Gas phase fluorination of benzene, fluorobenzene, m-difluorobenzene, and trifluoromethylbenzene by reactions of thermal fluorine-18 atoms

Hot Atom Chemistry of 18 F in Liquid Fluorobenzenes

“…It is considered unlikely that an excited complex as suggested for hot replacement of a halogen substituent in the aromatic ring [7] is the precursor of this aliphatic F-replacement. Since thermal 18 Fatoms have been shown [10] unable to do this, a hot onestep process seems the most obvious explanation in this case. We further notice that the replacement yield for aromatic bound F-atoms (2 to 3%) is comparable with the yield for replacement of aliphatic F-atoms.…”

“…Fluorine replacement CRAMER and ROWLAND [10] have shown that 18 F-for-F-replacement in highly moderated gas systems (95% SF 6 ) accounts only for < 0.3% in C 6 H 5 F and for 0.52% in m-C 6 H 4 F 2 . This means that thermal 18 F-for-F-replacement is a very minor process if possible at all.…”

“…Furthermore, gasphase thermal 18 F-for-H-replacement yields are available for comparison [10]. As is the case for 18 F-for-F-replacement, the 18 F-for-Hreplacement yield per atom is not very different for the liquid benzenes investigated and amounts to 1.5 to 2.5% per atom (Table 1).…”

“…In gaseous C 6 H 5 F or C 6 H S CF 3 thermalized 18 F-atoms do replace hydrogen atoms, but not fluorine or the CF 3 group as a whole [10]. Replacement of H and F by non-thermalized 38 CI in gaseous C 6 H S F has been explained by a hot "single" substitution mechanism [11].…”

“…Gasphase atomic beam experiments at low pressures [12,13] with thermal non-radioactive F-atoms suggest a high reactivity in aromatic systems. Also CRAMER and ROW-LAND [10] report that the thermal reactivity of 18 F-atoms for benzene is very high to the extent that almost every collision leads to reaction. These results strongly suggest a high reaction probability for hot 18 F-atoms with aromatic compounds in both the gas and the liquid phase.…”