C Preparation of Benzilic Acid

Weigh the dry benzil and calculate the percentage yield. Determine the melting point. The melting point of pure benzil is 95°C. Submit the benzil to the instructor unless it is to be used to prepare benzilic acid (Experiment 32C). Obtain the infrared spectrum of benzil using the dry film method. Compare the spectrum to the one shown. Also compare the spectrum with that of benzoin. What differences do you notice  

In this experiment, benzilic acid will be prepared by causing the rearrangement of the a-diketone benzil. Preparation of benzil is described in Experiment 32B. The rearrangement of benzil proceeds in the following way  

The driving force for the reaction is provided by the formation of a stable carboxy-late salt (potassium benzilate). Once this salt is produced, acidification yields benzilic acid. The reaction can generally be used to convert aromatic a-diketones to aromatic a-hydroxyacids. Other compounds, however, also will undergo benzilic acid-type of rearrangement (see questions). 

This experiment works best with pure benzil. The benzil prepared in Experiment 32B is usually of sufficient purity after it has been crystallized. 

Pour all of the aqueous filtrates into the waste bottle designated for aqueous waste. Ethanolic filtrates should be put in the nonhalogenated organic waste bottle. 

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