Phenol on oxidation with chromic acid gives

The correct answer is Option (4) → Benzoquinone

Phenol is easily oxidised because its ring is made electron-rich by the –OH group. With a strong oxidising agent such as chromic acid (sodium dichromate in sulphuric acid), the –OH carbon and the para carbon are both oxidised to C=O groups, and the product is a conjugated diketone, p-benzoquinone (cyclohexa-2,5-diene-1,4-dione):

    \( \displaystyle \ce{C6H5OH} \xrightarrow[\ce{H2SO4}]{\ce{Na2Cr2O7}} \ce{O=C6H4=O}\ (p\text{-benzoquinone}) \)

Even air slowly oxidises phenols to dark coloured mixtures containing quinones, which is why old samples of phenol turn pink or brown.

Why the other options are incorrect

• Option (1) Benzene — Incorrect. Phenol gives benzene on heating with zinc dust, which removes the oxygen; that is a reduction, not an oxidation.

      \( \displaystyle \ce{C6H5OH + Zn} \xrightarrow{\Delta} \ce{C6H6 + ZnO} \)

• Option (2) Benzaldehyde — Incorrect. A –CHO group on the ring needs a carbon side chain. Benzaldehyde is made by oxidising toluene, for example with chromyl chloride (Etard reaction); phenol has no side-chain carbon to oxidise.

• Option (3) Quinol — Incorrect. Quinol (benzene-1,4-diol) is the reduced form of p-benzoquinone. Chromic acid is a strong oxidising agent and takes the oxidation of the ring as far as the quinone.

NCERT (2026-27 edition):
Oxidation

Note: The same/similar question has been asked in different years by NTA as per the details given below: