Redox Interactions of Cr(VI) and Substituted Phenols: Products and Mechanism

Article abstract of DOI:10.1021/es00008a010

The mechanisms of aqueous oxidation-reduction interactions between Cr(VI) and substituted phenols (RArOH) were characterized by kinetic analysis and determinations of reaction products and intermediates. A rapid, peroxidative equilibrium between HCrO4(-) and RArOH forms chromate ester intermediates, as verified by spectroscopy. The subsequent rate-limiting ester decomposition proceeds via innersphere electron transfer. The overall rate dependence on [H(+)] is well accounted for by three parallel redox pathways involving zero, one, and two protons. The two-proton pathway dominates at pH <= 2, the single-proton pathway for 2 < pH < 5, and the proton-independent pathway at pH >= 5. The parallel reaction rate expression was fitted to data for 4-methyl-, 4-methoxy-, 2,6-dimethoxy-, and 3,4-dimethoxyphenol for pH 1-6. Beside accurately predicting rates for the calibrated conditions, the model predicts a sharp decline in rates at pH >= 6. Rates subsequently measured at pH 7 agreed well with those calculated a priori. Such predictions suggest that the proposed mechanism is robust and accurate. Rate constants were correlated with Hammett-type substituent parameters. Reaction products indicated both one- and two-electron pathways.