Figure ↑: The phenols 1,2- and 1,4-benzenediol (for which the respective common names catechol and hydroquinone are retained by IUPAC) are oxidized to the corresponding diketones, ortho- and para-benzoquinone, by a variety of oxidizing agents, such as sodium dichromate or silver oxide. Yields can be variable when the resulting diones are reactive, as in the case of o-benzoquinone, which partly decomposes under the conditions of its formation.

Figure ↑: The redox process that interconverts hydroquinone and p-benzoquinone can be visualized as a sequence of proton and electron transfers. Initial deprotonation gives a phenoxide ion, which is transformed into a phenoxy radical by one-electron oxidation. Proton dissociation from the remaining OH group furnishes a semiquinone radical anion, and a second one-electron oxidation step leads to the benzoquinone. All of the intermediate species in this sequence benefit from considerable resonance stabilization (two forms are shown for the semiquinone).

Note ↓: Phenol is a very weak acid and the position of equilibrium lies well to the left. Phenol can lose a hydrogen ion because the phenoxide ion formed is stabilised to some extent. The negative charge on the oxygen atom, resulting from loss of hydrogen, is delocalised around the ring. The more stable the ion is, the more likely it is to form.

Figure ↓: Phenols have pK_a values that range from 8 to 10. Even though they are less acidic than carboxylic acids (pK_a = 3–5), they are stronger than alkanols (pK_a = 16–18). The reason is resonance: The negative charge in the conjugate base, called the phenoxide ion, is stabilized by delocalization into the ring. Phenols are acidic because the corresponding anions are resonance stabilized.