CHAPTER XI

ESSENTIAL OILS AND RESINS

Included in this group are all those substances to which the characteristic odors of plants are due, along with others similar in structure and possessing characteristic resinous properties. They have no such uniformity in composition as is exhibited by the oils which are included among the fats and waxes; but belong to several widely different chemical groups. Furthermore, there is no sharp dividing line between the essential oils and certain esters of organic acids on the one hand and the fats on the other. For example, if an aromatic fluid essence is a light fluid, non-viscid, and easily volatile, it is usually classed with the organic esters; denser liquid substances, of oily or waxy consistency, and with comparatively slight odor and taste are usually fats, while oils of similar physical properties but possessing strong characteristic odors are classed as essential oils, regardless of their chemical composition.

Included in this general class are compounds having a great variety of chemical structures; e.g., hydrocarbons, alcohols, phenols, organic sulfides and sulfocyanides, etc. Many of these compounds are crystalline solids at ordinary temperatures, but melt to oily fluids at higher temperatures. The characteristic property which assigns any given plant extract to this group is that it has a strikingly characteristic odor or taste, often accompanied by some definite physiological effect, or medicinal property.

These compounds may be either secretions or excretions of plants, sometimes normally present in the healthy tissue, and sometimes produced as the result of injury or disease.

The essential oils and the resins often occur associated together in the plant; or, the resins may develop from the oily juice of the plant after exposure to the air.

THE ESSENTIAL OILS

These may be divided, according to their chemical composition, into two major groups; (1) the hydrocarbon oils, or terpenes, and (2) the oxygenated and sulfuretted oils.

The =terpenes= are of three different types, namely: (_a_) the hemiterpenes, C_{5}H_{8}, unsaturated compounds of the valerylene series, of which _isoprene_ (found in crude rubber) is the best-known example; (_b_) the terpenes proper, C_{10}H_{16}, which constitute the major proportion of the whole group; and (_c_) the polyterpenes (C_{5}H_{8})_{_n_}, of which _colophene_ and _caoutchouc_ are the most common examples.

Eleven different terpenes having the formula C_{10}H_{16} have been isolated from various plant juices, and their molecular arrangement carefully worked out. The following three examples will serve as typical of the general structural arrangement of these hydrocarbons:

Limonene Camphene Pinene

H | C /|\ / | \ / | \ / | \ / | \ CH_{3} H / | \ | | / | \ C C /CH_{3} | CH_{3}\ / \\ CH_{3} / | \ H_{3}C \|/ CH_{2} / \\ \ / | \ | C | H_{2}C CH C | CH_{2} | / | | | /| | | | / | | | CH_{3} | HCH | | / | | | | | | | / | H_{2}C CH_{2} CH_{2}=C | CH_{2} | / | \ / \ | / | / | \ / \ | / | / | CH C |/ | | | HC CH C H \ // // \ \ // H_{2}C CH_{3} \ // \ // \ // \ // \ // \ // C | CH_{3}

A discussion of the evidence which supports these formulas as properly represented the molecular arrangements of the various isomeric forms would be out of place here, as its only particular interest is in connection with the medicinal effects of the different compounds. It is clear, however, that they are six-membered hydrocarbon rings, with additional hydrocarbon groups attached to one or more of the carbon atoms in the ring.

Different modifications, or varieties, of the terpenes constitute the main proportions of the oils of turpentine, bergamot, lemon, fir needles, eucalyptus, fennel, pennyroyal, etc.

The =oxygenated essential oils= may be either alcohols, aldehydes, ketones, acids, esters, or phenols, derived from either five-membered or six-membered closed-ring hydrocarbons. They are usually present in the plant oil in mixtures with each other or with a terpene. Since most of them have pronounced physiological or medicinal properties, their structure has been well worked out, in most cases; but it seems to be hardly worth while to present these matters in detail here, as they are of interest chiefly on account of their medicinal properties rather than their botanical functions.

_Borneol_, C_{10}H_{17}OH, and _menthol_, C_{10}H_{19}OH, are typical _alcohols_. The latter is a crystalline substance, which melts at 42°, which is present in peppermint oil, both as the free alcohol and as an ester of acetic acid.

Amyl acetate, CH_{3}·COOC_{5}H_{11}, and linalyl acetate, CH_{3}·COOC_{10}H_{17}, the latter occurring in the oils of lavender and bergamot, are typical esters classed as essential oils.

As examples of the _aldehyde_ oils, benzoic aldehyde, C_{6}H_{5}CHO, "oil of bitter almonds," and cinnamic aldehyde, C_{6}H_{5}CH=CHCHO, found in the oils of cinnamon and cassia, may be cited.

