The principal hydrocarbons occurring in volatile oils are alicyclic. The composition of by far the greater number is represented by the formula C10H16, hence they belong to that class of the terpenes in which the carbon atoms are arranged in a cycle. As to their formation in the plant organism nothing definite is known. Possibly they are genetically related to oxygenated chain compounds C10H18O, since these yield artificially hydrocarbons C10H16 upon dehydration. Possibly they are formed from carbohydrates or proteins. It is noteworthy, however, that the terpene content of an oil is greater, the less developed the plant at the time of distillation.

The majority of the known hydrocarbons of the terpene group are found ready formed in nature. Thus the presence of a- and &-pinene, camphene, limonene, dipentene, terpinolene, a-and y-terpinene, a- and B]-phellandrene, sylvestrene, and sabinene has been definitely established. With the exception of the inactive terpinene and terpinolene, these hydrocarbons exist, for the most part, in both optically active forms.

In as much as it is frequently necessary to work with small amounts, it is often impossible to isolate the terpenes in pure form by means of fractional distillation. As a rule it suffices to remove oxygenated compounds from the fraction 150 to 180° by repeated distillation over sodium. A further Separation can be effected by distillation since the bicyclic hydrocarbons boil between the temperatures of 150 and 170°, the monocyclic between 170 and 180°. Both classes of terpenes also reveal appreciable differences in their molecular refraction. The physical constants having been determined, more or less definite conclusions can be drawn as to the nature of the terpenes under investigation, thus leading to the preparation of derivatives characteristic of the hydrocarbons in question. These derivatives are described in connection with the respective hydrocarbons.

Owing to the complicated structure of their molecules, also to the readiness with which these are inverted into isomeric forms, the synthesis of the terpenes and sesquiterpenes has been coupled with difficulties. A single compound is obtained but seldom, the principal product being associated with a mixture of several isomers. The following compilation of the synthesis, or more correctly artificial preparation, of the hydrocarbons makes no claim as to completeness. In each group the best known representatives only are enumerated.

I. Dehydration of terpene- and sesquiterpene alcohols.

a-Terpineol --> Dipentene1) y-Terpineol -> Terpinolene2)

Terpinenol-4 --> Terpinene3)

Dihydrocarveol -► Limonene, Isolimonene,4) Terpinene5)

. -> Camphene*5)

Isoborneol )

Fenchylalcohol I Fenchene)

Lsofenchylalcohol J

Pinocampheol -> a-Pinene8)

Cedrol -> Cedrene9)

Maalialcohol -► Maalisesquiterpene10)

Caryophyllene alcohol > Clovene.11)

II. Dehydration of ketones. Camphor - p-Cymene12) Fenchone -> /77-Cymene.13)

1) Wallach, Liebig's Annalen 230 (1885), 265; 275 (1893), 104; 291 (1896), 361.

2) Wallach, Ibidem 368 (1909), 10.

3) Report of Schimmel & Co. October 1909, 72.

4) Tschugaeff, Berl. Berichte 33 (1900), 735.

5) Wallach, Ibidem 24 (1891), 3984; Liebig's Annalen 275 (1893), 113.

6) Wallach, Liebig's Annalen 230 (1885), 233; Bertram and Walbaum, Journ. f. prakt. Chem. II. 49 (1894), 8.

7) Wallach, Liebig's Annalen 284 (1895), 331; Bertram and Helle, Journ. f. prakt. Chem. II. 61 (1900), 298; Tschugaeff, Chem. Ztg. 24 (1900), 542.

8) Tschugaeff, Journ. russ. phys. chem. Ges. 30 (1908), 1324; Gildemeister and Kohler, Wallach-Festschrift, Gottingen 1909, p. 136.

9) Report of Schimmel & Co. October 1904, 24.

10) Ibidem November 1908, 137.

11) Wallach, Liebig's Annalen 271 (1892), 294.

12) Fittica, Ibidem 172 (1874), 307.

13) Wallach, Ibidem 275 (1893), 157; 284 (1895), 324.

III. Deammoniation of bases.

Carvenylamine -> Terpinene1)

Thujylamine Thujene2)

Carylamine -> Carvestrene3)

Phellandrene diamine -> p-Cymene.4)

IV. By splitting off hydrohalogen.

Dipentene dichlorhydrate Dipentene5)

Terpinene dichlorhydrate -> Terpinene6)

Sylvestrene dichlorhydrate - Sylvestrene7)

Bornylchloride -> Camphene8)

Monochlorocarvenene -> Terpinene9)

Cadinene dichlorhydrate -* Cadinene10)

. , ., Limonene11)

Limonene tetrabromide -->- '

Ip-Cymene1-)

Pinene dibromide Cymene.13)

V. By splitting off carbon dioxide from acids.

a-Pinene14) Nopinolacetic acid - B-Pinene v2-Isopropylhexenone -> a-Phellandrene1) o-Cresol -> a-Terpinene.2)

Fenchene Acid from sabinaketone and bromoacetic ester-> B-Terpinene15) Bromocamphane carboxylic acid - Bornylene.16)

1) Harries and Majima, Berl. Berichte 41 (1908), 2516.

2) Wallach, Liebig's Annalen 272 (1893), 111; 286 (1895), 99.

3) Baeyer, Berl. Berichte 27 (1894), 3486; Baeyer and Villiger, Ibidem 31 (1898), 1402; comp. Semmler, Ibidem 34 (1901), 717.

4) Wallach, Ibidem 324 (1902), 276.

5) Wallach, Terpene und Campher. Leipzig 1909, p. 84.

6) Wallach, Liebig's Annalen 350 (1906), 148.

7) Wallach, Ibidem 230 (1885), 243, 270.

8) Wallach, Ibidem 230 (1885), 233.

9) Semmler, Berl. Berichte 41 (1908), 4474; 42 (1909) 522. 10) Wallach, Liebig's Annalen 238 (1887), 84; 271 (1892), 297. 11) GodlewskyandRoshanowitsch, Journ.russ.phys.chem.Ges.31(1899),209. 12) Wallach, Liebig's Annalen 264 (1891), 21. 13) Wallach, Ibidem 264 (1891), 9. 14) Wallach, Ibidem 363 (1908), 1; 368 (1909), 1. 15) Wallach, Ibidem 357 (1907), 68; 362 (1908), 285. 16) Bredt, Ibidem 366 (1909), 46.