Hydrochloric Acid → 3-Methyl-4-(2-pyridylazo)-1-(p-sulfophenyl)-2-pyrazolin-5-one

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41.0 g (0.10 mole) of 4,4-dibromo-3-methyl-1-(4-sulfophenyl)-2-pyrazolin-5-one, which can be prepared according to the method of Machiele, U.S. Pat. No. 3,952,009 (Example 1), and 16.0 g (0.15 mole) of 2-hydrazinopyridine were combined in 180 ml of water. The mixture was heated on the steam bath a total of six hours. After a few minutes of heating, the solution goes to a very thick slurry. The mixture was cooled somewhat, 10 ml of concentrated HCl were added, and the orange solid was isolated by suction filtration. After washing once with tetrahydrofuran, twice with ether, and drying, the yield of dye was 17.8 g (50 percent).

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