The relative rate of dehydration of alcohols is given by:

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This question was previously asked in

DSSSB PGT Chemistry (Female) Official Paper (Held On: 06 Jul, 2018 Shift 1)

Answer 1

CONCEPT:

Dehydration of Alcohols

Dehydration of alcohols refers to the elimination of a water molecule (H₂O) from an alcohol to form an alkene.
This reaction typically occurs in the presence of a strong acid catalyst such as concentrated sulfuric acid (H₂SO₄) or phosphoric acid (H₃PO₄).
The rate of dehydration of alcohols depends on the stability of the carbocation intermediate formed during the reaction.
Carbocation stability increases in the order:
3° (tertiary) > 2° (secondary) > 1° (primary)

EXPLANATION:

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During the dehydration process:
- The alcohol first undergoes protonation by the acid to form an oxonium ion (R-OH₂⁺).
- This is followed by the loss of water (H₂O), forming a carbocation intermediate.
The stability of the carbocation intermediate is a key factor determining the rate of the dehydration reaction.
The order of carbocation stability is:
3° (tertiary carbocation) > 2° (secondary carbocation) > 1° (primary carbocation)
As tertiary alcohols form the most stable carbocations, they dehydrate the fastest. Secondary alcohols are next, followed by primary alcohols, which form the least stable carbocations.

Therefore, the relative rate of dehydration of alcohols is: 3° > 2° > 1°.