Organic Syntheses, Coll. Vol. 1, p.394 (1941); Vol. 3, p.75 (1923).
In a 3-l. round-bottomed flask
are mixed 501 g. (3 moles) of pure p-nitrobenzoic acid (p. 392)
and 626 g. (3 moles) of pure phosphorus pentachloride (Note 1)
. The flask is provided with a calcium chloride tube
from which emerges a tube leading to the surface of a flask of water, in order to absorb the hydrogen chloride
formed during the reaction (Note 2)
. The flask is placed on a water bath
and heated with occasional shaking until the reaction starts (fifteen to forty-five minutes). Vigorous evolution of hydrogen chloride
takes place, and the heating is continued until the reaction is complete (fifteen to thirty minutes after the reaction starts). There is thus formed a light yellow, homogeneous liquid.
The reaction mixture is now transferred to a Claisen flask
connected with a water-cooled condenser
, and the phosphorus oxychloride
is removed at ordinary pressure by raising the temperature of the oil bath (Note 3)
gradually to 200–220°. The water condenser is then replaced by a short air-cooled condenser
, and the residual liquid is distilled under reduced pressure (Note 4)
. A small quantity of phosphorus oxychloride
first distils over, after which the receiver
is changed and the temperature rises rapidly to the boiling point of p-nitrobenzoyl chloride
, 197°/73 mm.
). During this distillation the oil bath should be kept at a temperature of about 230–250° (or at 210–215° if 20 mm. pressure is used). The yield is 500–534 g.
per cent of the theoretical amount). The distillate solidifies to a yellow crystalline mass melting at 71° (Note 5)
. The product may be recrystallized from ligroin or carbon tetrachloride
, from which it separates in fine yellow needles melting at 73° (Note 6)
The yield of p-nitrobenzoyl chloride
depends to a great extent upon the quality of the reagents used. With impure phosphorus pentachloride
and pure p-nitrobenzoic acid
, yields of 70–80
per cent of the product are obtained. With pure phosphorus pentachloride
and a less pure grade of p-nitrobenzoic acid
, yields of 85–90
per cent are obtained. When neither reagent is of high grade, yields may fall as low as 40–50
per cent of the theoretical amount. In all such cases the color of the original reaction mixture varies from a deep yellow to a black.
purchased as pure is occasionally of inferior quality and gives poor results. It is not a difficult matter to prepare in the laboratory a product suitable for this reaction. To 1 kg. (620 cc., 7.3 moles) of phosphorus trichloride
is added until the increase in weight is 500 g. The gas should be added above the surface of the liquid, which is stirred occasionally during the addition. The stirring should not be continuous, however, as this tends to allow the formation of the pentachloride in the tube, which thus becomes clogged. At the end of the reaction the mixture becomes practically solid.
If the reaction mixture is heated with a free flame during the distillation under reduced pressure, there is considerable danger of superheating and consequent decomposition (sometimes violent) of the p-nitrobenzoyl chloride
Experience with many preparations of this acid chloride has shown that it is desirable to have a safety water bottle between the receiving flask and the manometer
to use during the distillation under reduced pressure. This avoids the passage of vapors of phosphorus oxychloride
or acid chloride into the pump
At the beginning of the distillation under reduced pressure the air condenser is warmed gently with a free flame to prevent solidification of the first portion of the distillate.
The product is best placed while hot in small wide-mouthed bottles
and allowed to solidify. This prevents any moisture from the air from decomposing more than the surface layer of the acid chloride.
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