The Kinetics of the Soft Metal Ion Promoted Hydrolysis of Phenyl Isothiocyanates in Aqueous Solution. The Effects of Thallium(III) Ions and of Substituents

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.