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.