Gas-phase eliminations. Part IX. Pyrolysis of αα- and γγ-dimethylallyl chlorides and their interconversion

Abstract

The kinetics of the pyrolyses of αα- and γγ-dimethylallyl chlorides to yield isoprene and hydrogen chloride, together with the isomerisations αα⇌γγ have been investigated over the temperature ranges 255–315°C and 245–305°C respectively. The reactions are homogeneous and unimolecular and obey the following scheme [graphic omitted] with k₁≃k₂k₃≃k₄. Arrhenius equations for the elimination reactions are: αα: k₂= 10^13·30 exp (–42,610/RT)sec.^–1, γγ: k₁= 10^12·03 exp (–38,280/RT) sec.^–1 while the rate constant ratio k₂/k₁ at 294° is ca. 2. This is explained in terms of the γγ-compound undergoing elimination through a six-centred transition state. This is the first time such a transition state, well known in the case of esters and related compounds, has been invoked to explain the mechanism of halide pyrolysis.