Cooxidation of S-phenylmercaptoacetic acid and oxalic acid with pyridinium chlorochromate

The kinetics of oxidation of the mixture of S-phenylmercaptoacetic acid (PMA) and oxalic acid with pyridinium chlorochromate (PCC) in 30% (v/v) acetic acid-water exhibits a first-order dependence each in [PCC] and [PMA], at low concentrations of the latter. At high concentration of the substrate, a fractional order in [PMA] is observed. The rate increases linearly with [oxalic acid]. Increased reactivity in the presence of oxalic acid with carbon dioxide as one of the products is well explained by Rocek's mechanism for cooxidation, involving a ternary complex. Formation of a ternary complex at low concentrations and decomposition of the complex at higher concentrations of PMA is considered to be the respective rate-limiting step.