The kinetics and mechanism of the reaction between mercury(II) ions and thiobenzamides in aqueous solution

Abstract

Thiobenzamide, N-cyclohexylthiobenzamide, and N-thiobenzoylpiperidine form complexes with the mercury(II) ion in dilute aqueous perchloric acid. The predominant complex in each case has a 2 S-amide: 1Hg²⁺ stoicheiometry, but in the presence of a large excess of S-amide there is some evidence for the existence of a highly insoluble 4:1 complex. With an excess of mercuric ions, the 2:1 complexes are formed effectively quantitatively and any small amounts of 1:1 complex are not detectable. At 25°, the dissolved 2:1 complexes are relatively stable when [H₃O⁺]≳ 0·1M and in the absence of added mercury(II) ions. In the presence of free mercury(II) ions, the complexes decompose to mercury(II) sulphide and an organic product in a process which, for thiobenzamide and N-cyclohexylthiobenzamide, is kinetically first order in the 2:1 complex and in mercury(II) ions. Increases in [H₃O⁺], at a fixed value of [Hg²⁺], retard the reaction, the rate falling eventually to a constant value. For thiobenzamide, the organic product is benzonitrile; for the N-substituted thiobenzamides, it is the corresponding O-amide. Mechanisms are suggested in which, for thiobenzamide and N-cyclohexylthiobenzamide, the slow steps are bimolecular reactions between free mercury(II) ions and the 2:1 complex or one of its deprotonated forms. The transiently formed 1:1 complexes decompose rapidly to the products in the absence of free S-amide. The mechanisms give a good account of the detailed dependence of the rate on [H₃O⁺]. For thiobenzamide, the neutral species [PhC(:NH)S]₂Hg is found to react ca. 600-fold faster with Hg²⁺ than does {[PhC(:S)NH₂]₂Hg}²⁺. The overall sequence of reactivity at fixed [Hg²⁺], and at any value of [H₃O⁺], is N-cyclohexylthiobenzamide > thiobenzamide N-thiobenzoylpiperidine. This finding is also rationalised.