Alkylperoxy radicals with oxygen

The photodissociation of N02 is the only definitely established process for the formation of ozone in the troposphere and must be held responsible also for the generation of ozone in photochemical smog. The reaction of alkylperoxy radicals with oxygen, ROO + 02— R0 + 03 has occasionally been invoked—for example, by Cadle and Allen (1970)—but according to... 

Alkylperoxy radicals are generated by the reactions of carbon-centered radicals with oxygen and in the induced decomposition of hydroperoxides (Scheme 3.82). Their reactions have been reviewed by Howard452 and rate constants for their self reaction and for their reaction with a variety of substrates including various inhibitors have been tabulated.453... 

Small radicals such as tert-butylperoxy and ethylperoxy can, however, react via 1,4 H-transfer only the strain energy involved in O-heterocycle formation is 28 kcal. per mole. In this case, k.4(x — 106 sec."1 whereas krta = 10r> 4 sec. 1 and when [02] = 200 mm. of Hg, ko[02] = 105,3 sec. 1, so that k.4ct < < (tkr,a + k [02]). The result is that in the oxidation of small alkyl radicals, the route via alkylperoxy radicals will be blocked because reverse Reaction —4 competes successfully with Reaction 5. Reaction 2 will thus be a more effective mode of reaction of alkyl radicals with oxygen and the conjugate alkene will be a major product. 

Percent product distribution propanal 50.6 1.2, 2-hydroxy-butanal 7.0 0.3, l-hydroxy-butan-2-one 24.2 0.7, 1,2 dihydroxybutane 18.2 0.7. Computer assisted analysis of the product distribution showed that addition of the OH radical occurs to 26 % at the inner and to 74% at the outer position of the double bond. These reactions produced the corresponding primary and secondary hydroxy-alkylperoxy radicals. The branching ratio for the radical propagating channel of the self-reaction of the secondary peroxy radicals was determined to be issa/ iss = 0.75 0.02 28 % of the hydroxy-alkoxyl radical thus formed reacted with oxygen to produce hydroxyketone. If it is assumed that the rate coefficient for the reaction of the hydroxy-alkoxyl radical with oxygen is 8 x 10 cm molecule s the rate coefficient for the decomposition of this radical to produce propanal is 1 x 10 s V... 

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). 

Oxygen-centered radicals are arguably the most common of initiator-derived species generated during initiation of polymerization and many studies have dealt with these species. The class includes alkoxy, hydroxy and aeyloxy radicals and tire sulfate radical anion (formed as primary radicals by homolysis of peroxides or hyponitrites) and alkylperoxy radicals (produced by the interaction of carbon-centered radicals with molecular oxygen or by the induced decomposition of hydroperoxides). 

Alkyl radicals generated from azoalkanes as in (7) react with oxygen added to argon matrices giving alkylperoxy radicals. In this manner radicals... 

Bauer G (2000) Reactive oxygen and nitrogen species efficient, selective and interactive signals during intercellular induction of apoptosis. Anticancer Res 20 4115-4140 Beckwith AU, Davies AG, Davison IGE, Maccoll A, Mruzek MH (1989) The mechanisms of the rearrangements of allylic hydroperoxides 5a-hydroperoxy-3p-hydrocholest-6-ene and 7a-hydro-peroxy-3(1-hydroxycholest-5-ene. J Chem Soc Perkin Trans 2 815-824 Behar D, Czapski G, Rabani J, Dorfman LM, Schwarz HA (1970) The acid dissociation constant and decay kinetics of the perhydroxyl radical. J Phys Chem 74 3209-3213 Benjan EV, Font-Sanchis E, Scaiano JC (2001) Lactone-derived carbon-centered radicals formation and reactivity with oxygen. Org Lett 3 4059-4062 Bennett JE, Summers R (1974) Product studies of the mutual termination reactions of sec- alkylper-oxy radicals Evidence for non-cyclic termination. Can J Chem 52 1377-1379 Bennett JE, Brown DM, Mile B (1970) Studies by electron spin resonance of the reactions of alkyl-peroxy radicals, part 2. Equilibrium between alkylperoxy radicals and tetroxide molecules. Trans Faraday Soc 66 397-405... 

The participation by metal catalysts in autoxidations may be divided into four main groups (a) reaction with peroxides (h) reaction with substrate (c) reaction with oxygen (d) reaction with alkoxy and alkylperoxy radicals. The latter (d) leads to inhibition rather than to catalysis. Each of these types of participation will be discussed in the following sections. 

