In the presence of air, phenols are slowly oxidised to dark coloured mixture containing-
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → benzoquinone
Phenol is easily oxidised. With chromic acid it gives a conjugated diketone, benzoquinone (cyclohexa-2,5-diene-1,4-dione):
\( \displaystyle \ce{C6H5OH} \xrightarrow{\ce{Na2Cr2O7}/\ce{H2SO4}} \ce{O=C6H4=O} \)
Even oxygen of the air oxidises phenols slowly in the same direction. That is why a bottle of phenol, colourless when pure, turns pink and then dark on standing: it becomes a dark coloured mixture containing quinones.
NCERT (2026-27 edition):
Oxidation – “… In the presence of air, phenols are slowly oxidised to dark coloured mixtures containing quinones.”