Aromatic hydrocarbons reactions with nitrate radical

Chemical/Physical. Under atmospheric conditions, the gas-phase reaction of o-xylene with OH radicals and nitrogen oxides resulted in the formation of o-tolualdehyde, o-methylbenzyl nitrate, nitro-o-xylenes, 2,3-and 3,4-dimethylphenol (Atkinson, 1990). Kanno et al. (1982) studied the aqueous reaction of o-xylene and other aromatic hydrocarbons (benzene, toluene, w and p-xylene, and naphthalene) with hypochlorous acid in the presence of ammonium ion. They reported that the aromatic ring was not chlorinated as expected but was cleaved by chloramine forming cyanogen chloride. The amount of cyanogen chloride formed increased at lower pHs (Kanno et al., 1982). In the gas phase, o-xylene reacted with nitrate radicals in purified air forming the following products 5-nitro-2-methyltoluene and 6-nitro-2-methyltoluene, o-methylbenzaldehyde, and an aryl nitrate (Chiodini et ah, 1993). 

Gasoline hydrocarbons volatilized to the atmosphere quickly undergo photochemical oxidation. The hydrocarbons are oxidized by reaction with molecular oxygen (which attacks the ring structure of aromatics), ozone (which reacts rapidly with alkenes but slowly with aromatics), and hydroxyl and nitrate radicals (which initiate side-chain oxidation reactions) (Stephens 1973). Alkanes, isoalkanes, and cycloalkanes have half-lives on the order of 1-10 days, whereas alkenes, cycloalkenes, and substituted benzenes have half- lives of less than 1 day (EPA 1979a). Photochemical oxidation products include aldehydes, hydroxy compounds, nitro compounds, and peroxyacyl nitrates (Cupitt 1980 EPA 1979a Stephens 1973). 

Nitrations are highly exothermic, ie, ca 126 kj/mol (30 kcal/mol). However, the heat of reaction varies with the hydrocarbon that is nitrated. The mechanism of a nitration depends on the reactants and the operating conditions. The reactions usually are either ionic or free-radical. Ionic nitrations are commonly used for aromatics many heterocycHcs hydroxyl compounds, eg, simple alcohols, glycols, glycerol, and cellulose and amines. Nitration of paraffins, cycloparaffins, and olefins frequentiy involves a free-radical reaction. Aromatic compounds and other hydrocarbons sometimes can be nitrated by free-radical reactions, but generally such reactions are less successful. 

As seen in Table 6.1, the reactions of the nitrate radical with the simple aromatic hydrocarbons are generally too slow to be important in the tropospheric decay of the organic. However, one of the products of the aromatic reactions, the cresols, reacts quite rapidly with NO,. o-Cresol, for example, reacts with N03 with a room temperature rate constant of 1.4 X 10 " cm3 molecule-1 s-1, giving a lifetime for the cresol of only 1 min at 50 ppt N03. This rapid reaction is effectively an overall hydrogen abstraction from the pheno-... 

However, near the Earth s surface, the hydrocarbons, especially olefins and substituted aromatics, are attacked by the free atomic O, and with NO, produce more NO2. Thus, the balance of the reactions shown in the above reactions is upset so that O3 levels build up, particularly when the Sun s intensity is greatest at midday. The reactions with hydrocarbons are very complex and involve the formation of unstable intermediate free radicals that undergo a series of changes. Aldehydes are major products in these reactions. Formaldehyde and acrolein account for 50% and 5%, respectively, of the total aldehyde in urban atmospheres. Peroxyacetyl nitrate (CH3COONO2), often referred to as PAN, and its homologs, also arise in urban air, most likely from the reaction of the peroxyacyl radicals with NO2. 

