Variation of the temperature‐dependent substituent effects with solvent in alkaline hydrolysis of substituted phenyl and alkyl benzoates and phenyl tosylates

Abstract

AbstractThe second‐order rate constants k for the alkaline hydrolysis of eight substituted alkyl benzoates have been measured spectrophotometrically in aqueous 5.3 M NaClO4 and 0.5 M n‐Bu4NBr at various temperatures. Variation of the substituent effect with temperature in alkaline hydrolysis of ortho‐, meta‐, and para‐substituted phenyl benzoates, phenyl tosylates, and alkyl benzoates in various solvents (water, aqueous 0.5 M Bu4NBr, 80% (v/v) DMSO, 2.25 M Bu4NBr, and 5.3 M NaClO4) was studied. The susceptibility to temperature variation of the meta and para polar substituent effect, the ortho inductive effect, and the alkyl polar effect for various media showed good correlation with the solvent electrophilicity, ES, which characterizes the hydrogen‐bond donating power of the solvent. The variation of the temperature‐dependent ortho inductive effect with solvent hydrogen‐bond donor capacity (electrophilicity) was found to be nearly twice smaller than that for meta and para polar effect. The temperature‐dependent alkyl polar substituent effect was found to vary with ES nearly by the same extent as the polar effect of meta and para substituents. The dependences of the ρ values (altogether 109 values of ρ) on the (1/T) term for various media were found to cross nearly at the same isosolvent temperature (1/βisosolv ≈ 2 × 10−3) for meta‐, para‐, ortho‐, and alkyl‐substituted esters. At T = βisosolv the difference (ρ)S − (ρ)Water becomes zero for all polar substituent effects in all media considered and the additional inductive effect from the ortho position (compared with para derivatives) disappears for all solvents studied. Copyright © 2007 John Wiley & Sons, Ltd.