Kinetics and mechanism of oxidation of captopril by diperiodatocuprate(III) in aqueous alkaline medium

Captopril is a sulfur containing drug which inhibits angiotensin-converting enzyme. Its kinetics of oxidation were studied spectrophotometrically using diperiodatocuprate(III) in aqueous alkali. Major oxidative product of captopril was identified as captopril disulfide along with trace amount of the hydrolyzed product, l-proline. Kinetics of oxidation was found to be first order each in [oxidant] and [reductant]. Rate of reaction was decreased by increasing [OH-], whereas added periodate retarded the rate. Other kinetic parameters viz., ionic strength, dielectric constant, temperature effect, and intervention of free radical were also studied. Activation parameters like E (A), Delta H (#), Delta S (#), Delta G (#), and log A were calculated. A suitable mechanism was proposed. Rate law for proposed mechanism was derived and verified. Reaction constants involved in various steps of mechanism were evaluated and used to regenerate first-order experimental rate constants at various experimental conditions.