Specific solvent effects on rates of alkaline hydrolysis of acetylmandelate ion

Abstract

The effect of changing dielectric constant (D) and specific solvent effect on the rate of alkaline hydrolysis of acetylmandelate ion have been studied. For measurements of the effect of the dielectric constant, two series of mixed solvents were used: water–ethanol (at 25°, 35°, and 45°) and water–ethylene glycol (at 35°). Plots of log k₀ against 1/D for water–ethanol mixtures yield good straight lines with negative slopes up to a dielectric constant of ca. 50, beyond which the line bends upwards. With water–glycol media, the rate constant first increases with increasing proportions of glycol, attaining a maximum at ca. D= 72, beyond which the plot is a good straight line with a negative slope. The radius of the activated complex for water–ethanol media, as obtained from the slopes of the plots, decreases with increasing temperature. The rate constants at 35° at a particular dielectric constant (52·2) in different aquo-organic mixtures are found to lie in the order dioxan > acetone > ethanol ethylene glycol.