Benzoic Acid → 2-Phenylbenzimidazole-5-Sulfonic Acid

Participants

Reaction Conditions

Reaction Steps

A 2 l four-necked flask is initially charged with 182 ml of distilled water, and 1533 g (15 mol) of 96% sulfuric acid are carefully added. 367 g (3.0 mol) of benzoic acid are introduced into the sulfuric acid, which at this point has a temperature of about 100° C., over a period of about 10 minutes while stirring vigorously (during this the temperature drops to about 80° C.). The mixture is subsequently heated to an internal temperature of 100° C., the benzoic acid dissolving almost completely. 325 g (3.0 mol) of 1,2-phenylenediamine are subsequently metered in at such a rate that the temperature of the mixture does not rise above 125° C. After completion of the addition the mixture is heated to reflux temperature (about 178° C.) and stirred for a further 6 hours at this temperature, during which a small proportion of the benzoic acid sublimes. The mixture is then stirred into 6 l of water at a temperature of 100° C. and then stirred for a further 30 minutes at 100° C. and the precipitate is then filtered off with suction and washed with 2 l of water at a temperature of 80° C. until free of sulfate. The dried crude product (580 g, 70% of theory) is suspended in 2.5 l of water and the pH is adjusted to 8 with 32% sodium hydroxide solution. After addition of 20 g of activated carbon the mixture is stirred for a further 60 minutes at room temperature and subsequently clarified by filtration. The filtrate is heated to 80° C. and the pH is adjusted to 3 with concentrated sulfuric acid. The precipitated product is filtered off with suction while hot, washed with 4 l of water at a temperature of 90° C. and dried to constant weight in a vacuum drying cabinet at 75° C. and 20 mbar. 493 g (1.8 mol, 60% of theory) of 2-phenylbenzimidazole-5-sulfonic acid are thus obtained.

Provenance & Evidence