Camphor, C_{10}H_{16}O, is a _ketone_, having the following structural formula:

H | C / | \ / | \ / | \ / | \ / | \ / | \ CH_{2} | CH_{2} | | | |CH_{3}-C-CH_{3}| | | | CH_{2} | C=O \ | / \ | / \ | / \ | / \ | / \ | / C | CH_{3}

There are a considerable number of essential oils which are _phenols_. _Thymol_, C_{6}H_{3}·(CH_{3})·(C_{3}H_{7})·OH, in oil of thyme, and carvacrol, its isomer, in oil of hops, are familiar examples.

O-C=O / | Coumarin, the anhydride of cinnamic acid C_{6}H_{4} | , \ | HC=CH

is an example of an acid substance which is classed as an essential oil, even though it is a solid at ordinary temperatures. It has an odor and flavor similar to that of _vanillin_, the essential flavoring material of the vanilla bean, and is often used as a substitute for the latter in the preparation of artificial flavoring extracts.

Of the =essential oils containing sulfur=, there are two common examples; oil of mustard, allyl isosulfocyanide, C_{3}H_{5}NCS, and oil of garlic, allyl sulfide (C_{3}H_{5})_{2}S. The latter is present in onions, garlic, water cress, radishes, etc., the difference in flavor of these vegetables being due to the fact that the allyl sulfide is united with other different groups in the glucoside arrangement, in the different plants. Similarly, mustard oil is not present in mustard seeds as such, but as a glucoside which, when hydrolyzed by the enzyme _myrosin_ which is always present in other cells of the same seeds, yields C_{3}H_{5}NCS, KHSO_{4}, and C_{6}H_{12}O_{6}.

THE RESINS

The resins were formerly supposed to be the mother substances from which the terpenes are derived. It is now known, however, that they are the oxidation products of the terpenes. Their exact structure is still a matter of some uncertainty, as their peculiar "resinous" character makes them very difficult to study by the usual methods of chemical investigations.

Resins are divided into two classes: (_a_) the balsams, and (_b_) the solid or hard resins. Canada balsam and crude turpentine are familiar examples of the first class. They consist of resinous substances, dissolved in or mixed with fluid terpenes. Ordinary resin, or _colophony_, consists chiefly of a monobasic acid having the empirical formula C_{20}H_{30}O_{2}, known as sylvinic acid, whose exact structure is not known. Its sodium salt is used as the basis for cheap soaps.

The hard resins are amorphous substances of vitreous character, which consist of very complex aromatic acids, alcohols, or esters, combined with other complicated structures, known as _resenes_, whose definite chemical nature is not yet known. Among the hard resins are many substances which are extensively used in the manufacture of varnishes, such as copal, amber, dammar, sandarach, etc.

There are also resinous substances, such as asaf[oe]tida, myrrh, gamboge, etc., which are mixtures of gums (see Chapter VI) and true resins. Some of these have considerable commercial value for medicinal or technical uses.

PHYSIOLOGICAL USES AND BIOLOGICAL SIGNIFICANCE OF ESSENTIAL OILS

No theory has yet been advanced concerning the possibility of the use of essential oils and resins by plants in their normal metabolic processes. The very great diversity in their chemical nature makes it impossible that they should all be considered as having the same physiological function, if indeed any of them actually have any such function.

It is evident that those aromatic compounds which occur as normal secretions of plants and which give to the plants their characteristic odors may act either as an attraction to animals which might utilize the plants as food and so serve to distribute the seed forms, or as a repellent to prevent the too rapid destruction of the leaves, stems, or seeds of certain species of plants whose slow-growing habits require the long-continued growth of these portions of the plant for the perpetuation of the species. The presence of these compounds in larger proportions in those species of conifers, etc., which grow in tropical regions, in competition with other rapid-growing vegetation, suggests the latter possibility. It must be admitted, however, that their presence in such cases may be the result of climatic conditions, as indicated by the fact that most spice plants are tropical in habit, rather than the result of their protective influence in the struggle for survival during past ages.

Many of the oils and resins which are secreted as the result of injury by disease or wounds have marked antiseptic properties and undoubtedly serve to prevent the entrance into the injured tissue of destructive organisms.

But apart from these possible protective influences which may have had an important effect upon the preservation and perpetuation of the species of plants which secrete them, there is no known biological necessity for the presence of these aromatic substances in plants.

REFERENCES

ABDERHALDEN, E.--"Biochemisches Handlexikon, Band 7, Gerbstoffe, Flechtenstoffe, Saponine, Bitterstoffe, Terpene, Aetherische Oele, Harze, Kautschuk," 822 pages, Berlin, 1912.

ALLEN'S Commercial Organic Analysis, Vol. IV, "Resins, Rubber, Guttapercha, and Essential Oils," 461 pages, 7 figs., Philadelphia, 1911 (4th ed.).

HEUSLER, F., trans. by Pond, F. J.--"The Chemistry of the Terpenes," 457 pages, Philadelphia, 1902.

PARRY, E. J.--"The Chemistry of Essential Oils and Perfumes," 401 pages, 20 figs., London, 1899.