Essay: Synthesize benzil through oxidation of benzoin by bleach in the presence of Stark’s catalyst

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  • Synthesize benzil through oxidation of benzoin by bleach in the presence of Stark’s catalyst
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Purpose: The purpose of this experiment was to synthesize benzil through the oxidation of benzoin by bleach in the presence of Stark’s catalyst. The addition of Stark’s phase transfer catalyst (Aliquat 336) to the reaction mixture was necessary to promote the occurrence of the biphasic reaction.. Thin-layer chromatography was utilized to monitor the progress of the reaction. Liquid-liquid extraction was used to isolate the crude benzil product from the bilayered reaction mixture. The crude product was subsequently identified using IR and 1H NMR analysis. Co-spotting is a technique often employed by chemists to ensure the reaction has gone to completion. The absence of a spot in the starting materials lane of the final TLC plate ran indicates the reaction went to completion.

Results: The experiment was the oxidation of benzoin by bleach to benzil. Oxidation reactions are notable because of a carbon atom’s ability to change its electron density – and thus oxidation state – when binding with more electronegative atoms. In this reaction the hydroxyl of benzoin was oxidized to H2O, a good leaving group, by the proton of bleach. Bleach was the active oxidant formed by a reaction between Sodium hypochlorite and water. Bleach was an ideal oxidizing agent for this experiment because it’s only slightly reactive in oxidation reaction and safe and inexpensive compared metal and other more caustic catalysts.(1) The resulting, negatively charged O-Cl- ion nucleophilically attacks the dihydrated oxygen atom, allowing H2O by breaking the leaving group bond at the reaction center. The chlorine of the hypochlorite group in the resulting intermediate was an excellent leaving group and allows the hydrogen in the alpha position to the hypochlorite group to be accepted by water thus forming the carbonyl of the benzil product along with hydronium and chloride.

The oxidation reaction requires the presence of Stark’s catalyst. A quaternary ammonium salt, soluble in both the aqueous and organic layers, Stark’s catalyst binds to bleach in the aqueous phase dragging it to the organic layer where it can oxidize benzoin to benzil and return salts and byproducts back to the aqueous phase. TLC co-spots were performed every fifteen minutes to compare the intensities of the benzoin and benzil in the reaction mixture and thus monitor the progression of the reaction to completion. TLC co-spotting was an effective technique for monitoring the reaction progression because the difference in polarity between the benzoin and benzil was significant enough to produce distinct spots on the TLC plate and both molecules are activated by ultraviolet light. Liquid-liquid extraction was subsequently performed on the completed reaction mixture, isolating the crude benzil product in the organic layer. Sodium hydroxide was used in the extraction to quench the reaction mixture by neutralizing the hypochlorous acid and therefore halting the reaction. Sodium hydroxide and 6% sodium hypochlorite have pkas of approximately 15 and 10.13 respectively. This indicates sodium hydroxide sufficiently strong as a base to protonate sodium hypochlorite but not strong enough to protonate benzoin with a pka of 15.4.(2,3,1) HNMR and IR analysis were used to finally confirm the identity of the crude benzil product.

All Perk

In preparation for the experiment, benzoin was determined to be more polar than benzil, mainly because it contains a hydroxyl group, capable of forming hydrogen bonds with silica in the stationary phase of TLC. The two compounds are otherwise very similar; benzoin and benzil are roughly the same size and weight each molecule contains electronegative oxygen atoms – in the form of carbonyls – which produce temporary dipole moments when polarized. Due to its increased polarity benzoin yields a lower Rf value than benzil. TLC is meant to indicate when a reaction goes to completion, this reaction went to completion as can be seen by no spot in the reaction lane of the TLC plate at t=60 minutes. The Rf values were calculated as; 0.26 for benzoin and 0.42 for benzil. Polarity differences between the benzoin starting materials and the benzil product make TLC an effective technique to monitor this reaction.

When the allotted time to run TLC plates was up, the nearly complete reaction mixture was used up. Sodium hydroxide was added to quench the reaction by neutralizing sodium hypochlorite, halting the reaction with benzoin. In the work up an additional sodium hydroxide wash was performed on the mixture to react with the remaining oxidizing agent. The sodium hydroxide from the wash shifted the reaction between sodium hypochlorite and water to prefer its reactants, producing the NaOCl salt and water present in the aqueous layer. The final TLC plate indicated no remaining benzoin, however if it did, the additional sodium hydroxide was not intended to extract any remaining starting materials.

Extraction of the aqueous layer by ethyl acetate was only performed once, resulting in a poor extraction of any potential benzil left behind in the aqueous layer. There was no way to remove starks catalyst from the reaction mixture because its soluble in both the aqueous and organic layers thus explaining its presence in the IR and NMR analysis.

