23.3 TERPENES AND TERPENOIDS Hydrocarbons known generally as terpenes and oxygen-containing compounds called terpenoids are the most important constitutents of essential oils Most terpenes have skeletons of 10.15.20 or 30 carbon atoms and are classified in the following way Monoterpenes 20—— DIterpenes 15—- Sesquiterpenes30—- Triterpenes Recognition of the isoprene unit as a component of the structure of terpenes has been a great aid in elucidating their structures23.3 TERPENES AND TERPENOIDS Hydrocarbons known generally as terpenes and oxygen-containing compounds called terpenoids are the most important constitutents of essential oils. Most terpenes have skeletons of 10,15,20 or 30 carbon atoms and are classified in the following way. 10 15 20 30 Monoterpenes Sesquiterpenes Diterpenes Triterpenes Recognition of the isoprene unit as a component of the structure of terpenes has been a great aid in elucidating their structures