INGOLDS NITRATION MECHANISM LIVES

The proposal that electrophilic aromatic nitration by nitronium ion takes place via an initial electron transfer, followed by recombination of an intermediate radical pair, is critically examined. In particular, recent experimental results from an alleged model reaction, photoexcitation of ArH-tetranitromethane charge transfer complexes to give a triad of ArH.+ trinitromethanide ion and NO2, are scrutinized, showing that the propensity fdr adduct formation from the triad invalidates any comparison with the ArH-NO2+ reaction. By conducting the photolysis in the presence of a protic acid, trinitromethanide ion can be removed from the triad by protonation, thus promoting the ArH.+-NO2 pathway. The isomer ratios of the nitro-substitution products obtained from the latter are different from those of the ArH-NO2+ reaction. Thus the radical pair cannot be an intermediate in electrophilic aromatic substitution, as concluded previously in 1980 from studies of the reaction between electrocrystallized radical cation salts and nitrogen dioxide.