Multistep Synthesis of Benzilic Acid

Multi-Step Synthesis of Benzilic Acid from Benzoin Abstract: The main purpose of this experiment was to convert a secondary alcohol to a ketone, utilizing a mild and selective oxidizing agent. In addition, this converted alpha diketone was then subjected to rearrangement to a carboxylate salt, then acidification, to produce an alpha-hydroxyacid. In this experiment, benzoin was used and converted into benzil, which was then used to synthesize benzillic acid.

The yields were not ideal: . 081g of benzil- a 27. 5% yield; . 038g of benzilic acid- a 34. 7% yield. The matching melting points and IR readings, however, confirmed a high degree of purity for each compound: 95. 6°C for benzil, and 104. 5°C for benzilic acid, meager . 632% and . 333% discrepancy from literature values, respectively. Finally, the IR absorption frequencies on the attached graphs illustrate the successful removal of the alcohol in the oxidation step and successive return of the alcohol/creation of carboxylic acid, following rearrangement. Multistep syntheses are essential to producing complex molecules.

This experiment illustrated the importance of verifying intermediary products are pure, by utilizing different techniques such as IR and melting point. Below is a diagram of the overall reaction. First Reaction: Second Reaction: Introduction: The multistep reaction from Benzoin to Benzillic acid involves multiple organic chemistry concepts, such as oxidation and rearrangement. The first part of the experiment involves the oxidation of benzoin to benzil, utilizing a mild oxidating agent. The process of oxidation is used in all organic chemistry labs and is essential to a wide variety of synthesis reactions.

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In addition, oxidation reactions are essential in the the biochemistry of most living organisms. This experiment also breaks down oxidizing agents into selective and non-selective agents. For this experiment’s purposes, nitric acid was used, as it is selective towards secondary alcohols, oxidizing them to ketones. The second part of this experiment involves the rearrangement of benzil to benzillic acid or, more generally, the reaction of an alpha-diketone to an alpha-hydroxyacid. This reaction was first conducted by Justus von Liebig in 1838 (1).

The basics of this reaction involve the formation of a carboxylate salt from an alpha-diketone; acid is then added to produce an aromatic alpha-hydroxyacid. The reaction conducted in this experiment is an essential base step in the synthesis of pharmaceuticals and certain hallucinogenic drugs. The synthesis of Benzil from Benzoin is shown below: The above reaction shows the condensed oxidation of benzoin to benzil. The following diagram shows the condensed reaction of benzil to benzillic acid. Rearrangement occurs to form a salt, then the salt is acidified to form benzillic acid.

Once the products of each step were obtained- benzil and benzillic acid- their melting points and IR readings were obtained. These two measurements were used to prove that the correct product was formed without any impurities. One of the major risks in this experiment is loss of product through multiple filtrations. To minimize this problem, the filtration steps should be carefully and slowly executed. This ensures that the least amount of reactants are lost. In addition, recrystallization can occur too quickly if a hot solution is directly placed in an ice bath, allowing impurities to be trapped within the precipitate’s crystal lattice.

To avoid this, the solutions should be given ample time to cool to room temperature before adding the solutions to an ice bath. These precautions were taken to try to reach our goal of a high percent yield of product with little to no impurities. There were no new techniques used in this experiment, however there were old techniques used to provide information/obtain a product. The first technique was crystallization which was useful to obtain a solid product that can then be purified. Another old technique utilized was filtration, via hirsch funnel and vacuum. This technique was used to obtain a purified product, removing impurities.

Once the final solid product was obtained in both reactions the melting point procedure was used to determine the level of remaining impurity of the final product, comparing the experimental and expected values found in literature. In conjunction with the melting point procedure, infrared spectroscopy was used to reveal the different functional groups of the products. In other words, the IR machine indicates whether our final product matches up with the desired one, matching carbonyl and alcohol absorption peaks (or the lack thereof) to their theoretical presence (either benzil or benzillic acid).

Procedure: 1. 5ml of nitric acid was added to . 30g of benzoin in a conical vial with a stir vane. The mixture was then heated in a 70 degree Celsius water bath, while stirred, for one hour. The mixture in the conical vial was then cooled to room temperature and, using a pipette, the solution was transferred to a beaker containing 4ml of ice water. The mixture slowly crystallized in the beaker and the crystallized product was filtered on a hirsch funnel with vacuum. 5 ml of cold water was used to wash the product and then the product was allowed to dry.

The solid product was then collected from the funnel and added to a hot 95% ethanol solution in an Erlenmeyer flask and completely dissolved. Once the solid was dissolved completely, the solution was allowed to cool to room temperature. Once yellow crystals formed, the solution was placed into an ice bath. The product was then collected and filtered again on a hirsch funnel with vacuum. The product was then washed with ice cold 95% ethanol. Once completely dried, the crystals were weighed and the final mass, percent yield, melting point, and IR reading of the crystallized product measured/calculated. 100g of benzil and . 30ml of ethanol were mixed in a conical vial.

