In a dry 500 mL reaction flask, NaNO2 ( 3.89 g , 56.3 mmol) and water ( 10 m L ) were sequentially added to an acetic acid solution ( 300 mL ) of 4-bromo-o-toluidine ( 10.5 g , 56.3 mmol). The resulting reaction mixture was stirred at room temperature for 5 h. The progress of the reaction was monitored by TLC dot plate, and when the reaction was finished, the reaction mixture was concentrated directly under vacuum, and then the residue was diluted with ethyl acetate, and the ethyl acetate layer was washed sequentially with saturated aqueous NaHCO3 solution and brine. The organic layer was separated and dried over anhydrous Na2SO4, the desiccant was removed by filtration and the resulting filtrate was concentrated under vacuum. The residue was purified by silica gel column chromatography (0-50% ethyl acetate in n-hexane) to obtain 5-bromoindazole, note that if the product obtained by isolation is liquid, it can be solidified by adding n-hexane to it, shaking it and spinning it dry.
