General procedure for the synthesis of 2,5-dibromo-m-xylene from 4-bromo-2,6-dimethylaniline: 4-bromo-2,6-dimethylaniline (6.00 g, 30 mmol) was suspended in a 48% aqueous hydrobromic acid solution (20 mL), cooled to -10 °C, then sodium nitrite (2.28 g, 33 mmol) dissolved in water was slowly added, and the mixture was kept at that temperature with continuous stirring for 1 hour. Subsequently, sulfamic acid (0.58 g, 6 mmol) was added to quench the excess nitrite. The reaction mixture was transferred to a 48% aqueous hydrobromic acid solution (20 mL) containing ferrous sulfate heptahydrate (4.17 g, 15 mmol), gradually warmed to 80 °C and stirred at this temperature for 1 hour. After completion of the reaction, water was added to the mixture and extracted with hexane. The organic phases were combined, dried with anhydrous sodium sulfate and concentrated under reduced pressure to remove the solvent. Finally, the target product 2,5-dibromo m-xylene (6.19 g, 72% yield) was purified by medium-pressure column chromatography (silica gel as stationary phase and hexane as mobile phase).