The solution in the conical vial was then heated to about 100 degrees celsius until the benzil dissolved. Then . 25ml of potassium hydroxide was added dropwise to the vial. The mixture was then heated to 110 degrees for 15 minutes, then allowed to slowly cool to room temperature. The product was transferred with a pipette to a 10ml beaker and cooled in an ice bath for 15 minutes. 1ml portions of ethanol were added once crystallized and filtered. The product was then transferred to a 10ml beaker with 70°C water, allowing the product to dissolve. 0. ml of HCL acid was added dropwise and the mixture was allowed to cool and then transferred to an ice bath. The crystals were collected on a hirsch funnel and washed with 4ml of cold ice water.

The crystals were dried and collected for final mass, percent yield, melting point, and IR reading. Results and Discussion: Table 1: Mass and Molar Quantity of Starting and Synthesized Materials; Percentage Yield and Both Experimental and Theoretical Melting Points of Products and Percent Discrepancy| Compound| Mass (g)| Quantity (mol x10-4)| % Yield| M. P. EXP (°C)| M. P. THEO (°C)| % D| Benzoin| 0. 298| 14| -| -| -| -| Benzil Yield| 0. 081| 3. 5| 27. 50%| 95. 6| 95. 0| 0. 632%| Benzil Start| 0. 1001| 4. 76| -| -| -|  | Benzilic Acid Yield| 0. 038| 1. 67| 34. 97%| 149. 5| 150| 0. 333%| The initial mass of benzoin and its yield of benzil in the first part of the experiment, as well as the starting mass of benzil and its yield of benzilic acid in the second part.

Note that the yield from part 1 was not the same amount used at the start of part 2. Also shown: the converted molar quantity of each mass and corresponding percent yield for the two synthesized compound, as well as their experimental and theoretical melting points and percent discrepancy between these values. As noted, the “Start” weight of Benzil- in row 3 of Table 1- differs from the “Yield” weight- in row 2. The yield, itself, was not used in the second part of this experiment: the synthesis of benzilic acid from benzil. Furthermore, an error occurred in the second portion of the experiment and very little acid product was salvaged; thus, the yield shown for benzilic acid is data that has been shared from another synthesis (this product yield was from _____ and his lab partner). Additionally, this alternative product was the one used in determining a melting point. Graph 1 illustrates the successful oxidation of benzil, as the alcohol has been eliminated. As expected, it also retains a strong peak at ~1657cm-1, indicating the carbonyl groups present in the diketone, although this is a slightly lower absorption than expected. Graph 2 also presents a successful synthesis, as a relatively strong and somewhat broad peak appears around 3390cm-1, suggesting the reemergence of an alcohol and potential presence of the carboxylic acid; that the peak at 1715cm-1 remains strong, confirms his.

SAMPLE CALCULATIONS

Calculation of Benzil Percent Yield: Moles BenzilMoles Benzoin=. 081g? 210. 23g/mol. 298g? 212. 24g/mol=3. 85? 10-4mol14. 0? 10-4mol=0. 275? 100%=27. 5% Calculation of Benzilic Acid Percent Yield: Moles Benzilic AcidMoles Benzil=. 0380? 228. 25g/mol. 1001? 210. 23g/mol=1. 67? 10-4mol4. 76? 10-4mol=0. 3497? 100%=34. 97% Calculation of Percent Discrepancy in Benzil Melting Point: %D=xTHEO-xEXPxTHEO? 100%= 95. 0? -95. 6? 95. 0? =. 00632x100%=0. 32% Calculation of Percent Discrepancy in Benzilic Acid Melting Point: %D=xTHEO-xEXPxTHEO? 100%= 150. 0? -149. 5? 150. 0? =. 0033x100%=0. 33% Both steps of this experiment’s synthesis are considered successful. Though not in the desired quantities, a product of benzil was obtained from benzoin and that of benzoic acid from benzil. The initial step, synthesis of benzil, resulted in a yield of 27. 5%, despite an encouragingly smooth synthesis. The synthesis may have called for a second, more thorough recrystallization to appeal this yield.

The recrystallization was performed correctly; however, the mixture was likely not allowed to cool at an ideally slow rate. It was removed from the hot plate and, shortly thereafter, transferred to the ice bath- likely, before it had calmed to room temperature. This could have interrupted the ability for the product to crystallize without impurities being trapped within its lattice. During the second portion of this experiment, a known, pure quantity of benzil was used to synthesize a 34. 97% yield of benzilic acid. This small yield is likely also due to factors similar to the aforementioned.

Additionally, the recorded yield was shared from another synthesis; the original synthesis performed yielded too small and impure an amount of product to effectively determine a melting point and I. R. spectra. This failure may certainly be attributed to an error during the recrystallization, prior to the initial filtration. Proper care was taken to allow the solution to cool very slowly during this second recrystallization. Unfortunately, once the solution was transferred to the ice bath, a large chunk of ice crystals somehow fell over the lip of the flask and into the mixture. Being that the intermediate was relatively water

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