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The large group of the essential oils, containing hydrocarbons similar to oil of turpentine, and classed under the generic term of terpenes, might, from their origin and mode of occurrence, be expected to be allied in constitution to the aromatic compounds; and such is found to be the case. The terpenes are isomeric hydrocarbons of the formula C10H16. They are found sometimes singly, at other times mixed, in a great variety of plants, associated with sesquiterpenes C15H24, and oxygenated substances, such as camphor, borneol, menthol, etc., some of which have long been known and valued for their medicinal properties and technical applications. The elucidation of their constitution has taxed the skill of many workers during the past thirty years; but, thanks to the labours of Wallach, Baeyer, Perkin, Tiemann, Bredt, Komppa, and others, an insight has been gained into their nature and analogies. They are apparently all cyclic compounds with certain attributes which connect them with hydrocarbons of the aliphatic series. Pinene, the characteristic constituent of oil of turpentine, obtained by distilling the resinous exudations of many species of pines, exists in two modifications, distinguished by differences in optical activity, known respectively as australene, found in American, Russian, and Swedish turpentine, and terebenthene, found mainly in French turpentine. It would seem from their empirical formulæ, as well as from their association in nature, that the terpenes and camphor, which Dumas first showed to have the composition C10H16O, should be closely allied in constitution, and that it ought to be readily possible to effect their mutual transformation. The constitution of camphor was long one of the standing problems of organic chemistry, and dozens of formulæ have been suggested at various times during the last twenty years in explanation of its structure. That it contained a benzene nucleus seemed to be proved by the ease with which it yielded toluene, cymene, and other benzene homologues. The first real insight into its structure was gained when Bredt ascertained the constitution of camphoronic acid, C6H11 (CO2H)4—a product, together with camphoric acid, of the oxidation of camphor—which he found broke up into trimethylsuccinic acid and isobutyric acid, and the structure of which was established by Perkin and J. F. Thorpe.
The result of the Japanese monopoly has been to greatly enhance the price of natural camphor; during the last ten years it has practically trebled. This has naturally stimulated endeavours to prepare this substance by synthetical means. Artificial camphor is now made from pinene by transforming the hydrocarbon into bornyl chloride by the action of hydrochloric acid. From this camphene is prepared; by treatment with glacial acetic acid it forms isobornyl acetate. On hydrolysis this is transformed into isoborneol, which by oxidation yields camphor, differing from the naturally occurring variety only in the fact that it is optically inactive. All so-called aromatic compounds are not necessarily cyclic systems, for it has been recognised within the past few years that some of the most valuable natural perfumes, such as that of the rose, lavender, and orange blossom, lemon-grass, geranium, ylang-ylang, neroli, etc., owe their characteristic aroma to the presence of terpenes and camphors, which are not strictly benzenoid or cyclic compounds, but “ruptured rings” behaving like open-chain or aliphatic substances. To judge from past experience, it may confidently be stated that, now the constitution of these substances is understood, their synthetical preparation on an industrial scale is practicable. The discovery by Cahours in 1844 that oil of wintergreen is substantially methyl salicylate led to its artificial production from synthetically prepared salicylic acid. Sir William Perkin in 1868 effected the synthesis of coumarin, the aromatic principle in woodruff and hay. Fittig and Mielck in 1869 synthesised heliotropin, and in 1871 Tiemann and Haarmann obtained vanillin, the characteristic aromatic body in the vanilla pod, by synthetic means, and established its manufacture on a commercial scale. The chemical nature of the characteristic odoriferous substances in oil of cumin, anise, rue, cinnamon, heliotrope, jasmine, violet, parsley, etc., has now been established and some of them are made industrially. The artificial essence of violets known as ionone, prepared by Tiemann in 1893, and now made commercially, is similar but not identical in structure with the true perfume—irone. What is known as artificial musk is a trinitro-butyl toluol. Artificial orange-flower oil is a methyl ester of anthranilic acid.
In Vol. I. a short account has been given of the early history of the large and important group of vegetable products known as the alkaloids. Many of these have long been valued on account of their powerful physiological action. As they all contain nitrogen and are generally basic, they were regarded by Berzelius, and subsequently by Liebig and Hofmann, as akin to ammonia in constitution, and were classed as amines. The first experimental evidence of their nature was obtained by Gerhardt, who found that, when strychnine and certain of the alkaloids belonging to the quinine group are treated with potash, an oil was obtained which he termed quinoline, and which was recognised by Hofmann as identical with a substance obtained in 1834 from coal-tar by Runge, and at that time known as leucol. By other modes of treatment certain alkaloids—e.g., nicotine and conine—are found to yield pyridine, a basic substance found by Anderson, in 1846, in the fœtid liquor obtained by distilling bones, and since found in coal-tar. Others of them—e.g., papaverine, narcotine, etc.—yield isoquinoline, an oil also discovered in coal-tar, by Hoogewerff, and Van Dorp, in 1885. These three substances—quinoline, isoquinoline, and pyridine—constitute so many nuclei in the constitution of a large number of alkaloids. Pyridine resembles benzene in being a cyclic compound, consisting of five carbon atoms and one nitrogen atom. Quinoline stands to pyridine in much the same relation that naphthalene stands to benzene. It can be obtained synthetically, as first shown by Koenigs and Skraup, and subsequently by Doebner and Von Miller, from benzene derivatives.
Isoquinoline, isomeric with quinoline, differs from that substance in the position of the nitrogen atom. It, too, has been synthetically prepared from benzene derivatives in a number of ways.
Among the naturally occurring pyridine alkaloids may be named piperine, found in black pepper, and conine, the poisonous principle of hemlock (conium maculatum). The latter alkaloid was prepared synthetically by Ladenburg in 1886; as first obtained it differed from the naturally occurring product, which is dextro-rotatory, in being optically inactive. Ladenburg surmised that the synthetic preparation stood to the naturally occurring compound in the same relation that racemic acid stood to tartaric acid, and that, by treatment in the manner employed by Pasteur, the racemic modification of conine might be separated into its dextro- and lævo-constituents. This was found to be the case; but the separated dextro component was now found to be distinctly more optically active than the pure, natural variety. It was, in fact, an isomeric modification—iso-conine. By heating this to 300° it was transformed into ordinary conine, identical in all respects with the natural alkaloid. Ladenburg has also effected the synthesis of piperine by condensing piperidine and piperinic acid.
Nicotine, the alkaloid of tobacco, was discovered by Posselt and Reimann in 1828. Its constitution was first ascertained by Pinner, and it was synthetically obtained by Amé Pictet, in 1904, as an inactive substance, capable of being resolved by the crystallisation of its tartrates into a dextro- and lævo-modification, the latter of which was identical with that found in the tobacco leaf.
Atropine and hyoscyamine—the poisonous principles of belladonna and henbane—are isomeric alkaloids, the former of which is optically inactive, and the latter is lævo-rotatory. Atropine is, in fact, the racemic modification. The constitution of both alkaloids is known, and their synthesis is now possible.
The successive steps may be thus indicated:
1. Synthesis of glycerin (Faraday, Kolbe, Melsens, Boerhave, Friedel, and Silva).