Sodium Borohydride
Formula: NaBH₄
Type: Mild hydride reducing agent
Common use: Reduction of aldehydes and ketones to alcohols.
What it reduces
Section titled “What it reduces”Sodium borohydride commonly reduces:
- aldehydes to primary alcohols
- ketones to secondary alcohols
What it normally does not reduce
Section titled “What it normally does not reduce”Under standard undergraduate laboratory conditions, sodium borohydride usually does not reduce:
- carboxylic acids
- esters
- amides
General mechanism
Section titled “General mechanism”- Hydride transfer from BH₄⁻ to the carbonyl carbon.
- Formation of an alkoxide intermediate.
- Protonation during solvent or work-up to give the alcohol.
Common conditions
Section titled “Common conditions”NaBH₄ is often used in:
- methanol
- ethanol
- aqueous alcohol solvents
Comparison with LiAlH₄
Section titled “Comparison with LiAlH₄”NaBH₄ is milder than LiAlH₄.
LiAlH₄ can reduce a wider range of carbonyl derivatives, including esters and carboxylic acids, while NaBH₄ is usually more selective for aldehydes and ketones.
Exam traps
Section titled “Exam traps”- Do not assume NaBH₄ reduces every carbonyl compound.
- NaBH₄ is milder than LiAlH₄.
- The final proton usually comes from solvent or work-up.
- Esters normally require stronger reducing agents such as LiAlH₄.