Metal-ion oxidations in solution. Part 16. The oxidation of α-hydroxycarboxylic acids by cerium(IV) in perchioric acid media

Abstract

The redox reactions between cerium(IV) ions and α-hydroxycarboxylic acids (HA) proceed via an inner-sphere mechanism. Investigations over the range 6.4–30 °C using stopped-flow methods have demonstrated the existence of intermediate complexes. The complex [CeA]³⁺is thermodynamically more stable than the protonated [Ce(HA)]⁴⁺ [graphic omitted] H⁺+[CeA]³⁺ [graphic omitted] Ce^III+ A·(i) form [Ce(HA)]⁴⁺and is more reactive kinetically. Formation constants for the complexes have been obtained both from initial optical-density changes and kinetic data. For reactions (ii) the formation constants at 25 °C and [Ce(OH)]³⁺+ HA =+ [graphic omitted] [CeA]³⁺(ii) thermodynamic parameters are: glycolic acid, K₂= 119 ± 13 dm₃mol^–1, ΔH₂=–14.8 ± 2.5 kcal mol^–1, ΔS₂, =–40 ± 5 cal K^–1mol^–1: lactic acid, K₂= 191 ± 14, ΔH₂, =–12.8 ± 0.75, ΔS₂, = 33 ± 4; 2-hydroxy-2-methyl-propanoic acid, K₂= 372 ± 46, ΔH₂, =–18.5 ± 3.3, ΔS₂, =–50 ± 6; phenylglycolic acid. K₂= 645 ± 120 (17.5 °C), ΔH₂=–14.7 ± 1.5. ΔS₂=–37 ± 5. Rate constants, k₂, for the intramolecular redox reactions at 25 °C for the four substrates are 0.37, 2.25, 2.57, and 92 s^–1 respectively, the corresponding ΔH^‡ values being 28.8 ± 1.2, 25.0 ± 0.8, 28.5 ± 0.4, and 23.4 ± 1.3 kcal mol^–1. No medium effects have been observed on substitution of lithium for sodium as the counter ion of the background electrolyte. Attempts to characterise the radical A· using flow-e.s.r. techniques have been unsuccessful. The rate data are compared with those in sulphuric acid media and possible similarities in the reaction paths are discussed.