The terpinenes are a group of

isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same number of atoms of each element – but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers. Iso ...

ic hydrocarbons that are classified as

monoterpene Monoterpenes are a class of terpenes that consist of two isoprene units and have the molecular formula C10H16. Monoterpenes may be linear (acyclic) or contain rings (monocyclic and bicyclic). Modified terpenes, such as those containing oxygen funct ...

s. They each have the same molecular formula and carbon framework, but they differ in the position of carbon-carbon double bonds. α-Terpinene has been isolated from

cardamom Cardamom (), sometimes cardamon or cardamum, is a spice made from the seeds of several plants in the genera ''Elettaria'' and ''Amomum'' in the family Zingiberaceae. Both genera are native to the Indian subcontinent and Indonesia. They are rec ...

and

marjoram Marjoram (; ''Origanum majorana'') is a cold-sensitive perennial herb or undershrub with sweet pine and citrus flavours. In some Middle Eastern countries, marjoram is synonymous with oregano, and there the names sweet marjoram and knotted mar ...

oils, and from other natural sources. β-Terpinene has no known natural source but has been prepared from

sabinene Sabinene is a natural bicyclic monoterpene with the molecular formula C10H16. It is isolated from the essential oils of a variety of plants including Marjoram, holm oak (''Quercus ilex'') and Norway spruce (''Picea abies''). It has a strained ...

. γ-Terpinene and δ-terpinene (also known as terpinolene) have been isolated from a variety of plant sources. They are all colorless liquids with a turpentine-like odor.

Production and uses

α-Terpinene is produced industrially by acid-catalyzed rearrangement of α-

pinene Pinene is a collection of unsaturated bicyclic monoterpenes. Two geometric isomers of pinene are found in nature, α-pinene and β-pinene. Both are chiral. As the name suggests, pinenes are found in pines. Specifically, pinene is the major compo ...

. It has perfume and flavoring properties but is mainly used to confer pleasant odor to industrial fluids. Hydrogenation gives the saturated derivative ''p''-menthane.

Biosynthesis of α-terpinene

The biosynthesis of α-terpinene and other terpenoids starts with the isomerization of

geranyl pyrophosphate Geranyl pyrophosphate (GPP), also known as geranyl diphosphate (GDP), is the pyrophosphate ester of the terpenoid geraniol. Its salts are colorless. It is a precursor to many natural products. Occurrence GPP is an intermediate in the isoprenoid ...

to linalyl pyrophosphate (LPP). LPP then forms a resonance-stabilized cation by loss of the pyrophosphate group. Cyclization is then completed thanks to this more favorable stereochemistry of the LPP cation, yielding a terpinyl cation. Finally, a 1,2-hydride shift via a Wagner-Meerwein rearrangement produces the terpinen-4-yl cation. It is the loss of a hydrogen from this cation that generates α-terpinene.

Plants that produce terpinene

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Cuminum cyminum Cumin ( or , or Article title
) (''Cuminum cyminum'') is a

Melaleuca alternifolia ''Melaleuca alternifolia'', commonly known as tea tree, is a species of tree or tall shrub in the myrtle family, Myrtaceae. Endemic to Australia, it occurs in southeast Queensland and the north coast and adjacent ranges of New South Wales wher ...

'' * ''

Cannabis ''Cannabis'' () is a genus of flowering plants in the family Cannabaceae. The number of species within the genus is disputed. Three species may be recognized: ''Cannabis sativa'', '' C. indica'', and '' C. ruderalis''. Alternatively ...

'' * ''

Origanum syriacum ''Origanum syriacum''; syn. ''Majorana syriaca'' (also ''Origanum maru'', although this primarily refers to a hybrid of ''O. syriacum''), bible hyssop, Biblical-hyssop, Lebanese oregano or Syrian oregano, is an aromatic perennial herb in the mint ...

'' *''

Coriandrum sativum Coriander (;

References

{{Reflist Monoterpenes Dienes Cyclohexenes Cyclohexadienes