Aromatic sulfonation XI: Sulfonation of benzene and toluene with sulfur trioxide; sulfone formation and sulfonic acid isomer distribution.

Abstract

AbstractThe sulfonation of benzene and toluene with sulfur trioxide without solvent has been studied extensively.With benzene at low conversion benzenesulfonic acid and diphenylsulfone are the only products. At 25° the sulfone: sulfonic acid ratio changes from 0.2 to 0.6 on varying the SO3 : C6H6 ratio from 0.13 to 1.3. At a SO3 : C6H6 ratio > 1.3 benzene‐1,3‐disulfonic acid and diphenylsulfonesulfonic acid are found among the reaction products as well. With toluene the sulfone formation is less than with benzene.The isomer distribution of the monosulfonic acids formed in the sulfonation of toluene has been determined at various temperatures between −44 and 65°. Under all conditions the degree of meta‐substitution is low (1‐5%) whereas that of para‐substitution is high (∼ 80%). The difference in activation enthalpy between para‐ and meta‐substitution is −2.7 ± 0.4 kcal · mole −1, that between para‐ and ortho‐substitution 1.3 kcal · mole −1. The latter enthalpy difference is explainable by an intramolecular hyperconjugative proximity effect in the transition state leading to ortho‐substitution. The entropy of activation is greater for para‐ than for ortho‐substitution, indicating steric hindrance for ortho‐substitution.The formation of sulfonic acid and sulfone is discussed in terms of the mechanism:magnified image