Hydrogen Bromide → 4-Bromo-2-hydroxybenzoic acid

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In water (22.5 ml) and a 47% aqueous solution of hydrobromic acid (22.5 ml), 4-amino-2-hydroxybenzoic acid (5.04 g) was dissolved. While the resulting solution mixture was maintained at 5° C. or lower, an aqueous solution (water: 15.0 ml) of sodium nitrite (2.26 g) was added dropwise thereto, followed by stirring for 30 minutes under ice cooling. The reaction mixture was added, in portions, to a solution of cuprous bromide (5.63 g) dissolved in a 47% aqueous solution of hydrobromic acid (15 ml) under ice cooling. The resulting mixture was stirred at room temperature for 150 minutes. Ethyl acetate was added to the reaction mixture for extraction. The organic layer so obtained was washed with water and then dried over anhydrous sodium sulfate. The residue obtained by distilling off the solvent under reduced pressure was purified by chromatography on a silica gel column (dichloromethane˜10% methanol—dichloromethane), whereby 4-bromo-2-hydroxybenzoic acid (5.51 g) was obtained as a crudely purified product.

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