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 1 H NMR and 13 C 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 °C under pseudo-first-order conditions, ( c [NaOH] = 0.001-1.0 mol l -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 (σ p ) with ρ = 2.44. Quantitatively comparable results have been obtained by application of Alternative Interpretation of Substituent Effects theory (AISE) using the σ i set of substituent constants.