Intermediates in the solvolysis of benzhydryl p-nitrobenzoate

Abstract

The solvolyses of ring-substituted benzhydryl p-nitrobenzoates in aqueous ethanol and aqueous trifluoroethanol have been investigated. Dissection of rate data into initial state and transition state free energy contributions indicates that the solvent dependence on the rate of solvolysis in aqueous ethanol can be attributed principally to destabilization of the Ar2CHOPNB initial state by more polar solvents. The most sensitive probe for detecting solvent assistance in solvolysis (the 'ethanol-TFE' probe) indicates that nucleophilic solvent assistance does not play a significant role in the solvolysis of Ph2CHOPNB. Enhanced electrophilic solvent assistance is indicated in aqueous trifluoroethanol. To elucidate further the nature of ion pair intermediates in the solvolysis of benzhydryl derivatives an attempt was made to generate the same ion pairs, or ion pair spectrum, by two independent pathways, i.e. the reaction of diphenyldiazomethane with p-nitrobenzoic acid and the solvolysis of benzhydryl p-nitrobenzoate in aqueous ethanol, and product proportions were examined. The selectivity of the intermediate ion pairs toward attack by ethanol and water was examined. The results indicate that the observed selectivity (kE/kW) depends strongly upon the solvent polarity, the leaving group, and the substituents in the aryl rings attached to the carbon atom undergoing substitution. A previously ignored intermediate in the reaction of diphenyldiazomethane with p-nitrobenzoic acid (the nitrogen separated ion pair) has been called upon to help rationalize enhanced selectivity in polar solvents. Results of product studies in acetone-ethanol-water systems were tentatively interpreted in terms of a contributing pathway involving nucleophilic solvent assistance.