Chromic acid oxidation of aromatic alcohols

The mechanism of the chromic acid oxidation of di- and tri-aryl carbinols has been studied in 80 wt% acetic acid containing sulfuric acid. The reactions are cleanly second-order and give benzophenones in the case of the secondary alcohols and benzophenones plus phenols in the case of the tertiary alcohols. Electron-donating substituents in the tertiary alcohol appear predominantly in the phenol component and the rate-controlling step is believed to be a 1,2-aryl shift. The reaction constant for the migration is ρ+ = −1.44 and that for the overall reaction is ρ+ = −0.879. The presence of manganous ions does not alter these values although it lowers the overall rate of reaction. Although an analogous 1,2-hydride shift mechanism can be written for the oxidation of the secondary alcohols, there are enough points of difference between the oxidations of the secondary and tertiary systems to make this appear unlikely.