Khan, Alisha-Post-Lab_ Experiment 2 Reduction of Benzil (ochem 2)

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Lone Star College System, Woodlands *

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3330

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Chemistry

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Feb 20, 2024

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Name: Alisha Khan PS ID: 2139203 TA: Cassidy M DiTirro; CHEM 2125 10580 Date: February 7, 2023 Part Vll. Calculations TLC: Solvent front = 5.4 cm (+/-)-benzoin Sample meso-hydrobenzoin R f = 3.9cm/ 5.4cm = 0.722 R f = 2.1cm/ 5.4cm = 0.389 R f = 2.1cm/ 5.4cm = 0.389 Preweigh filter paper = 0.165 g Filter paper with product = 1.036 g Actual mass of product = 1.036 g - 0.165 g = 0.871 g 4. Calculate the following quantities using your experimental data: (a) Limiting reagent: Mass of benzil = 1.138g MW of benzil = 210.23 g/mol Amount of benzil in moles = (1.138 g benzil)*(1 mol benzil/ 210.23 g benzil) = 0.005413 mol Mass of NaBH 4 = 0.203 g MW of NaBH 4 = 37.83 g/mol Amount of NaBH 4 in moles = (0.203 g NaBH 4 )*(1 mol NaBH 4 / 37.83 g NaBH 4 ) = 0.005366 mol , the limiting reagent. (b) The theoretical amount of product that could be produced: Moles of product = 0.005366 mol MW of product = 214.27 g/mol Mass of product = (0.005366 mol meso-hydrobenzoin)*(214.27 g/1 mol meso- hydrobenzoin) = 1.150 g (c) The percent yield: % yield = (0.871 g/1.150 g) * 100% = 75.74% Part Vlll. Results and Discussion After calculating the percent yield of my product, the reduction of benzil had a percent yield of 75.74%. This percent yield is greater than 50%, but less than 100%, which supports the idea that some of my product was lost. Loss of my product could have happened when adding the sodium borohydride portions into the flask, filtering the reaction mixture and when transferring my product onto a watch glass. The melting point of my unknown product was 136.2-136.5 . According to the lab manual, this range is between the melting point of (+/-)- ℃ benzoin and meso-hydrobenzoin. Therefore, TLC was performed to be able to distinguish which compound is my unknown. After calculating the R f values, my sample had the same retention

factor as meso-hydrobenzoin which means my product could be meso-hydrobenzoin. In order to conclude that my unknown product is meso-hydrobenzoin, IR spectroscopy was performed. According to the IR spectrum of my product, the carbonyl stretch at 1700 cm -1 is shown, indicating a subset of benzil, however, the -OH peak is shown at 3300 cm -1 , indicating meso- hydrobenzoin. Meso-hydrobenzoin is the major product formed because it is the most stable and probable compound based on the IR spectrum and the RF values confirmation. After sodium borohydride reduces one carbonyl group, the molecule arranges itself to the most stable conformation to allow the reduction of the second carbonyl. Part lX. Conclusion In conclusion, in this experiment, I was able to visualize the reduction of benzil with sodium borohydride. By measuring the melting point, performing TLC and using IR spectroscopy, I was able to conclude that my unknown product was meso-hydrobenzoin.The importance of this experiment was to understand how sodium borohydride is able to reduce only aldehydes and ketones. As sodium borohydride being one of the important reduction reagents introduced last semester, this experiment was helpful to visualize what we learned. For future modifications, replicating the experiment at least two more times would increase the accuracy of the results. Maybe trying a different reduction reagent with benzil to see how the selectivity of these reagents differs. But overall, I enjoyed doing this experiment and being able to identify my unknown product. Part X. Post-Laboratory Questions 1. Calculate the theoretical mass of NaBH 4 needed to reduce 1.138g of benzil to (+/-)- benzoin. (1.138 g benzil)*(1 mol benzil/210.23 g benzil)*(1 mol NaBH 4 / 1 mol benzil)*(37.83 g NaBH 4 / 1 mol NaBH 4 ) = 0.2048 g NaBH 4 2. Calculate the theoretical mass of benzil that can be reduced to hydrobenzoin using 0.203g of NaBH 4 . (0.203 g NaBH 4 ) *(1 mol NaBH 4 / 37.83 g NaBH 4 )*(1 mol benzil/ 1 mol NaBH 4 )*(210.23 g benzil/ 1 mol benzil) = 1.128 g benzil 3. Using your results, explain how you can use each of the following techniques to identify your product: (a) Melting point: The melting point of my product was 136.2-136.5 , which falls ℃ in the range for (+/-)-benzoin and meso-hydrobenzoin. (b) TLC: my sample had the same R f value to meso-hydrobenzoin compared to (+/-)- benzoin and this can be used to identify the unknown product as meso- hydrobenzoin. (c) IR spectroscopy: my product does show the carbonyl stretch at 1700 cm -1 , indicating benzil, and OH peak at 3300, which indicates meso-hydrobenzoin, but meso-hydrobenzoin is later confirmed after assessing the TLC and MP. 5. Compare the IR spectrum of benzil with that of your product. Considering carbonyl, OH, aromatic CH, and aliphatic CH absorption bands, describe the similarities and differences you find.

In the IR spectrum of benzil, there is the carbonyl stretch at 1700 cm -1 and more stretches at higher intensity up to 1700 cm -1 . The IR of my product shows a carbonyl stretch at 1700 cm -1 , but it does show an OH peak at 3300 cm -1 . Both IR spectroscopies show the aromatic C-H stretch around 3300 cm -1 . IR spectrum:

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