How to Convert Benzil to Benzoin
Many first- and second-year organic chemistry students transform benzil to benzoin as part of a laboratory experiment designed to introduce students to reduction reactions. Benzil represents a diketone, which means it contains a pair of C=O double bonds as its primary structural feature. In the presence of a reducing agent, the C=O double bonds transform to alcohol groups, or C-OH single bonds. This transformation requires a fairly strong reducing agent: sodium borohydride (NaBH4).
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Things You'll Need
- 1.5 g benzil
- Balance capable of measuring to 0.1 g
- 2 125-mL Erlenmeyer flasks
- 15 mL absolute ethanol
- 50-mL graduated cylinder
- Hot plate (optional)
- 0.3 g sodium borohydride
- Weighing paper
- Metal or plastic bowl
- Ice
- Glass stirring rod
- 1 mL concentrated hydrochloric acid
- Plastic or glass eyedropper
- Round filter paper
- Short-stem glass funnel
- Watch glass
Instructions
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1
Measure 1.5 g of benzil on a balance and transfer it to a clean, dry 125-mL Erlenmeyer flask. Also measure 15 mL of absolute ethanol in a 50-mL graduated cylinder, and add it to the flask. Swirl the mixture until the benzil has fully dissolved. If necessary, warm the solution on a hot plate set to low to facilitate dissolution.
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2
Weigh 0.3 g of sodium borohydride onto a piece of weighing paper, and add it to the reaction mixture in five or six separate portions over a period of 4 to 5 minutes. After you have added all of the sodium borohydride, swirl the flask for 15 minutes to ensure complete reaction.
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3
Fill a 50-mL graduated cylinder with about 30 mL of water, and cautiously add the water to the flask over a period of 1 to 2 minutes. When you have finished adding the water, immerse the flask in an ice bath and stir the solution with a glass stirring rod.
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4
Add concentrated hydrochloric acid by drops to the flask while stirring the flask between drops. The mixture will begin to foam as you add the acid. Continue to add the acid while stirring until the foaming ceases.
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5
Measure an additional 10 mL of water in a 50-mL graduated cylinder, add it to the mixture, and then stir for an additional 15 minutes with the flask still in the ice bath. The flask should by now contain white benzoin crystals.
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6
Fold a piece of round filter paper, and place it in a short-stem glass funnel. Position the funnel over an empty 125-mL Erlenmeyer flask. Pour the benzoin solution through the filter, and then rinse the crystals on the filter paper with 75 mL of cold water.
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7
Spread the filter paper containing the benzoin out on a watch glass, and allow the crystals to air dry.
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Tips & Warnings
Sodium borohydride reacts violently with water. Use only absolute, or 200 proof, alcohol in this experiment. Otherwise, any water in the alcohol will react with the sodium borohydride before it can reduce the benzil.
Hydrochloric acid is corrosive to human tissue and fumes when exposed to air. Wear eye protection and gloves, and work in a well-ventilated area.
Store sodium borohydride in a cool, dry location, and keep the bottle tightly capped when not in use.
