U.S. flag

An official website of the United States government

Dot gov

Official websites use .gov
A .gov website belongs to an official government organization in the United States.


Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.


Main content area

Parabola-Like Shaped pH-Rate Profile for Phenols Oxidation by Aqueous Permanganate

Du Juanshan, Sun Bo, Zhang Jing, Guan Xiaohong
Environmental Science & Technology 2012 v.46 no.16 pp. 8860-8867
Gibbs free energy, chlorine, chlorophenols, equations, hydrogen bonding, models, oxidation, oxygen, pH, phenol, protons
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' pKₐ. However, a monotonic increase or decrease is observed if phenols' pKₐ 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.