Cu(Ⅱ)氧化抗坏血酸的动力学和机理

Kinetics and Mechanism of the Oxidation Reaction of Ascorbic Acid by Cu (II)

本文在总离子强度I=1.00mol·dm^(-3)、[Cu^(2+)]》[H_2A]、[H^+]》[H_2A]、无氧及无缓冲剂存在的条件下,研究Cu(Ⅱ)氧化抗坏血酸(H_2A)的动力学和机理。发现Cu(Ⅱ)与H_2A不发生配位反应,但在Cl^-存在的情况下,确有Cu(Ⅱ)的H_2A配合物生成。Cu(Ⅱ)氧化H_2A反应的速率方程为r={a+b[Cl^-]}[Cu^(2+)]{[H^+]+K_a}^(-1),25℃时a和b值分别为4.08×10^(-4)s^(-1)和0.555dm^3·s^(-1)·mol^(-1)。Cu(Ⅱ)氧化H_2A反应的表观活化能为68.1kJ·mol^(-1)。根据动力学结果,提出了反应机理,并给出了配合物ClCuHA的结构形式。

The kinetics and mechanism of the oxidation of ascorbic acid (H_2A) by Cu(II) have been studied in I=1.00 mol. dm^(-3) under conditions of [Gu^(2+)]>> [H_2A], [H^+] >>[H_2A]. The effect of chloride anion on the kinetics and the mechanism has been examined at the [H^+]=1.00×10^(-3) mol·dm^(-3) and the chloride concentration up to 7.00×10^(-3) mol. dm^(-3). The empirical rate law has been established as r={a+b[Cl^-]}[Cu^(2+)A] [H_2A] {[H^+]+K_a}^(-1), where K_a is dissociation constant of ascorbic acid. The constants a and b are 4.08×10^(-4)s^(-1) and 0.555 mol·dm^(-3)·s^(-1) respectively at 25℃.The Arrhenius activation energy for the oxidation reaction of ascorbic acid by Cu(II) is 68.1 kJ·mol^(-1). The stopped-flow spectrum shows that no CuHA^+ complex is formed in the system of Cu(II)- ascorbic acid, but the complex ClCuHA is formed indeed in the presence of Cl^-. A possible mechanism and the structure of the complex ClCuHA are proposed.