Which of the following reactions of phenol takes place with cleavage of the carbon–oxygen (C–O) bond?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Heating with zinc dust, giving benzene
Alcohols react with hydrogen halides, phosphorus trihalides and dehydrating agents by breaking the C–O bond.
In phenol the –OH group is attached to an sp2 hybridised carbon of the ring and the C–O bond has partial double bond character, so these reagents do not break it.
Phenol shows this type of reaction with zinc: on heating with zinc dust the –OH group is removed and benzene is formed.
\( \displaystyle \ce{C6H5OH + Zn} \xrightarrow{\Delta} \ce{C6H6 + ZnO} \)
Why the other options are incorrect
Option (1) Treatment with phosphorus tribromide, giving bromobenzene — Incorrect.
PBr3 replaces the –OH group of an alcohol by bromine.
This method does not work for phenol: its carbon–oxygen bond has partial double bond character and is stronger than a single bond, so it is difficult to break, and bromobenzene is not formed this way.
Option (3) Oxidation with chromic acid, giving benzoquinone — Incorrect. This reaction does take place, but the oxygen stays on the ring: the product is a conjugated diketone, so the C–O bond is not broken.
Option (4) Treatment with bromine water, giving 2,4,6-tribromophenol — Incorrect. This is electrophilic substitution on the ring; bromine replaces three ring hydrogens and the –OH group is kept.
NCERT (2026-27 edition):
7.4.4 Chemical Reactions – “The reactions involving cleavage of C–O bond take place only in alcohols. Phenols show this type of reaction only with zinc.”
“4. Reaction of phenol with zinc dust”
“Phenol is converted to benzene on heating with zinc dust.”
6.4.1 From Alcohols – “The hydroxyl group of an alcohol is replaced by halogen on reaction with concentrated halogen acids, phosphorus halides or thionyl chloride.” … “The above methods are not applicable for the preparation of aryl halides because the carbon-oxygen bond in phenols has a partial double bond character and is difficult to break being stronger than a single bond.”