Q.Identify the product obtained when phenol is heated with chromic acid.

On standing in air, phenol is slowly oxidised to a dark coloured mixture containing quinones, which is why phenol samples turn pink/brown on exposure to air and light.

Phenol is comparatively easily oxidised because of the activating -OH group on the benzene ring. Even traces of atmospheric oxygen (and light) slowly oxidise phenol, producing a complex, dark-coloured mixture whose major component is a quinone (e.g. benzoquinone, formed by oxidation of the para position).

…

Chromic acid oxidises phenol to p-benzoquinone, a conjugated cyclic diketone.

Phenol undergoes oxidation with a strong oxidising agent such as chromic acid ($CrO_3/H_2SO_4$ or $Na_2Cr_2O_7/H_2SO_4$), which converts the aromatic ring bearing the $-OH$ group into a cyclic conjugated diketone:

$C_6H_5OH \xrightarrow{CrO_3/H^+} \text{1,4-benzoquinone (p-benzoquinone)}$

…