Prediction of ortho substituent effect in alkaline hydrolysis of substituted phenyl benzoates in aqueous acetonitrile

Abstract

The second-order rate constants k (dm3mol−1s−1) for alkaline hydrolysis of meta-, para- and ortho-substituted phenyl esters of benzoic acid, C6H5CO2C6H4-X, in aqueous 50.9% (v/v) acetonitrile have been measured spectrophotometrically at 25 °C. In substituted phenyl benzoates, C6H5CO2C6H4-X, the substituent effects log kX − log kH in aqueous 50.9% acetonitrile at 25 °C for para, meta and ortho derivatives showed good correlations with the Taft and Charton equations, respectively. Using the log k values for various media at 25 °C, the variation of the ortho substituent effect with solvent was found to be precisely described with the following equation: Δlog kortho = log kortho − log kH = 1.57σI + 0.93σ°R + 1.08EsB − 0.030ΔEσI − 0.069ΔEσ°R, where ΔE is the solvent electrophilicity, ΔE = ES − EH20, characterizing the hydrogen-bond donating power of the solvent. We found that the experimental log k values for ortho-, para- and meta-substituted phenyl benzoates in aqueous 50.9% acetonitrile at 25 °C, determined in the present work, precisely coincided with the log k values predicted with the equation (log kX)calc = (log kHAN)exp + (Δlog kX)calc where the substituent effect (Δlog kX)calc was calculated from equation describing the variation of the substituent effect with the solvent electrophilicity parameter, using for aqueous 50.9% CH3CN the solvent electrophilicity parameter, ΔE = −5.84. In going from water to aqueous 50.9% CH3CN, the ortho inductive term grows twice less as compared with the para polar effect. Copyright © 2013 John Wiley & Sons, Ltd.