Collection of Czechoslovak Chemical Communications - digital archive 

Abstract

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 ¹H NMR and ¹³C 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_Ac2 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 σⁱ set of substituent constants.

Keywords: Esters; Phenyl benzoates; Base-catalysed hydrolysis; Kinetic acidity function; Substituent effects; AISE; Hammett equation; Kinetics; Chemometrics.

References: 48 live references.