Temperature dependence of the rate of reaction of OH with some aromatic compounds in aqueous solution. Evidence for the formation of a π-complex intermediate?

Abstract

The rates of reaction of OH with benzene, chlorobenzene, nitrobenzene, ion and benzoic acid have been measured in aqueous solution up to 200 °C using pulse radiolysis to generate OH. The temperature dependence of the observed rate constant, k_obs, is essentially the same for each compound and k_obs changes by less than three-fold between 20 °C and 200 °C. The kinetic data are consistent with a mechanism whereby OH reversibly forms a π-complex with the aromatic compound, irrespective of the substituent on the ring, which then transforms to a σ-bonded hydroxycyclohexadienyl radical. The values of k_obs were determined from the rate of formation of this radical. There is no evidence for dissociation of the σ-bonded radical nor for H atom abstraction from the ring which have been reported for the gas phase. The apparent mechanistic differences between the two phases may be due to the different timescales over which the kinetics measurements were made.