Alkaline hydrolysis of aryl phenylacetates and aryl 4-nitrophenylacetates. Evidence consistent with an elimination–addition mechanism

Abstract

Hydrolysis of the substituted phenyl esters of phenylacetic acid is found to be first order each in the ester and hydroxide ion. Hydrolysis is catalysed by general bases and the catalytic coefficients for the substituted phenoxides obey the Brønsted relation with β+0.49. The rate of hydrolysis of the esters of 4-nitrophenylacetic acid is independent of [hydroxide] in the range employed. Both series of reactions exhibit low solvent isotope effect and high sensitivity to substituents in the leaving group [ρ 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 E1cB 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 on the rate of hydrolysis for variations in [aniline]. An 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 (E1cB)_anion 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 (E1cB)B_{ion pair} or (E1cB)_reversible mechanism for the hydrolysis.