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

Abstract

EC-produced 21 'At /Hot reactions / Aromatic halogenated compounds SummaryHalogen and hydrogen replacement reactions in gaseous, liquid, and solid benzene and halobenzenes by astatine atoms produced via the 211 Rn(EC) 211 At nuclear process were investigated. The halogen replacement increases in the order C 6 H 5 F < C 6 H 5 C1 < C 6 H 5 Br < C 6 H 5 1, while the hydrogen replacement decreases in the same order. The halogen substitution yields were found to be linearly dependent on the bond strength of the halogen to be replaced in both liquid and gaseous systems. The isomer distribution of hydrogen replacement-products is nearly statistical in all the halobenzenes, independent of the phase. In liquid and solid aniline the hydrogen substitution yields were found to be in the same range. The results lead to the conclusion that the replacement reactions of EC-produced 2 " At in the systems studied are hot homo lytic processes, similar to those of hot halogens formed via (n, 7) and (n, 2n) nuclear reactions.Astatine, the heaviest member of the halogen family, was initially characterized as a halogen with rather metallic features [1,2]. Therefore, organic compounds of polyvalent astatine were synthesized and investigated first [3]. Later, however, chemical reactions of monovalent astatine with the formation of covalent C-At bonds were also successfully studied in both aliphatic [4-7] and aromatic [8-10] systems. The recently developed methods for the production and reliable chromatographic identification of organic astatine compounds [7 -12] also made it possible to investigate the chemical reactions of energetic astatine atoms formed in nuclear transformations.The first experiments of this kind were preliminary studies of the chemical consequences of the 211 Rn(EC) 211 At process in benzene [13] and some aliphatic hydrocarbons [14], as well as the preparation of astatobenzene using the recoil astatine formed in the 209 Bi(a, 2n) 211 At nuclear reaction with triphenylbismuth as a target [15]. More systematic investigations of EC-induced 211 At reactions with aromatic compounds were begun recently in these laboratories [16,17].The aim of the present work was to extend the investigations of high energy halogenation to the heaviest member of the halogen series. 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. Furthermore these studies would supply new information about the reactivity of astatine towards organic substrates, a subject which has not been sufficiently explored.In the case of the EC-decay...

Replacement Reactions of Recoil Halogens with Benzene Derivatives

“…Somewhat unexpectedly this has been found to be true not only for halogens forming in (n, 2n) and (η, γ) nuclear processes with high initial kinetic energy [25,26] but also for those originating via EC with relatively low kinetic and high ionization and excitation energy [27,36,37], This indicates fast neutralization of originally ionized recoil species [36,37,39] followed by their reactions as neutral atoms that still possess definite excess energy.…”

“…[27]) and also by others [28][29][30][31][32][33][34][35][36][37][38], the isomer distribution is not exactly statistical. The deviation shows a definite trend (see Fig.…”

“…2. Based on the bond strength dependence we have assumed [26,27] that the replacement reactions of hot halogens with benzene derivatives in liquid systems occur in the energy range comparable to the bond energies (few electronvolts) via an identical reaction mechanism which involves the formation of a short lived excited intermediate. This assumption is well in line with the observed (however low) selectivity of the halogen-for-hydrogen replacement in the same systems.…”

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

“…It can be assumed that in consequence of the Rn(EC)^'' At nuclear process multicharged astatine ions are produced [3]. Düring the interaction with the molecules of the medium which have a different ionization potential a partial or complete neutralization of these ions can occur.…”

Radiochemical Research with the Linear Electron Accelerator MEA

“…Photon beams at maximum electron energy are a means to produce radionuclides such as 38 S and 211 Rn by the reaction 40 Ar (γ, 2n) 38 S [16] respectively by the spallation of thorium [17]. Both nuclides provide recoil particles in a radiation-less environment by radioactive decay: " S "283h 38C1 (ίι/2 = 37-2m)res P ectivel y ÌllRn lSh 2U At(f 1/2 =7.22h) and are of interest for bond rupture studies [18,19] and excitation chemistry.…”