The terpens form a group of compounds the majority of which occur in the plant kingdom; a few terpenes have been obtain from other sources. The simpler mono- and sesquiterpenes are the chief constituents of the essential oils; there are the volatile oils obtained from the sap and tissues of certain plants and trees, the essential oils have been used in perfumery from the earliest times. The di- and tri-terpenes, which are not steam volatile, are obtained from plant and tree gums and resins.

Most natural terpene hydrocarbons have the molecular formula (C5H8)n, such as:

1. Monoterpenes, C10H16
2. Sesquiterpenes, C15H24
3. Diterpenes, C20H32
4. Triterpenes, C30H48
5. Tetraterpenes, C40H64 (carotenoids)
6. Polyterpenes, (C5H8)n

Monocyclic terpenes contain a six-membered ring, the p-cymene structure, all-natural monocyclic monoterpenes are derivatives of p-cymene.

In general, there are four methods of extraction of the terpenes; (i) expression; (ii) steam distillation; (iii) extraction by means of volatile solvents; (iv) adsorption in purified fats. The essential oils so obtained usually contain a number of terpenes, and these are separated by fractional distillation.

MONOCYCLIC MONOTERPENES

• Myrcene, C10H16, is an acyclic monoterpene hydrocarbon that occurs in verbena and bay oils.
• Ocimene, C10H16, us and acyclic compound which contains three double bonds.
• Citral, C10H16O, is widely distributed and occurs to an extent of 60-80% in lemongrass oil. Citral is liquid which has the smell of lemons.
• Ionones, the ionones are related to irone, C14H22O, this occurs in the oil obtained from the orris root.
• Geraniol, C10H18O, is found in many essential oils, particularly rose oil.
• Linalool, C10H18O, is an optically active compound, the (-)-form occurs in rose oil and the (+)-form in orange oil.
• Citronellal, C10H18O, is an optically active compound which occurs in citronella oil
• Citronellol and Rhodinol, C10H20O, (-)-Citronellol occurs in rose and geranium oils, and is a mixture of the two forms: the （+）-form of citronellol is made commercially by reduction of citronellal with sodium or aluminum amalgam; it also occurs in Java citronella oil.
• Alpha terpineol, is an optically active monoterpene that occurs naturally in the (+), (-)- and (+-)-forms; when heated with sulphuric acid, alpha terpineol forms some p-cymene. There are two other terpineols, beta terpineol, and gamma terpineol, both occur naturally.
• Carvone, G10H14O, occurs in various essential oils, spearmint, and caraway oils, in optically active forms and also as the racemic modification.
• Limonene, C10H16, is optically active, the (+)-form occurs in lemon and orange oils, the (-)-form in peppermint oil, and the (+-)-form in turpentine oil, the racemic modification is also known as dipentene.
• Terpinolene, C10H16, occurs naturally, is not an optically active, and it may be prepared by dehydrating alpha terpineol with oxalic acid.
• Phellandrenes, C10H16, there are two phellandrenes, alpha- and beta-, both of which are optically active.
• 1.8 Cineole, C10H18O, occurs in eucalyptus oils, it is isomeric with alpha terpineol, but contains neither a hydroxyl group nor a double bond.
• Menthol and menthone, Menthol, C10H20O, is an optically active compound, but only the (-)-form occurs naturally in peppermint oils.
  Menthone, C10H18O, (-)-menthone occurs in peppermint oil, and it may readily be prepared by the oxidation of (-)-menthol with chromic acid.