In a dry
2-l. three-necked flask are placed
600 g. (4 moles) of freshly distilled ethyl benzoate and
60 g. (0.5 mole) of freshly distilled acetophenone (Note
1). The flask is fitted with a
mercury-sealed stirrer which must be rugged enough to stir the reaction mixture even after it becomes very viscous (Note
2). A condenser for downward distillation is attached to one of the necks, and a
500-ml. filter flask is used as the receiver. This receiver is connected to a
water pump through a
suction flask carrying a
two-holed rubber stopper, one hole of which is left open. The reaction flask is heated in an
oil bath kept at 150–160°, and, after the mixture is hot,
44 g. (0.65 mole) of sodium ethoxide (Note
3) is added through the third arm of the flask in 1- to 2-g. portions. The ethoxide addition can be conveniently accomplished with a spoon shaped to enter the arm of the flask or by placing a very
short-stemmed large funnel in the third neck and pushing the ethoxide through the funnel with a
small glass rod or wire spatula. The reaction mixture becomes orange immediately;
ethanol distils after the first few additions, and thereafter the evolution of
ethanol is quite vigorous. The additions are made as rapidly as evolution of the
ethanol permits. During the ethoxide addition a gentle stream of air is kept passing through the flask by means of a water pump attached to the receiver in order to prevent the
ethanol vapor from rising in the third arm of the flask and interfering with the addition (Note
4).
The oil bath is removed, and, with the stirring maintained, the reaction mixture is cooled to room temperature by running cold water over the flask. Then 150 ml. of water is added to dissolve the reaction mass, and both layers of the mixture are transferred to a
separatory funnel. An ice-cold solution of
25 ml. of concentrated sulfuric acid in 200 ml. of water is added, and the mixture is shaken vigorously. The ester layer is separated and washed with 200 ml. of water; it is then shaken with successive
200-ml. portions of 5% sodium bicarbonate solution until the evolution of
carbon dioxide ceases, after which it is washed with 200 ml. of water. The bicarbonate solution is separated and extracted with
100 ml. of ether (Note
6). The
ether extract, after washing with 50 ml. of water, is combined with the ester layer and the resulting ethereal solution dried with
40 g. of calcium chloride. The
ether is removed by distillation from a
water bath, and the excess
ethyl benzoate is removed by distillation under reduced pressure. The recovered ester, b.p. 80–83°/8 mm., amounts to 475–490 g. After the ester is removed the temperature of the oil bath is slowly raised to 180–185° while the system is maintained under 8 mm. pressure. A small amount of liquid distilling higher than the ester is thus removed. When no more distillate comes over at this temperature, the remaining oil, while still warm, is poured into a
500-ml. Erlenmeyer flask and allowed to crystallize. This crude
dibenzoylmethane weighs
92–108 g. and is usually brown. It is recrystallized by dissolving in
150 ml. of hot methanol, adding
1 g. of Norit, filtering, and cooling the filtrate to 0° (Note
7). The yield of yellow crystals of
dibenzoylmethane, m.p.
77–78° (Note
8), thus obtained is
70–80 g. (
62–71% based on the
acetophenone).