Which amongst the following will be most readily dehydrated under acidic conditions?
The options are

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Question

Which amongst the following will be most readily dehydrated under acidic conditions?
The options are different types of alcohols:
(A) Primary alcohol (e.g., CH₃CH₂OH or butan-1-ol)
(B) Tertiary alcohol (e.g., 2-methylpropan-2-ol, (CH₃)₃COH)
(C) Secondary alcohol (e.g., butan-2-ol)
(D) Phenol (C₆H₅OH)

Accepted Answer

Correct Answer: B — Tertiary alcoholStep-by-step Solution:Dehydration of alcohols under acidic conditions (conc. H₂SO₄ or H₃PO₄) involves two steps:Step 1: Protonation of the –OH group by H⁺ to form an oxonium ion (R–OH₂⁺).Step 2: Loss of water to form a carbocation (E1 mechanism), followed by removal of H⁺ to form the alkene.The rate-determining step is the formation of the carbocation. The more stable the carbocation, the faster the dehydration:Stability order of carbocations: 3° > 2° > 1°. Therefore:Ease of dehydration: Tertiary > Secondary > Primary > MethanolA tertiary alcohol forms a tertiary carbocation (most stable due to hyperconjugation and inductive effects of three alkyl groups). This makes tertiary alcohols most easily dehydrated even at lower temperatures and with dilute acids.Why other options are wrong:Option A (Primary alcohol): Forms a primary carbocation (least stable) — requires highest temperature and most concentrated acid; most difficult to dehydrate.Option C (Secondary alcohol): Intermediate between primary and tertiary.Option D (Phenol): The –OH group of phenol is strongly resonance-stabilised with the benzene ring (C–O bond has partial double bond character). Phenol does not readily dehydrate to form benzyne under normal acidic conditions — it requires very harsh conditions.Key Concepts:E1 mechanism: Unimolecular elimination; involves carbocation intermediate; favoured by polar solvents, tertiary substrates, and higher temperatures.Carbocation stability: 3° > 2° > 1° > methyl, due to hyperconjugation and inductive electron donation by alkyl groups.Dehydration temperatures (approx.): Primary alcohol ~443 K, Secondary ~413 K, Tertiary ~353 K (lowest temperature needed).Zaitsev's rule: The more substituted (more stable) alkene is the major product of dehydration.

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