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ReductionIntermediate
LiAlH₄ Reduction
Also known as: Lithium Aluminum Hydride Reduction, LAH Reduction
General Equation
RCOOR' + LiAlH₄ → RCH₂OH + R'OH (after H₃O⁺)
Description
A powerful, non-selective reducing agent that reduces virtually all C=O containing functional groups. Much stronger than NaBH₄. Must use anhydrous conditions (reacts violently with water). Reduces esters to two alcohols, acids to primary alcohols, amides to amines.
Reaction Scheme
Reactants:
- • Aldehyde, Ketone, Ester, Carboxylic acid, Amide, or Epoxide
Products:
- • Alcohol (or Amine from amides)
Reagents:
- • LiAlH₄
Conditions:
- • Anhydrous Et₂O or THF, then H₃O⁺ workup
Mechanism (2 steps)
1
LiAlH₄ delivers H⁻ to the carbonyl carbon. For esters and acids, the tetrahedral intermediate collapses and a second H⁻ is delivered.
Arrow: curved
Two hydride deliveries for esters and acids
2
Aqueous acid workup (H₃O⁺) protonates the aluminum alkoxide to give the free alcohol.
Arrow: curved
Careful workup — quench excess LiAlH₄ first
Key Points to Remember
- ✓Reduces everything: aldehydes, ketones, esters, acids, amides, epoxides
- ✓Ester → 2 alcohols (RCOOR' → RCH₂OH + R'OH)
- ✓Acid → primary alcohol (RCOOH → RCH₂OH)
- ✓Amide → amine (RCONR₂ → RCH₂NR₂)
- ✓MUST be anhydrous (violently reacts with H₂O, generates H₂ gas)
- ✓Cannot reduce C=C or isolated C-X bonds
- ✓Use DIBAL-H (-78°C) to reduce ester to aldehyde (stop midway)
Examples
Reactant:
Ethyl benzoate
Reagent:
LiAlH₄, Et₂O, then H₃O⁺
Product:
Benzyl alcohol + Ethanol
Yield: 90%
Reactant:
Benzoic acid
Reagent:
LiAlH₄, THF, then H₃O⁺
Product:
Benzyl alcohol
Yield: 85%
Reactant:
N,N-Dimethylbenzamide
Reagent:
LiAlH₄, THF, then H₃O⁺
Product:
N,N-Dimethylbenzylamine
Yield: 80%
Synthetic Utility
The "nuclear option" reducing agent. When you need to reduce a stubborn functional group, LiAlH₄ will almost certainly do it.
Limitations
- ⚠Not selective — reduces almost everything
- ⚠Violently moisture-sensitive (fire/explosion hazard with H₂O)
- ⚠Cannot use protic solvents
- ⚠Over-reduction (cannot stop at aldehyde from ester; use DIBAL-H)
#reduction#LiAlH4#powerful#non-selective#hydride