Secondary alkylperoxy radicals

This reaction is certainly exothermic and by analogy with similar hydrogen abstraction reactions might be expected to have an activation energy of anywhere from zero to 8 kcal. If it should be proved that this is the terminating interaction or a possible terminating interaction of primary and secondary alkylperoxy radicals, then we must begin to think anew about the entire mechanism for the over-all chain decomposition and oxidation of hydrocarbons. I believe it would also require us to review seriously our interpretation of the spin resonance work on alkylperoxy radicals at low temperatures. 

Furimsky E, Howard JA, Selwyn J (1980) Absolute rate constants for hydrocarbon autoxidation. 28. A low temperature kinetic electron spin resonance study of the self- reactions of isopropylperoxy and related secondary alkylperoxy radicals in solution. Can J Chem 58 677-680 Gebicki JM, Allen AO (1969) Relationship between critical micelle concentration and rate of radiolysis of aqueous sodium linolenate. J Phys Chem 73 2443-2445 Gebicki JM, Bielski BHJ (1981) Comparison of the capacities of the perhydroxyl and the superoxide radicals to initiate chain oxidation of linoleic acid. J Am Chem Soc 103 7020-7022 Gilbert BC, Holmes RGG, Laue HAH, Norman ROC (1976) Electron spin resonance studies, part L. Reactions of alkoxyl radicals generated from alkylhydroperoxidesand titanium(lll) ion in aqueous solution. J Chem Soc Perkin Trans 2 1047-1052... 

Kinetic, isotopic and product studies of autoxidation suggest that these reactions and reaction (42) proceed via a tetroxide intermediate (RO4R) which may be a transition state or a molecule of finite lifetime [151]. Russell [152] has suggested that the termination reactions of primary and secondary alkylperoxy radicals in fact involve the formation of a cyclic transition state, viz. 

Figure 4.10. Possible reactions of secondary alkylperoxy radicals. From von Sonntag (1987). Reprinted by permission of Taylor and Francis.

Only few data are available for self-reactions of secondary alkylperoxy radicals. One of the principal contributions of the project has been the study of the cyclo-alkylperoxy radicals C-C5H9O2 and c-CeUi 02 radicals. The corresponding rate constants are 40 times smaller than the only linear secondary peroxy radical studied so far, /-C3H7O2, and exhibit a slight positive temperature dependence. More data would be necessary for a better description of such reactions but, due to their relatively low rate constants (< 5 x lO" " cm molecule s ), their contribution to atmospheric chemistry is negligible. 

Two mechanisms have been considered for the self-reaction of primary and secondary alkylperoxy radicals, the Russell mechanism (lO) reactions (26), (27), and (28) and a mechanism involving the intermediacy of alkoxy radicals ( 3, kk) reactions (29), (30), (31), and (32). The reactions involved in these two mechanisms are presented in Scheme II. 

Table VII, Arrhenius parameters for self-reaction of some primary and secondary alkylperoxy radicals.

There is no doubt that all alkylperoxy radicals interact to give a tetroxide which decomposes to give either radical or nonradical products. Furthermore, it would appear that the structure of the tetroxide determines the overall rate and mechanism of the reaction. Di-t-alkyl tetroxides decompose either by a concerted or two step process to give t-alkoxy radicals, a fraction of which combine in the cage. This reaction pathway is also available to primary and secondary alkylperoxy radicals but seems to be preferred at higher temperatures. At temperatures below 373K these radicals appear to react principally by a non-radical... 

Although primary and secondary alkyl hydroperoxides are attacked by free radicals, as in equations 8 and 9, such reactions are not chain scission reactions since the alkylperoxy radicals terminate by disproportionation without forming the new radicals needed to continue the chain (53). Overall decomposition rates are faster than the tme first-order rates if radical-induced decompositions are not suppressed. 

Alkyl orthophosphate triesters, 79 41 terteAlkyl peroxycarbamates, decomposition of, 78 486 Alkyl peroxyesters, 78 478-487 chemical properties of, 78 480 487 physical properties of, 78 480 primary and secondary, 78 485 synthesis of, 78 478-480 synthetic routes to, 78 479 tert-Alkyl peroxyesters, 78 480 84, 485 as free-radical initiators, 74 284-286 properties of, 78 481-483t uses of, 78 487 Alkylperoxy radical, 74 291 Alkyl phenol ethoxylates, 8 678, 693 ... 

Co(acac)3 in combination with N-hydroxyphthalimide (NHPI) as cocatalyst mediates the aerobic oxidation of primary and secondary alcohols, to the corresponding carboxylic acids and ketones, respectively, e.g. Fig. 4.71 [205]. By analogy with other oxidations mediated by the Co/NHPI catalyst studied by Ishii and coworkers [206, 207], Fig. 4.71 probably involves a free radical mechanism. We attribute the promoting effect of NHPI to its ability to efficiently scavenge alkylperoxy radicals, suppressing the rate of termination by combination of al-kylperoxy radicals (see above for alkane oxidation). 

As the rate constants for the termination reaction of alkylperoxy radicals are normally in the sequence primary > secondary P tertiary, termination will be predominantly between the R OO radicals. 

