An Improved Large-Scale Preparation of Benzimidazole-2-Sulfonic Acids and 2-Chlorobenzimidazoles.‡

‡ Dedicated to Henry Koppel.

Abstract

An improved method for the preparation of benzimidazole-2-sulfonic acids from 2-mercaptobenzimidazoles has been developed which utilizes aqueous sodium percarbonate. 2-Chlorobenzimidazoles were obtained in high yield from the corresponding sulfonic acids upon reaction with PCl₅ in POCl₃ on a gram-mole scale.

‡ Dedicated to Henry Koppel.

Notes

‡ Dedicated to Henry Koppel.

The acid gas effluent can be scrubbed by continuous water flow as described in ref. 16

It was found that the best conditions for reduction were to use a minimum of 10 mL ethanol and 0.1 g of Raney nickel per gram of 4,5-dichloro-2-nitroaniline. By decreasing the volume of solvent the product precipitates on the catalyst

The reaction times for each compound respectively were 12 hr and 24 hr. It is recommended that potassium ethyl xanthate be prepared in situ and the benzimidazole-2-thiones be reprecipitated before use

The solvent system used was CHCl₃:CH₃OH 10:1 (V:V). Benzimidazol-2-thiones typically have very high R_f Values (>0.8) in this system whereas the R_f values for the benzimidazole-2-sulfonic acids are very low (<0.2)

This is needed to control the initial extremely exothermic reaction that occurs on heating

This is conveniently carried out by dividing the reaction mixture into two or three portions contained in 20 L plastic buckets. The time required for neutralization is reduced to approximately 2 hr