Which of the following alcohols is converted into the corresponding alkyl chloride just by shaking it with concentrated hydrochloric acid at room temperature, without any catalyst?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 2-Methylpropan-2-ol
The reactivity of alcohols with a halogen acid follows the order 3° > 2° > 1°.
Primary and secondary alcohols react with HCl only in the presence of ZnCl2 as a catalyst.
A tertiary alcohol is reactive enough to give the alkyl chloride on shaking with concentrated HCl at room temperature. 2-Methylpropan-2-ol, (CH3)3C–OH, is a tertiary alcohol:
\( \displaystyle \ce{(CH3)3C-OH + HCl} \xrightarrow{\text{room temperature}} \ce{(CH3)3C-Cl + H2O} \)
Why the other options are incorrect
Option (1) Ethanol — Incorrect. Ethanol is a primary alcohol, the least reactive class; it needs ZnCl2 to react with HCl.
Option (2) Butan-1-ol — Incorrect. Butan-1-ol is also primary; a longer chain does not change its class, so it too needs ZnCl2.
Option (3) Propan-2-ol — Incorrect. Propan-2-ol is a secondary alcohol. It is more reactive than a primary alcohol but still needs ZnCl2 with HCl.
NCERT (2026-27 edition):
6.4.1 From Alcohols – “…The reactions of primary and secondary alcohols with HCl require the presence of a catalyst, ZnCl2. With tertiary alcohols, the reaction is conducted by simply shaking the alcohol with concentrated HCl at room temperature. … The order of reactivity of alcohols with a given haloacid is 3°>2°>1°.”