Metal-ion oxidations in solution. Part XXI. Kinetics and mechanism of the reaction of ascorbic acid, hydroquinone, and catechol with 12-tungstocobaltoate(III)

Abstract

The mechanisms of oxidation of the three di-hydroxy substrates in the title reactions have been investigated in acid media [HClO 4 ] = 0.04–1.00 M, at an ionic strength of 1.0 M [LiClO 4 ] over the temperature range 5–35 °C using the stopped-flow method. In contrast to the reactions of hydroquinone (k 2 (25 °C) = 1.43 × 10 4 M −1 s −1 , ΔH ≠ = 3.6 ± 0.4 kcal mol −1 , ΔS ≠ = −34 ± 8 cal deg −1 mol −1 ) and catechol (k 2 (25 °C) = 9.5 × 10 2 M −1 s −1 , ΔH ≠ = 5.9 ± 0.9 kcal mol −1 , ΔS ≠ = 27 ± 8 cal deg −1 mol −1 ) where no hydrogen ion dependence is observed over the range studied, the rate variations of oxidation of ascorbic acid (H 2 A) are consistent with two reactions involving H 2 A (k 5 = 77.4 M −1 s −1 , ΔH ≠ = 4.5 ± 0.6 kcal mol −1 , ΔS ≠ = −35 ± 5 cal deg −1 mol −1 ) and HA − (k 6 = 2.41 × 10 5 M −1 s −1 , ΔH ≠ = 2.0 ± 1 kcal mol −1 , ΔS ≠ = −17 ± 6 cal deg −1 mol −1 ). The dissociation constant of ascorbic acid has been re-determined over the temperature range studied. Solvation effects are considered important in these outer-sphere systems, which may be discussed in terms of the Marcus relationship. Comparisons with related reactions are discussed.