Kinetics of the mercury(II) ion-promoted desulfurisation of thiocarbamates in aqueous solution

Abstract

In a 1% dioxane–water solvent in the presence of Hg²⁺ ions thiocarbamates p-RC₆H₄NHC(S)OEt (R = MeO, Me, H, F) rapidly form complexes of 2 thiocarbamate: 1 Hg²⁺ stoichiometry. In the presence of an excess of Hg²⁺ ions partial conversion to the 2 : 2-complexes probably occurs which decompose to the corresponding carbamates (p-RC₆H₄NHCO₂Et) and HgS. With R = NO₂ the 2 : 2 complex is dominant even at low Hg²⁺ ion concentrations, and desulfurisation is then independent of [Hg²⁺]. Electron-withdrawal by R favours the 2 : 2 complex but decreases its rate of decomposition to products. The effects of changes in the hydrogen ion concentration suggest that ionisation of N-bound protons in the complexes favours reaction. A mechanism is suggested. The kinetic behaviour is somewhat similar to that found previously for thiobenzamides, but the new results suggest the mechanism previously proposed may need revision.