Terpene

# Overview
Terpenes are a large and varied class of hydrocarbons, produced primarily by a wide variety of plants, particularly conifers, though also by some insects such as 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".
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.
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.

# Structure and biosynthesis
Terpenes are derived biosynthetically from units of isoprene, which has the molecular formula C5H8.  The basic molecular formulas 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 preferred 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.
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.

# Types
- 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 are: geraniol, limonene and terpineol.
- Sesquiterpenes consist of three isoprene units and have the molecular formula C15H24.  Examples of sesquiterpenes are: farnesol. The sesqui- prefix means one and a half.
- Diterpenes are composed for four isoprene units and have the molecular formula C20H32.  They derive from geranylgeranyl pyrophosphate.  Examples of diterpenes 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.
- 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.
- Tetraterpenes contain eight isoprene units and have the molecular formula C40H56.  Biologically important tetraterpenes 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.

# Agri-chemical use
Research into terpenes has found that many of them possess qualities that make them ideal active ingredients  as part of natural agricultural pesticides.
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