Gas phase aromatic substitution by positive bromine and iodine ions from 80mBr(IT)80Br and 125Xe(EC)125I

Abstract

Gas phase aromatic bromination and iodination by positive bromine or iodine species resulting from the nuclear decay of 80mBr(IT)80Br and 125Xe(EC)125I, respectively, have been studied in simple aromatic systems of the type C6H5X (X = F, Cl, Br, CH3). At high Aror Xe-moderator concentrations electrophilic substitution reactions of thermal positive halogen ions can be observed. The positional selectivity of the unsolvated Br+ or 1+ ions is very low by solution chemistry standards; not only hydrogen substitution but also replacement of the substituent X takes place. In the case of Br+ considerable rearrangement occurs in Br-for-H substitution in fluorobenzene, which increases with increasing concentration of Ar and decreasing total pressure. The results can be rationalized on the basis of excitation isomerization, which competes with collisional stabilization and proton transfer after the formation of an excited arenonium ion. For both bromine and iodine ions the ease of X replacement increases relative to that of hydrogen replacement from fluorobenzene to bromobenzene, and a concomitant increase of the relative amount of ortho substitution is observed. The formation of an aryldihalonium precursor is discussed.

Effect of Dilution on Substitution Reactions of EC-Produced 211At with Chiorobenzene

“…In Table 2 the gas phase aromatic astatination results are compared with the corresponding data for the decay induced bromination and iodination of chlorobenzene obtained by STÖCKLIN et aL [11 ]. It can be seen that the H to C1 replacement ratio for the EC-induced astatine is very similar to those obtained for gas phase aromatic replacement by ^"Br'^ and ions.…”

Decay-Induced Bromination via the 76Kr(EC)76Br and 77Kr (β+, EC)77Br Decay

“…The decay-induced brominations via the generator systems 76 Kr(EC) 76 Br and 77 Kr(0 + , EC) 7 7 Br have been examined in gaseous and solid aliphatic hydrocarbons [10]. As a continuation of our previous studies [3,4,6], we felt that aromatic bromination via the decay of 76 ' 77 Kr provides a tool for elucidating the reactivity and selectivity of unsolvated bromine cations in a cleaner manner than that via the isomeric transition of 80m Br using an 80m Br-labelled compound as the source of 80 Br. The first promising results obtained in our laboratory [11] encouraged us to continue these studies.…”

“…[4][5][6] and 211 Rn(EC) 211 At decay [7][8][9] was also used to study aromatic halogenation. The decay-induced brominations via the generator systems 76 Kr(EC) 76 Br and 77 Kr(0 + , EC) 7 7 Br have been examined in gaseous and solid aliphatic hydrocarbons [10].…”

Replacement Reactions of EC-Produced 211At in Benzene and Halobenzenes

“…The study of halogen and hydrogen replacement reactions in benzene and halobenzenes should provide additional data for a better understanding of the substitution mechanisms by energetic halogen atoms [18][19][20][21][22], Among these, the reactions induced by EC-or ß + -decay processes [23][24][25][26][27] are of special interest, because the nucleogenic atoms exhibit relatively small translational energies, compared to those generated via (η, γ) or (n, 2n) nuclear reactions. They may therefore provide valuable information on the energy and charge of the reacting species.…”

“…Thermal astatine ions show a significant intramolecular selectivity in aromatic substitution reactions [17], as do 'positive' halogens in solution (cf. [32]) and in the gas phase [24]. Hot neutral halogens were, however, found to replace hydrogen in monosubstituted benzene derivatives with nearly statistical distribution of ortho-, meta-and para-products [19 -22].…”