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 [HClO4] = 0.04–1.00 M, at an ionic strength of 1.0 M [LiClO4] over the temperature range 5–35 °C using the stopped-flow method. In contrast to the reactions of hydroquinone (k2(25 °C) = 1.43 × 104 M−1 s−1, ΔH≠ = 3.6 ± 0.4 kcal mol−1, ΔS≠ = −34 ± 8 cal deg−1 mol−1) and catechol (k2(25 °C) = 9.5 × 102 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 (H2A) are consistent with two reactions involving H2A (k5 = 77.4 M−1 s−1, ΔH≠ = 4.5 ± 0.6 kcal mol−1, ΔS≠ = −35 ± 5 cal deg−1 mol−1) and HA− (k6 = 2.41 × 105M−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.