The bicycle monoterpenes may be divided into three classes according to the size of the second ring, the first being a six-membered ring in each class.
1) Thujone and its derivatives, the members of this group which occur naturally are the following: alpha-thujene, thujyl alcohol, thujone, umbellulone, sabinene, sabinol.
2) Carane and its derivatives, it appears that only three carane derivatives occur naturally: car-3-ene occurs in Swedish pine needle oil, when treated with hydrogen chloride it forms a mixture of sylvestrene dihydrochloride and dipentene dihydrochloride. (+)-Car-4-ene, occurs in various essential oils. Car-3-ene-5:6-epoxide, occurs in certain essential oils.
3) Carone, is a sythetic compound, it can be prepared by the action of hydrogen bromide on dihydrocarvone, which was then treated with ethanolic potassium hydroxide, whereupon carone was obtained.
4) Pinane and its derivatives. Pinane, can be prepared by the catalytic hydrogenation of either alpha pinene or beta pinene. Pinane exists in two geometrical isomeric forms, cis and trans, and each of these exists as a pair of enantiomorphs.
5) alpha Pinene, this is the most important member of the pinane class, it occurs in both the (+)- and (-)- forms in all turpentine oils.
6) Camphor, C10H16O, this occurs in nature in the camphor tree and is optically active; the (+)- and (-)-forms occur naturally, and so does racemic camphor, which is the usual form of synthetic camphor from alpha pinene. Synthetic camphor is usually obtained as the racemic modification.
7) Borneols, C10H18O, the borneols occur widely distributed in essential oils, both borneols and isoborneol are produced when camphor is reduced.
8) Camphene, C10H16, occurs naturally in the (+)-, (-)-, and (+-)-forms. It can be prepared by the removal of a molecule of hydrogen chloride from bornyl and isobornyl chlorides by means of sodium acetate, or by the dehydration of the borneols with potassium hydrogen sulphate.
9) Fenchone, C10H16O, occurs in oil of fennel, it is optically active. When fenchone is reduced with sodium and ethanol, fenchyl alcohol, C10H18O, is produced.
The sesquiterpenes, in general, form the higher boiling fraction of the essential oils.
1) Nerolidol, C15H28O, occurs in the oils of neroli, etc. Nerolidol is isomeric with farnesol.
2) Cadinene, C15H24, occurs in the (-)-form in oil of cubebs, etc. On dehydrogenation with sulphur, cadinene forms cadalene, C15H18.
3) Selinenes, C15H24, occurs in celery oil.
4) Eudesmol, C15H26O, occurs in eucalyptus oil. Catalytic hydrogenation converts eudesmol into dihydroeudesmol, C15H28O.
5) Phytol, C20H40O, is an acyclic diterpene; it is produced from the hydrolysis of chlorophyll, and it also forms part of the molecules of vitamins E and K.
6) Abietic acid, C20H30O, is a tricyclic diterpene. The non-steam volatile residue from turpentine is known as rosin (or colophony) and consists of a mixture of resin acids which are derived from the diterpenes. Abietic acid is one of the most useful of these acids.