The synthesis of several complex organic compounds follows a multistep synthesis. “Multistep synthesis” refers to the procedure in which the product of one reaction serves as the starting material in the subsequent reaction. The multistep synthesis of benzilic acid begins with a conversion benzaldehyde to benzoin through a condensation reaction. The benzoin then oxidizes into benzil, which undergoes rearrangement to give benzilic acid.
When two benzaldehyde molecules condense in the presence of thiamine, it leads to the formation of a molecule of benzoin.
The thiamine behaves as a coenzyme catalyst. This step of the reaction involves the addition of ethanol and sodium hydroxide into an aqueous solution of thiamine hydrochloride and creating a reaction with pure benzaldehyde. When you heat this mixture to a temperature of 60 degrees Celsius for about 90 minutes and then cool it in an ice bath, the benzoin crystallizes out. Recrystallization of these crystals from hot ethanol yields pure benzoin as a colorless powder.
Benzoin undergoes oxidation in the presence of a mild oxidizing agent such as nitric acid to produce the alpha diketone known as benzil.
When you heat benzoin with concentrated nitric acid using a reflux condenser, evolution of reddish brown nitrogen dioxide occurs and then stops. When you add cold water to the cooled reaction mixture, benzil precipitates out as a yellow solid. You can then recrystallize this substance from hot ethanol.
When you reflux a solution of benzil in ethyl alcohol with potassium hydroxide for 15 minutes and then cool it, it forms the carboxylate salt potassium benzilate.
When you dissolve this salt in hot water in an Erlenmeyer flask and add hydrochloric acid to bring the pH down to 2, the salt becomes acidified to yield benzilic acid.
During the conversion of benzaldehyde to benzoin, you must maintain temperatures below 65 degrees Celsius to obtain benzoin. Take care when refluxing benzoin with nitric acid; the nitrogen dioxide fumes are extremely toxic and can cause lung damage. During the conversion of benzoin to benzil, some benzoin may remain unoxidized. Prevent this scenario by creating a reaction of an ethanolic solution of the benzil with 10 percent sodium hydroxide solution; if benzoin is present, a purple color develops.
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