General procedure for the synthesis of 6-(trifluoromethyl)-4,5-dihydropyridazin-3(2H)-one from 6-(trifluoromethyl)-4,5-dihydropyridazin-3(2H)-one: a mixture of 6-(trifluoromethyl)-4,5-dihydropyridazin-3(2H)-one (90.0 g), bromine (30.5 mL), and acetic acid (270 mL) were stirred and reacted for 1 hour at 80 °C. Upon completion of the reaction, ice water (500 mL) was added to the reaction mixture to quench the reaction. The precipitated crystals were collected by filtration, washed sequentially with aqueous sodium bicarbonate and distilled water, and dried to give 6-(trifluoromethyl)pyridazin-3(2H)-one (58.74 g, 66% yield) as white crystals. The melting point of the product was 129-130 °C. 1H-NMR (CDCl3) δ: 7.14 (1H, dd, J = 9.9, 0.5 Hz), 7.54 (1H, d, J = 10.0 Hz), 12.64 (1H, brs).
