The structure of the major monohalo product in the following reaction is ___________.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 4-(Chloromethyl)phenol
4-Hydroxybenzyl alcohol has two OH groups of different kinds:
The CH2OH group is a benzylic alcohol. With HCl on heating, the OH is replaced by Cl (the C–O bond breaks easily, the benzylic carbocation being stabilised by the ring).
The OH on the ring is phenolic. The carbon–oxygen bond in phenols has partial double bond character and is difficult to break, so this OH is not replaced by halogen.
Only the benzylic OH reacts:
\( \ce{HO-C6H4-CH2OH} \) + HCl \( \xrightarrow{\text{heat}} \) \( \ce{HO-C6H4-CH2Cl} \) + \( \ce{H2O} \)
The major monohalo product is 4-(chloromethyl)phenol, structure (3).
Why the other options are incorrect
Option (1) — Incorrect. It has the phenolic OH replaced by Cl as well, which does not happen; it is also a dihalo, not a monohalo, product.
Option (2) — Incorrect. HCl does not substitute chlorine on the ring, and the benzylic OH, the reactive group, is left unchanged.
Option (4) — Incorrect. It replaces the phenolic OH and leaves the benzylic OH, the reverse of the actual reactivity.
NCERT (2026-27 edition):
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.”