Alkoxy Radicals RO

The reactions of R02 with NO and with R02 generate alkoxy radicals (RO). Alkoxy radicals have several possible atmospheric fates, depending on their particular structure. These include reaction with 02, decomposition, and isomerization as we shall see, reactions with NO and N02 are unlikely to be important under most tropospheric conditions. Atkinson et al. (1995b) and Atkinson (1997b) have reviewed reactions of alkoxy radicals and /3-hydroxyalkyl radicals  

This reaction may proceed via the initial addition of 02 to give a trioxy intermediate (-C-OOO ) followed by rearrangement and elimination of H02 (e.g., Jungkamp et al., 1997). Atkinson (1997a, 1997b) recommends rate constants of 9.5 X 10 15 and 8 X 10 15 cm3 molecule-1 s-1 at 298 K for primary RCH20 and secondary R,R2CHO radicals, respectively. 

Scission to produce the larger alkyl radical dominates. For example, in the case of the 2-butoxy radical in Eq. (28), path (b) dominates over path (a) by more than two orders of magnitude (see Atkinson, 1997b, and references therein). 

Structure-reactivity relationships have also been developed for these isomerizations. For H abstraction from a CH3-X, CH2-XY, or CH-XYZ, respectively, through a 6-membered transition state, rate constants of 1.6 X 105, 1.6 X 10fl, and 4 X 106 s 1, respectively, are recommended (Atkinson, 1997a, 1997b). These are corrected for neighboring group effects using F(-CH3) = 1.0, F(-CH2-) = 

The latter reactions with NO and N02 are less likely than the first three possibilities. With kM) 31 3 X 10-11 cm3 molecule-1 s-1 at 298 K and 1 atm pressure, the pseudo-first-order rate of reaction, /c30[NO] or 3I[N02], is 75 s l at 100 ppb of NO or N02. For a species such as (CH3)3CO that can only decompose (and then only relatively slowly), these reactions could be responsible for 10% of the alkoxy radical reaction at the relatively high NO and N02 concentrations of 100 ppb. 

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Thermally unstable cycHc trioxides, 1,2,3-trioxolanes or primary o2onides are prepared by reaction of olefins with o2one (64) (see Ozone). Dialkyl trioxides, ROOOR, have been obtained by coupling of alkoxy radicals, RO , with alkylperoxy radicals, ROO , at low temperatures. DiaLkyl trioxides are unstable above —30° C (63). Dialkyl tetraoxides, ROOOOR, have been similarly produced by coupling of two alkylperoxy radicals, ROO , at low temperatures. Dialkyl tetraoxides are unstable above —80°C (63). 

Alkyl radicals, R, react very rapidly with O2 to form alkylperoxy radicals. H reacts to form the hydroperoxy radical HO2. Alkoxy radicals, RO, react with O2 to form HO2 and R CHO, where R contains one less carbon. This formation of an aldehyde from an alkoxy radical ultimately leads to the process of hydrocarbon chain shortening or clipping upon subsequent reaction of the aldehyde. This aldehyde can undergo photodecomposition forming R, H, and CO or, after OH attack, forming CH(0)00, the peroxyacyi radical. 

Further oxidation of an alkoxy radical (RO ), via H-atom abstraction at the carbon adjacent to the oxygen s radical center, leads to the formation of an aldehyde. 

While the relative importance of the various paths is not well established, it is expected that dissociation to the alkoxy radical, RO, and N02 will predominate. Luke et al. (1989) experimentally measured rates of photolysis of simple alkyl nitrates and compared them to rates calculated using the procedures outlined in Chapter 3.C.2. Figure 4.22 compares the experimentally determined values of the photolysis rate constants (kp) for ethyl and n-propyl nitrate with the values calculated assuming a quantum yield for photodissociation of unity. The good agreement suggests that the quantum yield for photodissociation of the alkyl nitrates indeed approaches 1.0. 

The alkoxy radical, RO, generated by R02 reactions can react with 02, decompose, or undergo an isomerization, depending on the structure of the radical. At very high NO, concentrations for a few alkoxy radicals, reactions with NO and N02 may also occur to a small extent. Figure 6.2 summarizes these possibilities. 

Aldehydes are easily oxidized (19) and are well-known sensitizer compounds benzaldehyde enhances oxidation of linear alkenes producing the corresponding epoxides (15). An interesting feature of radical epoxidation is the generation of the alkoxy radical RO, which is more reactive and less selective than the peroxy radical (16)... 

Alcohols are not only source of ketyl radicals generated by hydrogen abstraction from the a-C-H position (Eq. (7), Table 1). Oxidation of alcohols with Pb(OAc)4, PhI(OAc)2, and S2082 with Ag(I) as catalyst produces alkoxy radicals (RO-) which may further undergo /3-scission (Eq. 13), intramolecular hydrogen abstraction, or intra- and intermolecular addition to alkenes, generating a nucleophilic carbon-centered radical useful for heteroaromatic substitution (Scheme 6) [2]. 

Probably the most interesting series of radicals which have not been trapped and identified conclusively in the solid phase by any method are the alkoxy radicals, RO-. We have attempted to prepare the t-butoxy radical by the following reactions ... 

