Desulfonation of aromatic sulfonic acids in acidic media: the kinetic isotope effect

The acid-catalyzed substitution of the sulfo group in mesitylene- and p-toluene-sulfonic acids by hydrogen (deuterium), which takes place in sulfuric, deutero-sulfuric, and phosphoric acids at various temperatures and acidities, was investigated. It was found that the solvent-reagent (D/sub 2/O-D/sub 2/SO/sub 4/) has a primary kinetic isotope effect during the desulfurization of mesitylenesulfonic acid (k/sub H//k/sub D/ = 2.8 + 0.3), and this indicate that the reaction rate is controlled by proton (deuteron) transfer. The high value of the Broensted coefficient (..cap alpha.. = 0.88 +/- 0.04) indicates that the structure of the transition state is close to a sigma complex.