Reactivity of radiolytically-produced nitrogen oxide radicals toward aromatic compounds

The nitration of aromatic compounds in the gas phase is an important source of toxic, carcinogenic, and mutagenic species in the atmosphere and has therefore received much attention. Gas phase nitration typically occurs by free-radical reactions. Condensed-phase free-radical reactions, and in particular nitrite and nitrate radical chemistry, have been studied far less. These condensed-phase free-radical reactions may be relevant in fog and cloud water in polluted areas, in urban aerosols with low pH, in water treatment using advanced oxidation processes such as electron beam (e-beam) irradiation, and in nuclear waste treatment applications. This study discusses research toward an improved understanding of nitration of aromatic compounds in the condensed phase under conditions conducive to free-radical formation. The results are of benefit in several areas of environmental chemistry, in particular nuclear waste treatment applications;The nitration reactions of anisole and toluene as model compounds were investigated in gamma-irradiated acidic nitrate, neutral nitrate, and neutral nitrite solutions. Cs-7SB, 1-(2,2,3,3,-tetrafluoropropoxy)-3-(4-sec-butylphenoxy)-2-propanol, is used as a solvent modifier in the fission product extraction (FPEX) formulation for the extraction of Cs and Sr from dissolved nuclear fuel. The formulation also contains the ligands calix[4]arene-bis-(tert-octylbenzo-crown-6) (BOBCalixC6) for Cs extraction and 4,4',(5')-di-(t-butyldicyclohexano)-18-crown-6 (DtBuCH18C6) for Sr extraction, all in Isopar L, a branched-chain alkane diluent. FPEX solvent has favorable extraction efficiency for Cs and Sr from acidic solution and was investigated at the Idaho National Laboratory (INL) for changes in extraction efficiency after gamma-irradiation. Extraction efficiency decreased after irradiation. The decrease in solvent extraction efficiency was identical for Cs and Sr, even though they are complexed by different ligands. This suggests that radiolysis of the modifier (Cs-7SB), which solvates both metal complexes, is responsible for this change. These reactions presumably occur due to reactions with radiolytically-produced nitrogen-centered radicals like • NO, •NO2 and •NO 3.;Anisole (C6H5-OCH3) was used in this study as a surrogate for Cs-7SB, since both are aryl ethers. Toluene was used as a surrogate for Cs-7SB because of the alkyl group on the benzene ring in both molecules. Anisole, highly reactive in acids, is a small molecule compared to Cs-7SB and the nitration products are easier to identify compared to those for the larger Cs-7SB molecule. Toluene is less reactive than anisole. Therefore, the highly reactive anisole and the less reactive toluene were considered in this study as model compounds to compare the reaction mechanisms and the nitrated products in acidic media under irradiation.;Experiments were designed to elucidate the mechanism of the nitration of aromatic rings in gamma-irradiated aqueous nitric acid. Since a suite of radical and ionic reactive species are produced in this condensed-phase system, solutions of nitric acid, neutral nitrate and neutral nitrite were irradiated in separate experiments to isolate selected reactive species. Product nitration species were assessed by high performance liquid chromatography (HPLC) with a reversed phase C-18 column and photodiode array detector. The nitrated anisole product distributions were the same with and without radiation in acidic solution, although more products were formed with radiation. In the irradiated acidic condensed phase, radiation-enhanced nitrous acid-catalyzed nitrosonium ion electrophilic aromatic substitution followed by oxidation reactions dominated over radical addition reactions. In contrast, the distribution of nitrated derivatives for toluene showed nitronium ion electrophilic substitution in the unirradiated acidic medium as a result of thermal nitration only at elevated temperatures, and free-radical reaction products with irradiation. These ionic/free-radical mechanisms would predominate in acidic solutions such as urban atmospheric aerosols and nuclear fuel dissolutions for similar organic compounds.;Neutral nitrate anisole and toluene solutions were dominated by mixed nitrosonium/nitronium ion electrophilic aromatic substitution reactions, but with lower product yields. Solutions such as these might be encountered in water treatment by e-beam irradiation. Irradiation of neutral nitrite anisole and toluene solutions, on the other hand, resulted in a statistical substitution pattern for nitroanisole and nitrotoluene products, suggesting non-electrophilic free-radical reactions involving the •NO2 radical. Although often proposed as an atmospheric nitrating agent, •NO 2 radical is unlikely to have an important effect in the acidic condensed phase in the presence of more reactive, competing species such as nitrous acid due to its low reaction rate constants with aromatic compounds.;Cs-7SB was also irradiated in nitric acid solution. Its possible nitration reactions are discussed in comparison to the results for anisole and toluene.