It is also possible to employ mixtures of arylsulphaminobenzylsulphonic acids in acidified aqueous solution for tanning purposes. According to Ger. Pat., 297,188, such mixtures are obtained by nitrating benzylchloride and heating with an equimolecular amount of sodium sulphite; the sodium nitrobenzylsulphonate thus obtained is reduced to aminobenzylsulphonic acid with iron and acetic acid, and finally condensed with the calculated amount of _p_-toluenesulphonic chloride. A mixture _o_- and _p_-toluenesulphaminobenzylsulphonic acid [Footnote 1: Cf. also Ger. Pat, 319,713 and 320,613.] thus results.

Amongst _aromatic alcohols_ the dihydric alcohols show characteristic behaviour; the latter combine with sulphonic acids with the elimination of water, condensation taking place without formaldehyde, and the resulting products being soluble in water and possessing tannoid properties. [Footnote 2: Ger. Pat., 300,567, of 20th September 1917.] In addition to phenolic mono- and disulphonic acids (and higher sulphonation compounds), the homologues, cresols, xylenols, and naphthols enter into reaction. The two components condense with great ease, liberating heat; dilute solutions (of the components) are heated to about 100° C., the process being complete in a few minutes. The products obtained are exceedingly pure and are easily crystallisable. Employing 1, respectively 2, molecules of sulphonic acid, the reactions take place according to:--

OH CH_2.OH OH OH }C_6H_4 + HO.C_6H_3{ = H_2O + }C_6H_3-CH_2-C_6H_3{ HSO_3 CH_2.OH HSO_3 CH_2.OH

OH OH CH_2.OH CH_2.C_6H_3{ }C_6H_4 + HO.C_6H_3{ = 2(H_2O) + HO.C_6H_3{ HSO_3 HSO_3 CH_2.OH | OH CH_2.C_6H_3{ HSO_3

OH CH_2.OH OH OH }C_6H_3.CH_3 + HO.C_6H_3{ = H_2O + }C_6H_2.CH_3.CH_2.C_6H_3{ HSO_3 CH_2.OH HSO_3 CH_2.OH

OH OH CH_2.OH CH_2.C_6H_2.CH_3{ }C_6H_3.CH_3 + HO.C_6H_3{ =2(H_2O) + HO.C_6H_3{ HSO_3 HSO_3 CH_2.OH | OH CH_2.C_6H_2.CH_3{ HSO_3

OH CH_2.OH OH OH }(C_10)H_6 + HO.C_6H_3{ = H_2O + }(C_10)H_5.CH_2.C_6H_3{ HSO_3 CH_2.OH HSO_3 CH_2.OH

OH OH CH_2.OH CH_2.(C_10)H_5{ }(C_10)H_6 + HO.C_6H_3{ =2(H_2O) + HO.C_6H_3{ HSO_3 HSO_3 CH_2.OH | OH CH_2.(C_10)H_5{ HSO_3

The condensation products above enumerated were tested with regard to their tanning power, both non-neutralised and partly neutralised (1:10, 1:20, and 1:30 c.c. N/10 NaOH) samples being examined. In all cases rapid tannage was observed yielding firm and soft leathers of light brown colour and varying degrees of swollenness.

Relatively to their reactions, all the products strongly precipitate gelatine, whereas only the condensation products of phenol, cresol, and xylenol derivatives give a characteristic coloration with iron salts.

The tannin contents of the non-neutralised condensation products lie between 72-80 per cent.--figures which clearly indicate the purity and efficiency of these substances.

Notable amongst _aromatic acids_ is salicylic acid, C_6H_4.OH.COOH, which at higher temperatures is easily sulphonated with concentrated sulphuric acid; the sulphonation product represents a white solid, which easily dissolves in water forming a clear liquid. The sulphonic acid, when mixed with about one-third of its weight of water and heated to about 120° C., is easily condensed with formaldehyde. Towards the end of the reaction, considerable frothing sets in, but in spite of the high temperature required by this reaction no insoluble bakelites are formed. A reddish-brown fluid is obtained easily soluble in water, to which it imparts a brown colour. An aqueous solution of the product completely precipitates gelatine, gives a strong opalescence with aniline hydrochloride and a deep violet coloration with ferric chloride. Neutralised as usual, the product, in a 3° Bé solution, converts pelt within three days into a white, full leather of good tensile strength.

This process has been patented by the Deutsch-Koloniale Gerb und Farbstoff Gesellschaft (German-Colonial Tanning and Colour Extracts Ltd.) in Karlsruhe, the letters patent also including the ring homologues of salicylic acid. Similar results are obtained when cresotinic acid (hydroxy-toluic acid), OH.C_6H_3.CH_3.COOH, is employed as base.

If the phenyl ester of salicylic acid, _Salol_,

HO.C_6H_4.CO.O.C_6H_5

is sulphonated, a product is obtained which is easily soluble in water, but which is identified as a mixture of the sulphonation products of salicylic acid and phenol, the salol being dissociated on sulphonation. The temperature must not exceed 80° C. by condensation with formaldehyde, or insoluble bakelite will be formed from the phenol; the aldehyde must also be added gradually. An aqueous solution of the partly neutralised condensation product has a pronounced tanning effect on pelt, and converts the latter into leather in one to two days; the leather being very similar to that produced by the salicylic acid condensation product. The qualitative reactions of the product in aqueous solution are the same as those given by the salicylic acid condensation product.

Salicylic acid may, however, also be condensed with formaldehyde without first being sulphonated; in this case, a little hydrochloric acid should be present. A product slightly soluble in water is obtained, which may be looked upon as being methylenedisalicylic acid. In alkaline solution it is easily soluble,

[Footnote 1: Its solubility in alcohol and alkalies renders this product an effective and cheap substitute for shellac.--_Transl._]

the liquid possessing an intensely bitter taste. The sodium salt gives a deep violet coloration with ferric chloride, a slight precipitate with gelatine, and slight opalescence with aniline hydrochloride. In contact with pelt, however, it exhibits no tanning effect, but when dissolved in alcohol, a pickling effect may be observed.

[Footnote 2: A similar reaction is observable in the case of the sodium salts of METHYLENEDISALICYLIC acid brommated or iodised, which form a clear solution varying from red to reddish-brown.]

The attempt at preparing a condensation product from sodium-_m_-hydroxybenzoate by means of formaldehyde and bisulphite is worthy of attention. A dark brown, viscous liquid is obtained which is perfectly soluble in water, and the aqueous solution of which gives opalescence with gelatine, a precipitate with aniline hydrochloride, and a bluish-black coloration with ferric chloride. Its behaviour towards pelt is very similar to that of phenolsulphonic acid, and it yields a similar leather.

A very similar condensation product was obtained by condensing sodium-_p_-hydroxybenzoate with formaldehyde and subsequent sulphonation with sulphuric acid. From a practical standpoint, however, these substances cannot be employed, since their tanning action is only effective in acid solutions of such concentration of acid as would gelatinise the pelt.3

If, on the other hand, non-condensed methane derivatives of phenol, _e.g._, hydroxyphenylmethanesulphonic acid, are partly neutralised and a solution of the product thus obtained used for tanning experiments, no tanning action is observable. The acidified solution does not precipitate gelatine, and gives a dark brown coloration only with ferric chloride.

GALLIC ACID, C_6H_2(OH)_3COOH, when heated with sulphuric acid, is easily converted into the insoluble rufigallic acid, which is also insoluble in alcohol. If, however, gallic acid is heated with an excess of sulphuric acid, the product cooled and treated with formaldehyde, a deep brown condensation product is obtained which is soluble in alcohol, and in this state is capable of converting pelt into a substance similar to leather which, though rather hard, possesses good tensile strength. This water-insoluble condensation product is also soluble in alkalies, the solution exhibiting properties similar to that described above. Gallic acid, therefore, is not a suitable base for the production of synthetic tannins soluble in water.

