A Case of the Benz. Part IV

Benzil to Benzilic acid

The third step of the benzaldehyde to benzilic acid synthesis is the base catalyzed rearrangement of benzil to benzilic acid. An early study of this reaction was reported in a 1921 J. Am. Chem. Soc. article by Arthur Lachman. According to the procedure we followed, the reaction only required 15 minutes of reflux. I think the reaction yield is actually quite high but there is the potential of losing most of the product mass during the crystallization and recrystallization steps. The recrystallization is done with hot water. For some reason when the benzilic acid in this condition is heated in boiling water it prefers to clump together in an insoluble mass rather than dissolve in the water. At first I thought that this was due to unreacted benzil. However, I tested the waste solids that were generated in the procedure and they all tested as benzilic acid. IR and NMR can easily distinguish between the two reagents. UV-vis and TLC are also helpful to characterize the products. There is an interesting presumptive test for benzilic acid: Concentrated sulfuric acid added to dry or moist benzilic acid turns the resulting solution purple. A side note – for some reason when I type “benzil” my word processor wants to turn it into “benzyl.”

benzil2benzilicacid

 

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  1. […] well under the same conditions as the benzil to benzilic acid rearrangement as outlined in a previous post. The products were characterized by melting point, UV absorption, IR, 1H and 13C NMR. The […]

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