Kinetic study of hydrolysis of benzoates. Part XXIII—Influence of the substituent and temperature on the kinetics of the alkaline hydrolysis of alkyl benzoates in aqueous 2.25 MBu4NBr and 80% DMSO

Abstract

AbstractThe second‐order rate constantsk2(M−1s−1) for the alkaline hydrolysis of substituted alkyl benzoates, C6H5CO(O)R (R = CH3, CH2Cl, CH2CN, CH2C≡CH, CH2C6H5, CH2CH2Cl, CH2CH2OCH3), were measured in aqueous 2.25 Mn‐Bu4NBr and in 80% (v/v) DMSO solution at several temperatures. The logkvalues were analyzed using the equation logk = logk0 + ρσ + δEsB. TheEsBscale has been proposed for the steric effect of alkyl substituents in the alkyl part of esters:EsB = (logkR − logkCH3), wherekis the rate constant for the acidic hydrolysis of substituted alkyl benzoates or acetates in water. As polar substituent parameters, both Taft σ* and σIconstants were used. The dual parameter treatments of the logkvalues with σ andEsBconstants gave excellent correlations (R = 0.997). For 2.25 Mn‐Bu4NBr, 80% (v/v) DMSO and pure water at 25 °C, calculated susceptibilities to the inductive effect of alkyl substituents ρ* were found to be 2.07, 2.21 and 1.64, respectively. The corresponding ρIvalues were 4.64, 4.94 and 3.64. The dependence of ρIon solvent and temperature in the alkaline hydrolysis of substituted alkyl benzoates was similar to that observed earlier formeta‐ andpara‐substituents in the alkaline hydrolysis of substituted phenyl benzoates and tosylates. The substituent dependence of the activation energy,E, was found to be completely caused by the polar effect. Susceptibility to steric effect in the alkaline hydrolysis of alkyl benzoates (δ ≈ 1) appeared to be independent of the solvent and temperature. Copyright © 2002 John Wiley & Sons, Ltd.