A colourless gaseous hydrocarbon, CH2=CHCH=CH2; m.p. –109°C; b.p. –4.5°C. It is made by catalytic dehydrogenation of butane (from petroleum or natural gas) and polymerized in the production of synthetic rubbers. The compound is a conjugated diene in which the electrons in the pi orbitals are partially delocalized over the whole molecule. It can have trans and cis forms, the latter taking part in Diels-Alder reactions.