Kinetics and mechanism of the oxidation of a heterocyclic thioamide by silver(II)-cyclam

Abstract

The kinetics of the oxidation of 4,6-dimethyl-2-mercaptopyrimidine (DMP) by Ag(cyclam)2+ were studied in buffer solutions from pH 5.8 to 7.2 at constant ionic strength of 0.10 M (NaClO4). The reaction is observed to be first-order with respect to [Ag(cyclam)2+] and to [DMP]. However, the reaction rate is affected by the pH of the solution owing to the acid–base equilibrium of the thiol. The mechanism postulated to account for the kinetics includes an acid–base equilibrium and oxidation of thiol (RSH) and thiolate ion (RS−) by Ag(cyclam)2+ to RS· radicals which undergo rapid dimerization to form disulfide (RSSR). From the postulated mechanism and the observed kinetics a rate expression was derived, and second-order rate constants and activation parameters were calculated. The pK a values of the acid dissociation reaction of DMP were also determined at four temperatures using spectrophotometric methods, and thermodynamic parameters calculated from the K a values.