Substituent effects on the base-catalysed hydrolysis of phenyl esters of para-substituted benzoic acids

Seventeen model phenyl esters of 4-substituted benzoic acids were synthesised by the reaction of substituted benzoyl chlorides with phenol in aqueous alkaline solutions (Schotten-Baumann reaction), in pyridine (Einhorn reaction), or by the reaction of substituted benzoic acids with phosphorus oxychloride. Structures and purity of the model compounds were confirmed by H-1 NMR and C-13 NMR spectroscopy as well as by HPLC and elemental analysis. Phenyl 4-aminobenzoate was synthesised by reduction of phenyl 4-nitrobenzoate in methanol on palladium. Kinetics of base-catalysed hydrolysis of model phenyl esters occurring by the B(Ac)2 mechanism were measured by UV spectrophotometry in 50% (v/v) aqueous dimethyl sulfoxide solutions at 25 degreesC under pseudo-first-order conditions, (c[NaOH] = 0.001-1.0 mol 1(-1)). The addition of OH- to phenyl benzoates was used to establish the kinetic J(-)(E) acidity scale. Linear relation between J(-)(E) and log k(obs) with the slope near unity was found for all the model compounds. The kinetic constants of hydrolysis of phenyl esters of 4-substituted benzoic acids precisely obey the Hammett relationship (sigma (p)) with rho = 2.44. Quantitatively comparable results have been obtained by application of Alternative Interpretation of Substituent Effects theory (AISE) using the sigma (i) set of substituent constants.