Aromatic reactivity. Part L. Thiophen, benzo[b]thiophen, anisole, and thioanisole in detritiation. Substituent constants for use in electrophilic aromatic substitution

Abstract

Rates of detritiation in trifluoroacetic acid at various temperatures have been measured for [2-³H]- and [3-³H]thiophen, [2-³H]- and [3-³H]-benzo[b]thiophen, [p-³H]anisole, and [p-³H]thioanisole. The approximate rates, f, relative to that for [³H]benzene at 70 °C, are 2·4 × 10⁷, 1·9 × 10⁴, 2·3 × 10⁵, 3·1 × 10⁵, 1·8 × 10⁵, and 7·0 × 10⁴, respectively, and aspects of these results are discussed. Hydrogen-bonding interaction between the methoxy-group and the trifluoroacetic acid results in about a 15-fold lowering of the rate constant for [p-³H]-anisole below the value which would be expected in the absence of such interaction.

In a plot of log f against σ⁺ for the compounds studied, tritiated monosubstituted benzenes, and tritiated poly-nuclear aromatic hydrocarbons, the points mostly lie acceptably on a single straight line, but with a fairly large scatter. A set of σ⁺-constants, σ⁺_Ar, which may be generally applicable to electrophilic aromatic substitution, has been derived from detritiation data for a large range of aromatic systems.