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. Biochemical modifications such as oxidation or rearrangement produce the related monoterpenoids.

Acyclic

Biosynthetically, isopentenyl pyrophosphate and dimethylallyl pyrophosphate are combined to form geranyl pyrophosphate.

Geranyl pyrophosphate

Myrcene, a monoterpene

Elimination of the pyrophosphate group leads to the formation of acyclic monoterpenes such as ocimene and the myrcenes. Hydrolysis of the phosphate groups leads to the prototypical acyclic monoterpenoid geraniol. Additional rearrangements and oxidations provide compounds such as citral, citronellal, citronellol, linalool, and many others. Many monoterpenes found in marine organisms are halogenated, such as halomon.

Monocyclic

In addition to linear attachments, the isoprene units can make connections to form rings. The most common ring size in monoterpenes is a six-membered ring. A classic example is the cyclization of geranyl pyrophosphate to form limonene.

The terpinenes, phellandrenes, and terpinolene are formed similarly. Hydroxylation of any of these compounds followed by dehydration can lead to the aromatic p-cymene. Important terpenoids derived from monocyclic terpenes are menthol, thymol, carvacrol and many others.

Bicyclic

Geranyl pyrophosphate can also undergo two sequential cyclization reactions to form bicyclic monoterpenes, such as pinene which is the primary constituent of pine resin.

Other bicyclic monoterpenes include carene, sabinene, camphene, and thujene. Camphor, borneol and eucalyptol are examples of bicyclic monoterpenoids containing ketone, alcohol, and ether functional groups, respectively.

Role in climate

Monoterpenes are emitted by forests and form aerosols that can serve as cloud condensation nuclei (CCN). Such aerosols can increase the brightness of clouds and cool the climate.^[1]

Uses

Several monoterpenes have antibacterial activity, such as linalool.

References

1. ^ D. V. Spracklen, B. Bonn, K. S. Carslaw (2008). "Boreal forests, aerosols and the impacts on clouds and climate". Phil. Trans. R. Soc. A. 366 (1885): 4613–26. Bibcode 2008RSPTA.366.4613S. doi:10.1098/rsta.2008.0201. PMID 18826917. http://homepages.see.leeds.ac.uk/~eardvs/papers/spracklen08c.pdf.