An unsaturated aliphatic hydrocarbon that has the general formula C_nH_2n. They comprise one or more carbon–carbon double bonds. They were formerly known as olefins. The series starts with ethene (ethylene) with the formula C₂H₄, followed by propene (propylene) C₃H₆, butene (butylene) C₄H₈, etc. Isomerism occurs with the higher alkenes beginning with butane for which there are two isomers: but-1-ene and but-2-ene that differ by the position of the double bond. Alkenes can undergo polymerization to form thermoplastics such as polyethene (polyethylene).