Aryl halides cannot be prepared from phenols by the methods used to convert alcohols into alkyl halides (with halogen acids or phosphorus halides). Which of the following is the reason?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → The carbon-oxygen bond in phenols has a partial double bond character and is difficult to break.
To turn an alcohol into an alkyl halide, the C–O bond must break so that the halogen can take the place of –OH.
In phenols the oxygen is attached to a ring carbon, and its lone pair is in conjugation with the ring.
The C–O bond therefore has a partial double bond character; it is stronger than a single bond and is hard to break, so the halogen acid or phosphorus halide cannot replace the –OH group.
Option (1) The carbon atom bonded to the –OH group in phenols is sp3 hybridised and crowded by the ring. — Incorrect. That carbon is a ring carbon, so it is sp2 hybridised, not sp3.
Option (3) The halogen acid adds to the double bonds of the benzene ring instead of replacing the –OH group. — Incorrect. The benzene ring is aromatic and does not take up HX by addition as an alkene does. Nothing happens at the C–O bond because it is too strong.
Option (4) The aryl halide formed is at once hydrolysed back to the phenol by the water produced in the reaction. — Incorrect. Aryl halides are very hard to hydrolyse: chlorobenzene changes to phenol only with aqueous NaOH at 623 K and 300 atmospheres.
NCERT (2026-27 edition):
6.4.1 From Alcohols – “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.”