General procedure for the synthesis of 5-hydroxyquinoline from 5-aminoquinoline: to a stirred aqueous solution (100 mL) of 5-aminoquinoline (5.0 g, 34.67 mmol) was added sodium bisulfite (NaHSO3; 25.2 g, 242.1 mmol). The mixture was heated to reflux temperature and stirred for 36 hours at room temperature. After completion of the reaction, the solution was cooled to room temperature, sodium hydroxide (NaOH; 9.7 g, 242.5 mmol) was added and the mixture was continued to be stirred at reflux temperature for 8 hours. The progress of the reaction was monitored by thin layer chromatography (TLC). At the end of the reaction, the mixture was cooled to room temperature and the pH was adjusted to 7.0 with 6 equivalents (N) hydrochloric acid (HCl). the precipitate was collected by filtration, washed with water and dried under high vacuum to afford the target product, 5-hydroxyquinoline (3.2 g, 22.04 mmol, 64% yield) as a light yellow solid.1H NMR (500 MHz, CDCl3): δ 8.92 (s 1H), 8.58 (d, J = 8.5 Hz, 1H), 7.71 (d, J = 8.5 Hz, 1H), 7.53 (t, J = 8.0 Hz, 1H), 7.41 (dd, J = 8.5, 4.5 Hz, 1H), 6.88 (d, J = 7.5 Hz, 1H), 6.10 (br s, 1H). Mass spectrum (ESI): m/z 146 [M + H]+.
