Aromatic sulphonation VII: Orientation in the homogeneous sulphonation of toluene in aqueous and fuming sulphuric acid

The isomer distribution in the homogeneous monosulphonation of toluene in 77.6-98.8 wt- % aqueous sulphuric acid has been determined at various temperatures between 5 and 65°. Sulphone formation and disulphonation are negligible. The isomer distribution is independent of toluene conversion. The degree of meta-substitution is low under all conditions. At 25.0° the ratio of o- to p-toluene-sulphonic acid changes from 0.276 to 1.08 on varying the concentration of sulphuric acid from 77.6 to 98.8 wt-%; the meta-isomer shows only a small increase over this acid range. The decrease in the partial rate factor ratio, fp/fm, with increasing sulphuric acid concentration results mainly from an increase in the activation enthalpy difference between para- and meta-substitution. This decrease in fp/fm, is taken as an indication of increasing reactivity of the sulphonating entity. The entropy of activation is greater for para-substitution than for ortho-substitution. On varying the sulphuric acid concentration from 77.6 to 95.8 wt-%, the entropy difference decreases from 8.8 ± 0.2 to 4.8 ± 0.7 cal · degree-1 · mole-1 probably because the solvation of the transition state leading to ortho-substitution decreases with increasing acid concentration. The difference in activation enthalpy between para- and ortho-substitution is 1.3 ± 0.2 kcal · mole-1, whatever the acid concentration. This enthalpy difference is explained by an intramolecular hyperconjugative proximity effect in the transition state leading to ortho-substitution.