The general procedure for the synthesis of 1-methyl-5-(trifluoromethyl)-1H-pyrazol-3(2H)-one from ethyl trifluoroacetoacetate and methylhydrazine was as follows: 6 kg of ethyl ether and 6164.2 g (33.5 mol) of ethyl trifluoroacetoacetate were added to a reaction flask. Under continuous stirring, 1542.4 g (33.5 mol) of methylhydrazine was slowly added and the temperature was maintained at about 24 °C during the reaction. After continued stirring for 1 hour, the reaction was confirmed to be complete. Subsequently, most of the ether solvent was removed by vapor extraction and the residue was slurried in 12 L of chloroform. To the slurried mixture was added 50 mL of concentrated sulfuric acid with continuous stirring overnight. Next, another 100 mL of sulfuric acid was added, and the slurry was washed once with 3 L of water, followed by five washes with 3 L of 10% sodium bicarbonate solution to remove the 5-hydroxy isomer, and finally two washes with 3 L of water. The aqueous layer and undissolved crystals were sequentially extracted twice with 4 L of chloroform. The chloroform layers were combined and concentrated under reduced pressure on a rotary evaporator at 40 °C to give 1405.9 g (25.3% yield) of the target product with a melting point of 205 °C (literature value 130°-131 °C).