The antioxidant radical produced because of donation of a hydrogen atom has a very low reactivity toward the unsaturated lipids or oxygen therefore, the rate of propagation is very slow. The antioxidant radicals are relatively stable so that they do not initiate a chain or free radical propagating autoxidation reaction unless present in very large quantities. These free radical interceptors react with peroxy radicals (ROO ) to stop chain propagation thus, they inhibit the formation of peroxides (Equation 13). Also, the reaction with alkoxy radicals (RO ) decreases the decomposition of hydroperoxides to harmful degradation products (Equation 14). 

In the Barton reaction, an alkyl nitrite is converted into an alcohol-oxime. The nitrite is photoexcited to a 1,2-diradical. It fragments to NO and an alkoxy radical (RO-), and the latter abstracts H- from the nearest C-H bond. The resulting alkyl radical then combines with NO to give a nitroso compound, which then tautomerizes to the oxime, probably by a polar stepwise mechanism. The Barton reaction has been used for the remote functionalization of hydrocarbons, especially steroids. 

The hydroperoxide group decomposes to generate a very reactive alkoxy radical (RO ) and hydroxyl radicals (HO ), and finally aldehydes, ketones and esters are formed ... 

Hydroxylamines are easily oxidized at ambient temperature with molecular oxygen and with ROOH (Scheme 21) [65,134]. The last reaction is very fast [127]. In spite of formation of alkoxy radicals RO , deactivation of ROOH contributes to the photostabilizing effect of NOH (formed RO are effectively scavenged by phenolic AO, mostly used in combination with HAS). 

Alkoxy radicals (RO) are important chain carriers in many oxidation reactions especially at higher temperatures [36,98]. They are formed in several reactions including self-reaction of tertiary R02- (Sect. 4.4)... 

Absolute rate coefficients and parameters for C—C cleavage of alkoxy radicals (RO )... 

The second reaction increases in importance monotonically as the size of R increases. The alkoxy radical (RO) reacts rapidly with 02 ... 

Step 2 Hydrogen atom abstraction from hydrogen bromide by an alkoxy radical RO- " Br ------> r6=H -b -Br ... 

Figure 5.13 summarizes the general organic/NO, system from the point of view of the peroxy radicals (RO2), alkoxy radicals (RO), and acyl peroxyacyl radicals (RC(O)OO). At each step in the chain, propagation and termination steps compete with each other. The peroxy radicals efficiently convert NO to NO2 as long as NO levels are sufficiently high. The... 

Other Oxidants Other reactive species may be generated as a consequence of photolytic activity. It has been shown above how peroxy radicals, ROO, alkoxy radicals, RO, and atomic oxygen may be formed. Hydrogen peroxide, H2O2, superoxide, 02 would also be candidates, however, because of the complex series of reactions that can be involved, it is difficult to develop a systematic analysis of their involvement. 

The sources of alkoxy radicals, RO-, include the decay processes of peroxy radicals such as breakdown of Russell tetroxides, the direct or sensitized photolysis (or metal-catalyzed heterolysis) of hydroperoxides, and the reaction of peroxy radicals with NOx or olefins. 

The heats of formation of alkoxy radicals, RO, have been derived from bond dissociation energies, measured by kinetic methods, in nitrites, nitrates and peroxides, i.e. jD(RO— NO), Z>(RO—NOg) and i>(RO—OR). These bond dissociation energies, taken from Gray s paper 226) shown in Table 5. The heats of formation of... 

Not aU radicals show the same reactivity, broadly speaking the character of radicals is affected by (i) the nature of the atom that is the radical center and (ii) the electronic properties of the groups attached to the radical. The importance of the atom bearing the unpaired electron is nicely illustrated by the different reactivities of group 6 radicals, and is rationalized by the hard/soft nature of the radical center. Thus, alkoxy radicals (RO ) are small and hard they typically undergo H-atom abstraction and p-scission reactions. However, they rarely add to C=C. Thiyl (RS ) and selenyl (RSe") radicals, however, are larger and softer. They do not usually abstract H, but they do readily add to C=C. This is useful for (Z/E) isomerization of alkenes. 

Interestingly, it has been found that the formation of SOA is accelerated in the presence of SO2 and sulfuric acid (Jang et al. 2002, Edney et al. 2005, Surrat et al. 2007). The first step in atmospheric SO2 oxidation is OH addition (Chapter 5.5.2.1) and this radical can react with alkoxy radicals (RO) to form sulfonic acid and further with organic peroxo radicals to form dialkyl sulfates ... 

The particularity of harden phase oxidation of polyolefyne is reaction of chain transfer -interaction of alkyl (R ) or alkoxy radical (RO ) with polymer competitive to its reaction with oxygen ... 

The general mechanism for the atmospheric degradation of VOCs follows a pathway in which the initially produced RO2 radical is transformed into an alkoxy radical, RO. The fate of the alkoxy radicals determines the nature of the first stable products of oxidation, the degree of fragmentation of the original carbon chain and the potential for photo-oxidant formation. 

In the literature, it is conunonly admitted that alkoxy radicals RO are involved in three types of reactions, oxidation, isomerization, decomposition, which occur according to parallel pathways. It is the reason for which, the products generated by the reactions of the n-butoxy radical have been analysed and separated in three different groups corresponding to each pathway (1-butanol apart, because it is likely produced by RO + RH ROH + R) butyraldehyde (oxidation), other compounds in C4 resulting from the isomerization, and species in C3 formed by the... 

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