Any of a group of unsaturated hydrocarbons that have carbon atoms in chains linked by one or more double bonds and that have the general formula CnH2n. Lighter alkenes, such as ethylene, are derived from petroleum by cracking. Also called olefin. ♦ The group of alkenes as a whole is called the alkene series or the ethylene series. Its first five members are ethylene (or ethene), propylene (or propene), butylene (or butene), pentene, and hexene.