Carbonyl Reduction using Sodium Borohydride
Introduction
- Speaker: Leah from leah4sci.com
- Topic: Carbonyl reduction using sodium borohydride (NaBH4)
- Context: Part of a Redox series; next video covers lithium aluminum hydride
- Resources: Practice quiz and cheat sheet available on leah4sci.com/redox
Sodium Borohydride (NaBH4)
- Composition: Sodium ion (spectator, non-reactive), anion of BH4
- Key Component: Hydride ion (H⁻) with lone pair and negative charge
- Hydride: Think "IDE" suffix indicates negative (e.g., chloride, fluoride)
- Strong base but used for nucleophilic attack in reduction
Mechanism and Reactivity
- Boron Exception: Boron in BH3 usually has 6 electrons, becomes stable with 4 in BH4⁻
- Reactivity: NaBH4 is a weak reducing agent
- Slower and safer, selective reduction
- Reacts primarily with aldehydes and ketones
Reduction of Ketones and Aldehydes
- Ketone Reduction:
- Breaks the pi bond
- Adds hydrogen to carbon and oxygen
- Produces a secondary alcohol
- Aldehyde Reduction:
- Similar process
- Produces a primary alcohol
Selective Reduction
- Scenario: Molecules with multiple carbonyls (e.g., aldehyde and ester)
- NaBH4 Action:
- Reduces aldehyde but not ester
Resonance and Carbonyl Reaction
- Resonance:
- Pi bond between carbonyl carbon and oxygen
- Can resonate to place a negative charge on oxygen, positive on carbon
- Partial charges facilitate reaction
Example Mechanism: Acetone Reduction
- Reactants: Acetone (propanone), BH4⁻
- Process:
- Hydride from BH4⁻ attacks carbonyl carbon
- Pi bond electrons move to oxygen
- Generates BH3 and attaches hydride to carbon
- Negative oxygen neutralized by solvent (e.g., methanol)
- Final Product: Secondary alcohol (methanol as solvent)
Additional Notes
- Side Products: Sodium ethoxide when methanol is used
Conclusion
- Next Topic: Lithium aluminum hydride
- Resources: Visit leah4sci.com/redox for more content
Ensure to watch the accompanying video for detailed visual explanations of the mechanisms and additional context.