Borane Reduction of Carboxylic Acids
Borane Reduction of Carboxylic Acids: Borane (BH₃) reduces a carboxylic acid (–COOH) to a primary alcohol (–CH₂OH).
What happens
Borane (BH₃) reduces a carboxylic acid (–COOH) to a primary alcohol (–CH₂OH). The carbonyl carbon is retained and the carbon count is unchanged.
Reaction summary
Carboxylic acid + BH₃·THF → primary alcohol (COOH → CH₂OH)
Reagents and conditions
- Starting material: Propanoic acid (carboxylic acid)
- Reagents: Borane (BH₃·THF)
- Conditions: Aqueous workup
- Products: Propan-1-ol (primary alcohol)
Mechanism
- Borane Reacts with the Carboxylic Acid: BH₃ interacts strongly with the oxygen atoms of the carboxylic acid. The acidic O–H group reacts readily with borane.
- Reduction: Hydride is transferred from boron to the carbonyl system. Further reduction converts the carboxylic-acid carbon toward the alcohol oxidation state.
- Workup: Aqueous workup breaks the boron–oxygen bonds and releases the final R–CH₂OH.
- Product: The primary alcohol: the COOH group has become CH₂OH.
Selectivity
- Chemoselectivity: Borane is useful because of its chemoselectivity for carboxylic acids: it can reduce –COOH to –CH₂OH preferentially in molecules that contain some other carbonyl functional groups. That makes it especially useful when you specifically want to target a carboxylic acid.