The most common pathway for catalysis of autoxidations by transition metal complexes involves the decomposition of alkyl hydroperoxides. Another route that may be possible for chain initiation involves direct oxygen activation, whereby the complexation of molecular oxygen by a transition metal would lower the energy of activation for direct reaction with the substrate [reaction (9)]. For example, oxygen coordinated to a metal might be expected to possess properties similar to alkylperoxy radicals and undergo hydrogen transfer with a hydrocarbon ... 

The mechanism of radical generation initially occurs via UV irradiation of the oxide producing surface trapped holes, O". Electron transfer from the adsorbed ketone to the surface trapped hole occurs, producing an adsorbed cation radical. This quickly deprotonates by H transfer to the surface oxide, forming an intermediate radical which subsequently reacts with molecular oxygen to form the adsorbed alkylperoxy radical observable by EPR [48]. 

The alkyl radical initially formed reacts readily with oxygen to give the corresponding alkylperoxy radical, which may abstract hydrogen from a fuel molecule to form the alkylhydroperoxide or alternatively decompose to yield an aldehyde and an alkoxy radical. Some workers thought that this decomposition was preceded by an isomerization of the alkylperoxy radical, the activation energy of which had been estimated by Semenov [3] to be ca. 20 kcal. mole. Shtern was of the opinion that the major, if not the only, fate of the alkylperoxy radical was decomposition, but in contrast to other workers he believed that it must involve scission of a C—C bond and could not lead to the formation of a carbonyl compound and hydroxyl radical. 

In contrast to the alkene theory the predominant mode of oxidation of the alkyl radicals is by oxygen addition and the alkylperoxy radical so formed then undergoes homogeneous intramolecular rearrangement (reaction (14)). Decomposition of the rearranged radical (reaction (16)) usually leads to a hydroxyl radical and stable products which include O-heterocycles, carbonyl compounds and alcohols with rearranged carbon skeletons relative to the fuel and alkenes. The chain-cycle is then completed by unselective attack on the fuel by the hydroxyl radical (reaction (12)). 

The acronym BHT stands for butylated hydroxytoluene, the common name for 2,6-di-r-butyl-4-methylphenol. It is used widely as an antioxidant in foodstuffs and food packaging. The reaction of oxygen with unsaturated fats gives, after several steps, alkylperoxy radicals (ROO ) that, upon further reaction, give smaller odiferous molecules that can ruin the palatability of foods. BHT acts as a scavenger for alkylperoxy radicals, ROO , because hydrogen abstraction by ROO- gives the stable free radical, 5-4. The hydroperoxide ROOH, which also is formed in the reaction, is much less reactive than ROO and consequently causes much less oxidation. The new radical, 5-4, which is formed from BHT, is relatively unreactive for two... 

In the presence of dioxygen, the carbon radical R- produced by reactions (201) and (202) ar transformed into alkylperoxy radicals ROO, reacts with Co or Mn species to regenerate th Co " or Mn " oxidants, and produce primary oxygenated products (alcohol, carbonyl compounds which can be further oxidized to carboxylic acids. This constitutes the basis of several Industrie processes such as the manganese-catalyzed oxidation of n-alkenes to fatty acids, and the cobal catalyzed oxidation of butane (or naphtha) to acetic acid, cyclohexane to cyclohexanol-on mixture, and methyl aromatic compounds (toluene, xylene) to the corresponding aromatic monc or di-carboxylic acids. ... 

At atmospheric pressure, the rate coefficients for the R+02 association reactions are already close to their second-order limit. The high concentration of oxygen makes these reactions occur almost instantaneously, so that other conceivable reactions of R radicals are negligible by comparison. Reactions of ROO- with NO are known to be fast for H02 and CH302 (see Table A-4), and other alkylperoxy radicals are assumed to react with NO at similar rates. A justification for this assumption is derived from model... 

The abstraction of a hydrogen atom from an alkane first produces an alkyl radical. In the atmosphere, however, alkyl radicals have but little choice other than to combine with oxygen to yield an alkylperoxy radical. As mentioned previously, tertiary hydrogen atoms are abstracted more easily than secondary H atoms, and their abstraction, in turn, is more facile than that of primary H atoms. In the higher hydrocarbons the number of secondary H atoms usually exceeds that of primary or tertiary ones, so that secondary alkyl and alkylperoxy radicals are most frequently formed ... 

Alkoxy radicals that arise from the reaction of NO with alkylperoxy radicals also may enter into several competing processes. Four such reactions must be considered thermal decomposition, isomerization, reaction with oxygen, and addition to either NO or N02. Falls and Seinfeld (1978) have presented a brief review of the various possibilities. Table 6-13 summarizes current information on rate coefficients and projected rates in the atmosphere for several small alkoxy radicals. 

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