A Case of the Benz. Part III

The second step of the benzaldehyde to benzilic acid synthesis is the oxidation of a benzylic alcohol to a ketone. This oxidation could likely be effected by dozens of different reagents. Some of the early experiments do this step with nitric acid. This seems like overkill. An alcohol in that position should be easily oxidized. Most recent experiments do this step with copper(II) acetate, ammonium nitrate, and acetic acid. The origins of this reaction can be traced back to a 1948 J. Am. Chem. Soc. article by Marvin Weiss and Mildred Appel.  This reaction shows up in Journal of Chemical Education in 1988. I found one source that uses vanadium(V)-oxychloride as an oxidizing agent.

The reaction is heated under reflux and once it is cooled the crystals of benzil crash out of solution. The light yellow benzil can be recrystallized with 95% aqueous ethanol. The disappearance of the alcohol can be verified by IR and/or NMR. UV-vis spectroscopy is also useful with these reagents. Benzoin has a lambda max of 247 nm and benzil has a lambda max of 259 nm in ethanol. Of course, TLC is helpful in monitoring the reaction. benzoin2benzil


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