LiAlH₄ Reduction of Carboxylic Acids
LiAlH₄ Reduction of Carboxylic Acids: A carboxylic acid (–COOH) can be reduced with LiAlH₄ to a primary alcohol (–CH₂OH).
What happens
A carboxylic acid (–COOH) can be reduced with LiAlH₄ to a primary alcohol (–CH₂OH). The carbonyl oxygen is removed, but the carbonyl carbon stays in the molecule, so the carbon count is unchanged. LiAlH₄ is a very strong reducing agent and a powerful source of hydride (H⁻).
Reaction summary
Carboxylic acid + LiAlH₄, then workup → primary alcohol (COOH → CH₂OH)
Reagents and conditions
- Starting material: Propanoic acid (carboxylic acid)
- Reagents: LiAlH₄ (LAH)
- Conditions: 1. LiAlH₄ (LAH); 2. H₃O⁺ / H₂O workup
- Products: Propan-1-ol (primary alcohol)
Mechanism
- Acid–Base Reaction: Because carboxylic acids contain an acidic proton, a hydride from LiAlH₄ first reacts with the O–H proton. Hydrogen gas can be formed, and an aluminum-bound carboxylate species results.
- Hydride Reduction: Further hydride transfer reduces the carbonyl carbon. The carbonyl functionality is progressively reduced.
- Further Reduction: Reduction continues until the original carboxylic-acid carbon has effectively become –CH₂O⁻, bound within an aluminum-containing intermediate.
- Workup: The reaction mixture is then treated with H₂O / H₃O⁺. This protonates the oxygen and releases the final primary alcohol, R–CH₂OH.
Common mistakes
- The aqueous/acidic workup must come after LiAlH₄, because LiAlH₄ reacts violently with water and other protic substances.