Example 1 Synthesis of 5-chlorosalicylic acid isohydroxamic acid: In a 50 ml reaction flask, concentrated sulfuric acid was slowly added dropwise to a stirred slurry containing 200 g of 5-chlorosalicylic acid. Subsequently, 4-4 ml of methanol was added. The temperature of the reaction system was raised to about 35°C. The mixture was reacted under heated reflux conditions for about 20 hours while the resulting water was removed. Upon completion of the reaction, the methyl ester oily substance was isolated and combined with a solution of 144 g of hydroxylamine sulfate in 600 ml of water. The mixture was rinsed with methanol at room temperature. Next, 468 g of 50% w/w sodium hydroxide solution was added slowly and dropwise with stirring while controlling the reaction temperature in the range of 25°-35°C. After the base was added, 750 ml of water was added to dissolve the solids while heating to about 50°C. The resulting clarified solution was cooled to room temperature and neutralized with 140 g of concentrated sulfuric acid, keeping the temperature below 40°C during the process. The resulting precipitate was diluted with 200 ml of water, washed with water after filtration and dried at about 100°C. The final product was 5-chlorosalicylidene isohydroxamic acid with a melting point of 218°-220°C and a yield of 182 g, giving a yield of 84.6%.
