Hydrochloric Acid → 3-Chloro-1,2-benzisothiazole

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A mixture of 1,2-benzisothiazole-3(2H)-one (Example 1, 818 g, 5.414 moles) and POCl3 (1114 g, 7.26 moles) was heated to 120° over about two hours. HCl evolution began at about 70°. Heat was continued at 120° for another 1.5 hours. The hot solution was poured into 8 liter of H2O at 25°. The temperature was not allowed to exceed 50°. After 30 minutes, the mixture was cooled to 25° (ice addition) and extracted with methylene chloride (4 liter). A dark oil was obtained by evaporation of the methylene chloride in vacuo. This oil was extracted with Skelly B (3×1 liter and 2×500 mL). The turbid extract was treated with Darco G-60 (30 g) and Celite A-545 (20 g) before filtering. The filtrate was evaporated in vacuo to give 743.9 g, 81%, of a yellow oil which readily crystallized. Distillation of the oil at reduced pressure gave 707 g, 77%, b.p. 80°-85° at 0.75 Torr., of colorless distillate which readily crystallized, m.p. 39°-41°.

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