Kinetics and Mechanism of the Oxidation of 4-Methyl-2-Mercaptopyrimidine by Silver(II)-Cyclam and the Thermodynamics of its Acid-Base Reaction

The kinetics of the oxidation of 4-methyl-2-mercaptopyrimidine (MMP) by Ag(cyclam) 2+ was studied in buffer solutions from pH 5.68 to 7.33 at total ionic strength of 0.10 M (NaClO 4 ). The reaction is first order with respect to each of the reactants, and is strongly influenced by the pH of the solution. The mechanism postulated to account for the kinetics and the pH behavior involves the acid-base reaction of MMP, the oxidation of RSH and RS − to RS* and free radical dimerization to form RSSR. The reaction obeys the following rate law: The second-order rate constants calculated for the oxidation of RSH (k′ 2 ) and RS − (k 2 ) at 25°C are 3.3 and 188 M −1 s −1 , respectively. The activation parameters for the k 2 path are δH≠ = 33.6kJ mol −1 and ΔS≠ = -83.3J mol −1 K −1 . An inner-sphere electron-transfer mechanism, with the probable formation of a penta-coordinate species between silver(II)-cyclam and RS − , is postulated. The acid-base reaction of MMP was investigated by UV-Vis spectroscopy from pH 4 to 12 at ionic strength of 0.10M (NaCl) and at four temperatures. The pK a values determined for MMP at 20°C, 25°C, 30°C and 35°C are 7.85, 7.73, 7.61 and 7.53, respectively. The thermo-dynamic parameters for the acid-base reaction of MMP at 298 K are ΔH 0 = 38.0 kJ mol −1 and ΔS 0 = -20.5 J mol −1 K −1 .