Russell mechanism radicals

Methyl ethyl ketone, a significant coproduct, seems likely to arise in large part from the termination reactions of j -butylperoxy radicals by the Russell mechanism (eq. 15, where R = CH and R = CH2CH2). Since alcohols oxidize rapidly vs paraffins, the j -butyl alcohol produced (eq. 15) is rapidly oxidized to methyl ethyl ketone. Some of the j -butyl alcohol probably arises from hydrogen abstraction by j -butoxy radicals, but the high efficiency to ethanol indicates this is a minor source. 

In this reaction, a 5 -phosphate end group is formed and the base is released. The products that result from the C(l )-C(3 ) fragment are not yet fully established. Malonaldehyde is a potential one. For this product to be formed, the C(3 ) per-oxyl radical has to be reduced, e.g., via the Russell mechanism (Chap. 8.8). 

Termination reactions. A very common termination reaction, known as the Russel mechanism from its discoverer, is the recombination of two peroxy radicals to form an unstable tetroxide that decomposes through a concerted mechanism to yield a hydroxy moiety and a carbonyl moiety (13) ... 

Several mechanisms have been suggested to produce the energy required to populate an excited carbonyl, which is at least 290-340 kj mol-1 [8]. Direct homolysis of hydroperoxides [9, 10], disproportion of alkoxy radicals [11] and /2-scission of alkoxy radicals [12] are all exothermic enough. However, the most widely accepted mechanism has been the highly exothermic (460 kj mol-1) bimolecular termination of primary or secondary alkyl per-oxyl radicals, i.e. the Russell mechanism (Scheme 2). It proceeds via an intermediate tetroxide to give an excited carbonyl, an alcohol, and oxygen [13, 14]. 

Polypropylene, in which tertiary radicals predominate, nevertheless gives CL. This has been an argument against the validity of the Russell mechanism, which requires at least one of the peroxy radicals to be primary or secondary. However, Mayo and co-workers [15, 16] showed that termination reactions are accompanied by production of alkoxy radicals that will cleave to... 

In the bimolecular decay of peroxyl radicals, a short-lived tetroxide is an intermediate. When a hydrogen is present in /3-position to the peroxyl function, a carbonyl compound plus an alcohol and O2 [Russell mechanism, e.g. reaction (42)] or two carbonyl compound plus H2O2 (Bennett mechanism, not shown) may be formed in competition to a decay into two oxyl radicals plus O2 [e.g. reaction (43) for details of peroxyl radical chemistry in aqueous solution, see Refs. 2 and 39]. 

There are two bimolecular reactions of peroxy radicals that are of special significance. The first is a chain termination reaction, widely proposed as the principal chain termination reaction (eq. (6)) and known as the Russell Mechanism [8, 10, 12, 16] ... 

The predominant reaction for the formation of cyclohexanol and cyclohexanone is the Russell mechanism of decomposition of secondary cyclohexylperoxy radicals, vhich first yields the product of coupling and then reacts by a non-radical, six-center 1,5-H-atom shift (termination of the radical-chain sequence) ... 

Peroxy radicals can react by yet other competing routes. For example, evidence for lipid peroxy radical combination through a tetraoxide has been reported recently (8). Such tetraoxides could generate singlet oxygen and nonradical products by the Russell mechanism (9) as shown in Reaction D. 

It is assumed, what recombination of cumyl peroxo radicals ROO occurs through intermediate formation of tetroxide RO R, decomposition of which is realized on Russell mechanism [34], (through cyclic six-member transient state) and results to formation of decay products CH, a-methyl styrene and 02,on scheme ... 

At atmospheric pressure, termination (k ) occurs first by the combination of peroxyl radicals to an unstable tetroxide intermediate followed rapidly by its decomposition by the Russell mechanism, to yield non-radical products (12),... 

Lipid peroxidation in microsomal fraction initiated by hydroperoxides or iron/ascorbate, as well as lipid peroxidation of isolated hepatocytes under oxygenation, has been studied extensively by Cadenas et al. (1981) and Cadenas and Sies (1982). The available evidence points to 02 formation by the Russell mechanism (Russell 1957, Howard and Ingold 1968). Secondary lipid-peroxy radicals react via a transient tetroxide to yield an alcohol, a carbonyl and oxygen, the carbonyl or oxygen being in an electronically excited state. 

Bimolecular interaction of alkyl-peroxy radicals must initially produce a highly unstable linear tetroxide. Where one of the radicals is secondary, this species rapidly reacts via the so-called Russell mechanism (77) to give a carbonyl and a hydroxy group ... 

Thus, the calculations [13] have produced evidence in favor of the Kornblum-Russell mechanism elucidating certain details of the nature and structural conditions for realization of its individual steps. Of course, so far only the simplest models have been considered, and the effect of the simplifying assumptions may in some cases be quite substantial. For example, experimental studies of radical-anions of various halogennitrobenzyl compounds IV using the impulse radiolysis technique [28] show that, unlike the chloronitromethyl radical considered earlier, the compounds IV do exist in solution for some finite time. Even so, their decomposition is, in accordance with the theoretical prediction, speeded up upon weakening of the C—X bond. The reaction proceeds by the scheme of intramolecular, inner-sphere electron transfer whose relatively low rate constant (10 -10 s) can be accounted for by the small overlap of the Ti-orbital, which carries the unpaired electron in the initial form IV and is localized predominantly at the nitro group, and the (j -orbital C—X ... 

The Russell mechanism requires one of the terminating radicals to be either primary or secondary so that a six-membered transition state can be formed. Such a mechanism may be prevalent in PE and PA, but not in PP, where the chaincarrying radical is tertiary. For this polymer, more complex alternative routes for chemiluminescence-produdng reactions have been proposed [Ref. 10, p. 175]. 

The reaction of these radicals takes place according to the Russell mechanism resulting in ketones [146a],... 

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. 

The work of Lindsay et at provides good evidence that not all S-RO2 undergo bimolecular self-reaction at ambient temperatures entirely by the Russell mechanism. It would, however, appear from their work that the yield of alkoxy radicals is very dependent on the structure of the peroxy radical. This lead these workers ( 6) to propose that the tetroxide may decompose by multiple bond scission by a concerted but non-eyelie mechanism. 

Scheme 3.29 Decay processes of secondary peroxyl radicals (Russel mechanism) [107].

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