Two criticisms of this mechanism can be made. First, these activation energies are overall activation energies for a two-step process for the decomposition of different alkylperoxy radicals [106] see opposite page. For the formation of 2-methyltetrahydrofuran both steps will involve cyclization and will have pre-exponential factors [104] of ca. lO sec", whereas the formation of pent-2-ene involves only one such step and a second step for which [39] A = 10 sec". Since the strain energy involved in the isomerizations of each of the alkylperoxy radicals is the same (ca. 6.5 kcal. mole" ) the activation energies of this step will only differ by the difference in primary and secondary C—H strengths (ca. 3.5 kcal. mole" ). It is difficult, therefore, to see how the overall activation energies for the formation of pent-2-ene and 2-methyltetrahydrofuran can be approximately equal. 

The available evidence suggests, therefore, that alkylperoxy radical disproportionation reactions are important for low molecular weight alkanes, but reliable values of the Arrhenius parameters for reactions (42) and (14a ) are urgently needed to confirm this. However, since is greater than fei4a by ca. 10 this will not be the case for alkanes which can form secondary or tertiary alkylperoxy radicals capable of undergoing extensive isomerization involving 1 5 or 1 6 H-transfer from further secondary or tertiary carbon atoms. 

Thus, in the case of 3-ethylpentane initial attack at a secondary C—H bond may always be followed by oxygen addition and 1 5 H-transfer involving another secondary C—H bond. Furthermore, since the initial attack is unselective during cool-flame oxidation a considerable proportion of primary alkylperoxy radicals will be formed from this alkane and these may all undergo the relatively easy isomerization involving 1 5-hydrogen transfer from a tertiary C—H bond... 

As a consequence of the simplicity of the propyl radical, studies of propane oxidation throughout the temperature range embracing the ntc region present an unrivalled opportunity to explore the extent to which the kinetic mechanisms involving alkylperoxy radical chemistry are consistent with experiment. However, interpretation of data is made difficult because molecular intermediate products can be more reactive than the parent fuel. Thus the experimental results may be complicated by secondary oxidation of the intermediates. For this reason, studies are made which involve only the very earliest stages of reaction [149]. The kinetics discussed in Chapter 1 may be applied to propane oxidation to give a skeleton structure. 

Alkylperoxy radical isomerization has a rather higher probability when 1-propyl radicals are involved in a corresponding competition because the activation energy for the primary H atom abstraction is higher than that for a secondary H atom abstraction. 

This is not a termination reaction. It is one means of converting alkylperoxy radicals to alkoxy radicals. It is the dominant reaction when neither peroxy radical contains an a-hydrogen, but it even occurs to a significant extent (in one report about 40% of the time [17]) with peroxy radicals that do contain a-hydrogens. Alkoxy radicals are vigorous hydrogen abstractors [12]. This appears to be the main reaction for primary alkoxy radicals the products are primary alcohols. Secondary and tertiary alkoxys, however, tend to undergo a competitive 6-scission reaction to a major extent [18] ... 

Alkylperoxy radicals are selective, and abstraction of a secondary H-atom is favored over abstraction of a primary H-atom. Alkoxy radicals, on the other hand, are less selective and can abstract primary hydrogens ... 

Alkylperoxy radicals participate in the chain propagation step of oxidation. In photo-oxidized PP, most ROO terminate after a few propagation steps [190]. The few ROO that escape from this recombination propagate with a great rate. At the same time, they are easily scavenged by chain-breaking phenolic and aromatic aminic AO or HAS derived NOH. Secondary and tertiary HAS associate in... 

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 ... 

CH3O2 is the most abundant alkylperoxy radical in hydrocarbon-rich atmospheres. It is of interest to assess the role of RO2 + CH3O2 cross-reactions since a series of rate constants has been obtained for such reactions, either resulting from indirect determinations (secondary reactions of RO2 + RO2 reactions) or from direct measurements. Rate constants for this class of reactions are reported in Table 8. 

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... 

The alkyl radical formed in the initial oxidation reactions subsequently react with O2 resulting in the formation of alkylperoxy radical. The alkylperoxy radical is the key intermediate in the oxidation of VOCs by OH radical and further reacts with HO2, NO2 and NO in the atmosphere. This intermediate can also undergo self-reaction and epoxidation reactions. These reactions lead to the formation of hydroperoxide adducts, alkoxy radical and O3, and also OH radical regeneration. The alkoxy radical is the second key intermediate in the VOCs oxidation by OH radical. The alkoxy radical undergoes prompt decomposition resulting in the oxidation of NO. The characterization of these secondary reactions is studied extensively using theoretical methods, but only very few experimental studies are available for such reactions. 

ESR of paramagnetic free radicals can be used to check the efficacy of AOs and other stabilisers. ESR was used in the study of phenothiazines as antioxidants in PP aromatic secondary amines can retard polymer oxidation by reacting with alkylperoxy radicals [824]. Tkac [825] has described hydrogen and electron transfer reactions of AOs by ESR and has shown the efficiency of the ESR technique in elucidating the relationship between structure and reactivity of radicals formed from antioxidants possessing different H- and e-donor functional groups, including (hindered) phenols, amines, etc. 

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