Phthalic acid also is difficult to sulphonate: the sulphonated compound treated with formaldehyde gives only water-insoluble condensation products.

3. Condensation Of Naphthalene Derivatives

The simplest method of condensing [Greek: b]-naphthalene-sulphonic acid is to heat it at 135° C. at a pressure of 20 mm. for several hours.[Footnote: Austr. Pat., 61,061, of 10th September 1913.] The resulting product is a cheesy mass which reacts strongly acid. By reducing the acidity of the substance to 1 gm. = 10 c.c. N/1O NaOH, a grey, cheesy mass results, which easily dissolves in water, the solution being coloured a light yellow-brown and precipitating gelatine aniline hydrochloride; no coloration, however, appears on adding ferric chloride.

The condensation of [Greek: b]-naphthalenesulphonic acid, however, proceeds with much greater energy in the presence of formaldehyde. In practice, for instance, 10 kilos of naphthalene is heated with the same weight of concentrated sulphuric acid (66° Bé), when the mixture is converted into [Greek: b]-naphthalenesulphonic acid by heating for several hours at 150°-160° C; the sulphonation completed, the sulphonic acid is cooled to about 85° C., and 4 kilos of formaldehyde (30 per cent, by weight) slowly added; finally, the product is stirred at the temperature mentioned till all formaldehyde has combined.[Footnote: Austr. Pat., 69,194, of 25th June 1915; Ger. Pat, 290,965.]

Tanning experiments with this product yielded, in a short time, a nearly white coloured leather (see later).

In addition to formaldehyde, there are other substances which induce condensation of naphthalenesulphonic acid; if, for instance, sulphur chloride is allowed to act upon [Greek: b]-naphthalenesulphonic acid, a light brown solid of pronounced acidic character is obtained; if the latter is partly neutralised with caustic soda, a greyish-brown solid results, which dissolves in water with a light brown colour, the solution precipitating gelatine and aniline hydrochloride, but giving no coloration with ferric chloride.[Footnote: Austr. Pat., 96,194.]

Tanning experiments with this product in aqueous solution gave a light brown, rather soft leather, and this, in addition to the qualitative reactions of the substance, prove that this method of condensation hardly alters the character of the product from a tanning point of view. The brown coloration imparted to the leather tanned with this condensation product owes its existence to coloured intermediary products.

Attempts at condensing chloronaphthalenesulphonic acid and nitronaphthalenesulphonic acid resulted in soluble condensation products which gave some of the reactions given by the tannins (precipitation of gelatine and aniline hydrochloride), but which were incapable of tanning pelt, a light tannage being effected on the surface only.

[Greek: a]-Naphthol dissolved in hot concentrated sulphuric acid and heated for some time on the water bath, yields the light brown, water-soluble [Greek: a]-naphtholsulphonic acid. A dilute solution of the latter, when treated with formaldehyde in the cold, undergoes no change; on heating the mixture on the water bath a brown precipitate is thrown down. If gelatine solution is added to the opaque liquid, a yellow flocculent precipitate separates. If caustic soda is added to the opaque liquid containing the condensation product described above, a clear solution results from which no deposit separates on the addition of acetic acid. Gelatine is precipitated by this solution.

The concentrated hot a-naphtholsulphonic acid, upon addition of sufficient formaldehyde, effervesces strongly and yields a dark brown condensation product insoluble in water, but soluble in caustic soda. If acetic acid is added in excess to the alkaline solution, the resultant solution strongly precipitates gelatine.

A suspension in water of the insoluble condensation product does not precipitate gelatine.

b-Naphthol, dissolved in hot concentrated sulphuric acid and heated for some time, yields the light brown, viscous b-naphtholsulphonic acid. A dilute solution of the latter, mixed with formaldehyde, remains clear; when heated on the water bath, however, it assumes a dark, reddish-yellow colour, and remains soluble in water and precipitates gelatine strongly. This condensation product, on adding excess of caustic soda, assumes a deep blue coloration, the alkaline solution giving no precipitate with gelatine; on adding acetic acid the solution turns brown, remains clear, and now precipitates gelatine.

The concentrated b-naphtholsulphonic acid heated with formaldehyde on the water bath yields as condensation product a dark, reddish-yellow mass, soluble in water, which precipitates gelatine. A dilute solution, when allowed to act upon pelt, gave in a few days a light brown leather, the properties of which are very similar to those possessed by vegetable tanned leathers.

The use of naphtholsulphonic and aminonaphtholsulphonic acids for the manufacture of synthetic tannins is protected by Ger. Pats., 293,640, 293,693, 293,042, and 303,640. [Footnote: _Cf._ Austr. Pat., 70,162.]

It is a remarkable fact that non-condensed methane derivatives of naphthol, _e.g._, b-naphthol-a-methanesulphonic acid, dissolved in water and partly neutralised, are devoid of tanning character when allowed to act upon pelt. Neither does this substance precipitate gelatine, but it does give a deep blue coloration with ferric chloride.

The condensation product of b-naphthol above referred to precipitates gelatine and aniline hydrochloride and gives a brown coloration with ferric chloride.

Thionaphtholsulphonic acid, when acted upon by formaldehyde, yields a condensation product of the following constitution:--

HSO_4 ^ ^ SH SH ^ ^ HSO_4 | | | | | | | | |_____CH_2_____| | | v v v v

This is a light yellow powder which, dissolved in water, yields an opaque solution; the latter only exhibits any tanning properties when it is not neutralised and even slightly acidified and then precipitates gelatine, aniline hydrochloride and barium chloride; dissolved in alkali, it forms a clear, yellow solution devoid of tannoid properties. Leather tanned with the acidified solution is very similar to those tanned with the phenolsulphonic acid condensation products; its colour, however, is more pronouncedly yellow.

b-Naphthol condensed with hydrochloric acid and formaldehyde yields a methylenedinaphthol, which is insoluble in water; the sodium salt, however, easily dissolves. The same condensation, however, takes place in alkaline solution with direct formation of the sodium salt. The condensation product gives a slight precipitate with gelatine, and a bluish-grey precipitate with ferric chloride; acids re-precipitate the insoluble methylene compound. Towards pelt it exhibits tanning properties, whereby the insoluble product referred to above is deposited, and soft, full, and white leather is obtained, possessing, however, but little tensile strength.

4. Condensation of the Anthracene Group

Anthracene heated with excess sulphuric acid yields the water-soluble anthracenesulphonic acid; the latter, when heated with formaldehyde, yields water-soluble, reddish-brown condensation products, which remain soluble on prolonged heating with formaldehyde. The aqueous solution of the condensation product shows no particular reactions; it gives a flocculent precipitate with gelatine and a green precipitate with copper sulphate, soluble with blue colour in excess of the reagent.

The partly neutralised solution tans pelt--to which it imparts a brown colour--in eight days, but on the surface only; the inner layers are merely pseudo-tanned (white colour). When dried, pelt thus treated yields a full and soft leather with brown grain and flesh possessing but little tensile strength. Hence, this condensation product exerts a pickling rather than a tanning effect.

Anthraquinone heated with sulphuric acid and treated with formaldehyde in the usual manner, yields a substance which, when mixed with water, forms an opaque, milky solution. This is not altered by excess of caustic soda. The aqueous solution precipitates gelatine and aniline hydrochloride; all other tannin reagents give no reaction.

The partly neutralised solution of the condensation product exerts, in the main, a pickling action on pelt; only the surface of which is tanned, with brown colour, the remainder being merely pickled (white colour). During "tannage," bakelite is formed in the liquid, and practically all solubles originally present are deposited. The tannage completed, a light brown, fairly soft and full leather, possessing little tensile strength, results; this leather can be washed only with great difficulty and approaches more the character of a pickled pelt.

