In a
500-ml. three-necked round-bottomed flask fitted with a
liquid-sealed stirrer and
two reflux condensers (Note
1) is placed
40 g. (0.27 mole) of anhydrous, powdered d-tartaric acid (Note
2). A solution of
1.2 ml. of concentrated sulfuric acid in 136 g. (126 ml., 1.33 moles) of acetic anhydride is added, and the stirrer is started. The mixture warms up, and the
tartaric acid goes into solution. The solution is heated gently (Note
1) under reflux with stirring for 10 minutes. The solution is poured into a
beaker and cooled for 1 hour in an
ice bath. The crude crystalline product is collected on a
15-cm. Büchner funnel (Note
3), washed twice with
20-ml. portions of dry benzene, stirred mechanically with
175 ml. of cold absolute ether, filtered, and placed in a
vacuum desiccator over
phosphorus pentoxide and paraffin shavings for 24 hours. The yield of
diacetyl-d-tartaric anhydride is
41–44.5 g. (
71–77%), m.p.
133–134° (Note
4),
[α]D20 97.2° in dry
chloroform (
c = 0.47).