TERPENES, in chemistry, is the generic name of a number of unsaturated cyclic hydrocarbons, which are usually classified in three groups : terpenes proper, Cialio: sesquiterpenes, C,511,4: and polyterpenes, (C5H8).. Sometimes hemiterpenes, and also a series of so-called olefinic terpenes are included, but the latter are all open-chain compounds. (See CHEMISTRY : Organic.) Many terpenes and sesquiterpenes are widely distributed as constit uents of essential oils, particularly of those secreted by plants belonging to the families Coniferae and Myrtaceae and the genus Citrus. The essential or fragrant oils occur usually in the flowers, fruit, leaves and stems, and sometimes in the roots and seeds of the plants, and differ from the fatty oils, found generally in the seeds, by their volatility in a current of steam. The majority con tain a complex mixture of ingredients, but some are composed mainly of one constituent, as, for example, oil of turpentine which is chiefly pinene. In essential oils the terpenic hydrocarbons are
commonly accompanied by products which usually are either alcohols or ketones and are often of greater importance as per fumes or as medicinal agents than the associated terpenes.
The hemiterpene isoprene is related to the terpenes in several ways. Thus it can be obtained by heating the vapours of limonene or pinene to a high temperature, and on the other hand when isoprene is heated to 300° C in a sealed tube two molecules unite to form one molecule of dipentene, (See POLYMERIZATION.) Theoretically the carbon skeletons of the terpenes proper may be built up from two, and those of the sesquiterpenes from three isoprene nuclei