Metal-ion oxidations in solution. Part 17. The kinetics and mechanism of the oxidation of malonic acid by cerium(IV) in perchloric acid media

Abstract

The redox reaction between Ce^IV and malonic acid (H₂L) proceeds via an inner-spheremechanism. Using stopped-flow methods, the oxidation has been investigated over the range 2.4–35 °C. A notable feature of the reaction is that, whilst at low temperatures there is kinetic and spectroscopic evidence for intermediate complex formation. at higher temperatures (30 and 35 °C) the reaction order changes to unity with respect to the reductant concentration. The reaction is catalysed by hydrogen ions. The data are rationalized in termsof thefollowing reactionscheme [graphic omitted] in which the principal path involves reaction of [Ce(H₂L)]⁴⁺(K₅ < 1 mol dm^–3). The significance of the relative magnitudes and temperature dependences of the equilibrium constants is discussed and the incorporation of the kinetic term k₂ provides a general mechanism for cerium(IV) oxidations. Flow e.s.r. techniques have been used to characterize the radical R˙, and its stability compared with radical intermediates fromtheoxidationofother substrates is discussed.