TEMPERATURE-DEPENDENCE OF THE RATE OF REACTION OF OH WITH SOME AROMATIC-COMPOUNDS IN AQUEOUS-SOLUTION - EVIDENCE FOR THE FORMATION OF A PI-COMPLEX INTERMEDIATE

The rates of reaction of OH with benzene, chlorobenzene, nitrobenzene, benzoate ion and benzoic acid have been measured in aqueous solution up to 200 degrees C using pulse radiolysis to generate OH. The tempera ture 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 degrees C and 200 degrees C. The kinetic data are consisten t with a mechanism whereby OH reversibly forms a pi-complex with the a romatic compound, irrespective of the substituent on the ring, which t hen transforms to a sigma-bonded hydroxycyclohexadienyl radical. The V alues of k(obs) were determined from the rate of formation of this rad ical. There is no evidence for dissociation of the sigma-bonded radica l nor for H atom abstraction from the ring which have been reported fo r 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.