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. (
155°/20 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. (
90–96 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).