Metal-ion oxidations in solution. Part XI. Malic acid with cerium(IV) in perchlorate media

Abstract

The redox reaction between cerium(IV) ions and malic acid (HL) proceeds via an inner-sphere mechanism and has been investigated at an ionic strength of 1·50M over the temperature range 10·6–25 °C using the stopped-flow method. Two intermediate complexes have been identified [equation (i)], the reactive species [CeL]³⁺[Ce(HL)]⁴⁺ [graphic omitted] H⁺+[CeL]³⁺ [graphic omitted] Ce^III+ L˙(i) being somewhat more stable; K₃= 11 ± 4 mol l^–1 at 25 °C. Formation constants for the complexes have been obtained both from initial optical-density changes and kinetic data. Activation parameters for the intramolecular electron transfer involving the complex [CeL]³⁺ have been derived; k_β= 0·52 ± 0·06 s^–1 at 25 °C with ΔH^‡= 20·6 ± 1·5 kcal mol^–1 and ΔS^‡= 9·5 ± 5 cal K^–1 mol^–1. The first hydrolysis constant for cerium(IV)[equation (ii)] has been redetermined; K_h= 0·2 ± 0·02 mol l^–1 at 25 °C with ΔH= 5·2 ± 0·;7 kcal mol^–1 and ΔS= Ce⁴⁺(aq) [graphic omitted] [Ce(OH)]³⁺(aq)+ H⁺(ii) 14·3 ± 4 cal K^–1 mol^–1, and compared with other values. Possible paths for the rapid decomposition of the radical formed in the redox step are discussed.