Nitrate radical thermal decomposition

The products of the thermolysis of 3-phenyl-5-(arylamino)-l,2,4-oxadiazoles and thiazoles have been accounted for by a radical mechanism.266 Flash vacuum pyrolysis of 1,3-dithiolane-1-oxides has led to thiocarbonyl compounds, but the transformation is not general.267 hi an ongoing study of silacyclobutane pyrolysis, CASSF(4,4), MR-CI and CASSCF(4,4)+MP2 calculations using the 3-21G and 6-31G basis sets have modelled the reaction between silenes and ethylene, suggesting a cyclic transition state from which silacyclobutane or a trcins-biradical are formed.268 An AMI study of the thermolysis of 1,3,3-trinitroazacyclobutane and its derivatives has identified gem-dinitro C—N bond homolysis as the initial reaction.269 Similar AMI analysis has determined the activation energy of die formation of NCh from methyl nitrate.270 Thermal decomposition of nitromethane in a shock tube (1050-1400 K, 0.2-40 atm) was studied spectrophotometrically, allowing determination of rate constants.271... 

Under atmospheric conditions, the nitro-oxyalkyl peroxy radicals will probably form mainly the corresponding nitro-oxy alkoxy radicals. Thermal decomposition, yielding carbonyl compounds and NO2, and reaction with O2 giving carbonyl nitrates, appear to be the dominant reactions under most atmospheric conditions. The extent to which carbonyl nitrates can act as temporary reservoirs for NOx will largely depend on their photolysis rates or reactions with OH radicals. 

The kinetics of the various reactions have been explored in detail using large-volume chambers that can be used to simulate reactions in the troposphere. They have frequently used hydroxyl radicals formed by photolysis of methyl (or ethyl) nitrite, with the addition of NO to inhibit photolysis of NO2. This would result in the formation of 0( P) atoms, and subsequent reaction with Oj would produce ozone, and hence NO3 radicals from NOj. Nitrate radicals are produced by the thermal decomposition of NjOj, and in experiments with O3, a scavenger for hydroxyl radicals is added. Details of the different experimental procedures for the measurement of absolute and relative rates have been summarized, and attention drawn to the often considerable spread of values for experiments carried out at room temperature (-298 K) (Atkinson 1986). It should be emphasized that in the real troposphere, both the rates—and possibly the products—of transformation will be determined by seasonal differences both in temperature and the intensity of solar radiation. These are determined both by latitude and altitude. 

Ellis and coworkers studied the effect of lead oxide on the thermal decomposition of ethyl nitrate vapor.P l They proposed that the surface provided by the presence of a small amount of PbO particles could retard the burning rate due to the quenching of radicals. However, the presence of a copper surface accelerates the thermal decomposition of ethyl nitrate, and the rate of the decomposition process is controlled by a reaction step involving the NO2 molecule. Hoare and coworkers studied the inhibitory effect of lead oxide on hydrocarbon oxidation in a vessel coated with a thin fQm of PbO.P l They suggested that the process of aldehyde oxidation by the PbO played an important role. A similar result was found in that lead oxide acts as a powerful inhibitor in suppressing cool flames and low-temperature ignitions.P l... 

It is characteristic that the new entrant methyl group assumes the ortho position to the nitro group and thus a substitution occurs which is similar to nucleophilic attack. Recently it has been found by Jackson and Waters [72] that higher nitrated benzene derivatives such as m- dinitrobenzene, or 1,3,5-trinitrobenzene, become homolytic hydrogen acceptors at temperatures of 80-100°C especially in the presence of the 2-cyano-2-propyl radical, which is formed by thermal decomposition of a,a -azo-bis-isobutyronitrile... 

Chlorine nitrate and HCl are considered to be the most important chlorine reservoir species in the stratosphere. Iodine nitrate has also been considered as a reservoir species for iodine radicals that could destroy tropospheric ozone, but photodissociation of IONO2 to form iodine radicals is only effective at temperatures below 290 K, at higher temperatures thermal decomposition takes place which does not yield iodine radicals. ... 

The thermal decomposition of alkyl nitrates has received continuous attention over a number of years. The introduction of improved techniques has required the periodic re-evaluation of data on these very complex reactions . The substrate of choice has been ethyl nitrate. Early studies on the thermal decomposition suggested that the reaction became complex after the initial fission into ethoxy radicals and nitrogen dioxide . 

This conclusion is based on the observation of the similarity of the overall activation energies for the thermal decomposition of homologous alkyl nitrates (Table 32). Also, the calculated heat of formation of the alkoxy radical agrees satisfactorily with other values, and the activation energies are only 1-2 kcal.mole higher than the O-N bond dissociation energy . 

It has been observed that the activation energy for thermal decomposition of ethyl nitrate is substantially reduced in the presence of lead oxide or copper surfaces. Above 200° C approximately, the thermolysis of ethyl nitrate becomes much more complex and detonation in the gas phase is common. In the range 242-260° C the reaction was found to be half-order, with an overall rate coefficient" k = 10 - exp(—46,800)/J 7 mmole. l . sec apparently the initiation step is unaltered but the subsequent radical chain mechanism affects the overall rate of decomposition. 

