**3.1 Terpenoids**

Terpenes are formed from n multiples of the C5H8 isoprene; they are multicyclic structures that differ not only in the functional groups but also in the basic structure of their hydrocarbon skeletons. When n = 2, the terpene corresponds to monoterpenes (C10H16)2. Monoterpenes and their derivatives are therefore linear chains or cycles formed from two isoprene units.
