Allenes

In organic chemistry, allenes are organic compounds in which one carbon atom has double bonds with each of its two adjacent carbon atoms (R₂C=C=CR₂, where R is H or some organyl group). Allenes are classified as cumulated dienes. The parent compound of this class is propadiene (H₂C=C=CH₂), which is itself also called allene. A group of the structure R₂C=C=CR− is called allenyl, while a substituent attached to an allene is referred to as an allenic substituent (R is H or some alkyl group). In analogy to allylic and propargylic, a substituent attached to a saturated carbon α (i.e., directly adjacent) to an allene is referred to as an allenylic substituent. While allenes have two consecutive ('cumulated') double bonds, compounds with three or more cumulated double bonds are called cumulenes.