iso·butylene \| ̷ ̷ ̷ ̷ at iso-+\noun Etymology: International Scientific Vocabulary is- + butylene : the branched-chain gaseous butylene (CH3)2C = CH2 obtainable from isobutane by dehydrogenation and used chiefly in making butyl rubber and gasoline components and in alkylating aromatic hydrocarbons — called also 2-methylpropene