Beitrag zur Kenntnis der Sulfonierung von Aromaten mit Schwefeltrioxid

Abstract

The rate of the sulfonation of aromatic compounds with sulfur trioxide in aprotic mediums is initially very fast and proportional to [ArH][SO₃]², but exhibits a marked retardation after a conversion of 60-70% of the aromatic compound, if stoichiometric amounts of sulfur trioxide are used. A new explanation is given for this characteristic run of reaction, justified by experiments specially with 4-nitrotoluene : In concurrence to the desired sulfonation the aromatic sulfonic acid formed is dehydrated by sulfur trioxide to aromatic sulfonic anhydride. As the small amounts of sulfur trioxide remaining in this equilibrium can further react with the unconverted aromatic compound, the equilibrium reaction runs backwards reforming sulfur trioxide, so that the completion of the sulfonation is only slowly reached.