Submitted by Lucius A. Bigelow and Roy S. Hanslick.
Checked by W. W. Hartman and Lloyd A. Smith.
1. Procedure
(
A)
Dibenzoyldibromomethane.—In a
1-l. three-necked flask, equipped with a
mechanical stirrer, a
dropping funnel, and a
thermometer, are placed
56 g. (0.25 mole) of dibenzoylmethane (
Org. Syn. Coll. Vol. I, 1941, 205;
Org. Syn. 20, 32) and
14 cc. of chloroform. The flask is surrounded by an
ice bath, the stirrer is started, and a solution of
28.5 cc. (88 g., 0.55 mole) of dry bromine (Note
1) in
230 cc. of chloroform is added slowly from the dropping funnel during a period of about thirty minutes. The temperature of the mixture should not exceed 15° during the bromination; the
hydrogen bromide evolved is continuously removed by aspirating a gentle stream of air over the surface of the solution. After all the
bromine has been added, stirring is continued for about fifteen minutes. The solution is then transferred to a distilling flask, and the solvent is completely removed under diminished pressure at room temperature (Note
2). The slightly colored residue is crystallized from
125 cc. of hot 95 per cent ethyl alcohol. The
dibenzoyldibromomethane is obtained in the form of white crystals melting at
94–95°. The yield is
72.4 g. (
76 per cent of the theoretical amount) (Note
3).
(
B)
Diphenyl Triketone Hydrate.—A solution of
34.3 g. (0.42 mole) of fused sodium acetate in
142 cc. of hot glacial acetic acid is prepared in a
1-l. round-bottomed flask;
72.4 g. (0.19 mole) of dibenzoyldibromomethane is added, and the mixture is refluxed until the precipitation of
sodium bromide ceases (one and one-half to two hours). The mixture is then cooled to room temperature and diluted with 150–200 cc. of water with constant shaking to dissolve the inorganic salt and to precipitate the
triketone hydrate, which separates as a white, curdy mass (Note
4). This is separated by filtration, washed well with water, and dried in an
oven at 60°. The melting point varies from
65 to 90°, depending upon the extent of dehydration that occurs during the drying operation. The yield is
41.5 g. (
86 per cent of the theoretical amount based on the
dibenzoyldibromomethane).
(
C)
Diphenyl Triketone.—The
41.5 g. (0.16 mole) of triketone hydrate is distilled in vacuum from a
Claisen flask heated by means of a
sand bath. A
distilling flask is used as a receiver, and no condenser is necessary. The neck of the receiving flask must be warmed, however, to prevent clogging of the apparatus by crystallization of the distillate. The anhydrous triketone distils at
174–176°/2 mm. as a reddish oil that solidifies to a light yellow, crystalline mass. This is dissolved in
70 cc. of hot ligroin (b.p.
90–120°); it separates on cooling in light yellow needles which melt at
68–70°. The yield is
35 g. (
91 per cent of the theoretical amount based on the
triketone hydrate;
59 per cent based on the
dibenzoylmethane) (Note
5).
2. Notes
2. If the solvent is removed by heat at ordinary pressure, there is a decrease both in the yield and the purity of the product.
3. If the recrystallization is omitted, a lower yield of the
triketone hydrate is obtained.
4. If the hydrate separates in part in an oily condition, it may be dissolved in a small amount of glacial
acetic acid and reprecipitated with an equal volume of water.
5. The triketone is hygroscopic and must be kept in a
vacuum desiccator or sealed tube.
3. Discussion
The two general methods for the preparation of
diphenyltriketone involve the treatment of
dibenzoylmethane with
bromine1 and with
oxides of nitrogen.
2 De Neufville and v. Pechmann, who originated the first method, recommended
1 conversion of the diketone to
dibenzoylbromomethane and transformation of this to the acetate and then to
dibenzoylbromocarbinol acetate which was split to the triketone. This sequence has been used with success by Kohler and Erickson
3 to prepare the triketone in good yields, but the present method, mentioned but not described by De Neufville and v. Pechmann,
1 is simpler and more direct.
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