Overview
This lecture covers the history, discovery, and laboratory use of sodium borohydride as a reducing agent, culminating in a hands-on reduction reaction and workup procedures.
Historical Background & Significance
- Sodium borohydride was discovered in 1947 by Herman Schlesinger and Herbert C. Brown in a rocket lab.
- An accidental reaction with acetone revealed its strong reduction power, changing organic synthesis.
- Herbert C. Brown won the Nobel Prize in Chemistry in 1979 for this work.
- Brown’s students (Suzuki and Negishi) later won the Nobel Prize in 2010 for palladium-catalyzed cross-coupling reactions.
- Sodium borohydride plays a role in pharmaceuticals, hydrogen storage, and green energy.
Sodium Borohydride Reduction Experiment
- Reduce benzophenone to benzhydrol using sodium borohydride and methanol.
- Add 0.5 g benzophenone and 10 ml methanol to a flask with a stir bar.
- Cool mixture in an ice bath, then add 0.5 g sodium borohydride slowly.
- Observe hydrogen gas bubbles forming as a byproduct.
- Stir for 10 minutes at room temperature after initial cooling.
- Quench the reaction by adding dilute 3M hydrochloric acid until pH is about 2 (pH paper turns red).
- Optional: Add 5 ml water after acid to dissolve salts and aid extraction.
Extraction and Workup Procedure
- Transfer mixture to a separatory funnel and rinse flask with ether.
- Separate and collect the organic (top) layer and aqueous (bottom) layer.
- Extract the aqueous layer with ether three times to maximize product recovery.
- Combine all organic layers and wash with 15 ml tap water.
- Dry the organic layer with sodium sulfate, then filter with a Buckner funnel.
- Remove solvent with a steam bath to isolate the product.
- Analyze the product by NMR (nuclear magnetic resonance) and IR (infrared spectroscopy).
Key Terms & Definitions
- Sodium borohydride (NaBH₄) — A chemical reagent used to reduce carbonyl compounds to alcohols.
- Reduction — Chemical reaction gaining electrons, often converting ketones/aldehydes to alcohols.
- Quenching — Stopping a reaction by deactivating remaining reactants, often using acid.
- Separatory funnel — Lab glassware used to separate immiscible liquid layers.
- Aqueous layer — Water-based layer in a liquid-liquid extraction.
- Organic layer — The non-water (organic solvent) layer in extraction.
- NMR/IR spectroscopy — Analytical methods for determining chemical structure.
Action Items / Next Steps
- Prepare for a hands-on Suzuki reaction lab next week.
- Obtain and analyze NMR and IR data for the reduced product.
- Review the extraction and drying techniques for lab proficiency.