Experiment Report on the Oxidation Reaction of Benzoin to Benzil Through Thin Layer Chromatography

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The purpose of this experiment was to monitor the oxidation reaction of benzoin to benzil through Thin Layer Chromatography (TLC) and extract the product with liquid-liquid extraction. During the oxidation reaction a phase-transfer catalyst called Stark’s catalyst was used and then monitored by TLC until completion. From the oxidation reaction the product was purified through a liquid-liquid extraction. It is important to purify the products from a liquid-liquid extraction to ensure that impurities do not show up in the analysis and cause inaccurate identification or analysis. The benzil product was analyzed using H1 NMR and IR spectroscopy.

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In this experiment, an oxidation reaction was completed through the use of bleach and a phase-transfer catalyst. Hypochlorous acid (HOCl) was formed and Stark’s Catalyst (Aliquat 336) was used as the phase-transfer catalyst to transport HOCl into the organic layer. Within the organic layer, the HOCl protonates the alcohol on the Benzoin and can now leave as an H2O molecule. OCl- acted as a nucleophile to kick off the water leaving group. Finally, the acidic hydrogen was taken up by the water and the chloride ion was kicked off when reforming the carbonyl group leaving the benzil product. During the synthesis of benzil, the reaction was monitored through Thin-Layer Chromatography at the start of the reaction, 15 minutes, 30 minutes, and finally at 45 minutes. At each time, the intensities were noted and used to determine when the reaction reached completion. The spot with a smaller retention factor (Rf) was benzoin due to the ability to hydrogen bond unlike benzil which could not hydrogen bond.

When the reaction started there was about a 10% intensity of the benzil product showing compared to the benzoin spot and at the 15-minute plate the intensities were already at 50%. That indicated that the reaction was half way complete. On the 45-minute plate the Benzoin spot was no longer visible and the Benzil product was at 100% intensity. At this point, the conclusion was made that the reaction had gone to completion and was ready for the work-up. Upon completion of the reaction, liquid-liquid extraction was utilized to complete the work-up. The work-up is utilized to remove any remaining benzoin, Stark’s catalyst, and any hypochlorous acid.

From the TLC plate information there should not have been any benzoin leftover. Sodium hydroxide, a strong base was used to extract the organic layer and remove hypochlorous acid. The strong base was able to deprotonate the acid and move it into to aqueous layer. This extraction was completed twice. The remaining Stark’s catalyst and some acetone was not separated out of this mixture and was identified on the NMR and IR spectrums. After the extraction with sodium hydroxide, two distilled water washes were completed, along with a sodium chloride wash to remove any bleach and Stark’s catalyst left in the organic layer.

The final organic layer was separated and dried over sodium sulfate to remove any remaining water molecules. In the NMR spectrum, there was impurities of Stark’s catalyst and acetone. The extraction was successful at removing the hypochlorous acid, but was not successful at removing all of the Stark’s catalyst. On the H1 NMR spectral data of the benzil product there were peaks for the product and also a few impurities. As stated about the peaks that were found in the range of 0.761 to 1.253 and at 3.310 ppm were the phase-transfer catalyst that was not completely removed during the washes during extraction. The impurity found at 2.276 ppm was determined to be an acetone impurity, which could be avoided by making sure that glassware is dry before using. From the TLC plate data there should not have been any remaining starting material and the signal is weak on the spectrum which could indicate that it was a very small amount that was left over. Finally, downfield from the impurities were the peaks for benzil. From 7.270 to 7.630 ppm a multiplet signal was observed and was determined to be the aromatic hydrogens farthest away from the carbonyl groups. The integration value for this signal was 5.93 and was representing six of the hydrogens. The final peaks were a second multiplet found in the range of 7.883 to 8.068 ppm, with a set integration value of four. It was assumed that the peaks were correlated with the four hydrogens closest to the carbonyl group. The final analytical data that was taken was an IR analysis of the benzil product. In the spectrum a peak at 1659.25 cm-1 that represented the conjugated ketone groups found in the molecule. The peaks at 1449.04 cm-1 and 1586.94 cm-1 were within the range for the carbon double bonds found in the benzene ring. 2923.29 cm-1 and 2854.77 cm-1 were peaks representing the carbon hydrogen bond that is found in all IR spectrums. A carbon hydrogen stretch on the benzene ring was signaled at 3063.32 cm-1 and 3318.06 cm-1. During evaporation of the ethyl acetate, the stream of nitrogen drying the sample was too strong and some of the sample was splashed out. When completely dry, the sample was a milky yellow powder and matched the data found on the SDS sheet stating the color should be yellow crystals.

The percent yield calculated was very low at 22.2%. In human error the number is low from splashing the sample. However, on the scientific side the reaction has a lower percent yield due to the impurities of acetone and the phase-transfer catalyst. In conclusion, a benzil product was obtained through an oxidation reaction of benzoin, the progress of the reaction was monitored through the use of TLC, and finally the product was isolated using liquid-liquid extraction. The benzil product was not completely pure and had trace amounts acetone and the Stark’s catalyst impurities on the NMR and IR spectrums. Improvements could be made during the extraction procedure and have multiple, smaller extractions so that to remove more of the impurities. Accordingly, making sure that the sample is not splashed and split will ensure a higher percent yield.

In the experiment, benzil was successfully synthesized and extracted with a few impurities. Experimental Benzil. Benzoin (250 mg, 1.18 mmol) was dissolved in ethyl acetate (10 mL). Stark’s Catalyst (2 drops) and Bleach (6% NaOCl solution) were added to the reaction mixture. A TLC mobile phase (25% ethyl acetate/75% hexane) was used to develop plates that were spotted at set times throughout the reaction. Upon completion the reaction, the layers were separated and the organic layer extracted with ethyl acetate (10 mL). The organic layer was then washed twice with sodium hydroxide (10 mL, 1M). A distilled water (10 mL) wash and saturated sodium chloride (10 mL) wash were completed. It was dried over sodium sulfate and ethyl acetate was evaporated to yield a milky, yellow solid.

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