Metal-ion oxidations in solution. Part II. The oxidation of malic acid by cobaltic ions

Abstract

The rate of oxidation of malic acid by cobaltic ions has been studied in the range 0–21° by a stopped-flow technique. During the reaction, two changes in absorbance are seen, namely, an initial increase associated with the formation of an intermediate followed by a slower decrease when the intramolecular electron transfer occurs. The spectrum of the intermediate has been obtained in the range 250–300 mµ. The rate constants k₁ and k₂ for the reactions Co³⁺+ HA [graphic omitted] CoHA³⁺ and CoOH²⁺+ HA [graphic omitted] CoOHHA²⁺(where HA = malic acid) are 5·4 ± 1·2 and 70 ± 10 mole^–1 sec.^–1, respectively, at 7° and an ionic strength of 0·25M. The observed rate constant for the oxidation reaction may be expressed as k_obs=[0·11 + 0·20/(H⁺)] at 12·7° and an ionic strength of 1·0M. Temperature studies yield activation energies of 15·3 ± 1·0 and 25·1 ± 1·0 kcal./mole for the oxidations involving CoHA³⁺ and CoOHHA²⁺ respecitively. The mechanisms of the reactions are discussed.