Aromatic sulphonation IV Kinetics and mechanism of the sulphonation of benzene in aqueous sulphuric acid

The kinetics of the sulphonation of benzene have been measured at 5.0°, 25.0° and 45.0° in aqueous sulphuric acid solutions, varying from 77.6 to 99.2 wt‐% H 2 SO 4 In addition, sulphonation rate measurements have been made at 5.0° in 98.8 wt‐% H 2 SO 4 containing various concentrations of sodium bisulphate. All reactions were found to be first order with respect to benzene. The kinetics are discussed in terms of the mechanism: magnified image It is suggested that sulphur trioxide or, though less probable, the sulphuric acid solvate of sulphur trioxide SO 3 (H 2 SO 4 ) or disulphuric acid (H 2 S 2 O 7 ), is the sulphonating entity X and that k 1 < k 2 |B|+ k 3 No kinetic distinction can be made between the deprotonation by reaction [2] and the less probable intramolecular proton shift (reaction [3]). A comparison between the mechanisms of sulphonation in aqueous and fuming sulphuric acid furnishes additional evidence that SO 3 , instead of SO 3 (H 2 SO 4 ) or H 2 S 2 O 7 , should be regarded as the sulphonating entity.