RADICAL BROMINATION OF 1,1-DIPHENYLETHYLENES AND 1,2-DIPHENYLETHYLENES IN 1,2-DICHLOROETHANE

The radical bromination of 1,1- and 1,2-diphenylethylenes in 1,2-dichl oroethane was investigated on the basis of kinetic and product distrib ution data. Whereas the ionic process followed a third-order rate law (second order in Br-2), the radical bromination was second order in Br -2 and zero order in olefin in the reagent concentration range examine d. Significant inverse kinetic isotope effects were found for the brom ination of 3,4'-bis(trifluoromethyl)-1,1-diphenylethylene and cis-1,2- diphenylethylene under these conditions. At variance with the ionic br omination, which gave mixtures of meso- and d,l-dibromides only the me so-dibromides were obtained both from the cis- and trans-1,2-diphenyle thylenes. The kinetic and product distribution data for the radical pr ocess are discussed in terms of a mechanism in which caged radical int ermediates are formed. (C) 1998 John Wiley & Sons, Ltd.