Relative reactivity studies for olefin sulphonation with sulphur trioxide in Dichloromethane: evidence for concerted [2s+ 2s] cycloaddition

Relative rate studies are described for the sulphonation by sulphur trioxide in dichloromethane at –50 °C of nine alkenes bearing one, two, or three alkyl substituents on the olefinic carbon atoms. There is remarkably little variation of reactivity with alkene structure, the most reactive alkene studied, 3-methylpent-2-ene, combining with sulphur trioxide only 3.4 times faster than the least reactive, non-1-ene. The findings are interpreted in terms of a concerted [2s+ 2s] cycloaddition mechanism, which is shown by orbital correlation analysis to be allowed thermally.