GENERAL BASE CATALYSIS OF ESTER HYDROLYSIS

The hydrolysis of alkyl formates with leaving groups in the range pK(a ) = 12-16 is catalyzed by substituted acetate anions. There is an incr ease in the Bronsted beta value for general base catalysis with decrea sing pK(a) of the leaving alcohol and a complementary increase in -bet a(lg) with decreasing pK(a) of the catalyzing base, both of which are consistent with a value of p(xy) = partial derivative beta/-pK(lg) = p artial derivative beta(lg)/-partial derivative pK(BH) congruent-to 0.1 1. This result supports a class n mechanism of general base catalysis, in which a proton is abstracted from the nucleophilic water molecule by the base catalyst in the transition state; it is not consistent wit h the kinetically equivalent class e mechanism of electrophilic cataly sis by general acids of a reaction with hydroxide ion, by proton donat ion to the leaving alcohol. Solvent deuterium isotope effects in the r ange k(H2O)/k(D2O) = 3.6-5.3 for the buffer-independent reaction and 2 .5-2.8 for catalysis by CH3COO- support concerted proton transfer and O-C bond formation. The secondary isotope effect for catalysis of the hydrolysis of LCOOMe by acetate ion is k(D)/k(H) = 1.05. Both nucleoph ilic and general base mechanisms of catalysis by acetate anions are ob served for the hydrolysis of substituted phenyl formates with leaving groups of pK(a) = 7.1 -10.1. A small value of beta = 0.12 for general base catalysis of the hydrolysis of phenyl formate and p-methylphenyl formate represents catalysis of the addition of water by hydrogen bond ing of water to the base catalyst. On the other hand, a larger value o f beta = 0.35 and a decrease in k(H2O)/k(D2O) to 1.2 were observed for general base catalysis of the hydrolysis of p-nitrophenyl formate. It is suggested that the increase in beta with decreasing pK(lg) (an app arent ''anti-Hammond effect'') may be accounted for by a change in mec hanism, from catalysis of a stepwise reaction of phenyl and p-methylph enyl formates to concerted general base catalysis of formyl transfer t o water for the reaction of p-nitrophenyl formate.