Oxidation of phenols in polar solutions

Oxidation of phenols with 02 in aqueous solutions is faster than that in hydrocarbon solutions. Oxygen attacks both phenol molecules and phenolate ions. Therefore the rate of oxidation depends on the concentration of hydrogen ions. The rate of phenol oxidation in aqueous solutions (at 14—21°C) is [219] 

Photochemical oxidation of phenol in aqueous solution produces dimers, HOCftH fiH OH, and dihydroxyphenols [221], It is suggested that these products are formed by reactions of phenoxyl radicals. 

Oxidation of phenols is faster in alkaline than in neutral solutions because the reaction ArO + 02 is much faster them ArOH + 02. The dependence of oxidation rates in alkaline aqueous alcohol solutions on phenol structure was studied [222,223]. A linear correlation between log W and a was observed with p = 3.55 (40° C, m- and p-substituted phenols). 

When phenols react with oxygen in polar alkaline solutions, semiquinone radicals are formed the oxidation proceeds autocatalytically producing hydroperoxides and quinols. 

---