Multistep Synthesis of Benzilic Acid

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The objective of the experiment was to prepare Benzilic acid by multistep synthesis starting with benzaldehyde. In this setup however, product of the first step, Benzoin, is provided thereby omitting the first step involving the conversion of benzaldehyde. For this experiment, the microscale techniques of reflux, crystallization, and melting-point determination were used. Utilizing these techniques a product yield of 93% for benzil and 57% for Benzilic acid was obtained, as well as a melting point range of 94.8 – 95.4oC for benzil and 152.0 – 153.9oC for Benzilic acid. With the literature melting-point value of benzil being 95oC ( Pavia, 2012), the product was deemed of pure enough quality to be used in the preparation of Benzilic acid. The melting point of Benzilic acid was taken twice, the first being155.4 – 156.5oC, and the second being 152 – 153.9oC. Another melting point was taken due to suspicion of a wet product as suggested when compared to the literature value at 150oC (Pavia,2012). The second melting point having decreased from the first indicated that the product was indeed wet however of pure quality nonetheless, concluding the reaction was overall a success.

Overall Reaction:


The oxidation of Benzoin to Benzil and conversion of Benzil to Benzilic acid are two of the main steps that make up this multistep reaction. This being a multistep reaction meant that the success of synthesizing the end product Benzilic acid would be completely dependent on the success of every step that came before it. Since Benzil is required to form Benzilic acid, it makes sense then that the first step be the production of Benzil, which was formed from Benzoin. This reaction required the oxidation of the α – hydroxyketone, Benzoin, by Nitric acid. Both reactions in this experiment however required refluxing of the reaction to maintain the amount of solvent and allow the reaction to proceed at a high temperature (Pavias, 2011). For this first step however, it was crucial to assure the continual evolution of the NO gas out of the air condenser which would shift the equilibrium to the side of the products. To ensure the highest yield in product, the reaction was carried out until no more NO gas was produced which indicated the completion of the reaction. The second major step involved the conversion of Benzil into Benzilic acid which began with refluxing of Benzil with potassium hydroxide. The hydroxide ion converted Benzil to the Benzilate by a simple rearrangement mechanism where a phenyl group bonds to the double bonded carbon triggered by the delocalized pairs of electrons on the oxygens, being pushed back and forth. Formation of the salt acts as a driving force for the reaction for it is later combined with cold acid to protonate it to its Benzilic acid form. Cold acid was used as to aid in the precipitation of the crystal product as most solvents lose capacity at lower temperatures ( In order to analyze whether the reactions were successful, a melting point determination was taken following each step as well as infrared spectroscopy to confirm the identity of the end products and gauge its purity. Performing a infrared spectrum on a successful production of Benzil following the first step wouldn’t yield a very interesting IR and would simply include a strong peak around ~1660 indicating strong presence of carbon double bonded oxygens and a small spike around ~ 3000 indicating sp2 carbons. The infrared of a successful production of Benzilic Acid however would yield a more complex IR consisting of a hydroxyl peak ~3400, and broad peak at ~2800, and a strong peak at around ~1700 indicating a strong presence of carbon doubled bonded oxygen’s. Melting point determination of a successful product of Benzil and Benzilic acid would result in a melting – point range that is small and narrow around the literate point of 95oC for Benzil and 150oC for Benzilic acid. A broad...
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