Once dried, the crude product obtained from this experiment was a smooth yellow solid. The experiment resulted in a 59.7% yield. The reactant benzoin was the limiting reagent in the reaction, for which theoretical yield of benzil was calculated to be 247mg. In this experiment, 148mg of benzil product was the actual yield. The recovery yield, although sufficient, is significantly lower than the theoretical yield. Multiple factors could have affected the yield including; insufficient mixing of the reaction mixture, thus not enabling enough surface exposure between the two layers for the reactants products to be transported their correct phase, between the layers, by the Stark’s catalyst (1). Due to low distribution coefficients for benzil and benzoin, multiple extractions are necessary to extract as much benzil from the aqueous layer, into the organic layer, as possible. Considering ethyl acetate was only used once to extract the aqueous layer, insufficient washing of the aqueous layer with ethyl acetate may well have played a role in the low yield too, allowing some of the benzil product to remain in the aqueous layer. No noticeable emulsions were present in my reaction mixture at any stage of the work up, which would reduce actual yield by not obtaining all of the benzil containing organic layer, however this would have played a negligible role in the low yield for this reaction.

The crude product was identified as benzil via 1H NMR and IR spectra analysis and confirmed by the lack of a benzoin spot in the reaction lane of the final TLC plate. Benzil is a symmetrical compound and has three distinct hydrogen groups as a result. The first peak revealed by 1H NMR analysis, was a split peak from 8.039 ppm to 7.908 ppm. It showed a doublet splitting pattern with an integration value of 4. Given its proximity to the electron withdrawing carbonyl group – which actively participated in resonance with the aromatic structure – the Ha hydrogen was the most deshielded and therefore the most downfield and was thus responsible for this peak. The second peak revealed by 1H NMR analysis split from 7.629 to 7.262. It had an ambiguous splitting pattern that was labeled as a multiplet and an integration value of 5.93 which was rounded up to 6 for simplicity. Compared to a standard 1H NMR of benzil, the obtained multiplet peak should have more clearly resembled a doublet of doublets for Hb and a doublet for Hc. (1)The probable explanation for this is that the machines used in this experiment performed the1H NMR at too low a frequency. Peaks for Stark’s catalyst (Aliquat 336) appear at 3.308ppm, 1.253ppm and 0.881ppm. Additionally there is a tiny peak at 2.165 that may have been ethyl acetate or potentially some unknown impurity. Finally there was no evidence of any benzoin present in the 1H NMR analysis.

The data collected from the IR spectroscopy also clearly identifies benzil as the product of this experimental reaction. The three signals from 3062.91cm-1 to 2854.94cm-1 indicate the aromatic carbon-hydrogen bonds found on the two aromatic rings of benzil. A deep signal at 1656.32cm-1 was associated with the carbon-oxygen carbonyl bonds of the neighboring conjugated ketone functional groups. The two peaks from 1583.37cm-1 to 1446.73cm-1 represented the aromatic carbon-carbon double bonds in the two rings of the benzil molecule. Additionally there is a small peak at 3315.81cm-1 which is likely from trace amounts of Stark’s catalyst remaining in the crude product. All impurity peaks found in the IR spectra correlate with impurities found in 1H NMR analysis. No signals from IR spectra suggested the presence of benzoin which matches up with the data from the TLC plates and the 1H NMR analysis.

Conclusion

Benzoin was successfully, completely oxidized by bleach with the facilitation of Stark’s catalyst to the final product benzil. This oxidation reaction was monitored using TLC every 15 minutes over a 60 minute period until no benzoin starting materials appeared in the reaction lane. The mass of the crude benzil product obtained from this experiment was 148mg for a 59.7% yield. The obdurate reactions slightly low recovery yield was probably a result of insufficient mixing of the reaction mixture, thus not allowing all the products and reactants to be extracted to their respective layers. Additionaly too few extraction cycles were run in the work up, given the low distribution coefficient of benzoin and benzil. The crude product was identified as benzil using 1H NMR analysis, IR spectroscopy and finally confirmed by TLC.

Experimental

Benzil. Starting material, benzoin (250 mg, 1.2 mmol) was mixed with ethyl acetate (10 mL) and stirred until completely dissolved. Stark’s catalyst (2 drops) and 6% sodium hypochlorite (10 mL) were added and mixed in to the reaction mixture. The reaction progress was monitored via Thin-layer chromatography (75% hexanes, 25% ethyl acetate) every 15 minutes over a 60 minute period. The bottom, aqueous layer was removed from the reaction mixture. Ethyl acetate (10 mL) was added to the aqueous layer and mixed. The new aqueous layer was removed from this mixture and the new organic layer was added to the original organic layer. The combined organic layer was washed twice with sodium hydroxide (1M, 10 mL). The organic layer was washed twice with distilled water (10 mL). The organic layer was washed once with saturated sodium chloride (10 mL). The worked-up organic layer was dried over anhydrous sodium sulfate for 10 minutes and decanted. Evaporation led to a final solid, smooth and yellow product (148 mg, 59.7%). 1H NMR (40 MHz, CDCl3):(ppm) 7.908 (d,4H), 7.629 (dd, 4H), 7.494 (dd, 2H); IR (ATR) max (cm-1) 3315.81, 3062.91, 2924.48, 2854.94, 1656.32, 1583.37, 1446.73.

References

19.02.2019

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