M. L. Bender, B. Turnquest
Apr 1, 1957
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90
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Journal
Journal of the American Chemical Society
Abstract
The hydrolyses of ¿>-nitropheny¡ acetate, 2,4-dinitrophenyl acetate, phenyl acetate and ethyl thiolaeetate are catalyzed by a number of bases including pyridine, 3and 4-picoline, trimethylamine, imidazole, N-methylimidazole, quinoline and acetate ion. The hydrolysis of ^-nitrophenyl acetate meets the requirements of general basic catalysis since the rate is proportional to the summation of catalytic constants times catalyst concentrations at constant pH and ionic strength. The catalytic rate constants are related to the basicity of the catalyzing amines of constant steric requirement in a manner similar to the Brdnsted catalysis law. The slope of this linear free energy relationship is 1.62, the highest recorded for a general basic catalysis. The mechanism of this general basic catalysis, involving not a rate-determining proton transfer but rather the addition of the base to the substrate producing an unstable intermediate, offers an explanation for the deviation of the slope from its usual limitation. Imidazole catalysis of the hydrolysis of various esters demonstrates that the rate of hydrolysis increases with the acidity of the alcoholic residue of the ester. It appears that general basic catalysis of ester hydrolysis is of importance only for esters containing an alcohol that is a reasonably strong acid (pK& <11). Comparison of the imidazoleand or-chymotrypsin-catalyzed hydrolyses of ^-nitrophenyl acetate shows that the two types of hydrolysis differ by a factor of ca. 10s in the rate of formation of £-nitrophenol but are similar in the rate of formation of acetate from the acylimidazole and acyl-enzyme intermediates. The action of imidazole as a general basic catalyst offers only a partial explanation of the mechanism of enzymatic hydrolysis.