Aromatic sulphonation VI: A kinetic study on the sulphonation and dealkylation oft‐butylbenzene in concentrated aqueous sulphuric acid

Abstract t ‐Butylbenzene has been sulphonated homogeneously in aqueous sulphuric acid solutions varying from 72.4 to 91.0 wt‐% at 5.0°, 25.0° and 35.0°. Under these conditions dealkylation of t ‐butylbenzene occurs. Pseudo first order rate constants for the sulphonation and the dealkylation reaction have been determined. The overall rate constant for sulphonation of t ‐butylbenzene is smaller than that of toluene. It is suggested that this difference arises predominantly from increased steric hindrance for ortho substitution in the former case. The overall rate constant for dealkylation increases rapidly with increasing acid concentration: at 25° d(log k exp )/d (.‐ H R ) = 1.08, whereas d(log k exp )/d (‐ H 0 ) ∼ 2.0. The apparent activation energy for the dealkylation reaction decreases from 34.0 kcal · mole −1 in 77.5 wt‐% sulphuric acid to 19.5 kcal · mole −1 in 85.8 wt‐% sulphuric acid. Possible mechanisms for the dealkylation reaction are proposed and it is suggested that the C 1 ring protonation of t ‐butylbenzene is governed by the H R acidity function.