A
250-ml. three-necked flask is equipped with a
rubber-sealed mechanical stirrer,
two gas-inlet tubes, and a
mercury-sealed escape valve consisting of an outlet tube dipping into a test tube of
mercury. A solution of
25 g. (0.14 mole) of benzophenone in
125 ml. of 95% ethanol is placed in the flask, which is cooled in an ice-salt freezing mixture.
Hydrogen sulfide and
hydrogen chloride are passed simultaneously into the stirred solution for 3 hours (Note
1). Within 1 hour the solution becomes blue. After 3 hours, the flow of
hydrogen chloride is stopped and
hydrogen sulfide alone is passed for a further 20 hours, with continued ice cooling. Toward the end of the reaction, the contents of the flask assume an intense violet color. The solid
thiobenzophenone (
23–25 g.) is filtered from the ice-cold solution in an atmosphere of
carbon dioxide (Note
2), immediately dried under high vacuum (Note
3), and recrystallized twice from about
20 ml. of petroleum ether (b.p.
60–80°), giving long needles, m.p.
53–54°. The yield of purified product, is
18–21 g. (
66–77%).