Intramolecular and Intermolecular H-Abstraction Reactions

The H-abstraction reactions of peroxyl radicals are related to ET discussed above, as in both cases the same final product is formed, a hydroperoxide. Mechanistically, these two processes are, of course, different. Hydrogen-abstraction reactions by peroxyl radical, including H02-, are common (cf. the autoxidation of polyunsaturated lipids Hasegawa and Patterson 1978 Patterson and Hasegawa 1978 Patterson 1981 Porter et al. 1981 Gebicki and Bielski 1981 Barclay et al. 1989 Zhu and Sevilla 1990 Aikens and Dix 1991 Simic et al. 1992). This H-abstraction reaction may occur inframolecularly as well as infermolecularly [cf. reactions (29) and (30)]. 

Because of the often slow rate of reaction, even of intramolecular H-abstrac-tion and a favorable six-membered transition state such as in reaction (29) (k 1 s 1 Schuchmann and von Sonntag 1982 for further examples see, e.g., Ulanski et al. 1996a,b), the reaction is only observed at a low steady-state of peroxyl radicals, that is, when the lifetime of peroxyl radical is long. Kinetic parameters with pre-exponential factors near 108 and activation energies varying from 30-60 kj mol1 have been reported for the reactions of peroxyl radicals with various alcohols (Denisov and Denisova 1993). 

As a consequence of the reformation of the starting radical, a chain sets in 

If this mechanism is strictly followed the chain length and hence the value of 02-uptake (see below) increases linearly with the substrate concentration and (initiation rate) 1/2 (i.e., in radiolytic studies the dose rate) and in charged polymers also on the pH (cf. Ulanski et al. 1996a). In polymers, the chain reaction may mainly proceed intramolecularly (Ulanski et al. 1996a Janik et al. 2000). An example for an efficient intramolecular autoxidation is poly(acrylic acid) [reactions (34)-(36) Ulanski et al. 1996a], In these autoxidation reactions, hydroperoxides are formed which, in some cases, are quite unstable [e.g. reaction (37) see also Leitzke et al. 2001], 

In micellar systems, the chain peroxidation reaction only becomes effective at the critical micelle concentration where the substrate molecules aggregate thereby forming locally high concentrations (Gebicki and Allen 1969). 

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