Bimolecular Decay of Peroxyl Radicals

The reaction of peroxyl radicals with alkenes may give rise to epoxides [reactions (44) and (45) cf. Morgan et al. 1984 Sawaki and Ogata 1984]. 

Peroxyl radicals which do not decay by one of the unimolecular processes discussed above must disappear bimolecularly. In contrast to many other radicals, they cannot undergo disproportionation. Hence they are left to decay via the recombination process, the results of which is a tetroxide intermediate [reaction (46) an exception may be their reaction with 02- cf. reaction (7)]., 

The tetroxide intermediate is a well-established in organic solvents at low temperatures (Bartlett and Guaraldi 1967 Adamic et al. 1969 Bennett et al. 1970 Howard and Bennett 1972 Howard 1978 Furimsky et al. 1980). However at the temperatures accessible in aqueous solutions the tetroxide, owing to its low ROO-OOR BDE, estimated at 21-33 kj mol1 (Benson and Shaw 1970 Nangia and Benson 1979 Bennett et al. 1987 Francisco and Williams 1988), can only attain a very low steady-state concentration. Even at the high radical concentrations achievable in the pulse radiolysis experiment, it has not yet been detected. Various decay processes of the tetroxide limits its steady-state concentration the reverse reaction [reaction (-46)] and its decay into products [reactions (47)-(50), R = alkyl or H], Most primary (and also some secondary) peroxyl radical decay with rate constants around 109 dm3 mol1 s1 (Neta et al. 1990). 

Depending on the identity of the peroxyl radicals involved, reactions (47)-(50) may occur in differing proportions. In particular, the product-forming self reaction of tertiary peroxyl radicals is restricted to path (49) and (50), since path (47) and (48) require the existence of C-H a to the peroxyl function. The exact mechanism of these reactions is still controversial, that is, whether the productforming processes are sequential or concerted. Reaction (47) has been described by Russell (1957) as a concerted process, and this process bears his name. It is formulated as having a six-membered transition state. 

Concerted decay Concerted decay, without and with two water molecules 

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In peroxyl-free-radical chemistry, H02702 " elimination reactions play a major role (Chap. 8.4). In polymer free-radical chemistry, these reactions are of special interest, because they lead to a conversion of slowly diffusing polymer-derived radicals into the readily diffusing HCV/CV radicals. The H02 /02 "-elimina-tion typically proceeds from an a-hydroxyalkylperoxyl radical [reaction (22)]. In poly(vinyl alcohol), for example, such an structural element is formed by H-abstraction and subsequent 02 addition [reactions (18) and (19)]. The same structural element may also be formed during the bimolecular decay of peroxyl radicals which carry an H-atom in [3-position [reactions (20) and (21)]. 

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]. 

Scheme 10.—Base-induced Elimination of HOt, and Bimolecular Decay of the Peroxyl Radical at C-5 of D-Clucose. ...

The H02- elimination at reaction (22) is often slow, and at a high concentration of peroxyl radicals this reaction may compete with the bimolecular decay of the peroxyl radicals (leading to chain scission Ulanski et al. 1994). However in the presence of base, deprotonation speeds up the 02 elimination [reactions (23)... 

In acid solutions, but also in neutral solutions at a high steady-state radical concentration, CV "-elimination becomes too slow to be of importance as compared to the bimolecular decay of the peroxyl radicals. This leads to a very different product distribution (Table 10.15). 

In the presence of O2, it is converted into the corresponding peroxyl radical [reaction (266)]. The bimolecular decay of this peroxyl radical with the other peroxyl radicals present is this system leads to erythrose [reaction (267)] in 15% yield, i.e., the p-fragmentation reaction of a short-lived oxyl radical intermediate is of minor importance. 

In the presence of GSH, 5 -d(T4AT7) and 5 -d(T4A) are formed [reactions (20) and (21)]. In the presence of 02, the primary radical is trapped by 02. In a subsequent step, the C(4 ) peroxyl radical is reduced to the corresponding hydroperoxide (the source of the reduction equivalent is as yet unknown potentially 02 generated in side reactions), and treatment with NH3 increases the yield of the glycolate which is also formed upon the bimolecular decay of the peroxyl radial [reactions (23)-(25)]. 

Yet, the corresponding geminal chlorohydrines have a lifetime of less than a few ps (Mertens et al. 1994). The related (i-Pr0)2P(0)0C(CH3)20H decays with k > 3 x 104 s"1, and the data on the peroxyl radical chemistry of trimethylphosphate (Schuchmann, von Sonntag 1984) will have to be reinterpreted in so far as the decay of (CH30)2P(0)CH20H into dimethylphosphate and formaldehyde must have occurred during the bimolecular decay of the peroxyl radicals, i.e. on the submillisecond time scale. 

The rate constant for the bimolecular decay of the a-tocopheroxyl radical is only 3.5x 102M-1 s . Therefore, its half-life is several hours in chloroform at ambient temperature. This implies that vitamin E free radical can react with a second peroxyl radical. In biological membranes, a-tocopherylquinone is generally believed to be the major end-product, but the mechanism of its production remains controversial. It may arise either from the decomposition of a-tocopherone, or from dismutation of a-tocopheroxyl radicals. However, the steady-state concentration of a-tocopherylquinone is usually too low to be measurable ex vivo when tissue homogenization and extraction are performed in the presence of pyrogallol and butylated hydroxytoluene, respectively,... 

In acid solutions, but also in neutral solutions at high steady-state radical concentrations, the superoxide elimination becomes too slow compared with the bimolecular decay of these peroxyl radicals [reactions (28)-(31)]. This leads to a very different product distribution, as seen in Table 5. There is evidence that in their bimolecular decay peroxyl radicals can give rise to the formation of oxyl radicals which may undergo fragmentation (see, e.g., [37, 38]) [e.g., reaction (30)], leading to products with the pyrimidine cycle destroyed (e.g., l-N-formyl-5-hydroxyhydantoin. Other pyrimidine-ring cleavage reactions are conceivable but at present not supported by product data). 

He supposed that the decay proceeds as the decomposition of alkoxyl radicals formed in the bimolecular reaction of two tertiary peroxyl radicals (see Chapter 2). 

For DNA in cells and in the absence of O2, one has to take into account that the lifetime of the DNA radicals is not determined by their bimolecular decay but rather by their reaction with the cellular thiols, mainly GSH (Chap. 12.11). In the presence of 02, the situation becomes more complex, and the lifetime of the DNA peroxyl radicals is as yet not ascertained. It is expected to be consider-... 

Michaels HB, Rasburn EJ, Hunt JW (1976) Interaction of the radiosensitizer para-nitroacetophenone with radiation-induced radicals on nucleic acid components. Radiat Res 65 250-267 Mieden OJ, Schuchmann MN, von Sonntag C (1993) Peptide peroxyl radicals base-induced O2 elimination versus bimolecular decay. A pulse radiolysis and product study. J Phys Chem 97 3783-3790... 

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