Aryl halides can not be prepared by the reaction of aryl alcohols with \(PCl_3\), \(PCl_5\) or \(SOCl_2\) because :
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Carbon-oxygen bond in phenols has a partial double bond character
Alcohols are converted into alkyl halides by \( \ce{PCl3} \), \( \ce{PCl5} \) or \( \ce{SOCl2} \), because the C–O single bond of an alcohol breaks easily. In phenol, the lone pair of oxygen is delocalised into the benzene ring by resonance, and the C–O bond acquires partial double bond character. It is shorter (136 pm) and stronger, and is not broken by these reagents:
\( \displaystyle \ce{ROH + SOCl2 -> RCl + SO2 + HCl} \quad (\text{works for alcohols, not for phenols}) \)
Why the other options are incorrect
Option (1) Phenols are highly stable compounds — Incorrect. Phenols react readily in many other reactions (bromination, nitration); the reason is the strength of the C–O bond, not general stability.
Option (3) Carbon-oxygen bond is highly polar — Incorrect. A polar C–O bond would make substitution easier, not harder.
Option (4) Phenyl cation is stabilised by resonance — Incorrect. The phenyl cation is in fact not stabilised by resonance (its empty orbital is an sp2 orbital in the plane of the ring), and it is very unstable.
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.”