Metal-ion oxidations in solution. Part VI. Oxidation of thiourea and its N-substituted derivatives by cobalt(III)

Abstract

The cobalt(III) ion oxidations of thiourea, NN′-dimethyl-, NN′-diethyl-, and ethylene-thioureas have been examined over the acidity range [H⁺]= 0·4–1·0M in the temperature range 0–21° by means of a stopped-flow technique. The rates are consistent with a mechanism which involves the formation of free radicals which subsequently dimerise. The formation of a complex intermediate is considered to be the rate-determining step in each case. Rate constants k₁, and k₂ for the reactions (1) and (2) [graphic omitted] (where RSH⁺= a thiourea) have been evaluated for each system. The thermodynamic parameters for the reaction are: thiourea, E_a(1)= 8·6 ± 1·5 kcal./mole, ΔS*(1)=–29 ± 5 e.u., E_a(2)= 9·0 ± 0·5 kcal./mole, ΔS*(2)=–22 ± 3 e.u.; diethyl-, E_a(1)= 7·8 ± 1·6 kcal./mole, ΔS*(1)=–29 ± 5 e.u., E_a(2)= 10·6 ± 0·4 kcal./mole, ΔS*(2)=–12 ± 3 e.u.; dimethyl-, E_a(1)= 7·6 ± 1·5 kcal./mole, ΔS*(1)=–26 ± 4 e.u., E_a(2)= 9·8 ± 0·4 kcal./mole, ΔS*(2)=–14 ± 3 e.u.; ethylene-, E_a(1)= 8·2 ± 1·4 kcal./mole, ΔS*(1)=–30 ± 4 e.u., E_a(2)= 10·9 ± 0·5 kcal./mole, ΔS*(2)=–19 ± 3 e.u. The mechanisms of the reactions are discussed.