Oxidation

$$\text{Phenol} \xrightarrow{\text{Na}_2\text{Cr}_2\text{O}_7 / \text{H}_2\text{SO}_4} benzoquinone \quad (\text{conjugated diketone})$$

Concept: Phenol undergoes controlled oxidation when treated with strong oxidizing agents such as sodium dichromate ($$\text{Na}_2\text{Cr}_2\text{O}_7$$) or chromic acid ($$\text{H}_2\text{CrO}_4$$) in acidic medium, yielding $$p$$-benzoquinone (a conjugated cyclic diketone). In the presence of air or standing over time, phenols undergo slow oxidation to complex quinone-containing dark-colored mixtures.