A Publication
of Reliable Methods
for the Preparation
of Organic Compounds
Annual Volume
Org. Synth. 1940, 20, 47
DOI: 10.15227/orgsyn.020.0047
[Ketene, diphenyl-]
Submitted by Lee Irvin Smith and H. H. Hoehn.
Checked by Nathan L. Drake and Jonathan Williams.
1. Procedure
Fifty-six grams (0.25 mole) of benzil monohydrazone [Org. Syntheses, 15, 62 (1935)] (Note 1) is mixed in a mortar with 81 g. (0.38 mole) of yellow mercuric oxide and 35 g. of anhydrous calcium sulfate (Note 2). The mixture is introduced into a 1-l. three-necked flask fitted with a stirrer, a condenser, and a thermometer. The flask is placed in a water bath, 200 ml. of dry thiophene-free benzene is added, and the suspension is stirred at 25–35° (thermometer in solution) for 4 hours (Note 3) and (Note 4). The reaction mixture is filtered through a fine-grained filter paper, with slight suction, and the residue is washed with dry benzene until the washings are colorless.
The benzene solution of the diazo compound is poured into a separatory funnel protected with a drying tube and connected to a 125-ml. Claisen distilling flask provided with a condenser set for downward distillation and arranged so that it can be heated in a bath of Wood's metal. The temperature of the metal bath being maintained at 100–110°, the benzene solution is dropped slowly into the hot flask. Under these conditions, the benzene is removed by distillation and the diazo compound is transformed into diphenylketene. The residue is distilled under reduced pressure in an atmosphere of nitrogen, and the fraction boiling at 115–125° at 3–4 mm. (Note 5) is collected. The yield is 31 g. (64%) of a product which, on redistillation, yields 28 g. of diphenylketene boiling at 119–121° at 3.5 mm. (58%).
Diphenylketene is best stored in an atmosphere of nitrogen; the addition of a small crystal of hydroquinone serves to inhibit polymerization (Note 6).
2. Notes
1. Benzil monohydrazone can also be obtained in practically quantitative yield using hydrazine hydrate, a method first suggested by Curtius and Thun.1 Hydrazine hydrate (45 g., 0.75 mole, of an 85% solution of hydrazine hydrate in water) is slowly dropped into a hot solution of benzil (158 g., 0.75 mole) in alcohol (300 ml.) with stirring. The product begins to separate from the hot solution after three-fourths of the hydrazine hydrate has been added. The solution is heated under reflux for 5 minutes after all the hydrazine hydrate has been added. The flask is then cooled to 0°, and the hydrazone is filtered off and washed twice on the funnel with 100-ml. portions of cold ethanol. The product melts at 149–151° with decomposition.
2. Anhydrous calcium sulfate removes the water formed in the oxidation.
3. Considerable heat is generated at the beginning of the reaction, and ice must be used in the water bath to keep the temperature within the prescribed limits. After 10–15 minutes the ice is removed, and the temperature is maintained at 25–35° by the water bath.
4. Best results are obtained when the reaction mixture is stirred for 4 hours.
5. A viscous red residue always remains in the distilling flask, necessitating superheating to remove the last traces of diphenylketene.
6. According to the submitters, the preparation has been carried out using twice the quantities of material throughout with no loss in yield of diphenylketene.
3. Discussion
Diphenylketene has been prepared by action of tripropylamine on diphenylacetyl chloride,2 by treating diphenylchloroacetyl chloride with granulated zinc,3 and by the action of quinoline on diphenylacetyl chloride.4 It is most conveniently prepared by heating phenylbenzoyldiazomethane—a method first described by Schroeter5 and later used by Staudinger.6

References and Notes
  1. Curtius and Thun, J. prakt. Chem., (2) 44, 176 (1891).
  2. Staudinger, Ber., 44, 1619 (1911).
  3. Staudinger, Ber., 38, 1735 (1905).
  4. Staudinger, Ber., 40, 1148 (1907).
  5. Schroeter, Ber., 42, 2346 (1909).
  6. Staudinger, Ber., 44, 1623 (1911).

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

ethanol (64-17-5)

Benzene (71-43-2)

hydroquinone (123-31-9)

nitrogen (7727-37-9)

mercuric oxide (21908-53-2)

calcium sulfate (7778-18-9)

Benzil (134-81-6)

zinc (7440-66-6)

hydrazine hydrate (7803-57-8)

Quinoline (91-22-5)

Benzil monohydrazone (5344-88-7)

Diphenylacetyl chloride (1871-76-7)

Ketene, diphenyl- (525-06-4)

tripropylamine (102-69-2)

diphenylchloroacetyl chloride (2902-98-9)