Kinetics and mechanism of the metal ion-promoted hydrolysis of 2-phenyl-1,3-dithiane in aqueous dioxane solution

Abstract

In 1%(v/v) aqueous dioxane the hydrolysis of 2-phenyl-1,3-dithiane (1) is promoted by thallium(III), mercury(II), mercury(I), and silver(I) ions; the qualitative sequence of their efficiencies is TI³⁺ [graphic omitted] Hg²⁺ > Hg₂²⁺ Ag⁺. Kinetic studies suggest the reaction mechanism with the thallium and mercury ions involves the rapid formation of a 1:1-adduct between the metal ion and the dithiane (formation constant, K 54.5, 3.0 × 10⁴, and 630 dm³ mol^–1 for TI³⁺, Hg²⁺, and Hg₂²⁺, respectively at 25 °C) followed by a slow ring-opening step (k₂ 77, 0.20, and 0.07 s^–1, respectively) which may not involve water. Added chloride ions inhibit the mercury(II) ion promotion: HgCl⁺ and HgCl₂ have, respectively, ca. 90% and ca. 14% of the reactivity of Hg²⁺. An increase in the dioxane content of the solvent to 20%(v/v) has relatively little effect on the reaction. The general kinetic pattern, and level of reactivity, is similar to that observed previously for the TI³⁺- and Hg²⁺-promoted hydrolyses of 2-phenyl-1,3-dithiolane (4). Both (1) and (4) are less basic towards TI³⁺ than towards Hg²⁺, and the thallium adducts of (1) and (4) both react more rapidly than the corresponding Hg²⁺ adducts. However, (1) is less basic than (4) towards TI³⁺, whereas the reverse is true for Hg²⁺, and the TI³⁺ adduct of (1) is more reactive than that of (4), whereas the reverse is true of the reactivities of the corresponding Hg²⁺ adducts. These differences of detail suggest that the (unknown) conformations of the adducts control to some extent the values of K and k₂.