BENZIL

In a 12-l. flask, fitted with a mechanical stirrer and mercury seal, a reflux condenser and an inlet tube for the introduction of air, is placed a mixture of 4100 g. (16.4 moles) (Note 1) of crystalline copper sulfate (Note 2), 4000 g. of technical pyridine and 1600 g. of water. This is heated on a steam bath with stirring until the copper sulfate is completely dissolved and then 1696 g. (8 moles) of benzoin (p. 94; unrecrystallized material is satisfactory) is added and heating and stirring continued for two hours. The reaction mixture becomes dark green in color and the melted benzil forms the upper layer. After cooling, the copper sulfate-pyridine solution is decanted and the benzil washed with water and then heated with 3–4 l. of 10 per cent hydrochloric acid. After cooling, the benzil is filtered, washed with water, dried and recrystallized from carbon tetrachloride (2 l. of solvent per kg. of benzil). By concentration of the mother liquors a certain amount of benzil is always obtained. The total yield is 1450 g. (86 per cent of the theoretical amount) of recrystallized material melting at 94–95° (Note 3).

The copper sulfate-pyridine mixture is readily reoxidized by passing a current of air through it for thirty-six hours (Note 4). To this resulting solution is now added 200 g. of pyridine and it is then used for oxidizing another 1696 g. portion of benzoin.

1. In checking this preparation, runs about 25 per cent of the size described were made. The yields were about 3 per cent less than in the larger runs. For the reoxidization of the smaller amount of copper sulfate-pyridine solution, air was passed through for fifteen hours.

2. Copper hydroxide (or carbonate) does not dissolve in pyridine.

3. In comparing the copper sulfate-pyridine method with the nitric acid method (Org. Syn. 1, 25) it should be pointed out that the constants on the samples are as follows:

Method	M. P. Crude	M. P. Recryst.	Fehling's Test on Recryst.


Copper sulfate-pyridine............	94–95°	94–95°	Negative
Nitric acid.......................	88°	93–94°	Positive

In other words, by the nitric acid oxidation it is difficult to obtain a product completely free from benzoin. The yields by the nitric acid method are generally about 95–96 per cent, whereas with the copper sulfate-pyridine method the yield drops to approximately 86 per cent.

The melting temperatures of mixtures of benzil and benzoin show¹ that the maximum possible depression is 10°.

4. Fifteen hours was found not long enough to effect a complete oxidation.

Benzil is readily formed by the oxidation of benzoin with nitric acid,² chlorine,³ iodine,⁴ electrolytically⁵ and catalytically.⁶ The procedure described is based on the observation⁷ that benzoin reduces Fehling's solution in the cold. Pyridine was selected because it prevents precipitation of cuprous oxide, is not so volatile as ammonia, and acts as a partial solvent for benzoin. It has been shown⁸ that copper, pyridine and air oxidize benzoin to benzoic acid. The same oxidation also takes place in the absence of copper. Benzil can also be prepared from desoxybenzoin with selenium dioxide.⁹

References and Notes