cago through whose courtesy the mass spectrometer under AEC Contract At(ll-1)92 was made available. We thank the Simonize Co. for its contribution to the departmental fund for research.Chicago 16, III.

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The kinetics of the acetate ion-catalyzed hydrolyses of phenyl acetate and /Miitropheny 1 acetate have been determined at three temperatures. The activation parameters of these processes have been calculated. The hydrolysis of 2,4-dinitrophenyl benzoate at pH 5.5 using acetate-O18 as catalyst led to benzoic acid containing about 75% of one oxygen-18 atom of the original acetate ion catalyst. The result of this tracer experiment suggests acetylbenzoyl anhydride as intermediate in this acetate ion catalysis and suggests the existence of anhydride intermediates in other acetate ion-catalyzed hydrolyses. The possibility of acylimidazole, acid anhydride, ester and lactone intermediates in nucleophilic catalysis is discussed, both in intramolecular and intermolecular cases. A kinetic comparison between a number of corresponding cases of intermolecular and intramolecular catalysis of hydrolytic reactions indicates the powerful nature of intramolecular catalysis and further suggests that in a rigidly held enzyme-substrate complex nucleophilic and/or electrophilic catalysis is responsible for the powerful action of enzymes in hydrolytic reactions.

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