Kinetic studies on hydrolysis of acetylsalicylic acid in binary systems of water and organic solvents

Abstract

The hydrolysis rate constants of acetylsalicylic acid in water mixed with ethanol, propylene glycol, or N, N-dimethylacetamide were determined at various concentrations (30-90%, v/v) at pH regions between 6 and 13 (pH values being obtained from direct measurement by glass electrode). It was assumed that, in these solvent systems, hydrolysis of acetylsalicylic acid is due to the decomposition of its anion from the degree of dissociation determined by ultraviolet (UV) absorption spectra. The pH-independent region of decomposition rate constant, as is known at pH 5-8 in aqueous solution, was found at pH higher than 8 in these binary systems. The increase in decomposition rate constants of acetylsalicylic acid in ethanol and propylene glycol with the concentration of organic solvents can be explained from the scheme of cyclic intermediate of acetylsalicylic acid by Garrett. However, in the case of higher concentration of N, N-dimethylacetamide, the rate constants at pH 6 decreased with concentration of the solvent, while increased the rate constants at pH higher than 7 could not be explained from this study.