How to Reduce Benzil Using Sodium Borohydride

Chemists refer to compounds such as sodium borohydride as reducing agents because they donate electrons to other atoms, thereby "reducing" their overall charge (See References 1). In organic chemistry, sodium borohydride finds a variety of uses in synthesis, particularly in the conversion of ketones and aldehydes -- compounds that contain C=O bonds -- to alcohols, which contain C-OH bonds (See References 2). Benzil contains two C=O bonds, and sodium borohydride converts this compound to hydrobenzoin, which contains two C-OH groups in place of the C=O groups (See References 3 & 4).

Things You'll Need

• 1 g benzil
• 10 ml absolute ethanol
• 0.5 g sodium borohydride
• 1 ml of 6 M hydrochloric acid
• 50 ml Erlenmeyer flask
• 2 x 250 ml beakers
• 10 ml gradated cylinder
• Hotplate
• Balance capable of measuring to 0.1 grams
• Funnel
• Filter paper
• Ice

Instructions

1. Dissolve 1.0 g benzil in 10 ml of absolute ethanol in a 50 ml Erlenmeyer flask.

2. Prepare a hot water bath by heating 100 ml of water in a 250 ml beaker on a hotplate, then heat the flask containing the benzil in the hot water bath while swirling the flask.

3. Remove the flask from the hot water bath and continue to swirl the flask while running cool tap water over the outside of the flask. Be certain to not allow water into the flask.

4. Add 0.2 g of sodium borohydride to the flask in small portions over a 5 minute period. Continuously swirl the flask during this process.

5. Allow the flask to sit for 10 minutes, then add 15 ml of water followed by 0.5 ml, or about 10 drops, of 6 M hydrochloric acid, then return the flask to the hot water bath for another 5 minutes. Swirl the flask continuously.

6. Remove the flask from the hot water bath and continue to stir the contents with a glass stirring rod for 20 minutes. Then transfer the flask to a beaker filled with ice water.

7. Collect the product by pouring the mixture through filter paper.

Tips & Warnings

• Sodium borohydride is irritating to skin, eyes, and mucous membranes and is highly toxic. Use in a well ventilated area away from open flames and wear eye protection and gloves. Sodium borohydride is also flammable and reactive toward water. (See Resources)

• Ethanol is flammable and is an eye irritant.

• Hydrochloric acid is corrosive. Avoid skin and eye contact. Avoid breathing vapors.

References

• University of Southern Maine: Oxidizing Agents and Reducing Agents in Organic Chemistry
• Towson University: Reducing Agents
• University of West Florida: Sodium Borohydride Reduction of Benzil and Determination of Acetonide Chemistry by 1H NMR
• Eastern Nazarene College: Stereospecific Reduction of Benzil with Sodium Borohydride

Resources

• Sigma-Aldrich, Inc.: Material Safety Data Sheet--Sodium Borohydride