The kinetics of the mercury(II) ion-promoted hydrolysis of 2-phenyl-1,3-oxathiolan and the effects of pH and chloride ion concentration

Abstract

The kinetics of the mercury(II) ion-promoted hydrolysis of 2-phenyl-1,3-oxathiolan (1) have been studied at various pH values and ambient chloride ion concentrations using dioxan–water and ethanol–water solvents. The products are always benzaldehyde and 2-mercaptoethanol. In the absence of mercury(II) ions the hydrolyses are comparatively slow. Hg²⁺ and (1) rapidly form a 1 : 1 adduct (2) in solution with a formation constant K ca. 1.8 × 10³ at 25 °C. Depending upon the pH, either (2) or its protonated or deprotonated form reacts to give a hemiacetal whose subsequent hydrolysis is fast. The ring-opening process may involve intramolecular transfer of mercury-bond aquo species. In ethanol-water solvents (2) also reacts with ethanol to give a (small) quantity of an open-chain OO-acetal (3) whose subsequent hydrolysis is relatively slow. Added chloride ions reduce the competition from this ethanol reaction but also catalyse the hydrolysis of (3). Chloride ions have complex effects on the rate of the direct hydrolysis of (1)[via(2) and the hemiacetal] which suggest a reactivity sequence Hg²⁺ HgCl⁺ < HgCl₂ > HgCl₃^– HgCl₄^2–. Mechanistic implications are discussed in the light of earlier work.