What is the correct order of reactivity of alcohols in reactions involving cleavage of the C–O bond?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 3° > 2° > 1°
In reactions that break the C–O bond (reaction with hydrogen halides, dehydration), the protonated alcohol loses water and a carbocation forms. Alkyl groups release electrons and stabilise the positive charge, so carbocation stability is 3° > 2° > 1°, and the alcohols react in the same order.
Lucas test: a tertiary alcohol gives turbidity at once with conc. HCl and ZnCl2; a primary alcohol gives none at room temperature.
Dehydration: tertiary alcohols lose water under the mildest conditions, primary alcohols need the harshest.
Why the other options are incorrect
Option (1) 1° > 2° > 3° — Incorrect. This is the order for reactions that break the O–H bond (acid strength of alcohols), not the C–O bond.
Option (3) 1° > 3° > 2° — Incorrect. A primary alcohol would give the least stable carbocation, so it cannot be the most reactive.
Option (4) 3° > 1° > 2° — Incorrect. A secondary carbocation is more stable than a primary one, so 2° must come before 1°.
NCERT (2026-27 edition):
6.4.1 From Alcohols – “… The order of reactivity of alcohols with a given haloacid is 3°>2°>1°.”
7.4.4 Chemical Reactions – “… Thus, the relative ease of dehydration of alcohols follows the following order: Tertiary > Secondary > Primary”