The kinetics and mechanism of the thallium ion-promoted hydrolysis of thiol esters

Abstract

The kinetics of the thallium(III) ion-promoted hydrolysis of three para-substituted S-ethyl thiobenzoates have been studied. The hydrogen ion-catalysed reaction is very slow by comparison. The sequence of ester reactivity in the promoted hydrolysis is p-MeO > p-H p-NO₂. The substituent effects, the values of the activation parameters and the effects of changes in ionic strength all suggest that the hydrolysis mechanism is A_Ac2 for the p-NO₂ derivative, but A_Ac1 for the other esters. In the A_Ac2 mechanism the rate acceleration produced by Tl³⁺ ions is comparable with that produced by Ag⁺ ions and much smaller than those produced by Hg²⁺ or by Hg₂²⁺ ions. However, in the A_Ac1 route the effect of Tl³⁺ is greater and comparable with that of Hg₂²⁺. These effects suggest the A_Ac2 route is intramolecular.