DIFFERENT REVERSIBILITY OF BROMONIUM VS BETA-BROMOCARBONIUM IONS FORMED DURING THE ELECTROPHILIC BROMINATION OF SUBSTITUTED STILBENES - EVIDENCE FOR RATE DETERMINATION DURING THE PRODUCT-FORMING STEP

In order to check the relative reversibility of brominium and beta-bromocarbonium ion formation, the product distributions of the reactions of Br2 with cis- and trans-stilbene (1b and 2b) and with their p-methyl (1a and 2a), p-(trifluoromethyl) (1c and 2c), and p,p'-bis(trifluoromethyl) derivatives (1d and 2d) in 1,2-dichloroethane have been determined in the 10(-1)-10(-4) M concentration range. The rate constants (k3) for the bromination of these stilbenes at 25-degrees-C spanned 7 orders of magnitude. The observed dibromide ratios showed that an open beta-bromocarbonium ion was the intermediate of the bromination of 1a and 2a, whereas bridged or partially bridged ions were involved with all other olefins. In these cases, the extent of bridging increased, and equilibration to the more stable trans bromonium ion occurred at the lowest reagent concentrations to give the erythro (or meso) dibromide 3 as the main product. The increase in 3 was always accompanied by an increase in cis-trans olefin isomerization during the bromination of the cis olefin. This is attributed to a return of the trans bromonium ion to the corresponding olefin. This process was maximum with 1d, and minimum with 1a. It is concluded that, whereas the formation of the beta-bromocarbonium ion intermediate is completely rate-determining in the case of 2a, both the formation of the trans bromonium ion and the following dibromide formation are partially rate-determining in the case of 2d.