A
3-l., four-necked, round-bottomed flask is equipped with a
thermometer (−50° to 150°), a
dry ice-cooled reflux condenser (protected from the atmosphere through a T-tube that is also connected to a
nitrogen source and a Nujol bubbler), and a
gas-inlet tube above the liquid level. The flask and condenser are heated in an air
oven at 125° for several hours and flamed under
nitrogen with a
Bunsen burner.
Pyridine (1.2 l.), previously dried over
potassium hydroxide pellets, is added to the flask. A
nitrogen atmosphere is maintained in the system, the
pyridine is cooled to −40°, and
462 g. (2.78 moles) of hexafluoroacetone (b.p.
−28°) is added from a cylinder through the gas-inlet tube over 30 minutes (Note
1), (Note
2), and (Note
3). Liquid
ammonia (58.3 ml., 47.6 g., 2.80 moles), previously distilled into a cold trap and measured at −78°, is distilled into the
pyridine solution over a period of 1 hour ((Note
4)). During this addition the bath is held at −45° to −40°, keeping the solution at −25° to −30°.
As soon as the
ammonia has been added, the gas-inlet tube is replaced with a
250-ml. pressure-equalized dropping funnel and the reaction mixture is heated with a
heating mantle to 40° over 30 minutes or as quickly as possible (Note
5). The dry ice–cooled condenser is then replaced with a
24-in. water-cooled bulb condenser with Tygon tubing joining the top of the condenser to a
300-ml. cold trap protected from the atmosphere with a
calcium chloride drying tube and cooled in a bath maintained at −30°. The condenser is cooled with 18–20° water. The heating mantle is turned off, and the dropping funnel is charged with
394 g. (235 ml., 2.57 moles) of phosphorus oxychloride, which is added dropwise at a rate to maintain a gentle reflux. The imine (b.p.
16°) collects in the cold trap. When addition is complete, the reaction mixture is heated to 100° over 20 minutes and maintained at this temperature for 30 minutes. The cold trap collects
320–360 g. of crude liquid product (Note
6), which is distilled through a Podbielniak still with a reflux head temperature of about 0° (Note
7), yielding
254–291 g. (
55–65%) of the purified imine, b.p.
15.5–17° (Note
8). It can be stored indefinitely in a stainless-steel cylinder.