Kinetics and mechanism of oxidation of thiourea by Waugh-type enneamolybdomanganate(IV) in aqueous perchloric acid

Abstract

The Waugh-type polyoxometalate, enneamolybdomanganate (IV), oxidizes thiourea in aqueous perchloric acid, to give formamidine disulphide as confirmed by HRMS spectra. The rate of reaction increases linearly as hydrogen ion and thiourea concentrations increase, while it remains constant as the concentration of oxidant, enneamolybdomanganate(IV), is varied. The protonated form of the oxidant and the unprotonated form of thiourea were found to be active species of the reaction. The protonation and the interaction of thiourea with the oxidant have been supported by UV-vis spectral studies. The reaction is found to be initiated by nucleophilic attack of unprotonated thiourea on the protons attached to the oxygen atoms directly bound to the Mn(IV) centre. The interaction between the thiourea and the oxidant to form a precursor complex has been verified spectrophotometrically. The proposed mechanism, in accordance with the kinetic results obtained, involves formation of a precursor complex between thiourea and oxidant which decomposes in a slow step. The corresponding sulfenic acid is the intermediate formed due to the decomposition of the complex which reacts with another thiourea molecule giving the final product. The rate law derived as per the proposed mechanism is also verified kinetically. The results of effects of ionic strength, solvent polarity and the temperature also support the proposed mechanism.