The alkenes are a functional group characterized by the presence of a carbon-carbon double bond. Acyclic alkene hydrocarbons are represented by the empirical formula C_nH_2n. Alkenes were originally called olefins. This name came from olefiant gas, which is an old name for ethylene, the first member of the group. The modern names of the alkenes follow the standard numbering of the carbon atoms forming the chain (eth-, prop-, but-, etc.), with an ending of - ene. In addition, alkenes are also described as "unsaturated," reflecting their ability to react with H₂ gas (in the presence of a metal catalyst) and so to undergo reduction of their carbon-carbon double bond. The product of this reduction is an alkane. The term "polyunsaturated" indicates the presence of more than one carbon-carbon double bond in the molecule.

The alkene carbon-carbon double bond is made from...