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ReductionIntermediate
NaBH₄ Reduction
Also known as: Sodium Borohydride Reduction
General Equation
R₂C=O + NaBH₄ → R₂CHOH (after workup)
Description
A mild reducing agent that selectively reduces aldehydes and ketones to alcohols without reducing esters, amides, or carboxylic acids. Delivers hydride (H⁻) to the electrophilic carbonyl carbon.
Reaction Scheme
Reactants:
- • Aldehyde or Ketone
Products:
- • Primary alcohol (from aldehyde) or Secondary alcohol (from ketone)
Reagents:
- • NaBH₄
Conditions:
- • MeOH or EtOH, 0°C to RT
Mechanism (2 steps)
1
NaBH₄ delivers a hydride (H⁻) to the electrophilic carbonyl carbon, forming an alkoxide intermediate. One NaBH₄ can reduce up to 4 carbonyls.
Arrow: curvedR₂CHO⁻ (alkoxide)
Nucleophilic addition of H⁻ to C=O
2
Protonation of the alkoxide by the protic solvent (MeOH) gives the alcohol.
Arrow: curved
Protic solvent serves as proton source
Key Points to Remember
- ✓Selective: reduces aldehydes/ketones only (not esters, amides, or acids)
- ✓Mild: can use in protic solvents (MeOH, EtOH, even H₂O)
- ✓One NaBH₄ delivers 4 hydrides (reduces 4 equivalents)
- ✓Chemoselectivity: aldehyde > ketone (aldehyde reduced first)
- ✓Cannot reduce C=C double bonds, esters, or amides
- ✓Compare with LiAlH₄: reduces almost everything (much stronger)
Examples
Reactant:
Benzaldehyde
Reagent:
NaBH₄, MeOH
Product:
Benzyl alcohol
Yield: 95%
Reactant:
Cyclohexanone
Reagent:
NaBH₄, EtOH
Product:
Cyclohexanol
Yield: 92%
Synthetic Utility
The standard mild reducing agent for carbonyl reductions. Excellent chemoselectivity makes it the first choice when other reducible groups are present.
Limitations
- ⚠Cannot reduce esters, amides, or carboxylic acids (use LiAlH₄)
- ⚠Cannot reduce C=C bonds
- ⚠Less stereocontrol than asymmetric reductions (CBS, etc.)
#reduction#NaBH4#hydride#selective#mild