The Collaborative International Dictionary of English v.0.48:

Hexylene \Hex"yl*ene\, n. [Hex- + -yl + ethlene.] (Chem.)
   A colorless, liquid hydrocarbon, C6H12, of the ethylene
   series, produced artificially, and found as a natural product
   of distillation of certain coals; also, any one several
   isomers of hexylene proper. Called also hexene.
   [1913 Webster]