Investigating Solvent Effects on the Kinetics and Mechanism of Alkaline Hydrolysis of Ethyl Pyruvate in Aqueous Dimethyl Sulfoxide

Abstract

Solvent composition can markedly influence the kinetics and mechanism of ester hydrolysis by altering solvent polarity, dielectric properties, ionic solvation, and transition-state stabilisation. Ethyl pyruvate is an α-keto ester whose alkaline hydrolysis provides a suitable system for examining these solvent effects. However, systematic kinetic and thermodynamic evaluation of its alkaline hydrolysis across different aqueous dimethyl sulfoxide compositions remains limited. This study investigated the effect of aqueous dimethyl sulfoxide (DMSO) composition on the alkaline hydrolysis of ethyl pyruvate using conductometric measurements. Reactions were examined in 10–50% (v/v) DMSO over the temperature range of 20–50°C. Specific rate constants decreased progressively as the DMSO proportion increased, whereas increasing temperature accelerated the reaction. The solvent-composition dependence of the rate was evaluated together with iso-composition activation energy, iso-dielectric activation energy, water participation, solvation number, and thermodynamic activation parameters. The iso-composition activation energy decreased from 50.54 to 37.33 kJ mol⁻¹ as DMSO increased from 10 to 50% (v/v), while the iso-dielectric activation energy increased from 86.06 to 103.58 kJ mol⁻¹ across the examined dielectric constants. Linear relationships between log k and log[H₂O] were observed at all investigated temperatures, and the corresponding solvation number increased from 0.677 at 20°C to 1.561 at 50°C. The enthalpy of activation decreased with increasing DMSO concentration, whereas the entropy of activation became more negative and the free energy of activation increased slightly. These findings indicate that solvent composition influences hydroxide-ion solvation, transition-state organisation, and activation energetics during ethyl pyruvate hydrolysis. The results provide a consistent kinetic and thermodynamic description of solvent effects within the investigated aqueous DMSO range under the stated experimental conditions.