THE MECHANISM OF OXIDATION OF 3-MERCAPTOPROPIONIC ACID

The mechanism of the oxidation of 3-mercaptopropionic acid (3-MPA) by hydrogen peroxide was studied in the acidic pH range. The nucleophilic attack by sulphur on the peroxide bond controls the rate. Extrapolati on of the pH dependency suggests that the rate of attack by the deprot onated dianion is highest. Traces of Fe(III), at levels below 10(-7) m ol dm(-3), do not catalyze efficiently the process through one-electro n mechanisms; at higher concentrations, or on the surface of iron(III) oxides, this type of catalysis becomes important. The electrochemical oxidation of 3-MPA was also studied, using differential pulse polarog raphy and cyclic voltammetry techniques. The mechanism is of the EC2E type, the second electron transfer step corresponding to the oxidation of the disulphide RS-SR. The rate constant for the dimerization of th e RS. radicals was 1.8 x 10(3) mol(-1) dm(3) s(-1); the slowness of th is step agrees with the mechanisms observed in the course of one-elect ron oxidations by metal ions.