Ms. Elovitz et W. Fish, REDOX INTERACTIONS OF CR(VI) AND SUBSTITUTED PHENOLS - PRODUCTS AND MECHANISM, Environmental science & technology, 29(8), 1995, pp. 1933-1943
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.