Peroxyacyl radicals

Peroxyacyl radical (CH3C(0)00 ) C203 Phenoxy radical PHO... 

In terms of metal-promoted reactions in the presence of oxygen and aldehydes, a detailed mechanistic study of the conversion of alkenes to oxiranes suggested that the primary oxidant in these reactions is also an acyl peroxy radical derived from the aldehyde <1996IC1045>. A second investigation of the Mukaiyama epoxidation (oxygen, Ni(acac)2, and an aldehyde) suggested that reactions proceed via peroxyacyl radicals and a peracid <2004JOC3453>. 

In the gas phase formation of ozonide would be unlikely, and the intermediates should live long enough to react with NO2 as in Equations 11 and 12, with CgHe as in 13, 14, 15, and 16, and with O2 as in 17 and 18. However preliminary calculations using Equation 51 indicate that reactions 13, 14, 15, and 16 are too slow to be important in this process. Reactions of intermediates, as in 17 and 18, explain the non-stoichiometric ratio of olefin to O3 in the presence of O2 as observed by Cvetanovic (3) and others. Ozone may also be formed upon reacting with peroxyacyl radicals such as in Equation 31. 

The acyl radicals add oxygen to give the peroxyacyl radical, R—C—0—0. This reaction seems probable because, at the concentrations at which compound X is formed, the acyl radical collides with oxygen several thousand times more frequently than with any of the other possible reactants. The likelihood of the reaction is also indicated by the fact that the low temperature oxidation of aldehyde in pure oxygen produces peroxyacids. 

The peroxyacyl radical adds nitric oxide to produce R—C—0—O—NO. This is consistent with the fact that nitric oxide is itself a free radical with an unpaired electron and is commonly used as an inhibitor of free radical reactions. 

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 organic peracids and peroxides are produced by the same chemistry which forms the PANs and the alkyl nitrates the alkyl nitrates and PANs are formed from peroxyacyl radicals (RCO3) and peroxy radicals (RO2) under high-NOx conditions, whereas the peracids and peroxides are formed under low-NO conditions through reactions with hydroperoxy radical ... 

The data on the coefficients k12 = ki.i/ri and fe2 i = k2<2/r2 (Table 12) enables a comparison to be made between the reactivity of the H02-radical and that of peroxyacyl radicals with regard to the same substrate. It is quite surprising to note that ft1(1 is very different from k2%1 and ft1>2 from k2 2. In general, for hydrocarbons, ethers, and secondary alcohols [62], reactivity depends on the oxidized molecule and is independent of the active species. However, except that H02 is one radical compared with various hydrocarbon peroxidic radicals, it is possible that the cyclohexadiene oxidation mechanism is more complex than has been suggested and the interpretation of the results is uncertain. 

PANs can be considered to exist in chemical equilibrium according to reactions 5.90 and 5.91. Thus the concentration of a PAN compound at any location and time will depend on the temperature and the local levels of NO2 and NO, the latter molecule because of the competing reaction 5.96 that serves to remove peroxyacyl radicals from the PAN system. 

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... 

An alternative method for the preparation of diacylperoxides involves the autoxida-tion of aldehydes (Scheme 9.1). The diacylperoxide results from the combination of the acyl radical and a peroxyacyl radical (Equation 9.83), both generated as indicated in the Scheme 9.1 (repeated below). 

Addition to Op can be considered as the sole fate of alkyl (R) and acyl (RCO radicals, leading to peroxyalkyl and peroxyacyl radicals, respectively. The acylate radical (RC(O)O) rapidly dissociates yielding an alkyl radicsJ. suid C02 Hydroxy-peroxy-alkyl radicals are formed in olefin-OH reactions. We do not discuss the reactions of hydroxy-peroxyalkyl radicals here. 

The peroxyacyl radicals involved in the atmospheric oxidation of butane, particularly in atmospheric regions that are low in NO c, will lead in part to the generation of organic acids and peroxy acids through two channels of the disproportionation reaction with HO2 RC(0)02 + HO2 RC(0)OH + O3 RC(0)02 + HO2 RC(0)00H -t- O2. The atmospheric oxidation of butane is expected to give the following... 

Peroxy acyl nitrates dissociate quite quickly at 298 K, to regenerate peroxyacyl radicals. For example, PAN has a lifetime of about 50 min. The lifetime increases rapidly at the lower temperatures experienced at higher altitudes and is several months at the temperatures ( 250 K) of the upper troposphere. This long lifetime provides a mechanism for the transport of NOjc from polluted areas to less polluted areas, by transfer of peroxyacyl nitrates from the boundary layer to the free troposphere subsequent subsidence can return them to the boundary layer where they dissociate at the higher temperatures encountered there. The atmospheric reactions of the nitrates are discussed in detail in chapters VIII and IX. 

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