The sulfonation of 2,4-dichlorobenzoic acid (I) with chlorosulfonic acid (II) at 140 C gives 5-chlorosulfonyl-2,4-dichlorobenzoic acid (III), which by reaction with liquid NH3 is converted into 5-sulfamoyl-2,4-dichlorobenzoic acid (IV). Finally, this compound is treated with NaOH.