Kinetics and Mechanism of Oxidation of Metformin Hydrochloride by Hexamolybdocobaltate(III) in Acidic Medium

Abstract

The oxidation of metformin hydrochloride by Anderson-Evans type hexamolybdocobaltate(III) anion was investigated under pseudofirst- order condition in acidic medium at 298 K. The rate of reaction is accelerated by increase in the concentration of H+ ion. The decrease in the reaction rate with increase in the concentration of the oxidant [H6CoMo6O24]3- anion and added molybdate ion kinetically indicate existence of the prior equilibria between various forms of the oxidant. In present study, the oxidant exists in monomers [H6CoMo6O24]3- anion, [H5CoMo5O20]2- anion and dimer [H4Co2Mo10O38]6- forms between the pH 2 and 1. The active oxidant species is [H5CoMo5O20]2- anion. Under experimental conditions, the reaction involves direct electron-transfer from metformin center to oxidant anion generating free radical in rate determining step. The fast hydrolysis of formed free radical in presence of second oxidant molecule leads to formation of carbonyl imino functional group in the oxidation product. The ionic strength and solvent polarity had no significant effect on the rate of reaction. FT-IR spectra of metformin and its oxidation product sample were recorded and analyzed. The FT-IR spectra show the change in frequency of the functional groups of oxidation product than that of the pure MET. The formation of oxidation product was confirmed by high performance liquid chromatography associated with electron impact mass spectroscopy (LC/EI-MS). Thermodynamic parameters are evaluated by temperature variation kinetic data and are in support of the proposed mechanism. The probable mechanism is proposed leading to complicated rate law as a result of involvement of prior equillibria between various forms of the oxidant.