Bromine → 4-Bromo-5-methylthiophene-2-carbaldehyde

To a solution of 5-methylthiophene-2-carbaldehyde (50.0 g) in acetic acid (400 mL) was added dropwise a solution of bromine (25 mL) in acetic acid (200 mL) at room temperature over 6 hr. After the dropwise addition, the reaction mixture was stirred overnight, neutralized with saturated aqueous sodium hydrogen carbonate solution, and extracted with ethyl acetate. The ethyl acetate layer was washed with saturated brine, dried (MgSO4) and concentrated. The obtained residue was dissolved in toluene (50 mL), and the mixture was stirred at 110° C. for 2 hr and concentrated. The residue was subjected to silica gel column chromatography, eluted with ethyl acetate-hexane (1:5, volume ratio) and concentrated to give 4-bromo-5-methylthiophene-2-carbaldehyde (40.0 g, yield 50%) as yellow crystals. melting point 60° C.