The kinetics and mechanisms of the hydrolyses of 1,3-oxathiolanes and 1,3-dithiolanes promoted by mercury(II) and by thallium(III) ions, including the effects of pH and added anions

Abstract

The kinetics of the mercury(II) ion-promoted hydrolysis of 2,2-diphenyl-1,3-oxathiolane (2) and of 2-phenyl-1,3-dithiolane (4) and of the thallium(III) ion-promoted hydrolysis of 2-phenyl-1,3-oxathiolane (3) and of (4), have been studied at various pH values and ambient anion concentrations using dioxane–water solvents. In the absence of the metal ion the hydrolyses are relatively slow. The mercury-promoted hydrolysis of (2) involves complex kinetic dependencies upon [Hg²⁺] and [H₃O⁺] and upon the concentrations of added anions. The results support the tentative mechanism previously proposed for the mercury-promoted hydrolysis of (3) which involves at pH > 3 the slow intramolecular attack of mercury-bound aquo species on the C(2) atom of the rapidly formed 1 Hg²⁺: 1 O,S-acetal adduct (5). For (2), the extra phenyl group reduces the importance of a route found for (3) at pH < 3 which involves the O-protonation of (5). Also for (2) large values of [Hg²⁺] can lead to a change from the rate-determining hydrolysis of the Hg²⁺–O,S-acetal adduct to the rate-determining hydrolysis of the resulting O,O-hemi-acetal. The pattern of pH and anion effects found for the mercury(II) ion-promoted hydrolysis of (4) differ completely from those found for (2) and (3). They suggest a simple mechanism involving only a rate-determining intermolecular attack by water on the 1 Hg²⁺: 1 S,S-acetal adduct (formation constant, K 4.3 × 10³ l mol^–1). The neutralisation of one positive charge on this adduct leads to a loss of reactivity of ca. 25%; neutralisation of both leads to a great reduction in hydrolysis rate. The thallium(III) ion-promoted hydrolysis of (4) is mechanistically similar to the mercury reaction; the 1:1-adduct is less easily formed (K 2.9 × 10² l mol^–1) but reacts ca. 10-fold faster with water. The kinetic form of the thallium-promoted hydrolysis of (3) is especially simple : zero order in [TI³⁺] and first-order in [H₃O⁺]. It is argued that this should not be interpreted as implying rate-determining hemiacetal hydrolysis for this system.