General procedure for the synthesis of 3,5-dichloro-2-pyridinecarboxylic acid from 2-cyano-3,5-dichloropyridine: 3,5-dichloro-2-pyridinecarbonitrile (10 g, 57.8 mmol) was dissolved in 100 mL of 95% concentrated sulfuric acid, and the reaction mixture was heated to 115 °C and kept reacting overnight. Upon completion of the reaction, the mixture was cooled to room temperature, followed by slow pouring into ice water and vigorous stirring. The precipitated solid product was collected by filtration, washed thoroughly with cold water and finally dried under reduced pressure at 40 °C to give 9.4 g of white solid product in 85% yield.
