KINETICS AND MECHANISM OF THE OXIDATION OF PHENOLS BY THE OXOCHROMIUM(IV) ION

The oxidation of phenols by the pentaaquaoxochromium(IV) ion, (H2O)(5)CrO2+, in acidic aqueous solutions yields p-benzoquinone as a major product. Small amounts of the unstable 4,4'-biphenoquinone were also produced, as evidenced by an increase (fast stage) and then a decrease (slow stage) in absorbance at 400 nm where there is an intense absorption band. The fast stage is first-order in CrO2+ and first-order in phenol. The slow stage follows a first-order exponential decay. There is a large kinetic isotope effect for the first stage, k(H)/k(D) = 14.7, on deuteration of the hydroxylic hydrogen. There is, however, no kinetic isotope effect of deuteration of the C-H hydrogen, k(H)/k(D) similar to 1.0. The rate constants for different meta-substituted phenols follow the Hammett relationship with rho = -1.7. The activation parameters are Delta H double dagger = 15.2 +/- 1.5 kJ mol-(1) and Delta S double dagger = -144 +/- 15 J mol(-1) K-1. We propose a mechanism according to which the phenols are first oxidized by one electron to the corresponding phenoxyl radicals; this occurs by hydrogen atom abstraction. The superoxochromium(III) ion, (H2O)(5)CrO22+, which is also present in solution, then oxidizes the phenoxyl radical to benzoquinone. The rate constants for both stages increase with increasing ionic strength of the medium. The solution acidity was found to decrease the rate of the first stage and enhance the rate of the second, but both effects are relatively small. The activation parameters for the second stage are Delta H double dagger = 46 +/- 4 kJ mol(-1) and Delta S double dagger = -129 +/- 11 J mol(-1) K-1.