Kinetics and mechanism of bromination of phenols by sodium bromate-sodium bisulphite reagent in water-acetonitrile mixture

532-534Kinetics of bromination of phenols with HOBr produced in situ from sodium bromate-sodi um bisulphite reagent has been studied in water-acetonitrile mixture iodometrically. The order of the reaction is found to be unity in [HOBr] and fractional in [phenol]. HOBr has been established as the effective brominating species. A mechanism involving a six-membered cyclic adduct between phenol and HOBr, which decomposes in a slow step has been proposed. Thermodynamic parameters for the format ion of adduct and the activation parameters for the first order decomposition of the adduct have been evaluated and discussed. The influence of the substituents on the first order decomposition rate constant (k/s-1) of the adduct has been discussed. The sole reaction product has been identified as the corresponding o-bromophenol