CHAPTER XI

THE FOUNDATIONS OF ORGANIC CHEMISTRY

As the horizon of chemistry widened and its operations extended, it became necessary to treat its subject-matter methodically. Accordingly attempts were made in the various systematic treatises which began to appear in the seventeenth century to group its facts into an orderly and rational arrangement. One of the earliest of such systematic treatises was the _Cours de Chimie_ of Nicolas Lemery, published in 1675. Although this work was styled by Boerhaave “a tumultuary mass of pharmaceutical processes, without any certain design or coherence,” it is noteworthy as being the first of its kind to divide the science into its present main branches of inorganic and organic chemistry.

It may be desirable to indicate, as briefly as possible, the general state of knowledge respecting the chemistry of organic substances down to the early years of the last century. As already mentioned, such substances as acetic acid, turpentine, starch, sugar, certain dye stuffs, and oils, had long been known; and such processes as saponification and fermentation had been practised from very early times. The alchemists had prepared a variety of essential oils, aliphatic ethers, and esters; and the iatro-chemists had obtained benzoic and succinic acids, and acetic acid from wood. Milk sugar was first prepared by Fabrizio Bartoletti in 1619. Grape sugar was first mentioned as occurring in honey by Glauber in 1660. Boyle first detected the presence of a spirit among the products of the destructive distillation of wood. Few of the followers of Stahl occupied themselves with organic products; and it was only towards the end of the phlogistic period that attention was once more directed to products of animal and vegetable origin. Scheele isolated glycerin in 1784, and obtained _ethyl chloride_ by the distillation of a mixture of common salt, pyrolusite, oil of vitriol, and alcohol. _Ethyl acetate_ was first prepared by Lauraquais in 1759. Arvidson obtained _ethyl formate_ in 1777. _Oxalic ether_ was first made by Savary in 1773. What was long known as _oil of wine_ appears to have been first mentioned by Libavius, but its true nature was discovered by Hennel in 1826. The formation of _aldehyde_ was first recognised by Scheele in 1774, and it was in turn investigated by Fourcroy and Vauquelin, Döbereiner, and Gay Lussac; but it was first definitely isolated in 1835 by Liebig, who gave it its name.

The first organic acid known was vinegar (acetic acid), and for a long time all naturally occurring organic acids having a sour taste were regarded as identical with or as forms of vinegar. It was only during the second half of the eighteenth century that it was clearly ascertained that a variety of organic acids exist, perfectly distinct from acetic acid. _Glacial acetic acid_ was first obtained by Löwiz in 1789. Acetic acid, as a product of the destructive distillation of wood, was first obtained by Göttling in 1779. The acetic fermentation has been studied from very early times. Surmises as to the mode in which wine was converted into vinegar are to be met with in the works of Basil Valentine, Becher (1669), Lemery (1675), and Stahl (1667). Priestley, for a time, held the opinion that vinegar contained a vegetable acid air, but he subsequently discovered and corrected his error. The direct conversion of spirit of wine (ethyl alcohol) into acetic acid was studied by Lavoisier and Berthollet, who first clearly recognised that it was a process of oxidation. The quantitative composition of acetic acid was first established by Berzelius in 1814. Many of the acetates have been known from early times. _Verdigris_ is mentioned by Theophrastus, Dioscorides, and Pliny. _Zinc acetate_ was known to Geber, and _potassium acetate_ to Pliny, who mentions its use in medicine. _Ammonium acetate_ was also used in medicine as far back as the beginning of the seventeenth century, and was particularly recommended by the physician, Raymond Minderer. _Sodium acetate_ was prepared by Duhamel in 1736. _Lead acetate_ was known in the fifteenth century, and was styled by Libavius _saccharum plumbi quintessentiale_, in allusion to its sweet taste. What was called by the alchemists _lac virginis_ was a turbid solution of basic lead acetate, and it was frequently used in medicine, more particularly by Goulard in 1760. What we now call _acetone_ was first observed by Libavius, in 1595, and subsequently by Boyle, during the destructive distillation of lead acetate: its formation from other acetates was noticed by Trommsdorff, Derosne, and Chenevix, by whom it was termed pyroacetic spirit. Its true nature and composition were first ascertained by Liebig in 1831.

The formation of tartar in the manufacture of wine has been known from the earliest times. It was regarded as, and originally styled, the _faex vini_. The word “Tartarus” is first met with in alchemistic literature in the eleventh century, and is the Latinised form of an Arabic word. Marggraf, in 1764, recognised that the tartar of wine contained potash; but tartaric acid itself was first isolated by Scheele in 1769.

