Kinetics of the desulfonation of benzenesulfonic acid and the toluenesulfonic acids in aqueous sulfuric acid

The kinetics of the desulfonation of benzenesulfonic and the isomeric toluenesulfonic acids have been studied at 120-155° in 65-85 wt-% sulfuric acid with the aid of an inert gas (nitrogen) stripping technique. All reactions were found to be of the first order with respect to the initial sulfonic acid. At high temperatures and/or high sulfuric acid concentrations substantial isomerisation of the toluenesulfonic acids took place, especially at low nitrogen flow rates. The rate-constants of the desulfonation step proper - i.e. the rate-constants corrected for resulfonation - increase with increasing acid concentration and temperature. The dependence of the rate of desulfonation on acidity is compatible with the sulfonation mechanism recently proposed by Kort and Cerfontain40, if the sulfonate anion is supposed to be the active substrate species in the desulfonation reaction. Partial rate-factors for protodesulfonation of the toluenesulfonic acids and the tert-butylbenzenesulfonic acids have been calculated. For a given alkyl substituent fo > fp >> fm. The protodesulfonation obeys the Selectivity Relationship. It is, therefore, suggested that the protodesulfonation of arenesulfonic acids in aqueous sulfuric acid proceeds via a Wheland-type intermediate.