Oxidation of Phenols to Benzoquinones
Oxidation of Phenols to Benzoquinones: Mechanism
The redox process hydroquinone - quinone can be seen as a sequence of proton and electron transfers. In the first step, deprotonation leads to a phenoxide ion which is transformed into a phenoxy radical by a one-electron oxidation. Dissociation of the second OH group generates the radical anion semiquinone followed by a second one-electron oxidation to give benzoquinone. All intermediates are resonance stabilized. Similar redox processes are frequently observed in nature.
Para-benzoquinone and hydroquinone form a reddish-brown 1:1 addition compound (charge-transfer complex) called quinhydrone. The complex was already discovered by Wöhler in 1844. Both molecules are linked through hydrogen bonds.