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.