Alkoxy radicals reactions with

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. 

With no abstractable hydrogen atoms on the alkoxy radical, reaction with 02 cannot occur and it decomposes by scission of the C-C bond (Edney and Driscoll, 1992 Tuazon and Atkinson, 1993b Sehested et al.,... 

The results of LACTOZ have provided an extended kinetic data base for the following classes of reactions reactions of OH with VOCs, reactions of NO3 with VOCs and peroxy radicals, reactions of O3 with alkenes, reactions of peroxy radicals (self reactions, reaction with HO2, other RO2, NO, NO2), reactions of alkoxy radicals (reactions with O2, decomposition, isomerisation), thermal decomposition of peroxynitrates. Photolysis parameters (absorption cross-section, quantum yields) have been refined or obtained for the first time for species which photolyse in the troposphere. Significantly new mechanistic information has also been obtained for the oxidation of aromatic compounds and biogenic compounds (especially isoprene). These different data allow the rates of the processes involved to be modelled, especially the ozone production from the oxidation of hydrocarbons. The data from LACTOZ are summarised in the tables given in this report and have been used in evaluations of chemical data for atmospheric chemistry conducted by international evaluation groups of NASA and lUPAC. 

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). 

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. 

With current estimates of the rates of alkoxy radical reactions [27], isomerization is likely to be the more important of these latter two processes. Clearly, a better understanding of these reactions is required before their role in the atmospheric degradation of aromatic hydrocarbons can be assessed. 

Alkoxy radicals, 75 peroxyalkyl radicals reaction with, 101 photochemical preparation from alkyl nitrites... 

The alkoxy radicals formed in pathway Eq. 26a have very interesting atmospheric chemistry. The atmospheric fate of alkoxy radicals differs with the nature of the R group. Some alkoxy radicals (e.g., CH3O) are lost solely via reaction with 02, others undergo rapid decomposition via C-C bond scission. Long chain alkoxy radicals can undergo isomerization via intramolecular H-atom abstraction ... 

Before moving on to consider the fate of the carbonyl products, it is appropriate to discuss the atmospheric fate of CF30 radicals. The usual modes of alkoxy radical loss are not possible for CF30 radicals. Reaction with O2 and decomposition via F atom elimination are both thermodynamically impossible under atmospheric conditions. Instead, CF3O radicals react with NO and hydrocarbons. 

The right-hand side of Table 6-13 shows relative rates for alkoxy radical reactions in the atmosphere for boundary layer conditions. Comparison of the rates makes it immediately clear that reactions with N02 (or NO) are of little importance. For the smaller alkoxy radicals the reaction with oxygen is preponderant, whereas for alkoxy radicals largerthan butoxy, decomposition and isomerization reactions become competitive. Tertiary butoxy radicals have no abstractable hydrogen atom and thus cannot react with oxygen. In this case, decomposition is dominant. 

Using the preceding discussion of alkylperoxy and alkoxy radical reactions as a guide, we present now specific mechanisms for several individual hydrocarbons, starting with alkanes. 

The alkoxy radicals produced from peroxy radical reactions with NO or with each other have, in general, three atmospheric fates [7] reaction with O2, dissociation, or isomerization. The reaction with oxygen. 

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 rate of a or 3 scission of a phosphoranyl radical intermediate also can greatly effect the overall reactivity of a given radical towards a particular trivalent phosphorus derivative (26). Alkoxy radicals react with trialkyl phosphites with ki (reaction 4) about 10 sec" mol" and E 2 kcal/mol (24). For R equal jt-... 

The atmospheric lifetime of CH3OCF3 is estimated to be 4.3 years, assuming an average tropospheric temperature of 272 K and [OH] 10 molecule cm . Product studies (using Cl-atoms to initiate the oxidation) have been carried out by Christensen et al. (1999), Good et al. (1999), and Chen et al. (2001) the sole primary product observed is CF30CH=0, which derives from the reaction of the alkoxy radical CF30CH20 with O2, or possibly via H-atom elimination from this same radical ... 