Only unrearranged cyclopropyl products were reported for photochemical chlori-nation and vapor phase nitration of cyclopropane. The Hunsdiecker reaction of silver cyclopropanecarboxylate and the thermal decomposition of cyclopropanoyl peroxide also gave exclusively unrearranged product as did the di-t-butyl peroxide initiated decarbonylation of 1-methyl and 1-phenylcyclopropanecarboxaldehyde. In general one can predict that when a good radical scavenger, solvent or substrate, is present in the reaction, unrearranged product will result (i.e. see Tables 11 and 13). 

CsHjoOj Combustible liquid. Forms explosive mixture with air [explosion limits in air (vol %) 1.6 to uel unknown flash point 149°F/65°C Fire Rating 1]. Unless inhibited (200 ppm hydroquinone recommended), polymerization may occur avoid exposure to high temperatures, ultraviolet light, free-radical initiators. Reacts with water with release of heat may not be violent if not contained. Strong oxidizers may cause fne and explosions. Reacts violently with sodium peroxide, uranium fluoride. Incompatible with strong acids, nitrates. Incompatible with sulfuric acid, nitric acid, caustics, aliphatic amines, isocyanates, boranes. Thermal decomposition releases toxic acrid fumes of acrolein and acrylic acid. On small fires, use dry chemical powder (such as Purple-K-Powder), water spray, alcohol-resistant foam, or CO2 extinguishers. 

Because of their short lifetimes at room temperature, the peroxy nitrates have been assumed not to act as key storage modes for peroxy radicals and NO2 in the lower atmosphere. At middle latitudes in the wintertime these may have hfetimes that approach days. Further, like the PANs these might be reformed to actively transport NO2 and peroxy radicals over long distances, depending upon the NO, hydroperoxy radical, and NO2 concentrations. With the possible exception of very cold air masses, these compounds are typically not present in significant concentrations in the troposphere because of rapid thermal decomposition to form NO2 and RO2. At room temperature they would be lost in samphng lines or during analysis. 

The reaction with NO leads to the formation of CO2 and a methyl radical that is oxidized to formaldehyde by reactions (16)-(20). In addition, the oxidation of CH3 regenerates HO c so that the oxidation cycle continues. Association with NO2 produces peroxyacetyl nitrate (PAN). Its lifetime is longer than that of alkylperoxy nitrates, but strongly temperature dependent, ranging from 1 hr at 298 K to 140 d at 250 K. Thus, PAN can be transported over a great distance before undergoing thermal decomposition. Under conditions of lowNOj concentrations acetyl peroxy radicals interact also with HO2 radicals... 

For NO3 + dialkenes (butadiene), the identified products were CO (4%), HCHO (12%) acrolein CH =CH-CHO (12%), total nitrates (60%). For the NO3 + isoprene reaction, yields of products were CO (4%), HCHO (11%), methacrolein CH3=CH(CH3)-CHO (uncertain yield), and total nitrates (80%). The aldehydes formed in the reactions of NO3 with the dialkenes can be explained by the thermal decomposition of the related nitrooxy-alkoxy radicals as in reaction (Equation 4.85) for monoalkenes. The formation of small quantities of CO in both the NO3 + dialkenes reaction systems is difficult to explain. It is unclear whether the CO is formed directly in the reaction of NO3 with dialkenes or whether it is a product of secondary reactions of NO3 with acrolein or methacrolein. 

The atmospheric lifetime of peroxyacetyl nitrate with respect to the removal by reaction with OH radicals is estimated to be more than 1 year. The wet and dry depositions are minor removal processes (Roberts, 1990). The thermal decomposition remains the most important loss process up to around 7 km, above which photolysis takes... 

Oxalic and malonic acids, as well as a-hydroxy acids, easily react with cerium(IV) salts (Sheldon and Kochi, 1968). Simple alkanoic acids are much more resistant to attack by cerium(IV) salts. However, silver(I) salts catalyze the thermal decarboxylation of alkanoic acids by ammonium hexanitratocerate(IV) (Nagori et al., 1981). Cerium(IV) carboxylates can be decomposed by either a thermal or a photochemical reaction (Sheldon and Kochi, 1968). Alkyl radicals are released by the decarboxylation reaction, which yields alkanes, alkenes, esters and carbon dioxide. The oxidation of substituted benzilic acids by cerium(IV) salts affords the corresponding benzilic acids in quantitative yield (scheme 19) (Hanna and Sarac, 1977). Trahanovsky and coworkers reported that phenylacetic acid is decarboxylated by reaction with ammonium hexanitratocerate(IV) in aqueous acetonitrile containing nitric acid (Trahanovsky et al., 1974). The reaction products are benzyl alcohol, benzaldehyde, benzyl nitrate and carbon dioxide. The reaction is also applicable to substituted phenylacetic acids. The decarboxylation is a one-electron process and radicals are formed as intermediates. The rate-determining step is the decomposition of the phenylacetic acid/cerium(IV) complex into a benzyl radical and carbon dioxide. 

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