Nitrate radical alkane reactions

The kinetics and mechanisms of nitrate radical reactions with alkanes and a variety of other organics relevant to the atmosphere are discussed in detail in two excellent reviews by Wayne et al. (1991) and Atkinson (1991). The kinetics of the N03-alkane reactions are summarized in Table 6.3, where it can be seen that, with the exception of methane, they are in the range 10 lX-10 lf cm3 molecule-1 s-1. 

While these reactions are much slower than the corresponding OH reactions, the nighttime peak concentrations of NO, under some conditions are much larger than those of OH during the day, 400 ppt vs 0.4 ppt. Even given the differences in concentration, however, as seen from the lifetimes in Table 6.1, the nitrate radical reaction is still relatively slow. While the removal of the alkanes by NO, is thus not expected to be very significant under most tropospheric conditions, reaction (20) can contribute to HNO, formation and the removal of NOx from the atmosphere. 

The major reactions carried out by hydroxyl and nitrate radicals may conveniently be represented for a primary alkane RH or a secondary alkane RjCH. In both, hydrogen abstraction is the initiating reaction. 

Dinitrogen pentoxide reacts with alkanes in carbon tetrachloride at 0 °C via a radical mechanism to give nitration products which can include nitrate esters.Reactions of alkanes with dinitrogen pentoxide in nitric acid are complex and of little synthetic value. 1-Adamantyl nitrate is one of the products obtained from the photochemical irradiation of a solution of adamantane and dinitrogen pentoxide in methylene chloride. ... 

The principal polyolefins are low-density polyethylene (ldpe), high-density polyethylene (hope), linear low-density polyethylene (lldpe), polypropylene (PP), polyisobutylene (PIB), poly-1-butene (PB), copolymers of ethylene and propylene (EP), and proprietary copolymers of ethylene and alpha olefins. Since all these polymers are aliphatic hydrocarbons, the amorphous polymers are soluble in aliphatic hydrocarbon solvents with similar solubility parameters. Like other alkanes, they are resistant to attack by most ionic and most polar chemicals their usual reactions are limited to combustion, chemical oxidation, chlorination, nitration, and free-radical reactions. 

The usual way to achieve heterosubstitution of saturated hydrocarbons is by free-radical reactions. Halogenation, sulfochlorination, and nitration are among the most important transformations. Superacid-catalyzed electrophilic substitutions have also been developed. This clearly indicates that alkanes, once considered to be highly unreactive compounds (paraffins), can be readily functionalized not only in free-radical from but also via electrophilic activation. Electrophilic substitution, in turn, is the major transformation of aromatic hydrocarbons. 

There remains some doubt about the first step of the overall reaction in MeCN the final products are usually the A -alkylacetamides, as shown in Eq. (35). In neutral solution, at extreme anodic potentials, it is difficult to decide between direct [Eqs. (1), (2), and/or (3)] and indirect electron transfer [Eqs. (16), (17), and/or (18)]. For oxidation in MeCN-BF4 solutions the variation in potentials is best explained in terms of the direct mechanism. An indirect oxidation mechanism involving hydrogen abstraction by electrogenerated nitrate radical has recently been proposed for the electrolysis of linear alkanes in tert-BUOH/H2O mixtures containing HNO3 and saturated with O2 [25]. 

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). 

TABLE 6.19 Rate Constants at 25° C for Reactions of Hydroxyl and Nitrate Radicals with Alkanes... 

The rate data for reaction of NO3 with aliphatic esters show that the presence of the ester group in an organic molecule has little influence on the reactivity compared to the parent alkane. The reactivity trends exhibited by the nitrate radical for reactions with alcohols, ethers and esters are similar to those shown for the analogous reactions of hydroxyl radicals. The major products identified from the NO3 radical-initiated oxidation of alcohols, ethers and esters under atmospheric conditions were esters, carbonyls and alkyl nitrates. Similar products arise from the reactions of OH radicals with these molecules under atmospheric conditions. 

A number of esters [10], ethers [11, 12] and alcohols [13] were investigated with respect to reactivity with nitrate radicals. Both absolute and relative rate methods were employed. Rate coefficients for the reaction of NO3 are given in Table 1. The rate coefficients for aliphatic esters may be predicted from available group reactivity factors for alkanes provided that formate carbonyl hydrogen atoms are treated as primary hydrogen atoms. The rate coefficients with temperature dependence for ethers and alcohols are valid between 268 to 363 K. 

Temperature dependence of reactions of the nitrate radical with alkanes, in G. Restelli, G. Angeletti (eds). Fifth European Symp. on Physico-Chemical Behaviour of Atmospheric Pollutants, Kluwer Academic PubL, Dordrecht 1990, pp. 328-333. 

Temperature dependence of reactions of the nitrate radical with alkanes,... 

Thus, the rate-determining step of alkane nitration is the formation of free alkyl radicals in (Equation 5.1), as well as in reaction of alkanes with O, NO3, OH, Cl and other active radicals. The reactions of R with NO, NO, O, N O [19], Br [20] and other components of the reaction system (including solvents) results in a mixture of nitro compounds, nitrites, nitroso compounds, and nitrates. All these compounds with the exception of RNO depending on the conditions undergo further conversions, resulting in the different composition of the end products of nitration. 

Nitration of alkanes can be carried out in the gas phase at 400°C or in the liquid phase. The reaction is not practical for the production of pure products for any alkane except methane. For other alkanes, not only does the reaction produce mixtures of the mono-, di-, and polynitrated alkanes at every combination of positions, but extensive chain cleavage occurs. A free-radical mechanism is involved. ... 

Interestingly, this reaction could be performed with catalytic amounts of silver provided that the nitrate counterion was present. The latter could be obtained from silver nitrate or by addition of lithium nitrate to silver bromide. Mixtures of alkanes were obtained starting from two different organomagnesiums, suggesting radical formation. 

CgHg + NO2 QH5NO2 + H Free-radical substitution a free radical is the attacking substituent. Such reactions can be used with compounds that are inert to either nucleophiles or electrophiles, for instance the halogenation of an alkane CH4 + CI2 CH3CI + HCl The term substitution is very general and several reactions that can be considered as substitutions are more normally given special names (e.g. esterification, hydrolysis, and nitration). See also electrophilic substitution nucleophilic substitution. 

A solution of a stable nitronium salt (generally the hexafluorophosphate NOa PF but also the hexafluoroantimonateNO SbFft ortetrafiuoroborate NO2BF4) in methylene chloride-tctramethylene sulfone solution was allowed to react with the alkane (cycloalkane), with usual precautions taken to avoid moisture and other impurities. Reactions were carried out at room temperature (25°C) in order to avoid or minimize the possibility of radical side reactions and/or protolytic cleavage reactions (tertiary nitroalkanes particularly readily undergo protolytic cleavage, even if the system initially is acid free but nitration forms acid). Data obtained are summarized in Table XXI. 

Since during the Purex process TBP, alkane, and aqueous nitric acid solution are in mixture or contact condition, the radiation chemical transformations depend on the composition, concentration of nitric acid, contaminant metal ions, irradiation conditions, and oxygen concentration (Triphathi and Ramanujam 2003 Katsumura 2004). Under aerated condition, the organic radicals react with oxygen forming peroxy radicals. After successive reactions a variety of alcohols, ketones, peroxides, and carbonyl compounds form. The ratio of nitration products to oxidation products is 0.8, and the ratio increases if there is no sufficient supply of O2. 

---