In the dehydration of alcohols to alkenes by heating with concentrated H2SO4, the first step is:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Protonation of the alcohol molecule
Acid-catalysed dehydration (shown for ethanol) goes in three steps, and the acid acts first. The –OH group is a poor leaving group, so the alcohol first takes up \( \ce{H^+} \) from the acid on its oxygen; this turns –OH into –OH2+, which can leave as a neutral water molecule.
Protonation of the alcohol (fast):
\( \displaystyle \ce{CH3CH2OH + H^+ -> CH3CH2OH2^+} \)
Formation of the carbocation (slow, rate determining): the protonated alcohol loses water.
\( \displaystyle \ce{CH3CH2OH2^+ -> CH3CH2^+ + H2O} \)
Loss of a proton: the carbocation gives up \( \ce{H^+} \) and the alkene forms.
\( \displaystyle \ce{CH3CH2^+ -> CH2=CH2 + H^+} \)
Why the other options are incorrect
Option (1) Formation of an ester — Incorrect. No ester appears in this mechanism; the only intermediates are the protonated alcohol and the carbocation.
Option (3) Formation of a carbocation — Incorrect. This is the second step; the carbocation can form only after the alcohol has been protonated.
Option (4) Elimination of water — Incorrect. Water leaves in the second step, from the protonated alcohol, not from the neutral alcohol at the start.
NCERT (2026-27 edition):
7.4.4 Chemical Reactions – “The mechanism of dehydration of ethanol involves the following steps: …
Step 1: Formation of protonated alcohol.”