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EliminationIntroductory
Alcohol Dehydration
Also known as: Acid-Catalyzed Dehydration, Alcohol to Alkene
General Equation
R₂CHCR₂OH → R₂C=CR₂ + H₂O
Description
Elimination of water from an alcohol to form an alkene. Tertiary alcohols are easiest (E1 mechanism), primary alcohols require higher temperatures (E2-like). Follows Zaitsev's rule: more substituted alkene is the major product.
Reaction Scheme
Reactants:
- • Alcohol (R₂CHCR₂OH)
Products:
- • Alkene + H₂O
Reagents:
- • H₂SO₄ or H₃PO₄
Conditions:
- • Heat (> 150°C for 1°, ~100°C for 3°)
Mechanism (3 steps)
1
Protonation of the alcohol hydroxyl group by the acid, converting -OH (poor leaving group) to -OH₂⁺ (good leaving group).
Arrow: curved
2
For 3° alcohols (E1): Loss of water to form a carbocation. For 1° alcohols (E2-like): Concerted loss of water and β-proton.
Arrow: curvedR₃C⁺ (for E1)
3° = E1 mechanism, 1° = E2-like
3
Loss of a β-proton (for E1 pathway) to form the alkene.
Arrow: curved
Zaitsev product — most substituted alkene
Key Points to Remember
- ✓Reactivity: 3° > 2° > 1° alcohols (easier with more substituted)
- ✓Zaitsev's rule: more substituted alkene is major product
- ✓Reverse of acid-catalyzed hydration (equilibrium)
- ✓High temperature favors elimination (entropy)
- ✓Carbocation rearrangements possible for E1 pathway
Examples
Reactant:
2-Methylcyclohexanol
Reagent:
H₂SO₄, heat
Product:
1-Methylcyclohexene (major)
Yield: 85%
Reactant:
2-Methyl-2-butanol
Reagent:
H₃PO₄, 85°C
Product:
2-Methyl-2-butene (major)
Yield: 90%
Synthetic Utility
Simple method to prepare alkenes from readily available alcohols.
Limitations
- ⚠Carbocation rearrangements (especially with 2° alcohols)
- ⚠Primary alcohols require harsh conditions (high temperature)
- ⚠Competition with ether formation at lower temperatures
#elimination#dehydration#alcohol#Zaitsev#E1