Sodium Borohydride Reduction of Benzoin
Purpose:
The main goal of this experiment if to use sodium borohydride to reduce benzoin to 1,2-diphenylethane- 1,2- diol.
Reaction Scheme:
Experimental Procedure:
Benzoin(0.5g, 2.36 mmol) and ethanol (4 mL)were added to a 25 mL erlenmeyer flask. The mixture was swirled at room temperature until the benzoin was completely dissolved. Then sodium borohydride(0.1 g, 2.64 mmol) was added in several portions over 5 minutes. It was swirled at room temperature for 20 minutes. The mixture was cooled in an ice bath for 15 mins. Then water(5 mL) and HCl(0.3 mL, 6M) were added to the mixture. After 15 minutes, water(5 mL) was added to the mixture. The mixture was then vacuum filtered to collect the product, using small amounts of ice cold water to wash the flask. The filter was left running with the product to dry for 15 minutes. The crude product was weighed, and the melting point was determined. Around 1 mg was put aside for TLC later. The rest of the product was recrystallized using minimal acetone, it was washed with hexane during the vacuum filtration process. The crystals were dried overnight. The mass and the melting point of the 1,2 -diphenylethane-1,2-diol were determined during morning hours.
For the TLC analysis, benzoin(1 mg), crude product(1 mg) and recrystallized product(1 mg) were all dissolved in separate vials with a 9:1 solvent of ethyl acetate. The TLC plates were then marked and spotted accordingly:
The solvent travelled until around 1 cm from the top of the plate. The TLC plates were dried and spots were observed and marked under the UV light. The TLC papers were then set in the iodine chamber to stain the spots. The Rf values were then calculated and recorded.
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