Energetics of the homolytic C-H and C-Cl bond cleavages in polychlorobenzenes: The role of electronic and steric effects

Geometries of (poly)chlorosubstituted benzenes and phenyl radicals are optimized at the BLYP/6-311G** level of theory. The radicals, which are all planar and of the sigma-type, possess geometries that are influenced by both electronic and indirect steric effects. The total energies of the species under study are quantitatively analyzed with simple additive schemes involving the chlorine-chlorine and chlorine-trivalent carbon interactions. Comparisons with the few available experimental data reveal that the computed C-H and C-Cl bond dissociation energies (BDEs) of benzene and its chloro- derivatives are systematically too low by ca. 5 kcal/mol and that the experimental C-Cl BDE of 1,3-dichlorobenzene is most probably in error. The substituents are predicted to facilitate the homolytic C-Cl bond cleavage by up to 6.6 kcal/mol while making the C-H cleavage less favorable by as much as 3.8 kcal/mol. The trends in BDEs are readily accounted for by a superposition of electronic end steric effects. In all cases, the C-Cl bond cleavages are found to require significantly less energy than the C-H ones, implying kinetic control of the aryl radical formation in the course of pyrolysis of (poly)chlorobenzenes.