Kinetics of the vapour-phase bromination of cyclanes and n-butane. C—H bond dissociation energies in cyclanes

Abstract

The kinetics of the photobromination of cyclanes and n-butane in the vapour phase were studied using a competitive method. Absolute A and E were assigned to the reaction Br + n-butane from which the following were obtained for Br + c-RH →HBr + c-R. [graphic omitted] The maxima in A and E at cyclohexane are discussed in terms of variations in ring tightness and strain in cyclanes and cycloalkyl radicals. It is concluded that cyclohexane is generally less reactive towards free radicals than are cyclopentane of cycloheptane.

The above activation energies are used to derive C—H bond dissociation energies in cyclanes which are compared with values obtained by other methods. Recommended values are: D(c-C₄H₇—H)= 96.5, D(c-C₅H₉—H)= 94.2, D(c-C₆H₁₁—H)= 95.5 and D(c-C₇H₁₃—H)= 92.5 kcal mol^–1.