1-Hydroxyanthraquinone, 1,5-dichloroanthraquinone, l,5-diaminoanthraquinone, 1-methylaminoanthraquinone, 1-benzoylamino, 6-chloranthraquinone, 1-_m_-toluidoanthraquinone, when treated with sulphuric acid and formaldehyde, all yield condensation products which are but little soluble in water, and which do not at all precipitate gelatine. Tanning experiments with these condensation products in alcoholic solution yielded empty leathers of pronounced pickle character.

If, however, 1-methylamino-4-bromanthraquinone is condensed with sulphuric acid and formaldehyde, a condensation product is obtained which is but slightly soluble in water, but which precipitates gelatine.

When phenanthrequinone is heated with excess of sulphuric acid for some time, a water-soluble, reddish-yellow coloured condensation product results. The latter, when treated with formaldehyde in the cold and then finally heated, gradually fixes the formaldehyde and forms a substance soluble in water. If the heating, however, is prolonged, insoluble bakelites are formed, which are neither soluble in alkali nor in alcohol.

An aqueous solution of these condensation products gives no reactions with the usual tannin reagents, though it completely precipitates gelatine. When acting upon pelt, the partly neutralised dilute solution of the condensation product pickles the former, and after a few days the pelt is converted into a light brown, full, and rather soft leather possessing good tensile strength.

When the condensation product is acted upon by bromine in hot aqueous solution, an additive compound is formed and the resulting product is soluble in water. The aqueous solution of the brominated product gives no special reactions with the usual tannin reagents, but precipitates gelatine completely. Its tanning action upon pelt is much slower than that of the original condensation product; the surface of the pelt only is tanned with brown colour, the inner pelt being only pickled (light brown colour). When dried, a hard and empty leather of good tensile strength is obtained, possessing mainly the properties of a pickled pelt.

CO OH ^ ^ ^ QUINIZARENE, | | | | , treated with sulphuric acid | | | | v v v CO OH

and formaldehyde, yields a condensation product which is but little soluble in water and which does not precipitate gelatine.

QUINOLINE, when sulphonated and condensed with formaldehyde, yields a dark coloured condensation product, completely soluble in water; the solution does not precipitate gelatine.

OXYQUINOLINE exhibits similar behaviour.

On the other hand, the use of _retene_ (methylisopropylphenanthrene),

CH_3 ^ ___________ ^ | | | | | |___CH:CH___| | C_3H_7 v v

for the production of synthetic tannins, is protected by Ger. Pat., 290,965 [Footnote 1: _Cf_ Austr. Pat., 69,194]

5. Di- and Triphenylmethane Groups

If DIPHENYLMETHANE, (C_6H_5)_2CH_2, is heated with excess sulphuric acid, a dark blue mass, easily soluble in water, is obtained. The product gently heated with formaldehyde yields a brown, water-soluble condensation product; once condensation is complete, the product will stand stronger heat. If, on the other hand, more formaldehyde is added, brown, water-insoluble bakelites are formed. The water-soluble condensation product precipitates gelatine, but not aniline hydrochloride. Dissolved in water, it possesses tannoid properties: the pelt is, however, tanned on the surface only, the intermediary layers being merely pickled; after four days in the solution, the pelt after drying was found to be converted into a greyish-brown, badly coloured leather, which was empty, hard, and possessed but little tensile strength.

CARBAZOLE (dibenzopyrrole),

^ _____ ^ | | | | | |__ __| | v v v N_3

on the other hand, was found a suitable base for the commercial production of synthetic tannins; its use is protected by Ger. Pat, 290,965.

TRIPHENYLMETHANE, (C_6H_5)_3CH, heated with excess sulphuric acid, yields a nearly black mass which, when condensed with formaldehyde in the cold, and subsequently heated, yields a mass which is soluble in water. With gelatine and aniline hydrochloride it exhibits reactions similar to those given by the diphenylmethane condensation products; its tanning properties also are similar to those of the latter. The resultant leather is black, but is soft and full and possesses good tensile strength.

Baeyer's observation, [Footnote: _Ber_., 1872, 5, 280, 1096.] that pyrogallol on condensation with formaldehyde yields an amorphous body soluble in water, which precipitates gelatine and is very similar to tannin, was confirmed by Caro [Footnote: _Ibid_., 1892, 25, 947.] and Kahl. [Footnote: _Ibid_., 1898, 31, 114.] These investigators found that by the condensation of phenols and hydroxybenzoic acids with formaldehyde, diphenylmethane derivatives were formed; pyrogallol yields hexahydroxydiphenylmethane--

C_6H_2(OH)_3 CH_2{ C_6H_2(OH)_3

Nierenstein [Footnote: _Collegium_, 1905, 221.] repeated these experiments, and found that in addition to the insoluble diphenylmethanes, water-soluble bodies were formed, which latter precipitate gelatine. The condensation product yielded by gallic acid was identified as hexahydroxyaurinecarboxylic acid--

_C_6H(OH)_3COOH C{-C_6H(OH)_3COOH | }C_6H(OH)_2COOH O

which is formed in addition to hexahydroxydiphenylmethane-dicarboxylic acid--

C_6H(OH)_3COOH CH_2{ C_6H(OH)_3COOH

Baeyer's experiment with pyrogallol probaly also yields, according to Nierenstein, another compound of the following constitution--

C_6H_2(OH)_3 C{-C_6H_2(OH)_3 |_}C_6H_2(OH)_2 O

Nierenstein considers these bodies confirmation of his hypothesis of the existence of a "tannophor,"--CO--, in the tannins.

This supposition was adopted by Stiasny [Footnote: _Gerber_, 1905, 233.] and Kauschke [Footnote: _Collegium_, 1906, 362.] and the latter points out that these easily soluable substances exhibit tanning properties. Nierenstein [Footnote: _Ibid_., 1906 424.] was further able to show that by all processes of condensation between phenols (or hydroxybenzoic acids) and formaldehyde, compounds of the character of hydroxyaurine (or hydroxyaurinecarboxylic acid) were formed in addition to the insoluble hydroxydiphenylmethanes (or hydroxydiphenylmethanecarboxylic acids), the former possessing the characteristic tannophor group and hence precipitating gelatine, _i.e._, exerting tanning action. If the formation of leather is viewed in the light of Schiff's base, [Footnote: _Ibid_., 1905, 159.] one may consider the constitution of a hexahydroxyaurinecarboxylic acid leather as follows:--

_C_6H_2(OH)_3.COOH C{-C_6H_2(OH)_3.COOH |_}C_6H_2(OH)_2.COOH R-N

In the preparation of these and similar condensation products, Nierenstein and Webster [Footnote: _Ber_., 1908, 41, 80.] observed a peculiar steric effect of the carboxyl group. Each 2.5 gm. of the phenol or the acid in question were dissolved in 30 c.c. of water, the solution brought to boil and 5 c.c. formaldehyde (20 per cent.) and 2.5 c.c. hydrochloric acid added drop by drop; the precipitate formed was filtered off after twenty-four hours, dried at 110° C. to constant weight, extracted (in a Gooch crucible) freely with water, and the residue again dried at 110° C. till constant. The following values were obtained:--

Total Insol. Aq. Sol. Aq. Oxy- Precipitate Diphenylmethane aurinecarboxylic in Grammes. Derivatives Acid.

