State Key Laboratory of Pollution Control and Resources Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai 20092, PR China.
Environ Sci Technol. 2012 Aug 21;46(16):8860-7. doi: 10.1021/es302076s. Epub 2012 Aug 8.
Oxidation of phenols by permanganate in the pH range of 5.0-9.0 generally exhibits a parabola-like shape with the maximum reaction rate obtained at pH close to phenols' pK(a). However, a monotonic increase or decrease is observed if phenols' pK(a) is beyond the pH range of 5.0-9.0. A proton transfer mechanism is proposed in which the undissociated phenol is directly oxidized by permanganate to generate products while a phenolate-permanganate adduct, intermediate, is formed between dissociated phenol and permanganate ion and this is the rate-limiting step for phenolates oxidation by permanganate. The intermediate combines with H(+) and then decomposes to products. Rate equations derived based on the steady-state approximation can well simulate the experimentally derived pH-rate profiles. Linear free energy relationships (LFERs) were established among the parameters obtained from the modeling, Hammett constants, and oxygen natural charges in phenols and phenolates. LFERs reveal that chlorine substituents have opposite influence on the susceptibility of phenols and phenolates to permanganate oxidation and phenolates are not necessarily more easily oxidized than their neutral counterparts. The chlorine substituents regulate the reaction rate of chlorophenolates with permanganate mainly by influencing the natural charges of the oxygen atoms of dissociated phenols while they influence the oxidation of undissociated chlorophenols by permanganate primarily by forming intramolecular hydrogen bonding with the phenolic group.
高锰酸盐在 pH 值为 5.0-9.0 的范围内氧化酚类通常呈现抛物线形状，最大反应速率在接近酚类 pKa 的 pH 值处获得。然而，如果酚类的 pKa 超出 5.0-9.0 的 pH 值范围，则会观察到单调增加或减少。提出了质子转移机制，其中未离解的酚类直接被高锰酸盐氧化生成产物，而离解的酚类和高锰酸盐离子之间形成酚盐-高锰酸盐加合物中间体，这是高锰酸盐氧化酚类的限速步骤。中间体与 H(+)结合，然后分解为产物。基于稳态近似得出的速率方程可以很好地模拟实验得出的 pH-速率曲线。通过建模、Hammett 常数和酚类和酚盐中氧的自然电荷，建立了线性自由能关系（LFER）。LFER 表明，氯取代基对酚类和酚盐对高锰酸盐氧化的敏感性有相反的影响，并且酚盐不一定比其中性对应物更容易氧化。氯取代基主要通过影响离解酚类中氧原子的自然电荷来调节氯酚盐与高锰酸盐的反应速率，而它们通过与酚基团形成分子内氢键来主要影响未离解的氯酚与高锰酸盐的氧化。
