Kinetics and mechanism of oxidation of pyridoxine by enneamolybdomanganate(IV)

Oxidation of pyridoxine (vitamin B6) by enneamolybdomanganate (IV) in aqueous perchloric acid medium has been studied spectrophotometrically at 25 °C under pseudo-first-order conditions. The mechanism involves formation of a precursor complex between the reactants which decomposes in a subsequent slow step to give pyridoxal as the intermediate product. The pyridoxal is further oxidized to a final product, 4-pyridoxic acid, by another oxidant molecule in a fast step. The precursor complex formation is supported both kinetically and spectrophotometrically. The accelerating effect of hydrogen ions on the reaction is due to the formation of active hexaprotonated oxidant species. The protonated enneamolybdomanganate(IV) and the pyridoxine cation are found to be the active species of the oxidant and the substrate respectively. The reaction involves direct two-electron transfer step without any free radical intervention. The effects of ionic strength, solvent polarity and the activation parameters were also in support of the mechanism proposed.