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Hydrolysis of the substituted phenyl esters of phenylacetic acid is found t o be first order each in the ester and hydroxide ion. Hydrolysis is catalysed b y general bases and the catalytic coefficients for the substituted phenoxides obey the Brgnsted relation with p +0.49. The rate of hydrolysis of the esters of 4nitrophenylacetic acid is independent of [hydroxide] in the range employed. Both series of reactions exhibit low solvent isotope effect and high sensitivity t o substituents in the leaving group [p 1.40 for the esters of phenylacetic acid and 3.4 for the esters of 4-nitrophenylacetic acid in 80% DMSO-20% water (v/v)]. These data suggest an €1 cB mechanism for the hydrolysis. The keten intermediate envisaged for such a mechanism has been trapped as the anilide when the reactions are conducted in aniline buffers, without any effect o n the rate of hydrolysis for variations in [aniline]. A n increase in the DMSO content in the solvent decreases the rate of hydrolysis of the esters of 4-nitrophenylacetic acid, which is explained by an (€1 CB)~,,~~,, mechanism for the hydrolysis. Transfer to aqueous DMSO results in rate accelerations for the esters of phenylacetic acid which can be accounted for by either an (€1 cB) Bi on pair or (€1 CB)reyegsible mechanism for the hydrolysis. * The pK, of 5-nitrocoumaranone, the cyclic analogue of the 4-nitrophenyl ester of 3-nitrophenylacetic acid has been shown ' to be 9.8.

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