2-Bromo-4-hydroxybenzaldehyde (1.7 g, 8.46 mmol, 1.0 equiv) was used as starting material and dissolved in 30 mL of 2 M aqueous sodium hydroxide solution. Potassium permanganate (2.7 g, 16.92 mmol, 2.0 equiv) was added in batches under ice bath cooling conditions. The reaction mixture was then slowly warmed to room temperature with continuous stirring for 2 hours. After completion of the reaction, the insoluble impurities were removed by filtration and the filtrate was washed once with dichloromethane. To the filtrate, 2 M aqueous hydrochloric acid was slowly added and the pH was adjusted to 1. The aqueous phase was extracted with ethyl acetate, the organic phases were combined and dried over anhydrous sodium sulfate. The organic phase was concentrated under reduced pressure to give the white solid product 2-bromo-4-hydroxybenzoic acid (1.6 g, 88% yield).
