(A) Phosphorus Pentachloride Method.—In a 2-l. round-bottomed flask is placed a mixture of 250 g. (1.2 moles) of finely divided phosphorus pentachloride and 450 g. (2.5 moles) of sodium benzenesulfonate which has previously been dried for three hours at 140°. The mixture is heated in an oil bath at 170–180° (Note 1) for fifteen hours (Note 2). It is advisable to have a reflux condenser attached to the flask, although very little active refluxing takes place. Every four hours during the heating period, the flask should be removed from the oil bath, cooled for fifteen minutes, stoppered and shaken thoroughly until the mass becomes pasty (sometimes ten to fifteen minutes are required to accomplish this). At the end of the heating period, the mixture is cooled and 1 l. of water with 1 kg. of cracked ice are added. Benzenesulfonyl chloride sinks to the bottom, is separated, washed once with water, filtered if necessary, and distilled under reduced pressure for purification. The forerun, consisting of a little chlorobenzene, water, and a little benzenesulfonyl chloride, is discarded and the fraction boiling between 145–150° /45 mm. is collected. This weighs 330–360 g. (75–80 per cent of the theoretical amount) (Note 3).

The yields in this procedure are likely to vary widely on account of the variability in the different samples of sodium benzenesulfonate. With certain samples a solid impurity is left as a residue from the distillation of the crude benzenesulfonyl chloride.

(B) Phosphorus Oxychloride Method.—Benzenesulfonyl chloride may be prepared by the same procedure described above from a mixture of 270 g. (1.5 moles) of sodium benzenesulfonate and 180 g. (1.17 moles) of phosphorus oxychloride. It is advisable to observe the same precaution of shaking every few hours. The yield of product boiling at 145–150° /45 mm. is 195–230 g. (74–87 per cent of the theoretical amount) (Note 3).

In a 5-l. flask, fitted with stirrer, separatory funnel, exit tube, and thermometer, is placed 3.5 kg. (2 l., 30 moles) of chlorosulfonic acid (Note 1), and to it is slowly added, with continuous stirring, 780 g. (888 cc., 10 moles) of benzene (Note 2), keeping the temperature between 20° and 25° by means of cold water. The hydrogen chloride which is evolved is led to a hood or absorbed in water (Fig. 7, p. 97). The addition requires two to three hours; when this is complete, the mixture is stirred for one hour, and poured upon 6–7 kg. of crushed ice (Note 3). One liter of carbon tetrachloride is then added, the oil is separated as soon as possible (Note 4), and the watery layer is shaken with 500 cc. of carbon tetrachloride. The combined extracts are washed with dilute sodium carbonate, and the bulk of the carbon tetrachloride is distilled under atmospheric pressure (Note 5); the pressure is then reduced (Note 6) and the benzenesulfonyl chloride collected at 113–115° /10 mm. (118–120° /15 mm.). The yield is 1320–1360 g. (75–77 per cent of the theoretical amount) of a colorless liquid which on cooling sets to a crystalline solid melting at 13–14°.

A small amount (50–100 g.) of higher-boiling material remains in the flask. This is mainly diphenyl sulfone, which may be isolated by distillation under reduced pressure (b.p. 225° /10 mm.) and recrystallization from methyl alcohol. It forms colorless crystals which melt at 128°.