A Case of the Benz Part I

The following posts report on the three step synthesis of benzilic acid from benzaldehyde. The first step is the conversion of benzaldehyde to benzoin using a vitamin, thiamine, as a catalyst. In the second step, the benzoin is oxidized to benzil through the use of an oxidizing agent. The third step is the preparation of benzilic acid from benzil via a base-catalyzed rearrangement reaction. There are at least three alternatives to the third step: 1) the reduction of benzil to hydrobenzoin (1,2-diphenylethane-1,2-diol). 2) the condensation of benzil and 1,3-diphenyl-2-propane to form tetraphenylcyclopentadienone. 3) the condensation of benzil and diamine to form benzil quinoxaline. Performing multistep syntheses in the undergraduate laboratory stresses the importance of obtaining a sufficient yield in one step to use as starting materials in the next step. A multistep synthesis is also economical because starting materials are produced rather than purchased for two of the three steps. For this particular synthesis, the atom economy for each of these three reactions is quite high. In this case, all three products are crystalline solids which can be characterized by melting point and other analytical methods. All three products can be obtained commercially as well. The reactions featured in the first and third reactions are not usually covered in Organic Chemistry curricula. The mechanisms can be easily portrayed by arrow pushing, however.benzaldehyde2benzilic_acid

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