Aromatic sulphonation VIII: Partial rate factors in the sulphonation of toluene with aqueous sulphuric acid†

The ratio of the overall rate constants for the sulphonation of toluene and benzene in aqueous sulphuric acid at 25° decreases from 110 to 25 on varying the acid concentration from 78 to 92 wt-%. Similar trends are observed at 5° and 45°. The partial rate factors for ortho-, meta- and para-substitution in the sulphonation of toluene with aqueous sulphuric acid all decrease strongly with increasing acid concentration. The partial rate factor for para-substitution is temperature independent, indicating a negligible difference in activation enthalpy between substitution at the para-position in toluene and at a single position in benzene. The corresponding difference in activation entropies is 7 ± 2 cal · deg−1 · mole−1. The latter difference is ascribed to differences in solvation of the starting hydrocarbons or/and of the transition states. These differences in solvation may also be responsible for the small but apparent deviations from the Selectivity Relationship observed in the present study. It is suggested that the sulphonation of toluene and benzene in aqueous sulphuric acid proceeds via a Wheland type of intermediate.