Kinetics and mechanism of the thallium(III) ion promoted decomposition of thiobenzamides in aqueous solution

Abstract

In aqueous solution at 25 °C thallium(I) ions do not detectably accelerate the decomposition of S-amides whereas thallium(III) ions bring about a rapid reaction. With thiobenzamide the organic product is benzonitrile, whereas with N-cyclohexylthiobenzamide and N-thiobenzoylpiperidine it is the corresponding O-amide. Spectroscopic and kinetic examination shows that all three S-amides form complexes with thallium(III) ions both rapidly and stoicheiometrically. With the thallium ions in deficit, thiobenzamide forms a 2 : 1 S-amide–Tl³⁺ complex which is converted into a 1 : 1 complex as [Tl³⁺] is raised. N-cyclohexylthiobenzamide and N-thiobenzoylpiperidine form 4 : 1 S-amide–Tl³⁺ complexes with a deficit of thallium ions, these complexes being converted into complexes of lower stoicheiometry as the thallium concentration is increased. In the presence of a large excess of thallium ions the predominant species is the 1 : 1 complex. The decomposition of the S-amides to benzonitrile (or O-amide) proceeds via the complexes, the 1 : 1 complexes being much less reactive than the 2 : 1 complexes. At high thallium(III) ion concentrations the rate of decomposition is inversely related to [Tl³⁺] since increase in [Tl³⁺] proportionately decreases the small equilibrium concentration of 2 : 1 complex. Reaction mixtures normally contained [H₃O⁺] [graphic omitted] 0.2M to suppress the dissociation of protons from water molecules bound to the thallium ions. Changes in [H₃O⁺] above 0.2M have little effect on the rate which, however, exhibits a powerful negative ionic strength effect. The prior dissociation of protons from the various complexes is unimportant in determining the reaction rate and a mechanism of decomposition is suggested. Some differences between the thallium promoted reactions and those promoted by other soft metal ions are pointed out. From a preparative viewpoint thallium (III) and gold (III) salts are likely to be better promoters of the decomposition of NN-disubstituted S-amides then are salts of mercury(II) or silver.