N[C:2]1[S:3][C:4]2[CH:10]=[CH:9][C:8]([Cl:11])=[C:7]([Cl:12])[C:5]=2[N:6]=1.Cl.N([O-])=[O:15].[Na+]>O>[Cl:12][C:7]1[C:5]2[NH:6][C:2](=[O:15])[S:3][C:4]=2[CH:10]=[CH:9][C:8]=1[Cl:11]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.
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