Metal-ion oxidations in solution. Part V. Cerium(IV) oxidation of thiourea and its N-substituted derivatives

Abstract

The cerium(IV) oxidation of thiourea, NN′-dimethyl-,NN′-diethyl-, and ethylene-thioureas and thiosemicarbazide has been examined at [H⁺]= 0·5M over the range 5–25° using a stopped-flow technique. The rates are consistent with a mechanism involving the formation of free radicals which subsequently dimerise. The thermo-dynamic parameters for the reactions are: thiourea E_a= 7·96 ± 0·4 kcal./mole (ΔS*=–32·7 ± 3 e.u.), dimethyl-E_a= 9·1 ± 0·5 kcal./mole (ΔS*=–29·3 ± 5 e.u.), diethyl-E_a= 8·9 ± 0·5 kcal./mole(ΔS*=–28·5 ± 4 e.u.), ethylene-E_a= 9·6 ± 0·3 kcal/mole (ΔS*=–25·4 ± 3 e.u.), and thiosemicarbazide E_a= 9·8 ± 0·5 kcal./mole (ΔS*=–28·8 ± 3 e.u.). The overall heats of each reaction have been determined calorimetrically. Linear free-energy relationships obtained are discussed.