Indole → 3-Chloro-1H-indole

To a 3 L round bottom flask equipped with a mechanical stirrer, thermometer and a Gooch tube was added sieve-dried dimethylformamide (1.2 L) and indole (200 g). The solution was stirred under an atmosphere of nitrogen and cooled to about 10° C. N-Chlorosuccinamide (216.6 g) was added via the Gooch tube at a rate such as to maintain a reaction temperature of 10° -18° C. After the addition was complete, a second charge of N-chlorosuccinamide (34.2 g) was added. When the reaction was found to be >97% complete by high performance liquid chromatography, the reaction mixture was poured into a well-stirred, cold solution of aqueous sodium bisulfite (5.2 L of 0.1% sodium bisulfite in water and 640 g of ice). After about 15 min, the precipitate was collected and washed with water (2×1 L). The filtrate was partitioned between dichloromethane (1.3 L) and 0.1% aqueous sodium bisulfite solution (1 L). The phases were separated and the aqueous solution was extracted with dichloromethane (300 ml). The organic fractions were combined, washed with water (800 mL), dried over anhydrous magnesium sulfate, filtered and the filtrate was concentrated (at about 25° C. and about 50 mm Hg). The residue was dried in a vacuum oven at 25° C. and about 125 mm Hg for 2 to 4 hrs to yield 240.3 g (92.9%) of product, 3-chloroindole (91.6% pure).