Substituent Effects in Hot Replacement by Recoil Halogens

Abstract

Recoil ^'Cl/Hot atom chemistry IHot repiacement/Benzene derivatives SummaiyThe effect of the second substituent on the repiacement of Cl, F, NO, and OH in benzene derivatives by recoil "O was studied. The results show that the chemical nature as well as the relative Position of the second substituent in the benzene ring are of significance, except in the case of "Cl-for-F repiacement.A clear distinction between the different factors Controlling the energetic reactions of recoil halogens in Condensed systems cannot always be achieved. One of the drawbacks of comparing the absolute yields measured in individual systems is the consideration that different moderator properties of the given medium may result in different energy spectra of the reacting atoms [1]. This influence of the medium can be excluded by investigating intramolecular competition reactions, i. e. by comparing the yields of different reactions using one and the same molecule. Disubstituted benzene derivatives offer such a possibility to establish the main factors Controlling the hot repiacement of different atoms and groups in the aromatic ring. This idea was first used by WHITE and ROWLAND [2] to study the repiacement of various atoms and groups by recoil tritium in solid substituted benzoic acids. Though the results showed an increasing repiacement yield with decreasing bond energy of the replaced atom or group, the relative position of the substituents in the aromatic ring was not found to play any significant role. Later, SATO [3] showed that in liquid chlorotoluenes tritium replaces chlorine in a higher extent than the methyl group, conesponding to the lower bond energy of the former. have carried out extensive studies on T, "F and ^'Cl reacting with liquid difluorobenzenes, dichlorobenzenes and chlorotoluenes. They found a preference for Cl repiacement compared with that of CHa also in the case of recoU ^'Cl [6].The experiments mentioned above indicated that the relative position of the substituents in the aromatic ring had no significant effect on the reactions of T [2,3,5], "F [4] or on the ^'Cl-for-CHs repiacement [6]. A measurable difference was, however, found in the case of hot ^'a-for-Cl repiacement in dichlorobenzenes [7], Higher yields from ortho and para compared with meta isomers are well in line with preliminary results [8].In the present work we used liquid disubstituted benzene derivatives to study the repiacement of F and Cl atoms as well as NOz and OH groups by recoil ^'Cl. Our aim was to establish the role of the chemical nature of the second substituent and of its relative position in the aromatic molecule during the course of these reactions.The possibility of the secondary geminate recombinations between the thermalized recoil atom and the radical produced at its first encounter with the medium molecule is another factor greatly complicating the investigation of the liquid systems [9 -11 ]. It was, however, shown in our earlier studies [9,12,13] that the role of such recombinations in aromatic hydrocarbons can effectively...

Substituent Effects in Hot Replacement by Recoil Halogens III. Bromine Replacement by Recoil and Photolytic Chlorine Atoms

“…[1] some conclusions could be drawn with regard to the energy range and possible mechanisms of the high energy replacements studied. Thus we assumed the formation of a short-lived excited intermediate between the recoiling 38 C1, 125 I and 211 At -reacting in an energy range not higher than a few eV -and the aromatic molecule, a finding well in line with some earlier studies [6][7][8].…”

Scavenger Effects on Recoil-Tritium Reactions with 5,10,15,20-Tetraphenylporphyrin

“…Thus, the yields of labile tritium increase from 19% in CuTPP to 29% in free H 2 TPP, as free H 2 TPP contains the scavenger in a molecule. A similar function was observed in recoil halogen reactions [10] and was termed the self-scavenger (internal scavenger), where the OH group of phenol acts as a scavenger for the recoil 38 CI atom in the aromatic compounds and CC1 4 system.…”

Recoil 38CI Reactions with Halobenzenes: Substituent Effects

“…[8] who have observed almost the same *C1-for -CI substitution yields 55%) for C 6 H S C1 and C 6 F 5 C1 and also by those of BEREI etal. [9] for C 6 H 5 C1 and o-C 6 H 4 FCl 25%) systems (the dose received by the samples in the former case was 7.5 kGy compared with 0.1 kGy in the latter).…”

“…BEREI etal. [9], in their study using the intramolecular competition of recoil 38 CI atoms for heavy atoms or group replacement in substituted chlorobenzenes, clearly showed that the substitution of CI, NO2 or OH is strongly dependent upon the nature and the relative position of the second substituent, whereas the yields of F-for -CI and CI-for -F substitutions are independent. Since further investigations in this direction help in understanding the effect of the nature and position of the second substituent, the present study deals with the reactions of recoil 38 C1 atoms with different substituted halobenzenes.…”