Aromatic substitution. Part XIII. The polarity of aryl radicals. Further reactions of pyridine with substituted phenyl radicals

Relative rates of arylation of pyridine with o-methoxyphenyl and o-bromophenyl radicals produced by the Gomberg–Hey reaction have been measured by means of competitive experiments. The ratios of isomers, and total and partial rate factors, are compared with those obtained with the corresponding para-substituted aryl radicals and the unsubstituted phenyl radical. The results are in complete accord with the concept of polarised aryl radicals, the methoxyphenyl radicals being more nucleophilic and the bromophenyl radicals more electrophilic than phenyl itself. o-Methoxyphenyl radicals gave less α-isomer than expected, in agreement with the “ortho-effect” previously proposed. The results with the o-bromophenyl radical were not as clear-cut.