Aromatic reactivity. Part XLVIII. Hydrogen exchange in the methyl groups of 2-methyl-, 3-methyl-, and 2,3-dimethyl-benzo[b]thiophen

Abstract

2- and 3-[³H₁]Methylbenzo[b]thiophen and [2-Me-³H₁]-2,3-dimethylbenzo[b]thiophen undergo detritiation in anhydrous trifluoroacetic acid, the first-order rate constants at 70° being 14·1 × 10^–7, 2·6 × 10^–7, and 1·1 × 10^–7 s^–1, respectively. The rate for the least reactive compound is comparable to that for ring detritiation of [³H]benzene. The results are interpreted in terms of a mechanism involving reversible attachment of a proton to the ring carbon atom ortho to the [³H]methyl group, followed by loss of a proton from the latter to give an olefinic species.