Alkadienes

Alkadienes are open-chain hydrocarbons with two C=C double bonds; the general formula is CₙH₂ₙ₋₂ (n ≥ 3). They have four hydrogens fewer than the alkane: propane C₃H₈ and propadiene C₃H₄. The name is formed by replacing “-ane” of the alkane with “-diene” and giving the positions of the double bonds: propadiene CH₂=C=CH₂, butadiene-1,3 CH₂=CH–CH=CH₂, 2-methylbutadiene-1,3 (isoprene). Butadiene-1,3 is a gas at normal conditions, isoprene a volatile liquid. S. V. Lebedev made divinyl from ethyl alcohol over a catalyst: 2C₂H₅OH → CH₂=CH–CH=CH₂ + H₂ + 2H₂O; in industry it is also obtained by dehydrogenating n-butane or isopentane. Butadiene-1,3 adds bromine in two ways: 1,2-addition gives CH₂Br–CHBr–CH=CH₂, 1,4-addition gives CH₂Br–CH=CH–CH₂Br, and excess bromine gives 1,2,3,4-tetrabromobutane. Dienes polymerize to synthetic rubbers; combustion: C₄H₆ + 5.5O₂ → 4CO₂ + 3H₂O.