Solvent effect and proton inventory in the hydrolysis of p-methylphenyl trichloroacetate

Hydrolysis of p-methylphenyl trichloroacetate in water-acetonitrile mixtures was studied as a function of water concentration in the range 5.5-55.5 m. The proton inventory technique, in H2O-D2O mixtures, shows, for a value of D atom fraction in the solvent n = 0.5, deviations from the expected value (for a reaction with one proton being transferred) of 7.5 and 12.3%, for experiments in the presence of 16.6 and 33.3 m L2O (L=H or D), respectively. Theoretical treatment of the data obtained at [L2O] = 16.6 m using the Gross-Butler equation are consistent with a cyclic tran si tion- state structure with three protons involved. Conversely, similar experiments in the presence of [L2O] = 33.3 m show that multiple water molecules are involved in the transition state of the reaction. Copyright (c) 2006 John Wiley & Sons, Ltd.