The kinetics of the soft metal ion promoted hydrolysis of phenyl isothiocyanates in aqueous solution. The effects of thallium(III) ions and of substituents

Abstract

In the Hg²⁺ ion-promoted hydrolysis of RC₆H₄NCS a large increase in electron-release by R produces only a small increase in reaction rate. These substituent effects support the mechanism previously proposed. Tl³⁺ ions are surprisingly much (ca. 10³-fold) less effective in promoting the hydrolyses than are Hg²⁺ ions, but show the same kinetic form and similar substituent effects. A similar mechanism is suggested. The low reactivity of Tl³⁺ ions, the different pattern of their activation parameters, and the greater dependence of the Tl³⁺ ion-promotion on ionic strength, all suggest that these ions form adducts with isothiocyanates with special difficulty; the comparable reactivity normally found for Hg²⁺ and Tl³⁺ ions when promoting the decomposition of organosulphur compounds may require chelating substrates.