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]

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]