Kinetics and mechanism of oxidation of paracetamol by an Anderson-type 6-molybdocobaltate(III) in acidic medium

Abstract

Oxidation of paracetamol by the Anderson-type hexamolybdocobaltate(III) anion [H₆Co^IIIMo₆O₂₄]³⁻ was investigated at pH values of 1 and 2 in aqueous medium. The reaction is found to proceed through formation of a weak complex between the reactants. Under the reaction conditions, the oxidant exists in the [H₅Co^IIIMo₅O₂₀]²⁻, [H₆Co^IIIMo₆O₂₄]³⁻ and dimeric $\left[ {{\text{H}}_{4} {\text{Co}}_{2}^{{\rm III}} {\text{Mo}}_{10} {\text{O}}_{38} } \right]^{6-}$ forms, of which the first form is the most reactive. Inhibition of the reaction by the oxidant and also by molybdate ion suggests the existence of prior equilibria between various forms of the oxidant. The reaction involves direct electron transfer from neutral paracetamol to the anion, generating a free radical in the rate-determining step. The free radical undergoes further oxidation to N-acetyl-quinone imine as an intermediate, which then undergoes hydrolysis to give the final products p-benzoquinone and acetic acid. The ionic strength and solvent polarity had no significant effect on the rate of the reaction. A probable mechanism is proposed, leading to a complicated rate law as result of involvement of prior equilibria between various forms of the oxidant. The activation parameters were determined and are consistent with the proposed mechanism. The kinetic data were found to deviate from Marcus theory. This observation, together with the low and negative activation entropy, suggests the formation of a weak complex between the reactants.