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.