The general procedure for the synthesis of 2-chloro-4-hydroxypyridine from 2-chloro-4-aminopyridine is as follows: in a round-bottomed flask, 2-chloro-4-aminopyridine (3.0 g, 23.3 mmol) was dissolved in 40% sulfuric acid solution (50 g). The solution was stirred at 0 °C and sodium nitrite (1.93 g, 28.0 mmol) was added slowly. After addition, the reaction mixture was stirred continuously at room temperature for 24 hours. Upon completion of the reaction, the reaction solution was neutralized by dropwise addition of aqueous sodium hydroxide and aqueous sodium bicarbonate. Subsequently, the organic phase was extracted with ethyl acetate. The organic phase was dried over anhydrous magnesium sulfate, filtered, distilled to remove the solvent and dried under reduced pressure to give 2-chloro-4-hydroxypyridine (light brown solid, 2.78 g, 92% yield). The product was characterized by 1H-NMR (270 MHz, DMSO-d6): δ 11.20 (s, 1H, -OH), 8.09 (d, 1H, J = 5.4 Hz, ArH), 6.81 (s, 1H, ArH), 6.77 (d, 1H, J = 2.2 Hz, ArH).
