Alkoxy radicals decomposition

Garner, Long, and Temple (19), using a flow system so that large amounts of reaction products could be collected, observed a ratio of acetaldehyde to formaldehyde of about 4.5 to 1 in the oxidation of hexane at 310° C. in a stoichiometric mixture. Bailey and Norrish (2) studied the oxidation of hexane in the cool-flame region. The considerable quantities of formaldehyde in the products were presumed to arise from alkoxy-radical decomposition and a methyl radical-oxygen reaction. 

R. Mereau, M. T. Rayez, J. C. Rayez, P. C. Hiberty, Phys. Chem. Chem. Phys. 3, 3650 (2001). Alkoxy Radical Decomposition Explained by a Valence-Bond Model. 

Rate constants for alkoxy radical isomerizations can be combined with rate constants for alkoxy radical decomposition and reaction with O2 to predict the relative importance of the three pathways (Atkinson 1994). Alkoxy radicals can also react with NO and N02, but under ambient tropospheric conditions these reactions are generally of negligible importance. 

Alkoxy Radical Decomposition Explained by a Valence Bond Model. 

The behavior of the alkoxy radical (decomposition via loss of RCO rather than RCHOH) is consistent with that observed in the chemistry of structurally similar radicals e.g., that formed following OH addition to methyl vinyl ketone. 

One decomposition of the tetioxide is not terminating, producing alkoxy radicals and oxygen (eq. 14). 

Metal-Catalyzed Oxidation. Trace quantities of transition metal ions catalyze the decomposition of hydroperoxides to radical species and greatiy accelerate the rate of oxidation. Most effective are those metal ions that undergo one-electron transfer reactions, eg, copper, iron, cobalt, and manganese ions (9). The metal catalyst is an active hydroperoxide decomposer in both its higher and its lower oxidation states. In the overall reaction, two molecules of hydroperoxide decompose to peroxy and alkoxy radicals (eq. 5). 

Alkyl hydroperoxides give alkoxy radicals and the hydroxyl radical. r-Butyl hydroperoxide is often used as a radical source. Detailed studies on the mechanism of the decomposition indicate that it is a more complicated process than simple unimolecular decomposition. The alkyl hydroperoxides are also sometimes used in conjunction with a transition-metal salt. Under these conditions, an alkoxy radical is produced, but the hydroxyl portion appears as hydroxide ion as the result of one-electron reduction by the metal ion. ... 

However, reaction (32) is expected to be more facile since it required minimum energy. Gray et al. [115] have found that alkoxy radicals formed by the decomposition... 

Dw-butyl pcroxyoxalatc (DBPOX) is a clean, low temperature, source of t-butoxy radicals (Scheme 3.33).136 The decomposition is proposed to take place by concerted 3-bond cleavage to form two alkoxy radicals and two molecules of carbon dioxide. 

Transfer to initiator is generally of lesser importance than with the corresponding diacyl peroxides. They arc, nonetheless, susceptible to the same range of reactions (see 3.3.2.1.4). Radical-induced decomposition usually occurs specifically to give an alkoxy radical and an ester (Scheme 3.34). 

Some limitations of the method arise due to side reactions involving the nitroxide. However, such problems can usually be avoided by the correct choice of nitroxide and reaction conditions. Nitroxides, while stable in the presence of most monomers, may act as oxidants or rcductants under suitable reaction conditions.516 The induced decomposition of certain initiators (e.g. diacyl peroxides) can be a problem (Scheme 3.94).166 177 There is some evidence that nitroxides may disproportionate with alkoxy radicals bearing a-hydrogens,123 Side reactions with thiols have also been identified.4 18... 

