Solvent effect on activation parameters for the neutral hydrolysis of phenyl trifluoroacetates in aqueous acetonitrile

Abstract

Rates of neutral hydrolysis of phenyl, 4-nitrophenyl, and 2,4-dinitrophenyl trifluoroacetates in acetonitrile–water mixtures (x_MeCN= 0.39–0.97) have been measured. The free energies of activation increase smoothly upon increasing the acetonitrile content of the medium while the enthalpies and entropies of activation show a more complex but partially compensating behaviour. For phenyl trifluoroacetate both ΔH^‡ and ΔS^‡ pass through an inflexion point around x_MeCN= 0.6. For 4-nitrophenyl and 2,4-dinitrophenyl trifluoroacetates a minimum value possibly occurs both in ΔH^‡ and ΔS^‡ around x_MeCN= 0.9–0.95, but it is clearly observed in ΔH^‡ at x_MeCN= 0.92 for the former ester and in ΔS^‡ at x_MeCN= 0.94 for the latter ester. The activation parameters are discussed in terms of solvent structure. The extreme values are mainly attributed to changes in the thermodynamic functions of transfer of the transition state of the hydrolysis reaction from water to acetonitrile–water mixtures.