Oxidation of captopril by hexacyanoferrate(iii) in aqueous hydrochloric acid medium - A kinetic and mechanistic study

The oxidation of captopril by hexacynoferrate(III) in hydrochloric acid and 50% acetic acid-water(v/v) at a constant ionic strength (I = 3.10 mol dm ^-3) has been studied spectrophometrically at 25 °C. The reactions are initiated by mixing previously thermostated solutions of hexacyanoferrate(3) and captopril containing the required amounts of hydrochloric acid and sodium perchlorate. The kinetics have been followed under pseudo-first order conditions with captopril in excess.The stoichiometry is found to be 2:2, i. e., two moles of hexacyanoferrate(3) react with two moles of captopril. The reaction products have been identified as [Fe(CN ⁶)]^4- and captopril disulfide. The product, captopril disulfide, has been confirmed by IR and GC-MS analysis. The reaction is of first order with respect to hexacyanoferrate(3) and fractional order with respect to captopril concentration. Increase in hydrochloric acid concentration accelerates the rate of reaction at constant ionic strength and at other conditions constant. The order with respect to acid concentration is nearly two. The added product, [Fe(CN⁶)]^4-, does not show any significant effect on the rate of the reaction. The effects of ionic strength and dielectric constant on the rate of reaction have also been studied. Based on the experimental results, a suitable mechanism is proposed and the rate law is derived. The reaction constants involved in the different steps of mechanism are calculated. The activation parameters with respect to limiting step of the mechanism and also the thermodynamic quantities are determined and discussed.