Aromatic substitution. Part XXII. Polarity of aryl radicals

Abstract

The rates of substitution of anisole relative to benzene by aryl radicals generated from diazonium tetrafluoroborates and pyridine in homogeneous solution have been measured and follow, qualitatively, the order expected on the basis of the theory of polarised aryl radicals. The substituent in the radical affects its permanent polarisation. Apparent contradictions in the literature concerning the behaviour of anisole as a substrate and the polarity of the p-methoxyphenyl radical are resolved by the use of this source of free radicals. p-Methoxyphenyl is more nucleophilic than phenyl as expected. m-Methoxyphenyl is no more electrophilic than phenyl and an explanation is suggested. p-Tolyl is more nucleophilic than expected on the basis of σ_p-Me.