The purpose of this experiment is to use sodium borohydride to reduce benzil. However, stereochemistry allows for five possible products. If only one carbonyl group is reduced during the reaction a racemic mixture of benzoin will be the product that is produced. After the first reduction a chiral center forms causing the second reduction to occur from only one side of the ketone. Depending on which side the second reduction take place there are three possible products including: a racemic mixture of hydrobenzoin (R, R and S, S) or meso-hydrobenzoin (R, S). To determine which product the reduction has produced we will use melting point, IR, and TLC. Key Experimental Details and Observations:
To perform this experiment 1.005 g of benzil was placed in a 125 ml Erlenmeyer flask. We then added approximately 10.0 mL of 95% ethanol, a stir bar, and placed the mixture on a hot plate until the benzil was dissolved. The solution was allowed to cool to room temperature and it began to recrystallize. Once cooled, we placed the substance on a stir plate and added 0.203 g of sodium borohydride over approximately two minutes. By the end of the two minutes the substance went from the yellow color of the benzil to an opaque pale yellow; likewise, the substance also began to admit heat once the sodium borohydride was added. The substance was allowed to stir for two more minutes and then we let it cool to room temperature for about ten minutes so that the precipitate could dissolve. However, while cooling bubble began to form at the top of our substance. After cooling, we added approximately 10.0 mL of hot water (~76.5 ˚C) and allowed the solution to boil on a hot plate; adding the hot water also caused the solution to become a clear pale yellow solution. 15.0 mL of water was slowly added to the mixture, white foamy precipitate began to form almost immediately on top of the solution after adding the addition water. We then removed the stir bar and allowed the solution to cool to room temperature before placing it in an ice bath for approximately five minutes. To collect the product we used vacuum filtration and a Buchner funnel. After fully drying the product under heat lamps it was a white shiny flaky powder with a total mass of 0.816 g.
Our next objective was to determine what product had been formed from the benzil reduction. To determine this we first performed an IR and a melting point analysis. For the first trial of the melting point analysis our range was 130.0 – 134.0 ˚C while the second was 131.0 – 135.0 ˚C. Likewise, we performed thin layer chromatography (TLC) to help determine which product was formed. The developing solvent was a 2:1 mixture of hexane and ethyl, approximately 5 mL of the solution was poured into the watching glass. We used ethyl acetate to dissolve racemic benzoin standard (spot one), meso-hydrobenzoin standard (spot two), our product (spot three), and a co-spot (spot 4). We then spotted our plate with the four samples.
Table 1: Literature melting point (°C)
94.0 – 95.0 °C
135.0 – 137.0 °C
122.0 – 123.0 °C
Meso - hydrobenzoin
137.0 – 139.0 °C
Table 2: Experimental melting points (°C), unknown compound
Melting point (°C)
130.5 – 136.0 °C
132.0 – 137.0 °C
131.25 – 136.5 °C
Table 3: Weight and yield of product
Weight of product
Yield of product
Table 4: TLC
Discussion and Conclusion:
The first method used to determine the identity of the product was melting point. The first trial the range was 130.5 – 136.0 °C and for the second 132.0 – 137.0 °C; the average was 131.25 – 136.5 °C. Moreover, the expected value for benzoin is 135.0 – 137.0 °C, for racemic hydrobenzoin 122.0 – 123.0 °C, and 137.0 – 139.0 °C for meso-hydrobenzoin. In comparison, the...
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