Per Cent. Per Cent. Phloroglucinol 2.4002 100 ... Hydroquinone 2.3716 100 ... " 2.0542 100 ... Pyrogallol 2.5150 100 ... " 2.7940 100 ... Pyrocatechol 2.9805 100 ... " 2.9574 100 ... Resorcinol 2.9954 100 ... " 2.9725 100 ... Gallic acid 2.0706 78.84 21.16 " 1.2240 83.18 16.82 " 1.1405 59.94 41.06 [Greek: b]-Resorcylic acid 2.1040 51.08 48.92 " " 2.2008 47.12 52.88 Protocatechuic acid ... ... ... " " ... ... ... Vanillic acid ... ... ... Tannin 2.0599 ... Nearly all sol. Digallic acid 2.1042 80.16 19.84 Leucodigallic acid 2.0041 1.94 98.06

With the introduction of the carboxylic group the tendency of condensation to diphenylmethane derivatives is lessened; by protocatechuic acid the tendency is nil. Nierenstein considers this reaction analogous to the formation of cork, to the genetic relation of which with the diphenylmethane formation Drabble and Nierenstein have referred in an earlier publication. [Footnote: _Biochemical Jour._., 1907, 2, 96.] It is hence possible that the plants may employ formaldehyde as a methylation medium, and produce these insoluble condensation products for the purpose of ridding themselves of the poisonous phenols and aromatic hydroxy acids (and tannins), in addition to oxidising processes whereby phlobaphenes, ellagic acid, etc., are formed.

The reaction between phenols and aldehydes has been further studied by Michael, [Footnote: _Amer.Jour_., 5, 338; 9, 130.] who prepared a condensation product from phenol and resorcinol with benzaldehyde, and Russanow, [Footnote: _Ber_.9 1889, 22, 1944.] who also employed benzaldehyde and phenol. Lipp [Footnote: Diss., Bern., 1905.] investigated the action of benzaldehyde and piperonal on phenols, anisoles, cresols, cresylic ether, resorcinol, and the ether of the latter and phenol, and showed that when free phenols are condensed with benzaldehyde the hydroxyls occupy the same position as by the interaction between benzaldehyde and the corresponding phenolic ethers. The resulting dihydroxytriphenylmethane derivatives form beautiful crystals, which on oxidation are converted into benzaurines, the constitution of the latter probably being--

O= ^=_____ ^ OH | | | | | |== __| | =v v v C | C_6H_5

In alkalies, the hydroxylated triphenylmethanes dissolve without imparting any colour to the solution; by concentrated sulphuric acid they are taken up with intense coloration.

If the hydroxyls occupy the ortho-position to methyl, they may form xanthenes by splitting off water--

O ^ ^ ^ | | | | | | | | CH_3 v v v CH_3 CH | C_6H_5

In the benzene series this reaction is difficult to establish, and has to be induced by distilling the particular dihydroxy-diphenylmethane at ordinary pressure. In the naphthalene series, on the other hand, the ring closes up by, for instance, the condensation of [Greek: b]-naphthol with benzaldehyde or paraldehyde, and yields the following compounds:--

C_10H_6 C_10H_6 C_6H_5-CH{ }O CH_3-CH{ }O C_10H_6 C_10H_6

These xanthenes are white, silk-glossy needles, which are soluble in water and in alkalies. In concentrated sulphuric acid, they are taken up with beautiful fluorescence.

6. Summary

From the qualitative reactions of the different condensation products described it may be seen that their tannoid properties are not dependent on whether they precipitate gelatine or are adsorbed by hide powder or not. Hydroxynaphthylmethanesulphonic acid, for instance, precipitates gelatine but does convert pelt into leather; on the other hand, sodium dicresylmethanesulphonate does not precipitate gelatine, and neither does it tan pelt; nevertheless it is adsorbed by hide powder as "tanning matter". The author discovered that _o_-nitrophenol does not precipitate gelatine, but has some tanning action on both hide powder and pelt.

Relatively to the possibilities of forming condensation products possessing tannoid properties, the following may be stated:--

All mono- and polyhydric phenols may be converted into true tanning matters by either condensing them as such, or after their conversion into the corresponding sulphonic acids, by substances capable of eliminating the elements of water. It makes no difference to the final product whether the condensation is the first step followed by sulphonation and consequent solubilisation of the intermediary insoluble product, or whether, vice versa, the sulphonic acid is subjected to condensation. Alkaline solution of phenols may also be condensed, the reaction products, when condensed, constituting tanning matters soluble in water.

Among the substitution products of the phenols, the thio-, chloro-, bromo-, nitro-, and aminophenols as a rule yield tanning matters similar in character.

The quinones are as such--_i.e._, without being condensed--substances possessing tannoid properties.

The aromatic dihydric alcohols are easily condensed with the different sulphonic acids and yield valuable tanning matters.

Of aromatic acids all those which yield water-soluble sulphonation products seem suitable for the industrial production of tanning matters. If the acids themselves do not yield water-soluble sulphonation products, the alkali salts of the latter may be condensed with formaldehyde, and the resulting products then constitute tanning matters provided their solutions can be neutralised or faintly acidified without the solute being thrown out of solution in insoluble form.

The diphenyl derivatives of the above groups often possess tannoid properties.

The same holds good of those compounds with condensed nuclei (naphthalene, anthracene, etc.), and all their derivatives which satisfy the above conditions.

The choice of condensing agent is, as a rule, of little significance. Elimination of the elements of water by the mere application of heat succeeds in few cases only, since the high temperature required to induce reaction in many cases causes decomposition of the substances. This difficulty is overcome by heating _in vacuo_. Condensation with formaldehyde always succeeds, with acetaldehyde and benzaldehyde only partly.

The action on hide powder, pelt, and gelatine by these characteristic substances is tabulated below:-- Relative Behaviour towards Substance. Gelatine. Hide Powder Pelt Formaldehyde ... ... Tanning Phenol Ppte. ... ... Chlorophenol " ... ... Surface Tribromophenol Slight ppte Tanning tanning _o_ Nitrophenol No ppte " " Bromonitrophenol Slight ppte " " Trinitrophenol Ppte " Tanning Bromotrinitrophenol Slight ppte " " _p_ Aminophenol Ppte ... ... _m_ Dihydroxybenzene " ... ... Orcinol " ... ... _p_ Dihydroxybenzene " Tanning Tanning Monochloro _p_ dihydroxybenzene " ... ... _o_ Dihydroxybenzene " ... ... Pyrogallic acid " ... ... Surface Tribromopyrogallic acid " Tanning Tanning Gallic acid No ppte Not tanning Not tanning Bromophloroglucinol Ppte Tanning " Gallotannic acid " " Tanning Galloflavine Slight ppte " Not tanning Quinone " " Tanning Bromosalicylic acid " " Not tanning Dinaphthylmethanedisulphonic acid Ppte " Tanning Diphenylmethanedisulphonic acid " " " Dicresylmethanedisulphonic acid " " " Sodium dicresylmethanedisulphonate acid No ppte " Not tanning Dixylylmethanedisulphonic acid Ppte " Tanning Naphtholdisulphonic acid " Not tanning Not tanning Methylenedinaphthol " Tanning Tanning Hydroxyphenylmethanesulphonic " " " acid Not tanning Hydroxynaphthylmethanesulphonic Slight ppte " " acid Diaminonaphthylmethanedisulphonic Ppte Tanning Not tanning acid Dihydroxynaphthylmethanedisulphonic acid " " " Dichloronaphthylmethanedisulphonic acid " " Surface tanning Dinitronaphthylmethanedisulphonic acid " " " Dithionaphthylmethanedisulphonic acid " " Tanning Bromo _[Greek: b]_ naphthol [1] Slight ppte " " Rosolic acid_ [1] Ppte " "

[Footnote 1: In alcoholic solution.]

SECTION III

TANNING EFFECTS OF MIXTURES AND NATURAL PRODUCTS

1. Mixture of Phenolsulphonic Acid and Formaldehyde

The most important invention relatively to the search for new tanning materials was that of Weinschenk,[Footnote: Ger. Pat., 184,449.] who first showed that pelt may be converted into leather by the action upon it of mixtures of naphthols and formaldehyde. This process consists of two steps: the pelt is first immersed in a 0.25-0.50 per cent, formaldehyde solution, and secondly in an aqueous solution of -[Greek: a] or -[Greek: b] naphthol; this order may be reversed. If, on the other hand, a pasty mixture is made of formaldehyde and naphthol, and this is allowed to act upon the pelt, the latter is rapidly converted into leather, but the mixture must be administered very gradually or otherwise the insoluble methylenedinaphthol is formed outside the pelt and hinders any tanning effect.

