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Terpenes are a large and diverse class of organic compounds, produced by a variety of plants, particularly conifers,though also by some insects such as termites or swallowtail butterflies, which emit terpenes from their osmeteria. They are often strong-smelling. They may protect the plants that produce them by deterring herbivores and by attracting predators and parasites of herbivores.Many terpenes are aromatic hydrocarbons and thus may have had a protective function.The difference between terpenes and terpenoids is that terpenes are hydrocarbons, whereas terpenoids contain additional functional groups.

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 fragrances in perfumery, and in medicine 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 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.

The aroma and flavor of hops, highly desirable in some beers, comes from terpenes. Of the terpenes in hops myrceneβ-pineneβ-caryophyllene, and α-humulene are found in the largest quantities.

TYPES OF TERPENES

Terpenes may be classified by the number of isoprene units in the molecule; a prefix in the name indicates the number of terpene units needed to assemble the molecule.

Hemiterpenes consist of a single isoprene unit. Isoprene itself is considered the only hemiterpene, but oxygen-containing derivatives such as prenol and isovaleric acid are hemiterpenoids.

Monoterpenes consist of two isoprene units and have the molecular formula C10H16. Examples of monoterpenes and monoterpenoids include geraniol, terpineol (present in lilacs), limonene (present in citrus fruits), myrcene (present in hops), linalool (present in lavender) or pinene (present in pine trees).

Sesquiterpenes consist of three isoprene units and have the molecular formula C15H24. Examples of sesquiterpenes and sesquiterpenoids include humulene, farnesenes, farnesol. (The sesqui- prefix means one and a half.)

Diterpenes are composed of four isoprene units and have the molecular formula C20H32. They derive from geranylgeranyl pyrophosphate. Examples of diterpenes and diterpenoids are cafestol, kahweol, cembrene and taxadiene (precursor of taxol). Diterpenes also form the basis for biologically important compounds such as retinol, retinal, and phytol.

Sesterterpenes, terpenes having 25 carbons and five isoprene units, are rare relative to the other sizes. (The sester- prefix means half to three, i.e. two and a half.) An example of a sesterterpenoid is geranylfarnesol.

Triterpenes consist of six isoprene units and have the molecular formula C30H48. The linear triterpene squalene, the major constituent of shark liver oil, is derived from the reductive coupling of two molecules of farnesyl pyrophosphate. Squalene is then processed biosynthetically to generate either lanosterol or cycloartenol, the structural precursors to all the steroids.

Sesquarterpenes are composed of seven isoprene units and have the molecular formula C35H56. Sesquarterpenes are typically microbial in their origin. Examples of sesquarterpenoids are ferrugicadiol and tetraprenylcurcumene.

Tetraterpenes contain eight isoprene units and have the molecular formula C40H64. Biologically important tetraterpenoids include the acyclic lycopene, the monocyclic gamma-carotene, and the bicyclic alpha- and beta-carotenes.

Polyterpenes consist of long chains of many isoprene units. Natural rubber consists of polyisoprene in which the double bonds are cis. Some plants produce a polyisoprene with trans double bonds, known as gutta-percha.

Norisoprenoids, such as the C13-norisoprenoids 3-oxo-α-ionol present in Muscat of Alexandria leaves and 7,8-dihydroionone derivatives, such as megastigmane-3,9-diol and 3-oxo-7,8-dihydro-α-ionol found in Shiraz leaves (both grapes in the species Vitis vinifera) or wine (responsible for some of the spice notes in Chardonnay), can be produced by fungal peroxidases or glycosidases.