The _double tartrate of potash and soda_ was first prepared in 1672 by Peter Seignette, an apothecary of Rochelle, and was used by him in medicine. _Tartar emetic_ was discovered by Adrian von Mynsicht in 1631, and its true nature explained by Bergmann in 1773. _Racemic acid_ was first mentioned by a wine manufacturer named Kestner, and was recognised as an acid in 1819. Its relation to tartaric acid, with which it is isomeric, was first explained by Berzelius, who gave it its name.

The naturally occurring oxalates were long considered as identical with tartar. _Oxalic acid_ was obtained by Scheele in 1776 by means of the action of nitric acid upon sugar. This acid was further investigated by Bergmann, who observed its decomposition by heat with the formation of a gas burning with a blue flame. The identity of the naturally occurring oxalic acid with that prepared from sugar was established by Scheele in 1784. The quantitative composition of oxalic acid was first ascertained by Dulong in 1815. _Mucic acid_ was discovered by Scheele in 1780, and was studied by Fourcroy, who gave it the name it now bears. _Pyromucic acid_ was also known to Scheele, and was observed by Hermbstädt and Houton-Labillardière. _Camphoric acid_ was first recognised by Bouillon-Lagrange and Vauquelin. _Suberic acid_ was discovered by Brugnatelli in 1787.

That gum benzoin yielded a product (_benzoic acid_) by sublimation was known in the sixteenth century. It was introduced into medicine by Turquet de Mayerne as _flowers of benzoin_. Scheele showed how this acid might be obtained by wet methods from gum-benzoin. It was detected in Peru-balsam by Lehmann in 1709. Rouelle found it in the urine of the cow and the camel. Liebig, in 1829, detected the difference between _hippuric acid_ and benzoic acid. The characteristic acid in amber (_succinic acid_) was first detected by Pott in 1753.

_Formic acid_ was first isolated by Wray in 1676. _Lactic acid_ was discovered by Scheele in sour milk in 1780. For a time it was regarded as impure acetic acid, until it was detected in muscle juice by Berzelius, and its individuality established. Its true composition was ascertained by Mitscherlich and by Liebig in 1832. _Citric acid_ has been known since the thirteenth century, but it was first definitely isolated by Scheele in 1784. Apple juice was used in medicine in the sixteenth century, and the soda salt of its characteristic acid (_malic acid_) was prepared by Donald Monro in 1767.

It was known to the ancients that extract of gall nuts acquired a black colour when mixed with a solution of iron vitriol; and Boyle and Bergmann ascribed this phenomenon to the presence of a peculiar acid. _Gallic acid_ was first isolated by Scheele in 1785, and its composition established by Berzelius in 1814. _Tannic acid_ was definitely recognised as distinct from gallic acid by Seguin in 1795.

Mellite, or honey-stone, is mentioned in mineralogical treatises in the sixteenth century. That it consisted of the alumina salt of a special acid (_mellic acid_) was shown by Klaproth in 1799.

_Prussian blue_ was accidentally discovered in 1710 by a dyer named Diesbach. Its mode of manufacture was first made known by Woodward in 1724. The peculiar reaction by which it was obtained was made the subject of investigation by many chemists of the period without any decisive result. Scheele observed that, when the salt which occasioned the blue colour with vitriol was distilled with sulphuric acid, a volatile acid, inflammable and soluble in water, was obtained. This acid received from Bergmann the name of _acidum cœrulei berolinensis_, or “Berlin-blue acid,” subsequently shortened by Guyton de Morveau to _prussic acid_. Scheele also prepared the cyanides of silver and ammonium. That prussic acid was free from oxygen was established by Berthollet. Anhydrous prussic acid was first obtained by Von Ittner, who first established its highly poisonous nature. Bolim, in 1802, had previously observed the presence of prussic acid in oil of bitter almonds, the poisonous character of which was known to Dioscorides. Porret first definitely isolated _potassium ferrocyanide_, and subsequently discovered the _thiocyanates_, the quantitative composition of which was ascertained by Berzelius in 1820. That prussic acid was a compound of hydrogen and cyanogen was established by Gay Lussac in 1815.

_Cyanic acid_ was discovered by Wöhler in 1822, in which year also L. Gmelin discovered the _ferricyanides_.

