Alkenes are unsaturated hydrocarbons. Their molecules have two hydrogen atoms less than the corresponding members of alkanes. Thus, their general formula is CnH2n· The functional group in alkenes is carbon-carbon double bond (C = C). Alkenes are also known as olefins (derived from Greek word olefiant means oil forming). It is because of the fact that lower members of this series form oily products when treated with chlorine.