Oxidation by Co³⁺ ions in aqueous acidic media

Abstract

Kinetics of oxidation of a wide variety of organic compounds by Co³⁺ in aqueous acidic media (H₂SO₄, HClO₄, or HNO₃) at constant [H⁺] and ionic strength, µ, in the temperature range, 5-25 °C have been investigated, the organic compounds chosen being hydroxy-compounds such as benzyl alcohol, allyl alcohol, ethylene glycol, propylene glycol, propane-1,3-diol, pinacol, glycerol, sorbitol, mannitol; ketones such as acetone, ethyl methyl ketone, n-propyl methyl ketone, isobutyl methyl ketone, cyclopentanone; acids such as malonic, glutaric, adipic, glycolic, diglycolic, citric, aceturic, crotonic, and itaconic acids; sugars such as glucose, fructose, sucrose; and ethers such as tetrahydrofuran and dioxane. All the reactions invariably obeyed second order kinetics, the order with respect to [CO³⁺] and [substrate] each being unity. Michaelis-Menten kinetics were observed only in glutaric acid oxidation. Inverse dependence on [H⁺] was observed in all the oxidations (except sucrose). Retardation of rate with [HSO₄^-] and effects of µ, added CO²⁺, and temperature were some other features studied. Relative rates of oxidation in the various acid media followed the sequence, H₂SO₄ < HNO³ < HClO₄. Product analysis by VPC was carried out and the reaction stoichiometry for some oxidations, such as those of benzyl alcohol and pinacol, has been established. The rate laws have been derived and suitable reaction mechanisms were suggested. The second order rate constants (k₂), acid-independent and acid-dependent rates (a and b), the corresponding rate constant ratio, k_b/k_aand the kinetic parameters, ΔE, ΔS, and A have been evaluated and discussed.