Photochemical Interchange of Halogens in Aromatic Compounds. I

Ipso substitution in free-radical aromatic substitution reactions

“…l6 On the other hand, in the gas phase, phenyl radicals generated by pyrolysis of nitrobenzene (600 'C) do displace CI, Br, and I but not F in reaction with halobenzenes,17 and those generated by pyrolysis of azobenzene (500 "C) appear to replace CI in chlorobenzene to a small extentla Relative to the amounts of halobiphenyl produced in the pyrolysis reactions, the amounts of biphenyl produced (by displacement of halo, not by dimerization of Ph-) are approximately statistical with chloro and bromo, larger with i0d0.l~ As in the halogen exchange repentafluoroaniline and pentyl nitrite) replaced fluoro in both chloro-and bromopentafluorobenzene, but that strongly electrophilic radical failed to substitute into hexafluorobenzene. 22 Earlier attempts to study reactions of pentafluorophenyl radicals by decomposition of pentafluorobenzoyl peroxide in chloro-and bromobenzene produced, not the expected biphenyls, but phenyl pentafluorobenzoate (eq The initially formed pentafluoheat C&CI -k (c&c0)202 + C&OCOC6F5 (10) robenzoyloxyl radical, before losing C02 as usual, reacted by ipso substitution. By contrast, very little substitution product was obtained from pentafluorobenzoyl peroxide and hexafluorobenzene, although a high-boiling residue indicated that some addition of radicals to the hexafluorobenzene and subsequent reactions did occur.23 Phenyl radicals are essentially electroneutral, pentafluorophenyl radicals are strongly electrophilic,21 and alkyl radicals are nucleophilic.…”

Selectivity in Thermal 34mCl-for-CI Exchange with Chlorobenzenes

“…The exchange between atoms (produced by photolysis of CP^Cl) and the Cl atom of 0-CIC6H4COOH is a fast reaction that proceeds through the fomiation of a 7r-complex [18]. We proposed that thermal '"""-''Cl-for-a substitution in CeHsCl proceeds via the formation of an ipso a-complex [4,12], which is a well established intermediate in homolytic substitution reactions [19,20].…”

The Effect of High Radiation Doses on the Reactions of Recoil CI Atoms with Arenes

“…We have choosen C 6 H s N0 2 as a target compound, because reported 38 Cl-for-N0 2 substitution yields (about 13% in unscavenged systems, Table 3) are rather high when compared to substitution yields of other groups such as NH 2 , CH 3 , CF 3 (less than 2% [8,9,10], Thermal ipso substitution in photolytical experiments was observed for C 6 H s N0 2 , but the rate constants are very low: 60% C 6 H S C1 is formed by the irradiation of a C 6 H 5 N0 2 /Cl 2 solution for 16 days at room temperature [16] and 50% p-C 6 H 4 Cl 2 is formed by irradiation of p-C 6 H 4 ClN0 2 in refluxing CC1 4 for 3 days [17]. Slow exchange between Li 3 6 CI and nitroderivatives of chlorobenzenes was also found at elevated temperatures [18], However, no enhanced chlorodenitration is observed with 38 CI at high dose ( Fig.…”