Kinetic Isotope Effects in Aromatic Bromination Reactions

Abstract

Both bromodeprotonation and bromodesulphonation occur during aqueous bromination of sodium p-methoxybenzenesulphonate, A, and potassium l-methylnaphthalene-4-sulphonate, B. Extensive kinetic studies reported here suggest that bromodesulphonation of A proceeds by a two-step process with Br2 as the brominating species, but do not completely exclude Br+ (or H2OBr+) acting in either a one- or two-step process. For B, the kinetic data can be interpreted by either a one- or two-step process with Br2 as the brominating species. Kinetic sulphur isotope effects have been measured for the bromodesulphonation of A and B and found to vary with bromide-ion concentration, thus strongly supporting the two-step process involving molecular bromine. The kinetic results for the bromodeprotonation of A cannot distinguish between a one- and two-step process involving Br2l the two-step mechanism has been confirmed by the observation of a variation in kinetic hydrogen isotope effect with bromide-ion concentration.