1. Iodine (78 mg, 0.33 mmol) was added to p-xylene (6.13 mL, 50.0 mmol) at 0 °C, followed by the addition of bromine (5.20 mL, 101 mmol) dropwise over 10 min. After removal of the ice bath, the reaction mixture was stirred for 16 h at room temperature under light protected conditions. The reaction was quenched with 20% KOH aqueous solution and stirred for 15 min until the solution was colorless. The solid precipitate was collected by filtration and washed twice with 100 mL of water. Recrystallization by ethanol gave a white solid product (9.19 g, 70% yield).1H NMR (CDCl3) data: δ 7.40 (s, 2H), 2.34 (s, 6H).
2. 1,4-Dibromo-2,5-dimethylbenzene (5.93 g, 22.5 mmol) and KMnO4 (15.72 g, 99.5 mmol) were dissolved in 80 mL of t-BuOH:H2O (1:1) mixed solvent, and refluxed for 18 h after adding diatomaceous earth (14 g). The reaction mixture was cooled to room temperature, filtered through diatomaceous earth and the filter cake was washed with hot water (100 mL) and EtOAc (50 mL). The filtrate was acidified to pH=1 with concentrated hydrochloric acid and the white suspension formed was extracted three times with EtOAc (100 mL). The organic phases were combined, dried over MgSO4 and concentrated under reduced pressure to give a white solid product (3.06 g, 42% yield) without further purification.1H NMR (DMSO-d6) data: δ 8.01 (s, 2H).
3. 2,5-Dibromoterephthalic acid (2.04 g, 6.30 mmol) was dissolved in 35 mL of methanol and refluxed for 30 min, then thionyl chloride (9.00 mL, 124 mmol) was slowly added and refluxing was continued for 12 hours. The reaction mixture was cooled to room temperature, 50 mL of water was carefully added and extracted three times with Et2O (75 mL). The organic phases were combined, washed three times with saturated NaHCO3 solution (50 mL), dried over MgSO4 and concentrated under reduced pressure. The solid was recrystallized by methanol to give a white crystalline product (1.65 g, 74% yield) with a melting point of 134-136 °C (literature value 134-137 °C).1H NMR (CDCl3) data: δ 8.06 (s, 2H), 3.96 (s, 6H), in agreement with known spectra.
