[Cl:1][C:2]1[CH:24]=[CH:23][C:5]([NH:6][C:7]2[C:15]([S:16](=[O:19])(=[O:18])[NH2:17])=[CH:14][C:10]([C:11]([OH:13])=[O:12])=[CH:9][C:8]=2[N+:20]([O-])=O)=[CH:4][CH:3]=1.N.S(S([O-])=O)([O-])=O.[Na+].[Na+]>O>[NH2:20][C:8]1[CH:9]=[C:10]([CH:14]=[C:15]([S:16](=[O:18])(=[O:19])[NH2:17])[C:7]=1[NH:6][C:5]1[CH:4]=[CH:3][C:2]([Cl:1])=[CH:24][CH:23]=1)[C:11]([OH:13])=[O:12]A mixture of 4-(p-chloroanilino)-3-nitro-5-sulphamyl-benzoic acid (7.43 g), water (50 ml), and concentrated aqueous ammonia (15 ml) was stirred at 25°C, and a solution of sodium dithionite (13.5 g) in water (50 ml) was added dropwise. After 1 hour, the pH of the reaction mixture was adjusted to 2.5, and the precipitated reaction product was filtered off. After repeated recrystallizations from aqueous ethanol, and after drying for 4 hours in vacuo, 3-amino-4-(p-chloroanilino)-5-sulphamyl-benzoic acid was obtained with a melting point of 273°-274°C.
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