Electrophilic aromatic substitution. Part 33. Partial rate factors for protiodetritiation of benzo[b]thiophen; the resonance-dependent reactivity of the ring positions

Abstract

Rate coefficients for detriation of benzo[b]thiophen, specifically labelled with tritium at each position, have been measured in anhydrous trifluoroacetic acid, and, for [2-³H]benzo[b]thiophen, in a range of trifluoroacetic acid–acetic acid mixtures, all at 70°. The dependence of rate upon acidity is consistent with the effect of hydrogen bonding between sulphur and trifluoroacetic acid, as found previously for other sulphur-containing heterocycles. Partial rate factors, corrected for the effect of hydrogen bonding are (positions in parentheses) : 8.84 × 10⁵(2); 1.18 × 10⁶(3); 360 (4); 13 200 (5); 16 200 (6); and 26.2 (7); the corresponding σ⁺ values are –0.68, –0.695, –0.29, –0.47, –0.48, and –0.16. The positional reactivity order is the same as found in pyrolysis of 1-arylethyl acetates, and solvolysis of 1-arylethyl chlorides, but the σ⁺ values are more negative due to the greater demand for resonance stabilization of the transition states in hydrogen exchange. Comparison of the 2-: 3-isomer ratios in various substitutions leads to the unexpected conclusion that substitution at the 2-position is subject to steric effects, probably due to the proximity of the sulphur d-orbitals.