_Fulminating mercury_ was first prepared by Howard in 1800, and _fulminating silver_ by Brugnatelli in 1802. These were recognised by Liebig, in 1822, to contain a peculiar acid, which he termed _fulminic acid_, and which he showed to have the same composition as the cyanic acid discovered by Wöhler. _Uric acid_, so named by Fourcroy, was discovered in gall stones by Scheele in 1776. _Urea_ was first definitely isolated by Fourcroy and Vauquelin in 1799, and was synthetically prepared by Wöhler in 1828.

The bitter principles of plants and their medicinal virtues early attracted attention, but the first attempt to isolate them was made by Fourcroy and Vauquelin in the case of the Peruvian bark, long known for its power as a febrifuge. In 1806 Vauquelin obtained _quinic acid_. _Cinchonine_ was first isolated by Gomes in 1811.

The chemical nature of opium was the subject of numerous inquiries in the early years of the nineteenth century. In 1805 Sertürner detected the existence of _meconic acid_, and in 1817 that of _morphine_, which he recognised as an alkaloid. _Narcotine_ was discovered by Robiquet in 1835. The investigation of other bitter substances was undertaken by Pelletier and Caventou, who in 1818 discovered _strychnine_, _brucine_ (1819), and _veratrine_ (1820).

The contemporaries and immediate followers of Lavoisier were the first to make a systematic attempt to elucidate the chemical nature of organic products of animal origin. To this period belongs the work of Fourcroy and Vauquelin on animal chemistry. Chevreul, a pupil of Fourcroy worked on urine, adipocire, and the animal fats in the first decade of the last century. Kirchhoff in 1811, discovered the method of converting starch into sugar; and Döbereiner, in 1822, described a method of preparing formic acid artificially. Dumas and Boullay, in 1827–1828, prepared a number of new derivatives of ethyl alcohol; and in 1834 Dumas and Peligot studied in like manner the chemistry of methyl alcohol, and pointed out many analogies which their compounds possessed, not only among themselves, but also to inorganic substances.

Although a considerable amount of information as to the existence, modes of occurrence, and properties of bodies found in the animal and vegetable kingdoms had been accumulated by the end of the first quarter of the nineteenth century, no serious attempt was made to study them systematically until after that period. In fact, they were not even regarded as coming within the operations of laws found to be applicable to the products of the inorganic world, by the investigation, of which products, indeed, those laws had been discovered.

Down to 1828 it was considered that inorganic and organic substances were sharply differentiated by the circumstance that, whereas the former might be prepared by artificial means, and even built up from their elements by synthetic processes in the laboratory, the latter could only be formed in the bodies of animals and plants as the result of vital force. In that year Wöhler showed that urea, pre-eminently a product of animal metabolism, could be prepared synthetically from inorganic materials. Other instances of a similar kind were discovered in rapid succession; and the idea that organic substances could alone be formed by vital processes was proved to be invalid. Moreover, large numbers of substances of a character analogous to those produced by physiological action, but not known to occur in the animal or vegetable kingdom, were prepared. There is, therefore, no absolute distinction to be drawn between the chemistry of the inorganic and organic worlds.

At the present day we mean by “organic compounds” simply the compounds of carbon. These are so numerous, and frequently so complex, that it is convenient to group them together and study them as a special section of the science. At the outset it was supposed that only very few elements entered into the composition of organic substances. This, indeed, was held to be a point of fundamental distinction between organic and inorganic compounds. Lavoisier was of opinion that all organic bodies were combinations of carbon, hydrogen, and oxygen. Berthollet first discovered the presence of nitrogen in a product of animal origin. Sulphur and phosphorus were detected later. There is apparently no _à priori_ reason why any element should not be associated with carbon, and enter into the composition of an organic compound.

Lavoisier was one of the first to devise methods for ascertaining the composition of organic (carbon) compounds, and to indicate the general principles by which the proportion of the elements met with in these substances can be ascertained. So imperfectly, however, were these methods worked out that it was not established until the close of the first decade of the nineteenth century that organic compounds even obeyed the law of multiple proportions. Thanks to the efforts of Berzelius, Gay Lussac, and Thénard, and especially of Liebig, in 1830, methods of organic analysis were so far perfected that it became possible to ascertain the empirical composition of these compounds with certainty. This point reached, the development of this section of chemistry proceeded with unexampled rapidity. Not only was the composition of numbers of products, such as sugar, starch, the vegetable acids, certain alkaloids, etc., established, but altogether unlooked-for facts became manifest. One of the most surprising of these was that of _isomerism_.