Leather obtained through the action of [Greek: a]-naphthol is, when freshly tanned, pure white and sufficiently soft and firm, but quickly assumes a brown colour on storing; if, however, [Greek: b]-naphthol is employed, a cream-coloured leather results, the colour of which turns only slightly more yellowish even when exposed to the direct rays of the sun.

A similar process has recently (25, xii., 1915) been protected by Ger. Pat, 305,516, granted to the Deutsch-Koloniale Gerb--und Farbstofif Gesellschaft, in Karlsruhe. According to this patent, pelt is treated in separate solutions, one of which is formaldehyde, the other being that of such aromatic compounds or their salts which yield water-soluble condensation products with formaldehyde; for example, pelt is immersed in 2-5 per cent, solution of formaldehyde for a few days, and is subsequently treated with 1-2 per cent neutral or faintly acidified solutions of [Greek: a]-naphthylamine hydrochloride, resorcinol or sodium phenate or cresylate, for several days. The resultant leather is claimed to be soft and full and to possess good tensile strength.

The tanning properties of mixtures of phenolsulphonic acid and formaldehyde have been examined by the author with the following results:--

I. II. III.

Grammes formaldehyde 10 20 40 " phenolsulphonic acid 20 50 100 " caustic soda (sol, 40 per cent.) 10 20 40 " water 500 500 500

The above solutions were made up and allowed to act upon pelt pieces weighing 15 gm.; whereas Solution I. remained clear throughout the experiment, Solution II. became somewhat clouded, and Solution III. assumed a milky appearance. The pelts were tanned through in seven days and yielded leathers which, after drying and finishing, possessed yellow colour, long fibre, and good tensile strength, but a rather empty feel.

To prevent separation of insoluble matter during tannage, another experiment was carried out, in which the pelts were first submitted to the action of formaldehyde (10, 20, and 40 gm. in 500 c.c. water) for three days, being subsequently removed to fresh solutions of partly neutralised phenolsulphonic acid (_cf_. above). Similar results were obtained, but the leather felt even more empty than those obtained by the former experiment.

Attempts at converting pelt into leather by first immersing the pelt in a partly neutralised solution of phenolsulphonic acid, and subsequently transferring it to fresh solutions of formaldehyde, gave merely negative results; the phenolsulphonic acid effected pickling action upon the pelt, but was subsequently quickly replaced by the formaldehyde, before the latter had penetrated the pelt in sufficient quantity to induce condensation, thereby exerting tanning action.

To explain the tanning effects of these mixtures, the author analysed the leathers resulting from the effects of the latter, and was able to show, that in these cases also, condensation of phenolsulphonic acid and formaldehyde takes place _inside_ the pelt, since on the one hand the analyses left no doubt but that true tannage had been effected, and on the other hand an ammoniacal extract of the leathers gave the typical reaction for condensation products of phenolsulphonic acid, with aniline hydrochloride. [Footnote: _Collegium_ 1913, 516, 142.]

The leather analyses gave the following figures:--

Moisture - - - 18.30 per cent. Fats - - - - 0.47 " _ Ash - - - - 0.98 " Leather { Tannin - - - 26.37 " substance { Hide substance - - 53.88 "

A characteristic feature is the low value of tannin, which is considerably higher [Footnote: _Ibid_., 1913, 521, 478.] where condensation products of phenolsulphonic acids are used as tanning agents; the action effected by the separate constituents, therefore, is more that of pickling.

2. Mixture of Phenolsulphonic Acid and Natural Tannins

A piece of pelt was immersed in a half-neutralised solution, measuring 6° Bé., of phenolsulphonic acid, and left sixteen hours in the solution, which completely penetrated the pelt during this time; it was then transferred to a 12° Bé. solution of a mixture of quebracho and chestnut, which in two days converted the pelt into a light coloured leather possessing good tensile strength.

By using a bath composed of half-neutralised phenolsulphonic acid and quebracho extract in 7° Bé. solution, another piece of pelt was completely tanned in two days. The same result was obtained by first half neutralising the phenolsulphonic acid and then adding sulphited quebracho extract till a 5° Bé. solution was obtained.

A piece of pelt received a 2º Bé. liquor composed of 3 parts of phenolsulphonic acid and 1 part of formaldehyde for sixteen hours, and was then completely penetrated; it was subsequently transferred to a 10º Bé. liquor composed of chestnut and quebracho, being completely tanned in two days. The same result was obtained on adding sufficient sodium sulphate to the above mixture of phenolsulphonic acid and formaldehyde to raise the density from 2º-3º Bé.

Sixty grammes of phenolsulphonic acid were partly neutralised with 100 c.c. of a 10 per cent solution of caustic soda, and 10 c.c. formaldehyde added to 400 c.c. of the mixture (2º Bé.): a piece of pelt was completely penetrated by the solution in sixteen hours, and was subsequently tanned in two days, using an extract of 10º Bé. Similarly, by treating a pelt with 400 c.c. of a half-neutralised solution of phenolsulphonic acid (3º Bé.) plus 8 c.c. formaldehyde, and adding after eighteen hours sulphited quebracho extract to the same bath, strengthening the latter to 6º Bé., the pelt was converted into leather in two days; in this case, however, much of the tannin was precipitated by the formaldehyde present in the solution. If, on the other hand, a mixture of 80 gm. dilute phenolsulphonic acid (1:1 aq) and 14 gm. of formaldehyde were cooled for several hours and subsequently strengthened with sulphited quebracho extract to 7º Bé., no tannin was precipitated in the liquor, and a piece of pelt immersed in the latter was completely tanned in sixteen hours.

To prevent the precipitation of tannin caused by the formaldehyde, sulphite cellulose extract (wood pulp) was substituted for sulphited quebracho extract, and the following experiments carried out:--

To 200 c.c. of a 6º Bé. sulphite cellulose extract plus 200 c.c. of half-neutralised phenolsulphonic acid solution was added 15 c.c. formaldehyde, and this solution tanned pelt in four days; the resultant leather was light brown, firm, and possessed good tensile strength and long fibre.

Another piece of pelt was immersed in a solution of 400 c.c. phenolsulphonic acid of 3ºBé. plus 15 c.c. formaldehyde for eighteen hours, and was then tanned in a 6º Bé. solution of sulphite cellulose extract. The resultant leather was extremely light coloured, and possessed qualities similar to those described in the former experiment. Finally, pelt was immersed in a 6° Bé. solution composed of 140 gm. of a 15° Bé. sulphite cellulose extract, 10 gm. of formaldehyde, 400 gm. water, 15 gm. phenolsulphonic acid, and 30 gm. of a 10 per cent caustic soda solution, and was tanned in four days. This leather also was coloured light brown, of good tensile strength, and rather firm.

These experiments prove that when pelt is treated with formaldehyde, phenolsulphonic acid, and vegetable tannins, the two former components effect, more or less, actual tannage; it is admittedly a matter of some difficulty to establish whether the effect is one of pickling or pseudo-tannage, or whether the tannage may be considered a true one. The final effect, however, is nearly always that of a true tannage, _i.e_., by varying the composition of the tanning solutions leather is obtained with properties identical with those tanned with true tannins of vegetable origin. The only difficulty encountered in these combinations is the property of formaldehyde, of precipitating the natural tannins, and it is hence essential, for practical purposes, to so arrange the combination that their value is not reduced by the property referred to. The fact that not only compounds already existing may convert pelt into leather, but that a similar effect is obtained _inside the pelt_, by their components, is indeed of theoretical interest.

3. Tanning Effects of Different Natural Substances

In addition to the vegetable tannins, Nature has also provided other substances of vegetable origin, which, admittedly, do not effect tannage in their original state, but which may, by suitable treatment, acquire this property. The oldest information on this point is supplied by Resch, [Footnote: _Scherer's Jour_., 1801, 6, 495.] who carried out tanning experiments, using three parts of peat and one part of oak bark.

