Benzoic Acid → 2-Phenylbenzimidazole-5-Sulfonic Acid

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Reaction Conditions

Reaction Steps

A 1 1 three-necked flask is initially charged with 275 g (2.70 mol) of 96% sulfuric acid and subsequently 54.1 g (0.50 mol) of 1,2-phenylenediamine are carefully introduced. 85.5 g (0.70 mol) of benzoic acid are added to the suspension, which at this point has a temperature of about 120° C., and the mixture is heated while stirring (clear solution from about 130° C.) to about 200° C. for 5 hours, during which a small proportion of the benzoic acid sublimes. The mixture is then allowed to cool to about 150° C. and is stirred into 1 l of cold water, which warms to about 65° C. The mixture is stirred for a further hour at 65° C. and the precipitate is then filtered off with suction and washed with water at a temperature of 80° C. until free of sulfate. The dried crude product (83 g) is suspended in 830 ml of water and the pH is adjusted to 8 with 32% sodium hydroxide solution. After addition of 6.5 g of activated carbon the mixture is stirred for a further 30 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 hydrochloric acid. The precipitated Eusolex 232 is filtered off with suction while warm, washed with 2 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. 67.0 g (245 mmol, 49% of theory) of 2-phenylbenzimidazole-5-sulfonic acid are obtained, 410° C. (decomposition).

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