Influence of alkene structure on the stability of alkene–Br2 complexes: Effect of chlorine substitution

Abstract

The reaction of chloro substituted ethylenes with Br₂ stops at the stage of charge transfer complexes. These complexes were studied spectrophotometrically in the UV-vis region. DFT and ab initio calculations (B3LYP and MP2 levels) were carried out in order to clarify the structures and establish the factors affecting the stability of this kind of complex. The complexes are characterized by a T shape structure with a Br–Br unit lying perpendicular to the CC double bond. B3LYP calculations show a trend in the stabilization energies which fits well with the experimental results. At variance MP2 calculations overestimate the correlation energies of the bromine complexes with trichloroethylene and tetrachloroethylene. The stability of these elusive complexes is mainly influenced by the charge transfer contribution and by a destabilizing effect arising from bromine–chlorine interaction. The geometry and the relative stability of the corresponding ionic intermediates (bromiranium or β-bromocarbenium ions), which have been detected neither by UV nor NMR measurements, were also evaluated through B3LYP calculations.