Isoprene and some monoterpenes. Centers of asymmetry are starred. Sequiterpenes contain three isoprene units and diterpenes (including vitamin A) four. Tri- and tetraterpenes are common in nature and various natural rubbers can be considered polyterpenes, containing several thousand isoprene units. Many synthetic rubbers imitate this structure.
A terpene is a hydrocarbon that is an oligomer or polymer of isoprene, or a derivative of these. Most terpenes are odorous liquids, reactive, and unstable, and are found in essential oils, especially turpentine. They contain double bonds and usually one or more rings. They include camphor, carotenoids, menthol, and vitamin A. Latex (see rubber) is a polyterpene.