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The procedure of Van Allan, J. A. V. and Deacon, B. D., Organic Synthesis. IV: 569 was adapted. A mixture of 1,2-diamino-4,5-dichlorobenzene (510 mg, 2.88 mMol, Aldrich), potassium hydroxide (190 mg, 3.40 mMol), carbon disulfide (260 mg, 3.40 mMol), 95% ethanol (3 mL) and water (0.45 ml) was heated under reflux for 3 h. Activated charcoal (120 mg) was then added cautiously, and after the mixture has been heated at the refluxing temperature for 10 min the activated charcoal was removed by filtration. The filtrate was heated to 60°-70° C., warm water (3 mL) was added, and then acetic acid (0.25 mL) in water (0.5 mL) was added with good stirring overnight. The mixture was placed in a refrigerator for 3h to get two crystals (brown and white). The brown crystals were removed by washing with chloroform (5 mL). Recrystallization from hot EtOH/H2O gave pure 5,6-dichloro-2-mercaptobenzimidazole (545 mg, 86%) as white long needles. Mp was measured: the color of 5,6-dichloro-2-mercaptobenzimidazole changed to yellow at 303° C.; the decomposition of 5,6-dichloro-2-mercaptobenzimidazole is obvious at 305° C. and it melted to a black liquid at 308°-10° C. IR (KBr, cm-1): 3447; 3107; 3043; 1607; 1496; 1461. NMR (1H, DMSO-d6): δ 7.305 (s, 2H); 12.781 (s, 2H). HRMS: calcd for C7H4Cl2N2S (M+) m/z: 217.9425, Found: 217.9482.

1. custom, Organic Synthesis
2. temperaturewas heated
3. temperatureunder reflux for 3 h
4. customwas removed by filtration
5. temperatureThe filtrate was heated to 60°-70° C.
6. temperaturewarm water (3 mL)
7. additionwas added
8. additionacetic acid (0.25 mL) in water (0.5 mL) was added
9. customto get two crystals (brown and white)
10. customThe brown crystals were removed
11. washby washing with chloroform (5 mL)
12. customRecrystallization from hot EtOH/H2O