The kinetics of the reactions of many xenobiotics with hydroxyl and nitrate radicals have been examined under simulated atmospheric conditions and include (1) aliphatic and aromatic hydrocarbons (Tuazon et al. 1986) and substituted monocyclic aromatic compounds (Atkinson et al. 1987c) (2) terpenes (Atkinson et al. 1985a) (3) amines (Atkinson et al. 1987a) (4) heterocyclic compounds (Atkinson et al. 1985b) and (5) chlorinated aromatic hydrocarbons (Kwok et al. 1995). For PCBs (Anderson and Hites 1996), rate constants were highly dependent on the number of chlorine atoms, and calculated atmospheric lifetimes varied from 2 days for 3-chlorobiphenyl to 34 days for 2,2, 3,5, 6-pentachlorbiphenyl. It was estimated that loss by hydroxylation in the atmosphere was a primary process for removal of PCBs from the environment. It was later shown that the products were chlorinated benzoic acids produced by initial reaction with a... 

Three studies on radical cations discuss the characterization of polynuclear aromatic radical cation salts as organic metals (8), the reactions of cation radicals with neutral radicals (9), and the magnetic-electrical properties of perfluoroaromatic radical-cation salts (10). Chapters on polynuclear aromatic compounds in nonvolatile petroleum products (II) and in coal-based materials (12) present reviews of the subject and new findings. The remaining chapters in this book discuss the thermal conversion of polynuclear aromatic compounds to carbon (13), the nitration of pyrene by mixtures of N02 and N204 (14), the spectra, structures, and chromatographic retention times of large polycyclic aromatic hydrocarbons (15), the desulfurization of polynuclear thiophenes correlated with tt electron densities (16) and simple theoretical methods to predict and correlate polynuclear benzenoid aromatic hydrocarbon reactivities (IT). 

G.6.2.3 Aromatic Hydrocarbons Rate constants for the reaction of hydroxyl and nitrate radicals with some aromatic hydrocarbons are compiled in Table 6.23, and it is clear that with a few exceptions, that the hydroxyl radical is the more... 

TABLE 6.23 Rate Constants for the Reaction of Hydroxyl and Nitrate Radicals with Aromatic Hydrocarbons at Ambient Temper atnres ... 

Reductive Remediation of Nonhalogenated Molecules. Na/NHa treatments can also destroy nonhalogenated hazardous conqraunds. Three classes pollutants will be mentioned here polynuclear aromatic hydrocarbons (PNAs), nitro- and nitrate-type explosive wastes and chemical warfare agents. The treatment of neat sanq>les of PNAs leads to destmction efficiencies of 99.99% for many of these conq)ounds including such examples as acenaphthene, benzo[a]anthracene, benzo[b]fluoranthene, benzo[g,h,l]perylene, chrysene, fluorandiene, fluorine, naphdialene and phenanthrene. With the exception of naphthalene and anthracene, conq)lex product mixtures are formed. Radical anion formation followed by protonation occurs sequentially leading to dihydro, tetrahydro and further reduced products (see Scheme 3). Depending on the reaction conditions, dimerization of intermediate radicals can occur to give dimers in various states of reduction. 

Eberson and Radner (19-23) have explored some of the scope of the reaction of N02 with ArH +. They prepared the solid hexafluorophosphates of the naphthalene cation radical and some methylnaphthalene cation radicals, for example, C10H8,+PF6, and carried out reactions of the solid salts with N02 in dichloromethane at temperatures near-25 °C (19-23). Reaction was found to occur according to equation 12. The results of their reactions are summarized in Table I (19). This table also lists the results of nitrating the parent hydrocarbons by the conventional route (N02 + ) and with N204. The results prompted Eberson and Radner to dismiss the possibility that conventional nitration of these compounds was preceded by electron transfer (equations 11 and 12), because the proportions of isomers from the reaction of ArH + with N02 were quite different from those from the reactions of ArH with N02+ and with N204. Eberson and Radner set out to determine whether or not the conventional nitration of naphthalene and methylnaph-thalenes involved the electron-transfer step. In so doing they became, to our knowledge, the first to show that a neutral radical (N02) will react with an aromatic cation radical. 

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