Benzoin oxidation/rearrangement

Oxidation of benzoin, PhCH(OH)COPh (above) yields benzil, PhCOCOPh (133), and this, in common with non-enolisable 1,2-diketones in general, undergoes base-catalysed rearrangement to yield the anion of an a-hydroxy acid, benzilate anion, Ph2C(0H)C02e (134). This is, almost certainly, the first molecular rearrangement to be recognised as such. The rate equation is found to be,... 

A similar rearrangement occurs when a-epoxyketones are refluxed with 30% aqueous sodium hydroxide. Best directions ate those for benzilic acid (90%) from benzoin, sodium bromate, and sodium hydroxide. Oxidation of the benzoin to benzil and rearrangement of benzil to benzilic acid are accomplished in one step. a-Ketoaldehydes and potential a-ketoalde-hydes undergo a similar internal oxidation-reduction reaction in excellent yields, viz.,... 

A detailed Organic Synthesis procedure is available for the conversion of benzil into benzilic acid. It is advantageous to isolate the acid as its potassium salt, for this enables the removal of the more soluble potassium benzoate, which results from a competitive cleavage reaction. Benzilic acid is then obtained in 77-79% yield by acidifying an aqueous solution of potassium benzilate. Since benzil is usually obtained by the oxidation of benzoin, both this conversion and rearrangement can be performed in tandem by using alkaline sodium bromate. Thus, benzilic acid is obtained in 84-90% yield from benzaldehyde via benzoin. 

Oxidation and the benzilic acid rearrangement occur during the reaction leading from benzoin to 5,5-diphenyldithiohydantoin (88) (Scheme 9). The... 

The experiment demonstrates multistep synthesis of benzilic acid starting from benzaldehyde. In Experiment 32A, benzaldehyde is converted to benzoin using a thiamine-catalyzed reaction. This part of the experiment demonstrates how a "green" reagent can be utilized in organic chemistry. In Experiment 32B, nitric acid oxidizes benzoin to benzil. Finally, in Experiment 32C, benzil is rearranged to benzilic acid. The scheme on the next page shows the reactions. 

In the above example, benzoin is oxidized directly to benzilic acid via the benzil interniediate. A similar rearrangement occurs when a-epoxy ketones are refluxed with 30% aqueous sodium hydroxide. 

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