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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, preoxidative equilibrium between HCrOr and RArOH forms chromate ester intermediates, as verified by spectroscopy. The subsequent ratelimiting 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 singleproton pathway for 2 < pH < 5, and the protonindependent 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 suggestthatthe proposed mechanism is robust and accurate. Rate constants were correlated with Hammett-type substituent parameters. Reaction products indicated both oneand two-electron pathways.

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