Kinetic study of hydrolysis of benzoates. Part XXIV—Variation of the ortho substituent effect with solvent in the alkaline hydrolysis of substituted phenyl benzoates

Abstract

AbstractThe second‐order rate constants k2 (M−1 s−1) for the alkaline hydrolysis of meta‐, para‐ and ortho‐substituted phenyl benzoates, C6H5CO2C6H4X, in aqueous 0.5 M n‐Bu4NBr were measured spectrophotometrically. The dependence of substituent effects, especially ortho inductive, resonance and steric terms on different solvent parameters, was studied using the following equation: where Δlog k=log kX−log kH, σI, σR0 and EsB are the inductive, resonance and steric substituent constants and E, Y and P are the solvent electrophilicity, polarity and polarizability parameters, respectively. In data treatment $\Delta E = E_{\rm S} - E_{{\rm H}_2 {\rm O}}, \ \Delta Y = Y_{\rm S} - Y_{{\rm H}_2 {\rm O}} \ {\rm and} \ \Delta P = P_{\rm S} - P_{{\rm H}_2 {\rm O}}$ were used. The solvent electrophilicity was found to be the main factor responsible for changes in the ortho, para and meta polar substituent effects with medium. The variation of the ortho inductive term with the solvent electrophilicity ES was found to be ca threefold smaller than that for para substituents, whereas the ortho resonance term appeared to vary with solvent very similarly to that for para substituents. The steric term of ortho substituents was found to be approximately independent of solvent parameters. The ortho effect caused by the supplementary inductive effect from ortho position was found to disappear in a solvent whose electrophilic solvating power is comparable to that of 60% aqueous ethanol (E ≈ 13.3). Copyright © 2005 John Wiley & Sons, Ltd.