Synthesis of Benzilic Acid

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Synthesis of benzilic acid from benzoin utilizing a multi-step reaction.

Names: Arian Karim
TA Name: Sayantan Das
Lab Day & Time: Thursday 7:30-11:40
Lab Section #:


The main purpose of this experiment was to synthesize benzilic acid from benzoin. This requires a multistep synthesis with benzyl as an intermediate product. The first step required HNO3 as an oxidizing agent and the second step required KOH and HCl. The percentage yields of benzil and benzilic acid were 59.5% and 21.9% respectively. The calculated melting point of benzil was 90.1-91.6 which suggested some impurities, but the MP of benzilic acid was 149.3-151.5 which suggested a pure yield. The IR further confirmed the identity of benzylic acid with two OH peaks and the carbonyl peak.


The purpose of this experiment was to synthesize benzilic acid from benzoin. Part A was omitted from the experiment and benzoin became the starting reagent. For this experiment a multistep synthesis was required. In the first part (Part B) benzoin was oxidized using nitric acid to make benzyl as a product. This reaction is characterized by the orange gas of nitrogen oxide that is liberated as a byproduct of the reaction. In Part C the benzyl was combined first with potassium hydroxide to make the carboxylate salt intermediate and second with hydrochloric acid to from the benzylic acid. For both reactions the organic compound was the limiting reagent, so one mole of product is expected for each mole of organic reagent. Crystallization was performed in both parts in order to eliminate any impurities in the product. The identities of the products were tested using IR and melting points methods. At the end a percentage yield calculation of benzilic acid was done.

Figure 1. Benzylic acid synthesis reaction

The mechanism for benzylic acid from benzyl is showed in Figure 2. First the negatively charged hydroxide group from the potassium hydroxide attacks one of the partially positive carbonyl carbons while an electron pair from the carbon-oxygen double bond goes to the oxygen leaving it negatively charged. Then the pair of electrons from the negatively charged oxygen move to create again a double bond with carbon while kicking out the phenyl group. At the same time the phenyl attacks the other partially positive carbonyl carbon making an electron pair from the carbon-oxygen double bond migrate to the oxygen. The acidic hydroxide group is deprotonated and the other less acidic negatively charged oxygen picks up the hydrogen. The negatively charged carboxylic group pair up with the potassium ion to make the carboxylate salt potassium benzilate. Finally hydrochloric acid is added to protonate the carboxylic group of the potassium benzilate to make the benzylic acid.

Figure 2. Benzylic acid synthesis mechanism


A small amount of benzoin (0.3 g) was obtained and weighed before being added to 1.5 mL concentrated nitric acid, producing a yellow solution. The solution was then heated at approximately 70C for an hour; during this heating process orange gases evolved. Once the heating period was complete, the solution was cooled until crystals formed that were dried in a Hirsch funnel with a cold water rinse. The mass of this crude solid was obtained then the solid was re-crystallized using 3.18 mL 95% ethanol. The product of the re-crystallization, which was paler yellow in appearance than the crude product, was dried, weighed, and used to obtain melting point. After the benzil product was thoroughly dried, an IR spectra was obtained then the remaining 0.100 g of the solid was combined with 0.3 mL 95% ETOH. This mixture was heated until fully dissolved, then 0.25 mL of aqueous potassium hydroxide solution was added and the mixture was observed to immediately turned dark brown. This solution was heated for 15 minutes at 110C then cooled slowly. The crystals that formed were dried with a cold ETOH rinse, which removed some of...
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