Hydrochloric Acid → 5-Amino-1-(4-chlorophenyl)-4-(4-sulfophenylazo)pyrazole

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20.8 g (0.12 mol) of sulfanilic acid was diazotized and to the mixture was then added with continuing ice cooling 18.9 g (0.23 mol) of sodium acetate. The dimethylformamide solution from Step 10.1 was added dropwise over a period of 30 minutes. During the addition, the suspension changed color from gray to yellow-brown. The mixture was allowed to agitate an additional 2 h at 5° C. after which it was allowed to stand overnight at room temperature. The next day, the reaction product was filtered off, the moist filter cake was suspended in 350 mL of dimethylformamide and heated to about 80° C. Then, first 19.2 g (0.19 mol) of triethylamine and then 100 mL of water were added to dissolve the intermediate product. The clear solution was allowed to agitate in the presence of 3.5 g of activated carbon for about 15 minutes and was then filtered warm. The residue on the filter was washed with a small amount of water. To the still warm filtrate was then added 17.8 mL (0.21 mol) of concentrated hydrochloric acid, and the resulting yellow suspension was cooled to 0 to 5° C. The precipitated product was removed by suction filtration and washed with 15-mL portions of isopropanol. The residue was dried at 60° C. This gave 23.8 g (63% of the theoretical) of 5-amino-1-(4-chlorophenyl)-4-(4-sulfophenylazo)pyrazole. Melting point; 260° C. (decomp.).

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