The Termination Reactions of Alkylperoxy-Radicals

There is uncertainty about the nature of the termination reactions of alkylperoxy-radicals, although in all mechanisms the elimination of... 

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. 

In 1967 Benson ( ) noted that the most important area of disagreement, or perhaps uncertainty, which had been raised at the International Oxidation Symposi mi in San Francisco ( - ) was the nature of the termination reaction of alkylperoxy radicals. In the ten years that have elapsed since that meeting numerous papers have appeared on the kinetics and mechanisms of the self-reactions Of these radicals and it is true to say that we are no closer to a complete understanding of the mechanisms of these reactions than we were in 1967. 

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. 

The reaction of alkylperoxy radicals with H02 leads to the formation of stable alkylhydroperoxides. This is a radical chain termination process. 

Effectively, we must compare the symmetrical and crossed terminating or nonterminating reactions of alkyl radicals with alkylperoxy 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... 

Alkylperoxy radicals play vital roles in both propagation and termination processes. Hydroperoxides, R02H, are usually the primary products of liquid phase autoxidations [reaction (4)] and may be isolated in high yields in many cases. Much of the present knowledge of autoxidation mechanisms has resulted from studies of the reactions of alkylperoxy radicals30-33 and the parent hydroperoxides,348-d independently of autoxidation. Thus, the various modes of reaction of organic peroxides are now well-characterized.35 -39... 

Under normal autoxidation conditions, the termination step occurs exclusively by the self-reaction of two alkylperoxy radicals, which combine to form unstable tetroxides ... 

In recent years much emphasis has been placed on studies of co-oxidations, since they can provide quantitative data about fundamental processes (such as the relative reactivities of peroxy radicals toward various hydrocarbons48-50), which are difficult to obtain by other methods. Co-oxidations are also quite important from a practical viewpoint since it is possible to utilize the alkylperoxy intermediates for additional oxidation processes instead of wasting this active oxygen. That the addition of a second substrate to an autoxidation reaction can produce dramatic effects is illustrated by Russell s observation51 that the presence of 3 mole % of tetralin reduced the rate of cumene oxidation by two-thirds, despite the fact that tetralin itself is oxidized 10 times faster than cumene. The retardation is due to the higher rate of termination of the secondary tetralyl-peroxy radicals compared to the tertiary cumylperoxy radicals (see above). 

Kinetic parameters for the second-order termination reactions of some alkylperoxy radicals (From refs. 149, 150)... 

By analogy with other oxidations mediated by the Co/NHPI catalyst studied by Ishii and coworkers. Reaction 20 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 alkylperoxy radicals. The resulting PINO radical subsequently abstracts a hydrogen atom from the a-C-H bond of the alcohol to propagate the autoxidation chain (Reactions 21-23). 

The aUcyl radicals formed by the abstraction reaction react exclusively with O2 in the atmosphere to form alkylperoxy radicals (pathways (c) (h)), and most of them are then transformed to alkoxy radicals by oxidizing NO to NO2 in the presence of NOx (pathways (d) and (i)). However, some alkylperoxy radicals react with NO by the recombination isomerization reactions (e) and (j), to form alkyl nitrate (see Sect. 5.3.3). Although the yields of 2-butyl and 1-butyl nitrate are not large in the case of 2-butyl and 1-butyl radicals, 0.083 and <0.04, respectively, the production yields of alkyl nitrates increase with the increase of carbon number of alkyl radicals as described in Sect. 5.3.3, and it is as large as >0.2 for the Cg, and C7 secondary n-aUcyl radicals (2-hexyl, 3-hexyl, 2-heptyl, and 3-heptyl radicals) (Lightfoot et al. 1992 Arey et al. 2001). Since these reactions act as termination reactions for the OH chain reaction, they are important as parameters in determining the ozone formation efficiency in model calculations. The production yields of RONO2... 

This difficulty has now been overcome. Howard, Schwalm, and Ingold (24) show that the rate constant for reaction of any alkylperoxy radical with any hydrocarbon can be determined (by the sector method) by carrying out the autoxidation of the hydrocarbon in the presence of >0.1 M hydroperoxide corresponding to the chosen radical. All the absolute propagation and termination constants for the co-oxidation of cumene and Tetralin were thus determined. Our Tetralin-cumene work suggests that their results agree well with the best we have been able to get... 

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. 

At partial pressures of oxygen greater than approximately 100 Torr, chain termination occurs exclusively via the mutual destruction of two alkylperoxy radicals [reaction (6)]. The cross-termination reaction (5) may be neglected. The predicted rate expression, under steady-state conditions, is then given by... 

In the early stages of autoxidations, hydroperoxide concentrations are low and chain initiation is inefficient. Under these conditions, Mn(II) and Co(II) can act as inhibitors by scavenging alkylperoxy radicals [reaction (278)]. Competition in the termination step between the usual bimolecular termination of peroxy radicals and their reaction with metal complexes can affect the chain length of the autoxidation. The expression for the chain length in a process involving bimolecular termination of peroxy radicals is... 

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] ... 

If >>c(oo-)o-h = 75 kcal mole-1, then q = 30 kcal mole-1. Therefore, hyroxyperoxy radicals, in contrast to alkylperoxy radicals, display a dual reactivity. They can take part both in oxidation and in reduction reactions and they would be expected to react not only with radicals but with molecules of the oxidizing agent, with quinones for example. The kinetics of 2-propanol oxidation in the presence of benzoquinone has been studied [80], Quinones are known to terminate chains in hydrocarbon oxidation only by reactions with alkyl radicals [1]. In alcohol oxidation, quinone terminates chains by reaction with hydroxyalkyl as well as with hydroxyperoxy radicals [80]. At 71°C and PQl = 760 torr, 86% of chain termination is due to the reaction >C)0H)00- + quinone. The rate coefficient is M>C(0H)00- + quinone) = 3.2 X 1031 mole-1 s-1 and kQ/kp = 1.0 X 104. Just as in the case of aromatic amines, f> 2 f= 23 for quinone, i.e. quinone is regenerated in the reactions... 

The reaction of alkanes with a mixture of SO2 and O2 also occurs more readily than simple autoxidation of the hydrocarbon (reaction 30). Here the radical RSO2OO , formed as shown in Scheme 27, is apparently less prone to termination reactions than a simple alkylperoxy radicaF. ... 

Kinetic results were consistent with a bimolecular termination reaction whereas reaction products and mechanisms were something of a mystery. At that time it was known that the termination rate constant for autoxidation of cumene ( ) is about three orders of magnitude smaller than the termination rate constant for autoxidation of tetralin (7.). It was, however, generally accepted that the tennination rate constants for tertiary ( ) and secondary (9 ) alkylperoxy radicals are insensitive to the structure of the hydrocarbon residue in the radical. 

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