Submitted by G. R. Dutton and C. R. Noller.
Checked by John R. Johnson and Anthony Hunt.
1. Procedure
In a
2-l. round-bottomed flask, fitted with a
reflux condenser,
liquid-sealed mechanical stirrer,
dropping funnel (Note
1), and
thermometer, are placed
222 g. (274 cc., 3 moles) of dry n-butyl alcohol,
260 g. (265 cc., 3.3 moles) of pyridine, and
275 cc. of dry benzene (Note
2). The solution is stirred and the flask is cooled in an ice-salt mixture until the temperature has fallen to −5°. With efficient stirring (Note
3),
153 g. (91 cc., 1 mole) of phosphorus oxychloride (b.p.
106–107°) is added dropwise at such a rate that the temperature does not exceed 10°. After the addition is completed the reaction mixture is heated slowly to the reflux temperature and held there for two hours. The mixture is cooled to room temperature, and 400–500 cc. of water is added to dissolve the
pyridine hydrochloride (Note
4). The
benzene layer is separated, washed with 100–150 cc. of water (Note
5), and dried over
20 g. of anhydrous sodium sulfate.
The
benzene and other low-boiling materials are removed by distillation at 40–50 mm. pressure until the temperature of the distilling vapor reaches 90°. The
n-butyl phosphate fraction is collected at
160–162°/15 mm., or
143–145°/8 mm., and weighs
190–200 g. (
71–75 per cent of the theoretical amount) (Note
6).
2. Notes
1. The tip of the funnel should be placed sufficiently high above the surface of the reaction mixture of avoid encrustation with
pyridine hydrochloride. It is advantageous to use a thermometer on which the scale above −5° is visible above the
stopper; otherwise, the fog in the flask and the pyridine salt may obscure the scale.
2. The reactants and solvent were dried by distillation; fractions boiling over an interval of 1° were used.
3. The first
10–15 cc. of phosphorus oxychloride must be added very slowly to avoid vigorous reaction and overheating. It is essential to avoid an initial temperature so low that unreacted
phosphorus oxychloride accumulates and then suddenly reacts with violence. The mechanical stirrer should be of such dimensions and operated at such speeds that the heat of reaction is dissipated rapidly without throwing solid material (and occluded reactants) against the upper walls of the flask.
4. About
50 per cent of the pyridine may be recovered by concentrating this aqueous solution over a
steam bath, treating with strong
caustic soda solution, and distilling the
pyridine layer.
5. The solution should be neutral before distillation. The presence of
hydrogen chloride promotes decomposition of phosphoric esters.
1 The
benzene solution should not be washed with alkaline reagents, such as
sodium carbonate solution, since alkaline reagents also cause decomposition during distillation.
6. This is a general method for preparing alkyl phosphates. Using a similar procedure, the
n-propyl ester may be obtained in
60–65 per cent yields, the
sec.-butyl ester in
40–45 per cent yields, and the
n-amyl ester in
60–65 per cent yields.
2
3. Discussion
Copyright © 1921-2002, Organic Syntheses, Inc. All Rights Reserved