Acid-catalysed aromatisation of benzene cis-1,2-dihydrodiols: a carbocation transition state poorly stablised by resonance

Abstract

Acid-catalysed dehydration of 3-substituted benzene cis-1,2-dihydrodiols exhibits a Hammett plot with ρ=–8.2, consistent with reaction via a benzenonium ion-like intermediate; however, correlation of +M resonance substituents such as Me and MeO by σ_p rather than σ⁺ constants indicates a marked imbalance between resonance and inductive stabilisation of the transition state.