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Org. Synth. 1940, 20, 29
DOI: 10.15227/orgsyn.020.0029
trans-DIBENZOYLETHYLENE
[2-Butene-1,4-dione, 1,4-diphenyl-]
Submitted by R. E. Lutz
Checked by C. F. H. Allen and F. P. Pingert.
1. Procedure
Eighteen hundred milliliters of benzene (Note 1) and 350 g. of finely ground anhydrous aluminum chloride (2.6 moles) (Note 2) are placed in a 3-l. three-necked flask, fitted with a mechanical stirrer, a dropping funnel (Note 3) containing 153 g. (1 mole) of fumaryl chloride (p. 422), and a reflux condenser (Note 4). A trap for absorbing hydrogen chloride is attached to the condenser.
The mixture is well stirred and heated externally by hot water (50–60°) (Note 5), the water removed, and the fumaryl chloride admitted at a brisk rate, moderated only enough to avoid a too rapid evolution of hydrogen chloride; this requires 15–25 minutes (Note 6) and (Note 7). The mixture turns dark red and soon reaches the boiling point; hydrogen chloride is rapidly given off. The mixture is then refluxed gently for 10 minutes with stirring.
The pasty red mixture is then poured portionwise upon 4 kg. of cracked ice to which has been added 75 ml. of concentrated hydrochloric acid, the reaction mixture being thoroughly stirred before each pouring so that the aluminum chloride complex does not settle out and become concentrated at the bottom of the flask. The residue in the flask is decomposed by adding some of the ice and water. After standing 20–30 minutes, very hot water (Note 8) is added to melt any ice or frozen benzene and raise the temperature generally. The bulk of the aqueous layer is discarded by drawing it off with a glass tube connected to a suction flask. The benzene layer is next washed at least four times with hot water (Note 8) and (Note 9). Finally the warm benzene layer is transferred to a large separatory funnel, any crusts of dibenzoylethylene adhering to the various pieces of apparatus are dissolved in hot benzene, and the solutions combined; the small water layer is separated and discarded (Note 10). The hot benzene layer is then filtered, either by gravity using a large glass funnel or by suction on a Büchner funnel, into a 3-l. round-bottomed flask. After a few porcelain chips have been added, the bulk of the solvent is distilled, using a steam or boiling water bath. Most of the residual solvent is removed under diminished pressure, using a water pump, and heating until the syrupy liquid begins to crystallize suddenly (Note 11). At this point, the heating and suction are discontinued and 125 ml. of 95% ethanol is added rapidly, stirring with a wooden paddle to break up any lumps. The flask is cooled a few minutes under the tap, and the bright yellow product is collected on a 127-mm. Büchner funnel. The solid is triturated on the funnel with cold ethanol for 10 minutes to remove adhering mother liquor, and sucked as dry as possible. The yield is 186–197 g. (78–83%) (Note 11), and the melting point is 109–110° (Note 12) and (Note 13).
2. Notes
1. The checkers used thiophene-free benzene, m.p. ; it gave a product of better quality than the commercial hydrocarbon. A large excess is used to facilitate stirring.
2. Resublimed aluminum chloride is suitable; it requires no further grinding. The excess over the required 2 moles assures a complete reaction and a product of good color.
3. A dropping funnel is preferred to the ordinary separatory funnel, since the rate of addition of the fumaryl chloride is important.
4. A wide-bore condenser permits a more rapid reaction, favoring an increased yield and better product.
5. If the mixture is not heated before the fumaryl chloride is added the reaction is slow, and when the temperature finally rises the accumulated chloride and intermediates react so vigorously that frothing and boiling over occur.
6. If for any reason stirring is interrupted, the addition of fumaryl chloride must be stopped immediately, and the stirrer started again very cautiously.
7. The product and yield are better with the shorter time of addition.
8. Very hot water is desirable; unless the benzene layer is really warm (50–60°), the separation into layers is poor and the product does not readily dissolve. For the same reasons, hot water is used in the subsequent washings.
9. The first and second wash waters should be acidulated with 25 ml. of concentrated hydrochloric acid; this facilitates the formation of layers.
10. If the water used has been hot enough to keep the benzene really warm (50–60°), there are no crusty deposits.
11. Ordinarily there is but 8 g. in the second crop; however, in the event that not enough solvent has been removed, there may be a larger amount, with a correspondingly smaller first crop. The total yield is 194–205 g. (82–86%). If the first crop weighs over 190 g., it is not economical to work up the mother liquor.
12. In some runs, the product sinters slightly at 106°, but the melting point is unaltered.
13. Lower-melting material, whatever its source, can be recrystallized from 95% ethanol, using 3 ml. per gram.
3. Discussion
trans-Dibenzoylethylene has been prepared by the present method,1 by heating dibenzoylmalic acid,2 by condensing benzoylformaldehyde and acetophenone,3 and by the Friedel-Crafts reaction on benzoylacrylyl chloride.4 Analogs may be prepared using other aromatics such as mesitylene,1,4,5,6 with carbon disulfide as the solvent, and also using other acid chlorides such as mesaconyl chloride,7 dibromofumaryl chloride,8 and dimethylfumaryl chloride.9
A mixture of stereoisomers has been prepared by the action of alcoholic potassium hydroxide on phenacyl chloride.10

References and Notes
  1. Conant and Lutz, J. Am. Chem. Soc., 45, 1305 (1923); Oddy, J. Am. Chem. Soc., 45, 2159 (1923).
  2. Paal and Schulze, Ber., 33, 3798 (1900).
  3. Smedley, J. Chem. Soc., 95, 219 (1909).
  4. Lutz, J. Am. Chem. Soc., 52, 3432 (1930).
  5. Conant and Lutz, J. Am. Chem. Soc., 47, 891 (1925).
  6. Weygand and Lanzendorf, J. prakt. Chem., 151, 209 (1938).
  7. Lutz and Taylor, J. Am. Chem. Soc., 55, 1177 (1933).
  8. Lutz, J. Am. Chem. Soc., 52, 3421 (1930).
  9. Lutz and Taylor, J. Am. Chem. Soc., 55, 1599 (1933).
  10. Bogoslovskii, J. Gen. Chem. U.S.S.R., 14, 993 (1944) [C. A., 39, 4600 (1945)].

Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)

aluminum chloride complex

ethanol (64-17-5)

hydrogen chloride,
hydrochloric acid (7647-01-0)

Benzene (71-43-2)

Acetophenone (98-86-2)

aluminum chloride (3495-54-3)

potassium hydroxide (1310-58-3)

benzoylformaldehyde (1074-12-0)

carbon disulfide (75-15-0)

Mesitylene (108-67-8)

phenacyl chloride (532-27-4)

2-Butene-1,4-dione, 1,4-diphenyl-

Fumaryl chloride (627-63-4)

dibenzoylethylene,
trans-Dibenzoylethylene

dibenzoylmalic acid

benzoylacrylyl chloride

mesaconyl chloride

dibromofumaryl chloride

dimethylfumaryl chloride