Kinetics and Mechanism of Mn-II Catalyzed Periodate Oxidation of p-anisidine: Effect of pH

The stoichiometry for the initial part of the reaction, Mn-II catalysed periodate oxidation of p-anisidine (PMA), has been found to be 1 mol of PMA consuming 2 mol of periodate ion. The kinetic-mechanistic study of reaction in acetone-water medium was made spectrophotometrically by noting the increase in the absorbance of reaction intermediate. Reaction is first order in reactants and catalyst each. A decrease in dielectric constant of the medium results in decrease in the rate of reaction suggesting an ion-dipole type interaction. Free radical scavengers do not affect the reaction rate. A special type of ratepH profile shows a maximum at pH = 7.0. This pH effect also suggests the involvement of at least three differently reactive reactant species in the reaction and this fact has been considered by us while deriving the rate law. Under pseudo first order conditions [IO4-] >> [PMA] and in agreement with the derived rate law, the 1/k(cat) versus [H+] plot passes through the minimum and the results can be fitted to the equation: 1/k(cat) = (K-2 / kK(3)K(4) [H+]) + {(K-w+ K-b K-2) / kK(3)K(4) K-w} + K-b[H+] / kK(3)K(4) K-w where kK(3)K(4) is the empirical composite rate constant, K-w is ionic product of water, K-2 is acid dissociation constant of H4IO6-and K-b is base dissociation constant of PMA. The experimental value of [H+](min) is in good agreement with the value calculated by using the derived rate law equation and is characteristic of the substrate involved relating to the base dissociation constant of PMA. The value of thermodynamic parameters have been evaluated. (C) 2014 BCREC UNDIP. All rights reserved