Org. Synth. 1926, 6, 6
DOI: 10.15227/orgsyn.006.0006
BENZIL
Submitted by H. T. Clarke and E. E. Dreger.
Checked by Roger Adams and Gerald H. Coleman.
1. Procedure
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.
2. Notes
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 show
1 that the maximum possible depression is 10°.
4.
Fifteen hours was found not long enough to effect a complete oxidation.
3. Discussion
Benzil is readily formed by the oxidation of
benzoin with
nitric acid,
2 chlorine,
3 iodine,
4 electrolytically
5 and catalytically.
6 The procedure described is based on the observation
7 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 shown
8 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
This preparation is referenced from:
Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
copper sulfate-pyridine
Fehling's solution
Desoxybenzoin
hydrochloric acid (7647-01-0)
ammonia (7664-41-7)
nitric acid (7697-37-2)
carbon tetrachloride (56-23-5)
copper sulfate (7758-98-7)
Benzoic acid (65-85-0)
copper (7440-50-8)
iodine (7553-56-2)
Benzil (134-81-6)
pyridine (110-86-1)
Benzoin (119-53-9)
Copper hydroxide (20427-59-2)
chlorine (7782-50-5)
cuprous oxide
selenium dioxide (7446-08-4)
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