Alcohols are the compounds containing hydroxyl group attached to the alkyl group, whereas phenols are hydroxyl derivatives of aromatic hydrocarbons in which hydroxyl group is directly attached to carbon atom of aromatic ring. Alcohols are versatile compounds and can behave both as nucleophiles as well as electrophiles.
Phenols, too are reactive compounds and are known to undergo reactions on : phenolic group, benzene ring as well as various other name reactions like Reimer-Tiemann reaction, Kolbe's reaction etc.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → benzoquinone
Phenols are easily oxidised. Chromic acid (\( \ce{K2Cr2O7} \) in \( \ce{H2SO4} \)) is the oxidant: phenol is oxidised at the carbon para to the –OH group, and the product is a conjugated diketone, p-benzoquinone:
\( \displaystyle \ce{C6H5OH + 2[O]} \xrightarrow{\ce{K2Cr2O7/H2SO4}} \ce{C6H4O2 + H2O} \)
So A is benzoquinone. The same oxidation goes on slowly in air, which is why old samples of phenol darken.
Option (1) diphenyl ether — Incorrect. Diphenyl ether, \( \ce{C6H5-O-C6H5} \), is an ether. An oxidising agent adds oxygen to the ring; it does not join two phenol molecules through oxygen.
Option (2) adipic acid (hexanedioic acid) — Incorrect. It comes from strong oxidation of cyclohexanol or cyclohexanone (ring opening by hot \( \ce{HNO3} \) or \( \ce{KMnO4} \)); chromic acid with phenol stops at the quinone.
Option (4) No reaction takes place — Incorrect. Phenol is readily oxidised; the ring is activated by the –OH group.
NCERT (2026-27 edition):
Oxidation – “Oxidation of phenol with chromic acid produces a conjugated diketone known as benzoquinone. In the presence of air, phenols are slowly oxidised to dark coloured mixtures containing quinones.”