Multistep Synthesis of Benzilic Acid

Topics: Alcohol, Carboxylic acid, Redox Pages: 5 (2119 words) Published: November 19, 2012
Multi-Step Synthesis of Benzilic Acid from Benzoin

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 34.97% 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:

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. 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...
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