Either of two isomers with the formula C4H8: but-1-ene (CH3CH2CH=CH2), which is made by passing 1-butanol vapour over heated alumina, and but-2-ene (CH3CH=CHCH3), which is made by heating 2-butanol with sulphuric acid. They are unpleasant-smelling gases used in the manufacture of polymers. The isomer 2-methylpropene ((CH3)2C=CH2) was formerly called isobutene or isobutylene. See alkenes.