STEREOSELECTIVITY AND REVERSIBILITY OF ELECTROPHILIC BROMINE ADDITION TO STILBENES IN CHLOROFORM - INFLUENCE OF THE BROMIDE TRIBROMIDE PENTABROMIDE EQUILIBRIUM IN THE COUNTERANION OF THE IONIC INTERMEDIATES

Two equilibria were found in chloroform solutions of Bu4N+Br- and Br2, leading to tribromide and pentabromide salts. The electronic spectra and formation constants of both (K3 = 2.77 (0.13) x 10(4) M-1 and K5 = 3.51 (0.35) X 10(5) M-2 at 25-degrees-C) were computed from spectrophotometric data. The stability of the Br3- species in chloroform resulted to be at least 3 orders of magnitude lower than in 1,2-dichloroethane. A change from prevalent formation of d,l-1,2-dibromo-1,2-diphenylethane to prevalent formation of meso dibromide, accompanied by a cis-trans isomerization of the unreacted olefin, has been observed in the bromination of cis-stilbene with decreasing reagent concentrations, when the bromide-tribromide-pentabromide equilibrium in the counteranions of the ion pairs intermediates is shifted in favor of the Br- form. The results show that these intermediates are reversibly formed even when the anion is Br-.