TERPENES, in organic chemistry, the generic name of a group of hydrocarbons of the general formula (C 5 H 8) n, and the more important oxygen derivatives, mainly alcohols, aldehydes and ketones, derived from them. They may be classified into several distinct groups: hemiterpenes, . C 5 H 8; terpenes proper, CroHis; sesquiterpenes, C15H24; and polyterpenes (C5H8)n. In addition to these, a series of open-chain olefine terpenes is known. The chief sources of the terpenes and their derivatives are the essential oils obtained by the distillation or extraction by pressure of various plants, chiefly of the Coniferae and different species of Citrus. Certain of these oils consist very largely of hydrocarbons; for example, those of turpentine, citron, thyme, orange, pine-needle, goldenrod (from Solidago canadensis) and cypress, while others contain as their chief constituents various alcoholic and ketonic substances. With the exception of camphene, all the terpenes are liquids, boiling approximately between 160° and 190° C., so that it is almost impossible to separate them from the various essential oils by fractional distillation. In order to prepare the individual members pure, advantage is taken of the different physical properties of their derivatives. The terpenes all possess a characteristic odour and are fairly stable to alkalis, but are easily decomposed by acids or by heating to a sufficiently high temperature. Many polymerize readily, or are transformed into isomers by boiling with dilute alcoholic sulphuric acid. Some oxidize rapidly on exposure to air, passing into resinous substances. The formation of addition compounds with the halogens, halogen hydrides, and with nitrosyl chloride, is characteristic of many, whilst others unite readily with nitrogen peroxide. According to A. v. Baeyer (Ber., 1895, 28, p. 648; 1896, 29, p. 10) the nitrosochlorides are not simple addition products, but bimolecular compounds or bisnitrosochlorides.
Hemiterpenes The best known is Isoprene, C 5 H 8, which is obtained on distilling caoutchouc or gutta-percha. It was synthesized by W. Euler (Ber., 97, 30, 1989) by distilling the addition compound of methyl iodide and 2 3 5-trimethylpyrollidine with caustic potash. It is an unstable liquid which boils at 33.5° C., and on heating rapidly polymerizes to dipentene, the same change being effected by hydrochloric acid. In ethereal solution it combines with bromine to form an unstable liquid dibromide; it also unites with one molecule of hydrobromic acid to form the same tertiary bromide as dimethylallylene; this points to its being Q-methyldivinyl, CH 2: C(CH 3)CH: CH 2 (V. A. Mokiewsky, Jour. Soc. Phys. Chim. Russ., 1900, 32, p. 207).
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