Reactivity of Polycyclic Aromatic Aryl Radicals

Article abstract of DOI:10.1021/ja00186a041

Results of experimental and theoretical studies of the properties and reactions of polycyclic aromatic aryl radicals are reported.Reactions of phenyl, 1- and 2-naphthalenyl, and 9-anthracenyl radicals with toluene and naphthalene were examined in the gas phase at 400 and 450 deg C.Arylation rates for each radical were measured relative to hydrogen abstraction from toluene (k_ar/k_abs).For reactions with toluene of both phenyl and 2-naphthalenyl radicals, this ratio was 0.20-0.25.For the 1-naphthalenyl and 9-anthracenyl radicals, these ratios were significantlylower (0.05 and 0.01, respectively).Relative rates for arylating the different available positions in toluene and naphthalene, however, were not nearly as different.Differences in arylation/abstraction rates of the different radicals are explained in terms of differing degrees of reversibility for the initial addition step.Results are consistent with literature dissociation rate constants measured by Ladaki and Szwarc for aryl bromides.MNDO calculations on a range of arene-aryl radical pairs suggest that these differences originate primarily from differences in radical stabilities.Calculations also suggest that, on the basis of bond strenghts, aryl radicals can be roughly divided into three groups, which depend on the nature of the two neighboring aromatic carbon atoms and are independent of the size of the aromatic cluster.