REDOX INTERACTIONS OF CR(VI) AND SUBSTITUTED PHENOLS - PRODUCTS AND MECHANISM

The mechanisms of aqueous oxidation-reduction interactions between Cr( VI) and substituted phenols (RArOH) were characterized by kinetic anal ysis and determinations of reaction products and intermediates. A rapi d, preoxidative equilibrium between HCrO4- and RArOH forms chromate es ter intermediates, as verified by spectroscopy. The subsequent rate-li miting ester decomposition proceeds via innersphere electron transfer. The overall rate dependence on [H+] is well accounted for by three pa rallel redox pathways involving zero, one, and two protons. The two-pr oton pathway dominates at pH less than or equal to 2, the single-proto n pathway for 2 < pH < 5, and the proton-independent pathway at pH gre ater than or equal to 5. The parallel reaction rate expression was fit ted to data for 4-methyl-, 4-methoxy-, 2,6-dimethoxy-, and 3,4-dimetho xyphenol for pH 1-6. Beside accurately predicting rates for the calibr ated conditions, the model predicts a sharp decline in rates at pH gre ater than or equal to 6. Rates subsequently measured at pH 7 agreed we ll with those calculated a priori. Such predictions suggest that the p roposed mechanism is robust and accurate. Rate constants were correlat ed with Hammett-type substituent parameters. Reaction products indicat ed both one- and two-electron pathways.