Kinetics and mechanism of the reaction between copper(II) ions and thiobenzamide in aqueous solution

Abstract

The reaction between thiobenzamide and copper(II) ions in aqueous perchloric acid leads to benzonitrile and copper(II) sulphide. With N-substituted thiobenzamides the organic product is the corresponding O-amide. At 60° and with [HClO₄]≲ 0·5M, the thiobenzamide reaction obeys the rate equation –d[S-amide]/dt=[S-amide][Cu²⁺](8.2 + 0·063/[H₃O⁺]) mol l^–1 h^–1. A mechanism is suggested which involves the rate-determining decompositions of both a 1 S-amide: 1 Cu²⁺ complex and of its deprotonated form. This mechanism is similar in outline, but different in detail, from that probable for the corresponding reaction involving mercury(II) ions, which is ca. 2 × 10⁵-fold faster under comparable conditions. N-Cyclohexyl thiobenzamide reacts faster than does thiobenzamide with Hg²⁺, but slower with Cu²⁺. The reasons for these differences are discussed. The copper(II) sulphide product can be precipitated in colloidal form and then leads to powerful heterogeneous autocatalysis. This effect is relatively unimportant at temperatures ≳ 50°.