Multistep Lab

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  • Topic: Potassium hydroxide, Base, Nitric acid
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  • Published : April 10, 2012
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Title: Oxidation of Benzoin and Rearrangement of Benzil to Produce Benzilic Acid

The objective of the lab was to produce benzilic acid from benzoin. Benzoin was successfully oxidized with nitric acid to form benzil. The percent yield of benzil was 59.26% and the melting point range was 94.1-95.5°C. The literature melting point of pure benzil is 95°C, which indicates the correct product was obtained. Benzil was then rearranged using potassium hydroxide to form benzilic acid. The percent yield of benzilic acid was 57.94% and the melting point range was 147.9-149.8°C. The literature melting point of pure benzilic acid is 150°C; this also indicates that the correct product was isolated. Crude products were formed through crystallization and then recrystallized to obtain pure products. Melting points were taken of each product to prove its identity.

The purpose of the experiment was to create benzilic acid from benzoin. This reaction would occur in a two-step process of oxidation of a α-hydroxyketone to form the α-diketone, benzil. Then, a rearrangement would occur to form the α-hydroxyacid, benzilic acid. Benzoin is an organic compound consisting of an ethylene bridge bound to phenyl groups and with hydroxyl and ketone functional groups. The nitric acid would readily oxidize benzoin to benzil, a diketone, itself would reduced to nitrous acid, which would decompose to oxides of nitrogen and water. Additional water was added after the initial reaction to act as a base and facilitate removal of the hydrogen on the carbon that initially held the hydroxyl functional group. This indicated the removal of NO2, and generated the carbonyl to complete the oxidation.

The conversion of benzil, an α-diketone, into the salt of α-hydroxyacid by means of base treatment is generally referred to as the benzilic acid rearrangement. As potassium hydroxide was added to benzil, the stable salt, potassium benzilate, was formed. The salt was acidified by the addition of hydrochloric acid until the pH of the solution became acidic and solid benzilic acid precipitated.

The ability to be able to produce benzilic acid is vital. Benzilic acid is used in organic synthesis as a base point for preparation of glycolate pharmaceuticals.

After the oxidation of benzoin, the melting point characteristic of the product would prove that the benzil was produced. If the melting point range were narrow, it would indicate that few impurities existed in the benzil product. Again, after the rearrangement of benzil, a melting point would be taken to prove that benzilic acid was produced and how pure the final product was. Crystallization through ice bath is important in this process, so that the solid crystal product could be obtained as a precipitate from the reaction solution. After the crystals are obtained, it is important to recrystallize to remove impurities. This allows the most accurate and precise melting point to be taken of the product and also gives the most accurate percent yield of the product, without impurity being weighed into the final product. To avoid errors, it would be best to make sure the benzil produced in the first step had a narrow melting point range. If the benzil contained many impurities, it would most likely produce inaccuracies in the next step of the process when benzil was rearranged to benzilic acid. Procedure

0.30 grams benzoin, 1.5 mL of concentrated nitric acid, and a spin vane were placed into a 5-mL conical vial with an attached air condenser. The mixture was heated in a 70°C water bath for approximately 90 minutes, while being stirred, until the red nitrogen oxide gases were no longer present in the air condenser. Afterwards, the mixture was removed from heat and transferred into a beaker that contained 4 mL of ice-cold water. The mixture was cooled in an ice bath and then crystals were collected by vacuum filtration using a Hirsch funnel...
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