By the action of nitric acid on substances of vegetable and animal origin, Hatchett, [Footnote: _Gehlen's Jour_., 1805, 1, 545.] Chevreul, [Footnote: _Ann. Chim_., 1810, 73, 36.] and Vogel [Footnote: _Jour. Chem. Phys_., 1812, 6, 101.] claim to have obtained tanning materials, whilst later, Buff [Footnote: _Ibid_., 1827, 51, 38.] obtained a material suitable for tanning purposes from indigo.

By subsequent treatment with lime and soot, or tar, Ashmore [Footnote: _Dingier's Jour_., 1833, 48, 67.] claims to have converted pelt into leather.

By treating peat with nitric acid, Jennings [Footnote: _Jahresber. d. Chem_., 1858, 666.] and Payne [Footnote: _ Chem. Centralbl_., 1908, ii. 554; Ger. Pat., 200, 539.] have produced artificial tanning materials.

Skey [Footnote: _Chem. News_, 1866, 206; _Zeits. f. Chem_., 1866, 753.] obtained a dark brown extract, soluble in water and precipitating gelatine, by treating bituminous coal or lignite with nitric acid; by extracting coal with alkalies, Reinsch [Footnote: _Pharm. Centralh_., 1887, 141.] isolated a substance (pyrofuscine) which, when partly neutralised with carbon dioxide, was capable of converting pelt into leather.

In addition to these tanning materials the recovery of a substance possessing tanning properties from the so-called acid rosins has been made the subject of a patent; [Footnote: _Ger. Pat_., 36,019.] this rosin is formed when crude oil is treated with concentrated sulphuric acid in the oil refineries. The greasy substance is partly neutralised with alkali and is claimed to produce a very springy leather.

The waste liquors obtained in the manufacture of cellulose, the so-called sulphite and sodium cellulose waste, have, however, been the subject of numerous investigations, and several hundred publications have appeared and a great number of patents [Footnote: "Literatur überiSulfitablauge" 1910-13. (Reprint from _WocheWochenblPapiePapierfabrikation_)] taken out, the first one being that of Mitscherlich [Footnote: _Jahresber. d. Chem_., 1893, 890; Ger. Pat., 72,161.] and Hönig [Footnote: _Chem. Centralbl_., 1902, ii. 174; Ger. Pat., 132,224.]

The waste liquors contain large quantities of acids and lime, and in order to utilise the liquors for tanning purposes, the excessive sulphuric and sulphurous acids as well as the lime must be removed. The active tannin is no doubt the ligninsulphonic acid, and those cellulose extracts containing the largest amounts of free ligninsulphonic acid may also be considered the most efficient.

According to the author,[Footnote: _Technikum_, 1912, 20, 156.] such sulphitecellulose extracts precipitate gelatine, aniline hydrochloride, ammoniacal zinc acetate, and basic coal-tar dyes, and give a greenish-black coloration with ferric chloride. These reactions indicate the presence of tanning matters in cellulose extracts.

The official shake method of analysis gives the following results:--[Footnote: _Ibid_.]

Tanning matters 23.0 per cent. Non-tannins 30.3 " Insoluble matters 0.7 " Water 46.0 " --------------- 100.0 per cent.

Ash 4.3 " Sulphurous acid 0.6 "

Many other substances have been used for tanning experiments, a number of them precipitating gelatine. Zacharias [Footnote: _Zeits. f. Ang. Chem_., 1907, 1645.] obtained leather by the action of many coal-tar dyes on pelt, similarly Herzog and Adler, by using Prussian blue, Neufuchsin, patent blue V, crystal violet, and colloidal gold.

Most inorganic substances possess tanning properties when in the colloidal state, _e.g_., sulphur, halogens, chromium salts, iron salts, silver oxide, and the salts of mercury, copper, bismuth, zinc, lead, platinum, cesium, vanadium, and the rare earths (salts of cerium, lanthanum, didymium, neodymium, thorium, and zerconium).

For practical purposes, however, only sulphur, chrome, and alum salts are used, the latter two being of the greatest importance.

SECTION IV

METHODS OF EXAMINING TANNING MATTERS

Whereas the evaluation of vegetable tanning matters necessitates determinations of their practical applicability in addition to qualitative and quantitative analyses, the latter two determinations are of practically no value when dealing with synthetic tannins. The way in which tanning matters obtained by chemical means exert their action, in addition to the intensity with which they convert pelt into leather, is the only criterion of their quality for practical (tanning) purposes; both may be demonstrated by experimental tests.

When dealing with the natural tanning materials it is desirable to know their contents of actual tanning matter, from which their special qualities as tanning agents may be deduced. Where the vegetable tanning materials have already been converted into extracts, it is essential to establish the identity of the original material used by the qualitative reactions of the extract in addition to the quantitative estimation of actual tannin contents. It is frequently necessary to examine whether the extract in question has been actually prepared from the material giving the extract its name, or whether the extract has suffered the addition of other extracts of tanning materials of but low quality. Such determinations may be undertaken by microscopical observations and by means of qualitative and quantitative reactions; for this purpose many colour reactions and precipitation methods are available in addition to the determination of the molybdenum figure (Lauffmann),[Footnote: Collegium, 1913, 10.] the alcohol and ethyl acetate figures and microscopical examination (Grasser).[Footnote: Ibid., 1911, 349.] Of other adulterants tending to reduce the quality of extracts may be mentioned sugars, mineral salts, and coal-tar dyes; [Footnote: Grasser, _Collegium_, 1910, 379.] for the determination of these, the special literature should be consulted. [Footnote: Grasser, "Handbuch f. gerbereichem. Laboratorien" (Leipzig, 1914); Procter-Paessler, "Gerbereichem. Untersuchungen" (Berlin, 1901).]

Two methods are devised for the purpose of quantitatively determining the tannin contents, both of which employ hide powder, and which are known as the "shake method" and the "filter bell method" respectively: the former is adopted as the official method of the "International Association of Leather Trades' Chemists" (I.A.L.T.C.). [Footnote: And also by the Society of Leather Trades' Chemists.-_Transl._]

The original method, [Footnote: _Leather Manufacturer_, 1894, No. 9 J.S.C.I.,1894, 494.] worked out in the laboratory of the Yorkshire College (now the University of Leeds), essentially consists in introducing 6-9 gm. of hide powder in a shaker, washing it at least twice with distilled water and carefully squeezing out the powder in a linen cloth between each washing. 100 c.c. of the solution to be examined, which may not contain more than 1 per cent, total solids, are introduced into the shaking bottle which is then weighed. About one-third of the washed hide powder is then added, and the bottle shaken ten to fifteen minutes; another third is then added and, after shaking, the third portion. The bottle plus contents is now weighed, and the amount of hide powder introduced ascertained by difference of the two weighings. The liquid is then filtered through filter paper, 50 c.c. of the clear filtrate evaporated in a basin, dried and weighed. The residue in the original solution is then obtained by multiplying the former by 100 (plus weight of water added with hide powder), and dividing by 100.

This method was closely investigated by a large number of leather trades' chemists, was considerably improved, and in its final form presented a method of the highest degree of accuracy; the method was therefore adopted as _The Official Method of Tanning Analysis_ by the I.A.L.T.C., which body, at the same time, gave precise instructions as to the details of the method. The latest instructions, which are reprinted below, permit of any method of analysis which observes the following conditions:--

1. The solution for analysis must contain between 3.5 and 4.5 gm. of tanning matter per litre, and solid materials must be extracted so that the greater part of the tannin is removed at a temperature not exceeding 50° C.

