Terpene(redirected from Hemiterpenes)
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any of a class of hydrocarbons whose molecules are composed of isoprene links, C5H8; that is, their composition is (C5H8)n where n = 2,3,4,
Terpenes belong to a broad class of natural compounds, the isoprenoids. Depending on the number of isoprene links per molecule, terpenes are divided into monoterpenes, C10H16 (usually called terpenes); sesquiterpenes, C15H24; diterpenes, C20H32; triterpenes, C30H48; and so on. Terpenes are found in almost all plant tissues—they are contained in essential oils, turpentine, resins, and balsams. They also occur in metabolic products of certain bacteria and fungi and in the secretions of insects. They are usually accompanied by their oxygen-containing derivatives, such as alcohols, ethers, aldehydes, ketones, and acids, which are often called terpenoids. Terpenes are classified according to molecular structure as acyclic or cyclic. The acyclic terpenes, such as myrcene, have an open chain of carbon atoms; the cyclic terpenes—for example, limonene, camphene, and pinenes—contain one or more nonaromatic rings.
Monoterpenes are colorless liquids with a characteristic odor and a boiling point of 150°–190°C. Sesquiterpenes are colorless, viscous liquids or fusible crystalline solids with a boiling point of 230°–300°C and with a weaker but more persistent odor. For example, the odor of ginger results from the presence of the sesquiterpene hydrocarbon zingiberene, and the smell of linden, from the presence of the sesquiterpene alcohol farnesol. The active component of Levant wormseed is the sesquiterpene santonin. The category of diterpene derivatives includes such resin acids as abietic and L-pimaric acids, which are contained in rosin, and the alcohol phytol, the ester of which is a constituent of chlorophyll. The triterpene squalene is found in shark liver. It has been established that sterols and hormones also have a triterpene structure; thus, the formation of the steroid hormone cholesterol from squalene has been shown. Certain tetraterpenoids, such as carotene and xanthophylls, are plant pigments. Gutta-percha and natural rubber may be regarded as polyterpenes.
Terpenes are virtually insoluble in water but are readily soluble in nonpolar organic solvents; they are readily oxidized, polymerized, hydrogenated, halogenated, and isomerized. Acyclic terpenes are readily converted into cyclic terpenes, for example, under the action of dilute mineral acids. The reverse transition is effected under more rigorous conditions. For example, myrcene is obtained commercially by the pyrolysis of p-pinene at 500°C. Terpenes and terpenoids are generally extracted from the starting material by rectification or, as in the extraction of menthol from oil of peppermint, by freezing. Many terpenoids are prepared commercially from more accessible terpenes or from chemical raw material. The terpenes contained in turpentine in quantities up to 95 percent are used for the preparation of camphor, terpineol, terpin hydrate, geraniol, and carvone. (Camphor is also extracted from the oil of the camphor tree.) Citral, which is extracted from certain essential oils, is also obtained by the oxidation of linalool and is synthesized in considerable quantities from isoprene, acetone, and acetylene.
Terpenes and terpenoids are widely used, both in the pure state and in the form of essential oils and resins, as constituents of perfume compositions, and food extracts. Menthol, geraniol, terpin hydrate, and others are used in medicine. Terpenes are also used to prepare lubricants, flotation oil, immersion fluids, and such insecticides as polychloropinene and polychlorocamphene.
REFERENCESNenijescu, C. D. Organicheskaia khimiia, vol. 2. Moscow, 1963. Page 811. (Translated from Rumanian.)
De Mayo, P. Terpenoidy. Moscow, 1963. (Translated from English.)
Goriaev, M., and I. Pliva. Metody issledovaniia efirnykh masel. Alma-Ata, 1962.
V. N. FROSIN