c4h6

from The Collaborative International Dictionary of English v.0.48

Crotonylene \Cro*ton"y*lene\ (kr?-t?n"?-l?n), n. [Crotonic +
   acet-ylene.] (Chem.)
   A colorless, volatile, pungent liquid, {C4H6}, produced
   artificially, and regarded as an unsaturated hydrocarbon of
   the acetylene series, and analogous to crotonic acid.
   [1913 Webster]

from The Collaborative International Dictionary of English v.0.48

butadiene \butadiene\ n.
   a gaseous hydrocarbon {C4H6}; -- it is used as a monomer unit
   in making synthetic rubbers.
   [WordNet 1.5]