2. The total solubles must be determined by the evaporation of a measured quantity of the solution previously filtered till optically clear, both by reflected and transmitted light. This is obtained when a bright object such as an electric light filament is distinctly visible through at least 5 cm thickness, and a layer of 1 cm. deep in a beaker placed on a black glass or black glazed paper appears dark and free from opalescence when viewed from above. Any necessary mode of filtration may be employed, but if such filtration causes appreciable loss when applied to a clear solution, a correction must be determined and applied as described in paragraph 6.

Filtration shall take place between the temperatures of 15° C. and 20° C. Evaporation to dryness shall take place between 98.5° C. and 100° C. in shallow, flat-bottomed basins, which shall afterwards be dried until constant at the same temperature, and cooled before weighing for not less than twenty minutes in air-tight desiccators over dry calcium chloride.

3. The total solids must be determined by drying a weighed portion of the material, or a measured portion of its uniform turbid solution, at a temperature between 98.5° C. and 100° C. in shallow, flat-bottomed basins, which shall afterwards be dried until constant weight at the same temperature, and cooled before weighing for not less than twenty minutes in air-tight desiccators over dry calcium chloride.

"Moisture" is the difference between 100 and the percentage of total solids, and "insoluble" the difference between "total solids" and "total solubles."

4. _Non-Tannins._--The solution must be detannised by shaking with chromed hide powder till no turbidity or opalescence can be produced in the clear solution by salt-gelatine solution. The chromed powder must be added in one quantity equal to 6.0-6.5 gm. of dry hide powder per 100 c.c. of the tanning solution, and must contain not less than 0.2 per cent. and not more than 1 per cent. of chromium calculated on the dry weight, and must be so washed that in a blank experiment with distilled water, not more than 5 mg. of solid residue shall be left on evaporation of 100 c.c. All water contained in the powder should be determined and allowed for as water of dilution.

5. _Preparation of Infusion_.--Such a quantity of material shall be employed as to give a solution containing as nearly as possible 4 gm. of tanning matter per litre, and not less than 3.5 or more than 4.5 gm. Liquid extracts shall be weighed in a basin or beaker and washed with boiling water into a litre flask, filled up to the mark with boiling water, and well mixed and rapidly cooled to a temperature of 17.5° C., after which it shall be accurately made up to the mark, again well mixed, and filtration at once proceeded with. Sumac and myrabolam extracts should be dissolved at a lower temperature.

Solid extracts shall be dissolved by stirring in a beaker with successive quantities of boiling water, the dissolved portions being poured into a litre flask, and the undissolved being allowed to settle and treated with further portions of boiling water. After the whole of the soluble matter is dissolved, the solution is treated similarly to that of a liquid extract.

Solid tanning materials, previously ground till they will pass through a sieve of sixteen meshes per square centimetre, are extracted in Koch's or Procter's extractor with 500 c.c. of water at a temperature not exceeding 50° C.; the extraction is then continued with boiling water till the filtrate amounts to 1 litre. It is desirable to allow the material to soak for some hours before commencing the percolation, which should occupy not less than three hours, so as to extract the maximum of tannin. Any remaining solubles in the material must be neglected or reported separately as "difficultly soluble" substances.

The volume of liquid in the flask must, after cooling, be accurately made up to 1 litre.

6. _Filtration_.--The infusion shall be filtered till optically clear (_vide_ 2). No correction for absorption is needed for the Berkefeld candle, or for S. and S. 590 paper [Footnote: Schleicher and Schüll, Düren (Rheinland), Germany.] if a sufficient quantity (250-300 c.c.) is rejected before measuring the quantity for evaporation, and the solution may be passed through repeatedly to obtain a clear filtrate.

If other methods of filtration are employed, the average correction necessary must be determined in the following manner:--About 500 c.c. of the same or a similar tanning solution is filtered perfectly clear, and after thorough mixing 50 c.c. is evaporated to determine "Total Soluble A." A further portion is now filtered in the exact method for which the correction is required (time of contact and volume rejected being kept as constant as possible), and 50 c.c. is evaporated to determine "Total Soluble B." The difference between "A" and "B" is the correction sought, which must be added to the weight of the total solubles found in analysis. An alternative method of determining correction, which is equally accurate and often more convenient, is to filter a portion of the tanning solution through the Berkefeld candle till optically clear, which can be generally accomplished by rejecting 300 or 400 c.c., and returning the remaining filtrate repeatedly; and at the same time to evaporate 50 c.c. of the clear filtrate obtained by the method for which correction is required, when the difference between the residues will be the correction sought. An average correction must be obtained from at least five determinations. It will be found that this is approximately constant for all materials, and amounts in the case of S. and S. 605, 150 c.c. being rejected, to about 0.005 gm., and where 2 gm. of kaolin are employed in addition to 0.0075 gm. The kaolin must be previously washed with 75 c.c. of the same liquor, which is allowed to stand fifteen minutes and then poured off. Paper 605 has a special absorption for a yellow colouring matter often contained in sulphited extracts.

7. Hide powder shall be of a woolly texture, thoroughly delimed, preferably with hydrochloric acid. It shall not require more than 5 c.c. or less than 2.5 c.c. of decinormal NaOH or KOH to produce a permanent pink colour with phenolphthalein on 6.5 gm. of the dry powder suspended in water. If the acidity does not fall within these limits it must be corrected by soaking the powder before chroming for twenty minutes in ten to twelve times its weight of water, to which the requisite calculated quantity of standard alkali or acid has been added. The hide powder must not swell in chroming to such an extent as to render difficult the necessary squeezing to 70-75 per cent. of water, and must be sufficiently free from soluble organic matter to render it possible in the ordinary washing to reduce the total solubles in a blank experiment with distilled water below 0.005 gm per 100 c.c. The powder, when sent out from the maker, shall not contain more than 12 per cent. of moisture, and shall be sent out in air-tight tins.

The detannisation shall be carried out in the following manner:--

The moisture in the air-dried powder is determined, and the quantity equal to 6.5 gm. actual dry powder is calculated, which will be practically constant if the powder be kept in an air-tight vessel. Any multiple of this quantity is taken according to the number of analyses to be made, and wet back with approximately ten times its weight of distilled water. Two grammes per 100 of dry powder of crystallised chromic chloride, CrCl_3.6aq., is now dissolved in water and made basic with 0.6 gm. of Na_2CO_3 by the gradual addition of 11.25 c.c. of normal Na_2CO_3, thus making the salt correspond to the formula Cr_2Cl_3(OH)_3. In laboratories where analyses are continually being made, it is more convenient to employ a 10 per cent stock solution, made by dissolving 100 gm. of Cr_2Cl_6.6aq. in a little distilled water in a litre flask and very slowly adding a solution containing 30 gm. of anhydrous sodium carbonate, with constant stirring, finally making up to the mark with distilled water and well mixing. Of this solution 20 c.c. per 100 gm., or 1.3 c.c. per 6.5 gm. of dry powder, should be used. This solution is added to the powder, and the whole churned for one hour. At the end of the one hour the powder is squeezed in linen to free it as far as possible from the residual liquor, and washed and squeezed repeatedly with distilled water, until, on adding to 50 c.c. of the filtrate one drop of 10 per cent. K_2CrO_4 and four drops of decinormal silver nitrate, a brick-red colour appears. Four or five squeezings are usually sufficient. Such a filtrate cannot contain more than 0.001 gm. of NaCl in 50 c.c.

The powder is then squeezed to contain 70-75 per cent, of water, and the whole weighed. The quantity Q containing 6.5 gm. dry hide is thus found, weighed out, and added immediately to 100 c.c. of the unfiltered tannin infusion along with (26.5-Q) of distilled water. The whole is corked up and agitated for fifteen minutes in a rotating bottle at not less than 60 revs. per minute. It is then squeezed through linen, the fitrate stirred and filtered through a folded filter of sufficient size to hold the entire filtrate, returning till clear. Sixty c.c. of the filtrate is then evaporated and calculated as 50 c.c., or the residue of 50 c.c. multiplied by 6/5. The non-tannin filtrate must give no turbidity with a drop of a solution of 1 per cent, gelatine and 10 per cent, common salt. [Footnote: It is convenient for technical purposes to employ the commercially obtainable chromed hide powder as prepared, for instance, by the German Experimental Station at Freiberg, Saxony.]

