Which of the following alcohol will react fastest with Lucas reagent?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 2-methylpropan-2-ol (tert-butyl alcohol)
The Lucas reagent is concentrated HCl with anhydrous \( \ce{ZnCl2} \). It converts alcohols into alkyl chlorides, which are insoluble and make the solution turbid. The reaction goes through a carbocation (SN1), so its speed follows carbocation stability, 3° > 2° > 1°:
Tertiary alcohols: turbidity appears immediately.
Secondary alcohols: turbidity appears in about 5 minutes.
Primary alcohols: no turbidity at room temperature.
Structure 3 has the –OH on a carbon attached to three other carbons: it is 2-methylpropan-2-ol, a tertiary alcohol, so it reacts fastest.
\( \displaystyle \ce{(CH3)3C-OH + HCl} \xrightarrow{\ce{ZnCl2}} \ce{(CH3)3C-Cl + H2O} \)
Why the other options are incorrect
Option (1) butan-1-ol — Incorrect. A primary alcohol; it gives no turbidity at room temperature.
Option (2) butan-2-ol — Incorrect. A secondary alcohol; it reacts, but only after a few minutes.
Option (4) 2-methylpropan-1-ol — Incorrect. It is branched, but the –OH is on a \( \ce{CH2} \) group, so it is a primary alcohol.
NCERT (2026-27 edition):
Reaction with Hydrogen Halides
