Pyrolysis of gem-dichlorides: 2,2-dichlorobutane and 2,2-dichloropropane

Abstract

2,2-Dichlorobutane decomposes at 308–370° to hydrogen chloride and a non-equilibrium mixture of chlorobutenes. In ‘seasoned’ reaction vessels the reaction is homogeneous and of the first-order, and for initial pressures within the range 27–320 mm. the rate is expressible as k₁= 10^14·49 exp(– 50,200/RT)sec.^–1 The results are consistent with a unimolecular mechanism for the elimination of hydrogen chloride, with negligible subsequent decomposition or isomerisation of the chlorobutenes. In contrast to previous results for other gem-dichlorides the Arrhenius parameters are closer to those reported for the corresponding monochlorides. This was also observed for the decomposition of 2,2-dichloropropane, which was re-examined at 322–385° with initial pressures from 30 to 300 mm., and for which the first-order rate coefficient is expressible as k₁= 10^14·52 exp(–51,400/RT)sec.^–1 Substitution of a β-methyl group in 2,2-dichloropropane increases the rate of elimination of hydrogen atoms both from the carbon centre to which the methyl group is attached and also from the carbon centre on the other side of the molecule.