One gramme of kaolin, freed from all soluble matter, may be added to the filtrate, or it may be used by mixing it with the hide powder in the shaking bottle.

The analysis of used liquors and spent tans shall be made by the same methods as are employed for fresh tanning materials; the liquors being diluted, are concentrated by boiling _in vacuo_, or in a vessel so closed as to restrict access of air, until the tanning matter is if possible between 3.5 and 4.5 gm. per litre, but in no case beyond a concentration of 10 gm. per litre of total solids, and the weight of hide powder used shall not be varied from 6.5 gm.

The results shall be reported as shown by the direct estimation, but it is desirable that in addition efforts shall be made, by determination of acids in the original solution and in the non-tannin residue, to ascertain the amount of lactic and other non-volatile acids absorbed by the hide powder, and hence returned as "tanning matters."

In the case of tanning materials it must be clearly stated in the report whether the calculation is on the sample with moisture as received, or upon some arbitrarily assumed percentage of water; and in that of liquors whether the percentage given refers to weight or to grammes per 100 c.c., and in both cases the specific gravity shall be reported.

All analyses reported must be the average result of duplicate determinations, which must agree in the case of liquid extracts within 0.6 per cent, and of solid extracts within 1.5 per cent, or the analysis shall be repeated until such agreement is obtained.

All reports shall be marked: Analysed in accordance with the rules of the S.L.T.C. (I.A.L.T.C.)--when the analyses have been carried out according to the method described above.

As has been repeatedly emphasised in this treatise, the synthetic tannins form a special class of substances, and the results obtained by either of the two hide-powder methods do not give figures which are always comparable to those of the natural tannins. An example of the inapplicability of the methods where synthetic tannins are concerned is illustrated by the behaviour towards hide powder of them when partly neutralised to varying degrees: commercial Neradol D of acidity 1 gm.= 10 c.c. N/10 NaOH contains 33 per cent. tanning matters, completely neutralised Neradol D, which exerts no true tanning action on pelt, still contains 20 per cent tanning matter when analysed according to the Official Method; a difference hence exists regarding the adsorption by hide powder of a tannin and the adsorption of the latter by hide. As, however, we are unable to make a distinction between these two different properties by using hide powder only, we are also unable to draw the factor into account.

Another source of error is the swelling influence on hide powder by acids; for instance, an acid extract of vegetable tannins would show higher tannin contents in the analysis than would the same extract when less acid. The free sulphonic acid, however, is the active principle in synthetic tannins, and since the latter always contain other acids (of organic and inorganic origin) devoid of tannoid character, a source of error is thus introduced, which we cannot eliminate by the present method of analysis.

Of other methods of estimating the quality of a tanning material or tanning extract the _determination of solubility_, _ash_, _colour_, and _weight-giving properties_ in addition to the _firmness imparted to the leather_ by the particular material are of importance. As regards the synthetic tannins they are as a rule very soluble and it will generally be found sufficient to subject them to the ordinary qualitative examination. The ash determination in synthetic tannins, on the other hand, is not of such value as in the case of natural tanning extracts. From their composition we know that synthetic tannins contain considerable quantities of mineral salts, the presence of some of which on the one hand emphasises their pickling effect, and that on the other hand the property of dissolving phlobaphenes exhibited by the synthetic tannins is closely connected with their salt contents.

A colour determination of synthetic tannins is not of much importance, since synthetic tannins nearly always impart a white or light brown colour to the hide. In those cases only where coloured decomposition products appear as a result of intermediary reactions, may the former impart greyish or dirty colorations of little beauty to the hide. This is easily ascertained by lightly tanning a pelt.

The determination of the weight and solidity-giving properties is important both for leathers tanned with vegetable tanning extracts and for those treated with synthetic tannins, but the results obtained when using animalised cotton are not directly convertible into figures required for practical purposes. Comparative figures are better obtained by actually tanning pieces of pelt on as practical a scale as is possible, and testing the weights and tensile strengths of the pieces as against those of the original pelts, whereby in the former case the yield (pelt --> leather) is obtained.

Its capability as a tanning agent may be ascertained by submitting the synthetic tannin to an actual test tannage. The latter is carried out by introducing the dilute extract into open glass jars, holding about 400 c.c. at a width of about 8 cm. [Footnote: Accumulator jars are excellent for the purpose.--_Transl_.] The concentration of the solution is chosen according to acidity and salt contents of the synthetic tannin, the most suitable being 1.5°-2.5° Bé. A piece of bated pelt is suspended in the liquor in such a way that the pelt is completely surrounded by liquor, without, however, being creased or touching the bottom. If the pelt were creased during tannage, the wrinkles would become fixed and would show in the finished leather. Thus an unfair judgment of the extract would be delivered, since similar results are produced by liquors which are either too concentrated or are not properly composed, and naturally this property of an extract would be greatly to its disadvantage.

The various stages of tannage may be judged from various standpoints when examining the pelt as tannage proceeds. On the one hand, the surface of the but slightly porous pelt is altered so as to present a more porous appearance, which is now rendered more capable of absorbing liquids. On the other hand, a similar alteration takes place _within_ the pelt, to the extent to which the tanning matter has penetrated it. How far the penetration has proceeded is easily determined by utilising the different adsorption of coal-tar dyes by untanned and tanned pelt (see p. 121). An indicator for those synthetic tannins, which are derived from the phenols, is ferric chloride, which only colours those parts of the pelt which have been penetrated by the synthetic tannins; clearer and better results are, however, obtained when the dyestuffs referred to above are employed.

As soon as the tanning matter has completely penetrated the pelt, the total time of tannage is noted, and the velocity with which the tanning matter converts the pelt into leather at that particular concentration is thus obtained. The tannage completed, the leather must be well washed in running water to remove excess of synthetic tannin and then dried. On examining the dry leathers, the colour may then be observed, and a cut will give an idea of the tensile strength and the length of fibre of the leather. The tensile strength is, however, not of much value in such a barely tanned leather and cannot be compared with that obtained in leathers tanned on a practical scale. The length of fibre is, however, of some importance, since a special feature of finished leathers tanned with synthetic tannins is the beautifully long fibre--a property which manifests itself when the leather is torn and in which an expression of the quality of the synthetic tannin may be found.

Similarly, tanning experiments combining synthetic and natural tannins may be carried out, the most interesting features of these being the different proportions in which the two products are mixed. Such experiments may be done, for instance, by preparing 2° Bé. solutions of each extract and then mixing them in proportions of, say, 10:90, 20:80, 30:70, etc. Here it is again possible to infer the _tanning intensity_ of the synthetic tannin from the concentration and the time used for tannage.

A further determination of the quality of a synthetic tannin is the capability of the latter of dissolving or precipitating the natural tannins. As is well known, synthetic tannins frequently possess the practically important property of rendering natural tannins easily soluble in water. In some cases, however, synthetic tannins appear to solubilise natural tannins in concentrated solutions; when, however, the latter are diluted, the natural tannin is precipitated with varying completeness, the reason of which is often the presence of excessive acid or the presence of such salts as have no phlobaphene-solubilising properties.

For practical purposes this determination may be carried out by mixing, in different proportions, concentrated tannin solutions and the synthetic tannin; heating the mixture on the water bath for a short time, cooling and finally diluting 10, 20, and 30 gm. of the mixture to 100 c.c., which are then left in measuring cylinders for twelve to twenty-four hours; the amount deposited will then be an indication of the solubilising or precipitating effect exhibited by the synthetic tannin.

Other properties of the synthetic tannins connected with their practical application will be discussed in Part II. of this treatise.