A number of reports on the thermal decomposition of peroxides have been published. The thermal decompositions of f-butyl peroxyacetate and f-butyl peroxypivalate, of HCOH and a kinetic study of the acid-induced decomposition of di-f-butyl peroxide in n-heptane at high temperatures and pressures have been reported. Thermolysis of substituted f-butyl (2-phenylprop-2-yl) peroxides gave acetophenone as the major product, formed via fragmentation of intermediate alkoxy radicals RCH2C(Ph)(Me)0. A study of the thermolysis mechanism of di-f-butyl and di-f-amyl peroxide by ESR and spin-trapping techniques has been reported. The di-f-amyloxy radical has been trapped for the first time. jS-Scission reaction is much faster in di-f-amyloxyl radicals than in r-butoxyl radicals. The radicals derived from di-f-butyl peroxide are more reactive towards hydrogen abstraction from toluene than those derived from di-f-amyl peroxide. 

Many polymerizations are initiated by free radicals, especially alkoxy radicals formed by thermal decomposition of peroxides. A general mechanism for olefin free radical polymerization with initiation, propagation, and termination is given in Fig. 14.1. 

The simplest hydrocarbon, methane, has posed a wealth of challenges to experimentalists and theoreticians seeking to discern its combustion mechanism. Methane s reactions have been explored in a wide variety of contexts over the past several decades. We have discussed these briefly the interested reader is referred to the reviews cited in our previous discussion for further details. Due to the scope of this review, we are primarily interested in these reactions insofar as they provide useful benchmarks for the reactions of larger alkylperoxy (RO2 ) and alkoxy (RO ) systems. With respect to the reactive intermediates present in methane combustion and their implications for larger systems, Lightfoot has published a review on the atmospheric role of these species, while Wallington et al. have provided multiple overviews of gas-phase peroxy radical chemistry. Lesclaux has provided multiple reviews of developments in peroxy radical chemistry. Batt published a review of the gas-phase decomposition reactions available to the alkoxy radicals. ... 

A recent report by Bartlett and Guaraldi (5) provides convincing evidence for the existence of the tetroxide as an intermediate in the selfreactions of ferf-butylperoxy radicals. They estimate AH for the formation of tetroxide by dimerization of peroxy radicals to be —6 kcal. per mole and AEact for decomposition of the tetroxide to alkoxy radicals and oxygen to be 11 kcal. per mole. 

For example, the decomposition of a hydroperoxide to generate an alkoxy free radical can result in the reaction of the alkoxy radical with an olefin. A carbon radical then forms. Olefin chain propagation and polymerization can follow to yield high-molecular-weight deposits. 

The alkoxy radical originates in the oxidation of a VOC to an alkyl radical by any of the oxidants described here or, alternatively, in the thermal decomposition of species such as PAN which gives the CH, radical. In short, H02 is a natural consequence of the oxidation of organics. 

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 ... 

Table 6.7 compares the estimated rates of reaction with 02, decomposition, and isomerization for some alkoxy radicals with different structures. It is important to recognize that there is a great deal of uncertainty in many of these estimates, and this is an area that clearly requires more research. However, given these caveats, it is clear that where isomerization is possible, it usually predominates at room temperature. (Note, however, that this will be slower at the lower temperatures found at higher altitudes.) When isomerization is not feasible, e.g., for the smaller alkoxy radicals or for branched species, reaction with 02 is always significant and usually predominates. 

In the case of the stabilized alkoxy radical formed in the ethene reaction, decomposition and reaction with... 

For the /3-hydroxyalkoxy radicals formed from alkenes C5 and larger, there is experimental evidence that isomerization starts to dominate (e.g., see Atkinson et al., 1995d Kwok et al., 1996b). Thus, isomerization followed by reaction with 02, NO, etc., ultimately leads to the formation of dihydroxycarbonyl compounds. For the reaction of OH with 1-butene, for example, isomerization of one of the alkoxy radicals ultimately leads to 3,4-dihydroxybutanal in competition with its decomposition and reaction with 02 ... 

The alkoxy radical CF3CHFO has, in principle, two fates reaction with 02 and decomposition. For this particular radical, both are important under tropo-... 

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