C4H8

from The Collaborative International Dictionary of English v.0.48

Tetramethylene \Tet`ra*meth"yl*ene\, n. [Tetra- + methylene.]
   (Chem.)
   (a) A hypothetical hydrocarbon, {C4H8}, analogous to
       trimethylene, and regarded as the base of well-known
       series or derivatives.
   (b) Sometimes, an isomeric radical used to designate certain
       compounds which are really related to butylene.
       [1913 Webster]

from The Collaborative International Dictionary of English v.0.48

butene \butene\ n.
   any of three isomeric singly unsaturated hydrocarbons {C4H8};
   all are used in making synthetic rubbers; -- called also
   {butylene}.
   [WordNet 1.5]

from The Collaborative International Dictionary of English v.0.48

Butylene \Bu"ty*lene\, n. [From {Butyl}.]
   1. (Chem.) Any one of three metameric hydrocarbons, {C4H8},
      of the ethylene series. They are gaseous or easily
      liquefiable; -- called also {butene}.
      [1913 Webster]

   2. the hypothetical divalent radical {-(CH2)4-}; -- used in
      combining forms; as, butylene diamine.
      [PJC]

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