In the same year, Tilden3) found that the hydrocarbon of turpentine oil combines with nitrosylchloride to form a well crystallizing compound. Together with Stenhouse, he applied this reaction to the terpenes from the oils of sage, orange, lemon and bergamot. On the behavior of these hydrocarbons to nitrosyl-chloride he based a new classification, concerning which he makes the following statement:
"The natural terpenes are colorless limpid liquids which vary in specific gravity from about 0.84 to about 0.86. They are divisible into two groups as follows:
1. Turpentine group: b. p. 156° to 160°; m. p. of nitroso-derivative 129°; form solid crystalline hydrated terpin C10H20O2H2O.
2. Orange group: b. p. 174° to 176°; m. p. of nitroso-derivative 71°; form (by Wigger's process) no solid crystalline terpin hydrate.4)
The liquids included in each group are allotropic modifications of the same hydrocarbon distinguished one from another by their various rotatory action on the polarized ray. It will, however, be found I believe that the terpenes from several different plants will on further examination be conclusively proved to be really identical and not simply isomeric. This, I believe, to be the case with the terpenes from French turpentine and sage, also with the terpenes from orange peel, bergamot and lemon."
Tilden's prediction, that the number of terpenes would be shown to be much smaller than assumed in his days, has proven itself true. His classification, however, was insufficient, for it included only a small number of terpenes. Indeed the material at hand was not sufficiently sifted for an attempt of that kind. It consisted of a large number of disconnected observations, the study of which was rendered difficult by an arbitrary nomenclature. Only by a systematic exploration of this disorderly realm could a clear insight into the subject be gained.
1) Kekule's formula for camphor was based on the same consideration.
2) Compt. rend. 80 (1875), 1446.
3) Journ. chem. Soc. 28 (1875), 514; Ibidem, 31 (1877), 554; Pharm. Journ. III. 8 (1877), 191.
That we are able to-day to distinguish sharply between so many terpenes and their derivatives is due primarily to the excellent experimental researches of Otto Wallach, the founder of modern terpene chemistry.
Inasmuch as it was impossible to isolate the numerous ter-penes boiling between 155 and 185° by fractional distillation, methods had to be sought which enabled the characterization of these hydrocarbons, even in mixtures, by means of crystalline derivatives. Only after the characterization of the numerous isomers was accomplished was it possible to study successfully the relation of one terpene to another, the relation of the ter-penes to their oxygenated derivatives, and the problem of their constitution.
These problems have been solved in so far that it is now possible to identify many if not most terpenes without great difficulty. The inversions, or changes from one to the other, are also better understood. The problem of their constitution, however, is still far from a satisfactory solution, though structural formulas have been proposed for a number of the terpenes.
In 1884 Wallach1) began his researches on this subject with the investigation of the oil of wormseed (Oleum cinae). Three years later he was in a position to characterize eight terpenes by means of crystalline derivatives (tetrabromides, hydrochlorides, hydrobromides, nitrosites etc.), viz. pinene, camphene, limonene, dipentene, sylvestrene, terpinolene, terpinene and phellandrene.
1) Wallach's contributions are to be found in the following volumes of Liebig's Annalen: 225, 227, 230, 238, 239, 241, 245, 246, 252, 253, 258, 259, 263, 264, 268, 269, 270, 271, 272, 275, 276, 277, 278, 279, 281, 284, 286, 287, 289, 291, 296, 300, 302, 305, 306, 809, 312, 313, 314, 315, 319, 323, 324, 327, 329, 331, 332, 336, 339, 340, 343, 345, 346, 347, 350, 353, 356, 357, 359, 360. - Several papers from his pen are to be found in the Berl. Berichte 23 (1890) to 40 (1907).
To these fenchene was added later. The sesquiterpenes were also included in his investigations. Three of these were well characterized, viz., cadinene, caryophyllene and clovene. The reactions that were applied were for the most part first studied in connection with simpler substances. Thus a series of contributions resulted including amylene, indene, methylindene, ane-thol, isosafrol etc.
Hand in hand with these investigations, researches were conducted with the oxygen derivatives of the terpenes which have a greater capacity for reaction. Of these even less was known. Like the terpenes, they were characterized by crystalline derivatives.
The genetic relationship between the terpenes themselves and also that existing between the terpenes and the oxygenated constituents of the volatile oils was revealed by transforming the one into the other. These changes were of the greatest importance in the study of the chemical constitution of these compounds. Moreover, this study yielded numerous new compounds, among them such as methylheptenone, which are found in volatile oils. Mention may here be made of the relation existing between the terpenes, terpin hydrate and the terpineols; between the members of the carvone group; of cineol; and of the first synthetic oxide of a terpene, the pinol and its hydrate. Mention may also be made of the more recent work on terpinene, the terpinenol, terpinene terpin and terpinene cineol and their relation to sabinene and thujene. As a chapter by itself, mention should be made of the study of the camphor and fenchone groups, which were made possible by the discovery of the latter ketone and its optical isomers. These studies led to the preparation of numerous new derivatives as did also those connected with the discovery of thujone.
When the cleavage of pulegone made 1,3-methylheptenone a readily accessible substance, the study of alicyclic compounds resulted in a considerable number of papers. These substances are mostly closely related to the terpenes and their derivatives. Hence many a reaction was first studied in connection with these simpler substances in order to be applied later to the more complicated terpenes. These simpler substances, e. g. isopropyl hexenone, nopinone, sabinaketone having been obtained from other terpene derivatives, were in turn used for synthetic pur- poses. In this connection methods of condensation were studied, especially those with brom substitution products of the fatty acids. Thus were accomplished the syntheses of menthene, phellandrene, terpinene, &-pinene, fenchene, and of homologues of the terpenes.