The atmospheric lifetime of CH3OCF2CHF2 is estimated to be 2.3 years, assuming an average [OH] 10 molecule cm" and an average tropospheric temperature of272 K. The mechanism of the OH-initiated oxidation of this species has not been studied. However, the Cl-atom-initiated oxidation has been studied by Nolan et al. (1999), who showed formation of the formate ester, HC(0)0CF2CHF2, which is obtained from the reaction of the alkoxy radical, OCH2OCF2CHF2 with O2. 

Mn (IT) is readily oxidized to Mn (ITT) by just bubbling air through a solution in, eg, nonanoic acid at 95°C, even in the absence of added peroxide (186). Apparently traces of peroxide in the solvent produce some initial Mn (ITT) and alkoxy radicals. Alkoxy radicals can abstract hydrogen to produce R radicals and Mn (ITT) can react with acid to produce radicals. The R radicals can produce additional alkylperoxy radicals and hydroperoxides (reactions 2 and 3) which can produce more Mn (ITT). If the oxygen feed is replaced by nitrogen, the Mn (ITT) is rapidly reduced to Mn (IT). 

The reactions of alkyl hydroperoxides with ferrous ion (eq. 11) generate alkoxy radicals. These free-radical initiator systems are used industrially for the emulsion polymerization and copolymerization of vinyl monomers, eg, butadiene—styrene. The use of hydroperoxides in the presence of transition-metal ions to synthesize a large variety of products has been reviewed (48,51). 

Apparently the alkoxy radical, R O , abstracts a hydrogen from the substrate, H, and the resulting radical, R" , is oxidized by Cu " (one-electron transfer) to form a carbonium ion that reacts with the carboxylate ion, RCO - The overall process is a chain reaction in which copper ion cycles between + 1 and +2 oxidation states. Suitable substrates include olefins, alcohols, mercaptans, ethers, dienes, sulfides, amines, amides, and various active methylene compounds (44). This reaction can also be used with tert-huty peroxycarbamates to introduce carbamoyloxy groups to these substrates (243). 

Other miscellaneous compounds that have been used as inhibitors are sulfur and certain sulfur compounds (qv), picryUiydrazyl derivatives, carbon black, and a number of soluble transition-metal salts (151). Both inhibition and acceleration have been reported for styrene polymerized in the presence of oxygen. The complexity of this system has been clearly demonstrated (152). The key reaction is the alternating copolymerization of styrene with oxygen to produce a polyperoxide, which at above 100°C decomposes to initiating alkoxy radicals. Therefore, depending on the temperature, oxygen can inhibit or accelerate the rate of polymerization. 

The same products may be made from primary alkoxides by the violent reaction with elementary chlorine or bromine. A radical mechanism has been proposed to account for the oxidation of some of the alkoxy groups (54) ... 

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. 

With a radical-scavenging compound present in the reaction mixture, an alkyl radical species like 5 can be trapped, thus suggesting a fast conversion of the alkoxy radical 3 by intramolecular hydrogen abstraction, followed by a slow intermolecular reaction with nitrous oxide. 

Minato ct a/.1(12 proposed that the transition state for disproportionation has polar character while that for combination is neutral. The finding that polar solvents enhance kJkK for ethyl170 and /-butyl radicals (Section 2.5.3.5), the very high kjktc seen for alkoxy radicals with a-hydrogens,171 and the trend in kJkK observed for reactions of a scries of fluoroalkyl radicals (Scheme 1.13, Table 1.7) have been explained in these terms.141102... 

Dialkyl peroxydicarbonates have been reported as low temperature sources of alkoxy radicals (Scheme 3.30)lfMJfb and these radicals may be formed in relatively inert media. However, it is established, for primary and secondary peroxydicarbonates, that the rate of loss of carbon dioxide is slow compared to the rate of addition to most monomers or reaction with other substrates.186,187 Thus, in polymerizations carried out with diisopropyl peroxydicarbonate (47), chains will be initiated by isopropoxycarbonyloxy (48) rather than isopropoxy radicals (49) (see 3.4.2.2).188... 

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... 

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