Terpenes are the most common hydrocarbons found in the essential oil of dry brown plants.
A wide variety of cyclic terpene hydrocarbon are known and, as multiples of (C5H8), these have fewer double bonds than the open-chain terpenes.
The chemical structure of the terpenes was established by Wallach in 1887, by proposing the so-called "isoprene rule", by which the terpenes are considered to be fusions of two or more isoprene (C5H8) unities. The general formula of terpenes is (C5H8)n. If n=2 the (C10) compounds are called monoterpenes; if n=3 they are known as sesquiterpenes and when n=4 as diterpenes. Some triterpenes (C30) and tetraterpenes (C40) are also known, as, for example, the squalene, an acyclic triterpene (C30H50) and the lycopene, an acyclic tetraterpene (C40H56), or the bicyclic tetraterpene, the b-carotene.
|dipentene (C10H16)||cadinene (C15H24)|
limonene ( d and l )
b-phelandrene ( d and l )
|Bicyclic||camphene ( d and l )
D3-carene ( d and l )
a-pinene ( d, l and dl )
b-pinene ( d and l )
sabinene ( d and dl )
Ruzika (1959) developed the terpenoid theory which predicts that the biogenesis of terpenes probably takes place in four phases: the formation of the C5 precursor units; the linking together to form the primary terpenoid structure; cyclization; and specific rearrangements involving oxidation, reduction, hydroxylation.