Aromatic sulfonation X: Determination of the kinetic isotope effect of hydrogen in the sulfonation and sulfonylation of benzene remarks on hydrogen exchange between benzene and benzenesulfonic acid†

The kinetic isotope effect of hydrogen for sulfonation with sulfur trioxide has been determined at 25.0° in a competitive study, using hexadeuterated and ordinary benzene, as kH/kD = 1.14 ± 0.06. Hydrogen exchange between benzene and benzenesulfonic acid is relativelys slow under these conditions and does not interfere with the determination of the isotope effect. Diphenylsulfone was found as a by-product in the sulfonation reaction. The kinetic isotope effect for the sulfonylation of benzene at 25.0° was found to be kH/kD = 0.86 ± 0.06. It is proposed that the hydrogen exchange between benzene and anhydrous benzenesulfonic acid proceeds according to a SE-2 type of mechanism.