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).