An organic compound A, molecular formula C6H6O, gives a violet colour with neutral FeCl3 solution. A on treatment with CO2 and NaOH at 400 K under pressure gives B, which on acidification gives a compound C. Compound C reacts with acetyl chloride to give D, which is a popular painkiller.
Compound (A) of the passage is oxidised to benzoquinone, a conjugated diketone. Which of the following reagents brings about this change?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Chromic acid (Na2Cr2O7 and H2SO4)
Compound (A), C6H6O, is phenol. Oxidation of phenol with chromic acid (sodium dichromate and sulphuric acid) gives benzoquinone, a conjugated diketone with two C=O groups at positions 1 and 4 of a six-membered ring:
\( \displaystyle \ce{C6H5OH} \xrightarrow[\ce{H2SO4}]{\ce{Na2Cr2O7}} \underset{\text{benzoquinone}}{\ce{C6H4O2}} \)
Even in air, phenols are slowly oxidised to dark coloured mixtures containing quinones.
Why the other options are incorrect
Option (1) Zinc dust, on heating — Incorrect. Zinc dust removes the oxygen: phenol is converted to benzene, and zinc becomes ZnO. This is the opposite of oxidation.
Option (2) Bromine water — Incorrect. Bromine water brings about electrophilic substitution of the ring and gives a white precipitate of 2,4,6-tribromophenol.
Option (4) Dilute nitric acid at 298 K — Incorrect. Dilute nitric acid at low temperature nitrates the ring and gives a mixture of ortho and para nitrophenols.
NCERT (2026-27 edition):
7.4.4 Chemical Reactions – “5. 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.”
\( \displaystyle \ce{C6H5OH} \xrightarrow[\ce{H2SO4}]{\ce{Na2Cr2O7}} \underset{\text{benzoquinone}}{\ce{C6H4O2}} \)