Alcohol dehydration is the acid-catalysed elimination of water from an alcohol to form an alkene. The reaction proceeds via an E1 mechanism for secondary and tertiary alcohols, and the product distribution follows Zaitsev's rule — the more substituted alkene is the major product.
The mechanism in three steps (for 2° and 3° alcohols):
Ease of dehydration: 3° > 2° > 1° alcohols. Tertiary alcohols dehydrate at lower temperatures (~443 K) because the tertiary carbocation intermediate is most stable. Primary alcohols require higher temperatures (~453 K) and may proceed via an E2-like pathway.
Zaitsev's rule applied: When multiple β-hydrogens are available, the alkene with the greater number of alkyl substituents on the double bond predominates. For example, butan-2-ol dehydration gives but-2-ene (major) over but-1-ene (minor).
Carbocation rearrangement — the trap NEET exploits: If the initial carbocation can rearrange via a 1,2-hydride or 1,2-methyl shift to a more stable carbocation, the rearranged product forms. Example: 3,3-dimethylbutan-2-ol → 2,3-dimethylbut-2-ene (via methyl shift from secondary to tertiary carbocation). NEET questions often give a substrate where the "obvious" product is the non-rearranged alkene — pick the rearranged one.
Key conditions (NCERT Class 12 Chemistry Chapter 7, page 8): concentrated H₂SO₄ at 443 K is the standard dehydration condition. Alumina (Al₂O₃) at 623 K is the alternative heterogeneous catalyst.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
The ease of acid-catalysed dehydration of alcohols follows the order:
Which reagent/condition is the standard laboratory method for dehydration of alcohols as described in NCERT?
What is the role of H₂SO₄ in the first step of the E1 dehydration mechanism of a secondary alcohol?
Butan-2-ol undergoes acid-catalysed dehydration. According to Zaitsev's rule, the major product is:
3,3-Dimethylbutan-2-ol is heated with concentrated H₂SO₄. The major product is:
Which of the following alcohols requires the highest temperature for acid-catalysed dehydration?
During the dehydration of neopentyl alcohol (2,2-dimethylpropan-1-ol) with H₂SO₄, a rearranged alkene is obtained. The rearrangement occurs because:
When 2-methylcyclohexanol undergoes acid-catalysed dehydration, two alkene products are possible. Applying Zaitsev's rule, the major product is:
Given
3,3-Dimethylbutan-2-ol is heated with concentrated H₂SO₄.
Required
Identify the major dehydration product.
Concept
Acid-catalysed dehydration follows E1 mechanism for secondary/tertiary alcohols. If a carbocation intermediate can rearrange to a more stable cation via a 1,2-shift, the rearranged product predominates. Zaitsev's rule then selects the most substituted alkene from the rearranged carbocation.
Formula / Principle
Zaitsev's rule: the more substituted alkene is the major product. Carbocation stability: 3° > 2° > 1°.
Substitution / Setup
- Substrate: CH₃–C(CH₃)₂–CH(OH)–CH₃ (3,3-dimethylbutan-2-ol) - Step 1: –OH protonated → –OH₂⁺ - Step 2: H₂O leaves → secondary carbocation at C-2: CH₃–C(CH₃)₂–C⁺H–CH₃ - Assess: Can C-2 (2°) rearrange? Adjacent C-3 has three methyl groups. A 1,2-methyl shift moves one methyl from C-3 to C-2.
Calculation / Reasoning
After 1,2-methyl shift: - C-2 now has two methyl groups and is bonded to C-3 → tertiary carbocation at C-3: CH₃–C(CH₃)=C(CH₃)–CH₃ path available. - The rearranged tertiary carbocation: (CH₃)₂C⁺–CH(CH₃)–CH₃ is equivalent to the 2,3-dimethylbutyl cation at C-2 (tertiary). - β-Hydrogen elimination by Zaitsev's rule: remove H from the most substituted side → tetrasubstituted alkene.
Final answer
**Major product: 2,3-dimethylbut-2-ene** (tetrasubstituted alkene from the rearranged tertiary carbocation).
Common trap
Selecting 3,3-dimethylbut-1-ene (the product from direct elimination of the unrearranged secondary carbocation). NEET distractors exploit students who forget to check for possible 1,2-shifts before applying Zaitsev's rule.
Similar NEET-style question
"When neopentyl alcohol (2,2-dimethylpropan-1-ol) is treated with H₂SO₄ at 443 K, the major product is: (a) 3,3-dimethylbut-1-ene, (b) 2-methylbut-2-ene, (c) 2-methylbut-1-ene, (d) 3-methylbut-1-ene." Answer: (b) — 1,2-methyl shift converts the primary carbocation to a tertiary one, then Zaitsev elimination gives the trisubstituted alkene. ---
Nitrogen (first element of Group 15) is anomalous: (1) No d-orbitals — covalence limited to 4; cannot form pentahalides. (2) Forms strong pπ–pπ multiple bonds (N≡N, N=O); heavier homologues form dπ–pπ bonds. (3) N–N single bond weaker than P–P due to lone-pair repulsion (small bond length). (4) No tendency for catenation compared to phosphorus. (5) Unique ability to form hydrogen bonds (NH₃, HNO₃).
-- NCERT Class 12 Chemistry, Ch. 7, p. 3Stronger acid than phenol due to more effective resonance over two equivalent oxygens. EWG substituents (Cl, NO2) increase acidity.
| Symbol | Quantity | SI Unit |
|---|---|---|
| pKa | -log Ka | - |
Phenols ~10⁶× more acidic than aliphatic alcohols due to resonance stabilisation of phenoxide ion. Electron-withdrawing substituents lower pKa further.
| Symbol | Quantity | SI Unit |
|---|---|---|
| pKa | -log Ka | - |
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Organic Reaction Conditions
1° alcohol: PCC/PDC → aldehyde (stops). KMnO4/K2Cr2O7 in acidic → carboxylic acid (continues). 2° alcohol: any oxidiser → ketone. 3° alcohol: not oxidised by ordinary reagents.
Question gives 1° alcohol oxidation with specified reagent.
PCC, PDC, Swern, DMP: mild → stop at aldehyde. KMnO4, K2Cr2O7, CrO3, jones: strong → carboxylic acid. Reagent choice matches desired product.
Root cause: concept gap
Aldehydes more reactive: less steric hindrance, less +I from one R group. Order: HCHO > RCHO > R₂CO.
Root cause: concept gap
Iodoform test ONLY positive for: methyl ketones (CH₃-CO-R), ethanol, secondary alcohols of form CH₃-CH(OH)-R, ethanal.
20 questions from NEET 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.
Identify the suitable reagent for the following conversion.
The correct order of decreasing acidity of the following aliphatic acids is
Identify the correct reagents that would bring about the following transformation.
In which of the following equilibria, K and K are NOT equal? p c
Taking stability as the factor, which one of the following represents correct relationship?
The right option for the statement "Tyndall effect is exhibited by", is :
Which of the following reactions is the metal displacement reaction? Choose the right option. → ↑
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
inverts aldehyde ketone reactivity
Believes ketones more reactive
includes non methyl ketones
Treats all ketones as positive
ignores resonance vs induction balance
Uses inductive only without considering resonance
Test yourself on this topic with real past-paper questions:
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