Up to the close of the first quarter of the nineteenth century it seemed self-evident that substances of the same percentage composition are necessarily identical. In 1823 Liebig showed that the silver cyanate of Wöhler had the same composition as silver fulminate. Faraday, in 1825, found a hydrocarbon in oil gas, which had the same composition as olefiant gas, but was otherwise different from it; and in 1828 Wöhler discovered that urea and ammonium cyanate—perfectly dissimilar substances—were identical in elementary composition. Lastly, Berzelius found this to be true of tartaric and racemic acids; and he thereupon proposed the term _isomerism_ to denote the general fact. He further pointed out that the phenomenon could only be explained by supposing that the relative positions of the atoms in isomeric compounds are different.

But the influence of molecular or atomic grouping in determining the specific character of a substance is not confined to compounds. The same phenomenon is observed to occur among the elements. It was conclusively established by Lavoisier that the diamond and charcoal are chemically the same thing—both forms of carbon. Scheele showed that graphite was a third form of carbon. Phosphorus, sulphur, and oxygen were subsequently shown to be each capable of existence in various modifications. Instances of this character were grouped together in 1841 by Berzelius under the term _allotropy_.

The recognition of the fact of isomerism exerted a great influence on the development of organic chemistry. It ultimately led to the assumption that particular groups of elements or atomic complexes, so-called _radicals_, were to be found in organic compounds—a conception based originally on Gay Lussac’s discovery of _cyanogen_, a combination of carbon and nitrogen, which was found to behave like a simple substance, such as chlorine, and to give rise to compounds analogous to the corresponding chlorides. This idea of the existence of compound radicals was greatly strengthened by a memorable investigation by Liebig and Wöhler, in 1832, on oil of bitter almonds and its derivatives, in which they showed that these substances might be represented as containing a special group or radical termed _benzoyl_, which behaved like an element. The idea of groups of elements going in and out of combination like a simple substance was not new to chemists: there was not only the case of cyanogen, discovered by Gay Lussac in 1815. The attempt had been made by Dumas and Boullay in 1828 to classify the derivatives of alcohol and ether as compounds containing a common radical _etherin_. Gay Lussac had pointed out that the vapour density of ethyl alcohol seemed to show that it consisted of equal volumes of ethylene and water. Robiquet had also shown that ethyl chloride might be assumed to be a compound of hydrochloric acid and ethylene; and Döbereiner had regarded anhydrous oxalic acid as a combination of carbonic acid with carbonic oxide.

But the investigation of Liebig and Wöhler served to give precision to the conception. It thereby exercised a profound influence on the development of organic chemistry by demonstrating, in effect, that this branch of the science might be regarded as the chemistry of the compound radicals, in contradistinction to inorganic chemistry—the chemistry of the simple radicals. Additional support for this view was afforded by the remarkable research by Bunsen on the so-called _alkarsin_, the “fuming liquor of Cadet”—an evil-smelling substance long known as being formed when an acetate is heated with arsenious oxide. Bunsen showed that this liquid contained a compound radical having arsenic as a constituent; and he prepared a series of derivatives, all of which might be formulated as combinations of this radical, which he termed _cacodyl_. The study of the electrolytic decomposition of the acetates by Kolbe and the discovery of _zinc-ethyl_ by Frankland afforded powerful support to the doctrine of combined radicals.

Although there can be no doubt that this doctrine greatly stimulated the pursuit of organic chemistry, it was gradually perceived that to regard inorganic and organic chemistry as the chemistry respectively of the simple and of the compound radicals was an imperfect and misleading conception of the true relations of the two main divisions of the science. Facts showed that the properties of a substance depend more on the arrangement of its atoms than on their nature. The doctrine of compound radicals was implicitly an attempt to extend the dualistic conceptions of Berzelius to the facts of organic chemistry; and as such it was welcomed by the great Swedish chemist. But dualism was found to have its limitations, even in inorganic chemistry; and these were still more apparent when it was sought to apply it in the other main branch of the science. Attempts were therefore made—notably by the French chemists Laurent, Dumas, and Gerhardt—to formulate organic substances by methods in which the electro-chemical and dualistic conceptions of Berzelius and his followers had no part. How these attempts developed, and how they subsequently grew into the organic chemistry of to-day, will be shown in the second part of this work.

It will be convenient also to delay any account of the personal history of the creators of the science of organic chemistry—Liebig, Wöhler, Dumas—until we are in a position to give a fuller statement of their labours, and of the results which flowed from them. Although the foundations of organic chemistry may be said to have been laid during the closing years of the first half of the nineteenth century, the superstructure was not erected until the second half.