pH Dependent Effect of β-Cyclodextrin on the Hydrolysis Rate of Trifluoroacetate Esters

The kinetics of the hydrolysis of phenyl trifluoroacetate ( 1a ) and p -methylphenyl trifluoroacetate ( 1b ) is pH independent below pH 6 and shows first order dependence on HO - concentration above pH 8. The reaction is weakly buffer catalyzed. In the presence of β-cyclodextrin the rate increases in a nonlinear fashion at constant pH above pH 8 but it decreases at pH 6. At pH 9.91 the ratio of rate constants for the reaction in the presence and in the absence of 0.012 M β-cyclodextrin is 11 and 2 for phenyl acetate and phenyl trifluoroacetate, respectively. The different behavior of the two substrates is attributed to different modes of inclusion. It is suggested that the reacting complex in the case of substrates 1a and 1b has the trifluoromethyl group inside the cavity. The inhibition of the reaction in neutral solution is attributed to a microsolvent effect and to protection of the carbonyl group toward water attack.