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

Abstract

Abstract 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, f p/ f m, with increasing sulphuric acid concentration results mainly from an increase in the activation enthalpy difference between para ‐ and meta ‐substitution. This decrease in f p/ f m, 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.