Linear free energy relationships for electrophilic substitutions at α- and β-positions of thiophen

Abstract

Isomer distributions have been determined for several electrophilic substitutions of thiophen: bromination by Br₂ and Br⁺, chlorination, tin tetrachloride, or iodine-catalysed acetylation by acetic anhydride, trifluoroacetylation, reaction with acetyl trifluoroacetate, and Vilsmeier formylation. The α : β ratios vary from 100 to over 1000, according to the ‘selectivity’ of the electrophilic agent. The results obtained together with other data from the literature permit us to test the applicability of linear free energy treatments to electrophilic substitutions at the α- and β-positions of thiophen. Plots of log α_f and log β_f against ρ for nine reactions are linear; from the slopes values of σ_α⁺=–0·79 and σ_β⁺=–0·52 are obtained. The linearity obtained argues for a similarity in the character of the transition states for substitution at thiophen and benzene rings.