Terpenes (pronounced /ˈtɜrpiːn/ TUR-peen) are a large and varied class of hydrocarbons, produced primarily by a wide variety of plants, particularly conifers,[1] though also by some insects such as termites or swallowtail butterflies, which emit terpenes from their osmeterium.
They are the major components of resin, and of turpentine produced from resin. The name "terpene" is derived from the word "turpentine". In addition to their roles as end-products in many organisms, terpenes are major biosynthetic building blocks within nearly every living creature. Steroids, for example, are derivatives of the triterpene squalene.
When terpenes are modified chemically, such as by oxidation or rearrangement of the carbon skeleton, the resulting compounds are generally referred to as terpenoids. Some authors will use the term terpene to include all terpenoids. Terpenoids are also known as Isoprenoids.
Terpenes and terpenoids are the primary constituents of the essential oils of many types of plants and flowers. Essential oils are used widely as natural flavor additives for food, as fragrances in perfumery, and in traditional and alternative medicines such as aromatherapy. Synthetic variations and derivatives of natural terpenes and terpenoids also greatly expand the variety of aromas used in perfumery and flavors used in food additives. Vitamin A is an example of a terpene.
Terpenes are released by trees more actively in warmer weather, acting as a natural form of cloud seeding. The clouds reflect sunlight, allowing the forest to regulate its temperature.[2]
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Terpenes are derived biosynthetically from units of isoprene, which has the molecular formula C5H8. The basic molecular formulae of terpenes are multiples of that, (C5H8)n where n is the number of linked isoprene units. This is called the isoprene rule or the C5 rule. The isoprene units may be linked together "head to tail" to form linear chains or they may be arranged to form rings. One can consider the isoprene unit as one of nature's common building blocks.
Isoprene itself does not undergo the building process, but rather activated forms, isopentenyl pyrophosphate (IPP or also isopentenyl diphosphate) and dimethylallyl pyrophosphate (DMAPP or also dimethylallyl diphosphate), are the components in the biosynthetic pathway. IPP is formed from acetyl-CoA via the intermediacy of mevalonic acid in the HMG-CoA reductase pathway. An alternative, totally unrelated biosynthesis pathway of IPP is known in some bacterial groups and the plastids of plants, the so-called MEP(2-Methyl-D-erythritol-4-phosphate)-pathway, which is initiated from C5-sugars. In both pathways, IPP is isomerized to DMAPP by the enzyme isopentenyl pyrophosphate isomerase.
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As chains of isoprene units are built up, the resulting terpenes are classified sequentially by size as hemiterpenes, monoterpenes, sesquiterpenes, diterpenes, sesterterpenes, triterpenes, and tetraterpenes.
Terpenes may be classified by the number of terpene units in the molecule; a prefix in the name indicates the number of terpene units needed to assemble the molecule.
Research into terpenes has found that many of them possess qualities that make them ideal active ingredients as part of natural agricultural pesticides.
Terpin hydrate is a derivative of turpentine. An expectorant and humectant, it is commonly used in the treatment of acute or chronic bronchitis and related conditions.
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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