Kinetic study of hydrolysis of benzoates. Part XXII. Variation of the ortho inductive, resonance and steric terms with temperature in the alkaline hydrolysis of substituted phenyl benzoates in aqueous 2.25 M Bu4NBr

Abstract

The second order rate constants k₂ (M⁻¹ s⁻¹) for the alkaline hydrolysis of meta-, para-, and ortho-substituted phenyl benzoates C₆H₅CO₂C₆H₄–X (X = H, 3-Cl, 3-CH₂, 3-NH₂, 3-NO₂, 4-F, 4-NO₂, 2-Cl, 2-F, 2-CH₃, 2-OCH₃, 2-NO₂) in 2.25 M aqueous n-Bu₄NBr have been measured spectrophotometrically at various temperatures (15, 25, 30, 40, 50 °C). The log k values were treated using both the modified Fujita–Nishioka and the Charton three-parameter correlation equations. The σ°, σ_I, σ_R°, E_s^B scales were used as the measure of the ordinary polar, inductive, resonance, and steric components of the substituent effects. E_s^B = (log k_X − log k_H)_H⁺, where k_X and k_H are the rate constants of the acidic hydrolysis for ortho-substituted and unsubstituted phenyl benzoates in water at 50 °C. The modified Fujita–Nishioka and Charton treatments gave the excellent and identical results. The susceptibility to the induction effect of ortho-substituents was found to be about 0.2 units of the ρ scale smaller [ρ_I)_ortho = 1.80] than that for meta- and para-substituents [(ρ°)_m,p = 2.03], in the alkaline hydrolysis of substituted phenyl benzoates in 2.25 M aqueous n-Bu₄NBr at 50 °C. Also in this solution, the variation of the susceptibility to the inductive and the resonance effects with temperature for the ortho-derivatives appeared slightly smaller than that for para-derivatives. During the transition from water to 2.25 M aqueous Bu₄NBr the susceptibility of the meta- and para-substituent polar effect (ρ°)_m,p was found to increase by about 1.0 units, but the susceptibility to the inductive effect of ortho-substituents was enhanced only by about 0.5 units of the ρ_I scale at 50 °C. The variation of the inductive term of ortho-substituents with both solvent and temperature was found to be half that for para-substituents. Variation of the inductive term of ortho-substituents with the solvent and temperature by the same extent had earlier been found for the alkaline hydrolysis of substituted phenyl tosylates, though the ratio of the susceptibilities to the polar influence of meta-, para-, and ortho-substituents in water differed two-fold. The steric term of ortho-substituents for 2.25 aqueous n-Bu₄NBr was found to be independent of temperature and nearly equal to the steric term for water.