B.
Diphenylcyclopropenone. A solution of
100 ml. of triethylamine (Note
2) in
250 ml. of methylene chloride is magnetically stirred in a 2-l. one-necked flask while
108 g. (0.29 mole) of the above dibromoketone in
500 ml. of methylene chloride is added dropwise over 1 hour. The mixture is stirred for an additional 30 minutes and then extracted with two
150-ml. portions of 3N hydrochloric acid; the aqueous extracts are discarded. The red organic solution is transferred to a
2-l. Erlenmeyer flask and cooled in an
ice bath. While this solution is swirled, a cold solution of
50 ml. of concentrated sulfuric acid in 25 ml. of water is slowly added. A slightly pink precipitate of
diphenylcyclopropenone bisulfate gradually separates (Note
3). This is collected on a
sintered-glass funnel and washed with two
100-ml. portions of methylene chloride. The solid is then returned to the flask (Note
4) along with
250 ml. of methylene chloride and 500 ml. of water, and
5 g. of solid sodium carbonate is added in small portions. The organic layers is collected and the aqueous solution extracted with two
150-ml. portions of methylene chloride. The combined organic layers are dried over
magnesium sulfate and evaporated to dryness. The impure
diphenylcyclopropenone is recrystallized by repeated extraction with boiling
cyclohexane (total 1.5 l.), the solution being decanted in each case from a reddish oily impurity. On cooling, the solution deposits
29 g. of white crystals, and an additional
1 g. can be obtained by concentrating the mother liquors to 150 ml. The combined
30 g., m.p.
119–120°, represents an overall yield of
44% based on
dibenzylketone.