Reactions of Recoil Fluorine in Liquid Halobenzenes

Abstract

Experimental investigations of halogen and hydrogen replacements in liquid halobenzenes by nucleogenic fluorine atoms from the le F(n,2n) 18 F process are reported. Liquid binary mixtures of hexafluorobenzene with each of the four monohalobenzenes were irradiated with a 14 MeV neutron generator. The hexafluorobenzene served both as the source of 18 F and as one of the participants in the intermolecular competition reactions. Iodine and bromine were used as scavengers. The products of le F-for-halogen and 18 F-for-hydrogen replacements were identified by gas chromatography. The results are compared with the earlier data on the reactions of hot chlorine, bromine and iodine with aromatic compounds. The 18 F seems to obey the same rules which were found earlier to govern the replacement reactions of the other nucleogenic halogen atoms. The results are discussed in terms of a mechanism which involves a highly inelastic atom-molecule collision and formation of an excited intermediate complex. ZusammenfassungEs wird berichtet über die Verdrängung von Halogen-und WasserstofFatomen durch Radiofluoratome der Reaktion le F(n, 2n) 18 F in flüssigen Halobenzenverbindungen. Binäre Mischungen von Hexafluorobenzen mit jedem der vier Monohalobenzene wurden mit einem 14 MeV Neutronengenerator bestrahlt. Das Hexafluorobenzen bildete sowohl das Ausgangsmaterial für die Bildung des 18 F als einer der Teilnehmer bei den konkurrierenden Reaktionen. Jod und Brom wurden als Radikalfänger verwendet. Die Produkte, worin 18 F an die Stelle eines Waeserstofifatoms oder eines Halogenatoms getreten war, wurden mittels Gaschromatographie nachgewiesen. Die Resultate werden verglichen mit früheren Daten für die Reaktionen von heißem Chlor, Brom und Jod mit aromatischen Verbindungen. Das 18 F scheint denselben Regeln zu folgen, die früher gefunden wurden für die Verdrängungsreaktionen von anderen nukleogenen Halogenatomen. Die Resultate werden besprochen an Hand eines Mechanismus, wobei stark inelastische Atom-Molekülzusammenstöße und die Bildung eines energiereichen intermediären Komplexes eine Rolle spielen.

Hot Atom Chemistry of 18 F in Liquid Fluorobenzenes

“…On the basis of the above considerations we conclude to a hot process for the 18 Ffor-F-replacement in the compounds studied. This conclusion is in agreement with that of others for similar systems [7,11,16]. F-atom of 1.6% to 2.6% [17].…”

“…On the average it does not effect the F-replacement yield. The observed increase in the Η-replacement yield for C 6 H S F (as already mentioned by BEREI and VASÁROS [7] and for m-C 6 H 4 F 2 will be given special attention. Fig.…”

“…F-atom of 1.6% to 2.6% [17]. 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.…”

“…Earlier work on the hot atom chemistry of 18 F with liquid fluorobenzenes has mainly been limited to the determination of the total organic yield [3][4][5][6]. Results given by BEREI and VASÁROS [7] and VAN DÜLMEN [8,9] for liquid C 6 H S F and C 6 F 6 are:…”

“…Yields for hot halogen replacement by recoiling 18 F, 38 CI, 82 Br and 128 1 atoms and for hot hydrogen replacement by 18 F have been discussed in terms of the formation of a highly excited intermediate complex [7]. Gasphase atomic beam experiments at low pressures [12,13] with thermal non-radioactive F-atoms suggest a high reactivity in aromatic systems.…”