PH DEPENDENT EFFECT OF BETA-CYCLODEXTRIN ON THE HYDROLYSIS RATE OF TRIFLUOROACETATE ESTERS

The kinetics of the hydrolysis of phenyl trifluoroacetate (1a) and p-m ethylphenyl trifluoroacetate (1b) is pH independent below pH 6 and sho ws first order dependence on HO- concentration above pH 8. The reactio n is weakly buffer catalyzed. In the presence of beta-cyclodextrin the rate increases in a nonlinear fashion at constant pH above pH 8 but i t decreases at pH 6. At pH 9.91 the ratio of rate constants for the re action in the presence and in the absence of 0.012 M beta-cyclodextrin is 11 and 2 for phenyl acetate and phenyl trifluoroacetate, respectiv ely. The different behavior of the two substrates is attributed to dif ferent modes of inclusion. It is suggested that the reacting complex i n the case of substrates 1a and 1b has the trifluoromethyl group insid e the cavity. The inhibition of the reaction in neutral solution is at tributed to a microsolvent effect and to protection of the carbonyl gr oup toward water attack.