Hydrochloric Acid → 4,5-Dichloro-2(3h)-benzothiazolone

33 g of 2-amino-4,5-dichlorobenzothiazole, 241 g of 30% hydrochloric acid and 33.9 g of water are introduced into a 750 ml pressure vessel at room temperature and subsequently heated to 50° C. After the mixture has been stirred for 30 minutes, 58.3 g of aqueous, 40% sodium nitrite solution are metered in at a uniform rate in the course of 3 hours at 50° C., with vigorous stirring. After metering in has ended, stirring is continued for 4 hours at 50° C. Excess nitrite is then removed by adding approximately 15 g of 18% aqueous urea solution. The reaction vessel is subsequently sealed, and the reaction mixture is heated for 4 hours at 140°-150° C. (pressure approximately 6 bar). After the mixture has cooled to 20° C., the crystalline product is filtered off with suction, washed with water and dried at 60° C. to constant weight. 27.9 g of 4,5-dichlorobenzothiazol-2-one with a pure substance content of 97.2% are obtained (corresponding to 81.8% yield) with a DSC melting point of 229.1° C.