General procedure for the synthesis of 6-chloro-5-nitro-2-pyridinecarboxylic acid from 6-chloro-5-nitro-2-methylpyridine: 6-chloro-5-nitro-2-methylpyridine (97 g, 560 mmol) was slowly added to a reaction flask pre-filled with 18 M sulfuric acid (400 mL) under stirring conditions. Subsequently, chromium trioxide (169 g, 1.69 mol) was added to the reaction mixture in small portions and the reaction temperature was controlled to be below 50 °C. The reaction mixture was stirred continuously at 15 °C for 20 hours. Upon completion of the reaction, the mixture was quenched by pouring it into 2 Kg of ice water and the resulting solid product was collected by filtration. The resulting solid was dried under vacuum to give 6-chloro-5-nitro-2-pyridinecarboxylic acid (103 g, 90% yield) as a pale white solid. The structure of the product was confirmed by 1H NMR (400 MHz, DMSO-d6) with chemical shifts of δ 8.70 (d, 1H), 8.24 (d, 1H).
