Alkoxyl Radicals in Peroxyl Radical Systems

In most peroxyl radical systems investigated so-far alkoxyl radicals play a certain, albeit often not dominating role [cf. reaction (49)]. As mentioned above and discussed in more detail in Chap. 7.2, primary and secondary alkoxyl radicals undergo in water rapid (k 106s ) 1,2-H-shift [reaction (51)]. In competition, P-fragmentation also occurs [reaction (60)]. 

The rate of this reaction (which is the main decay of tertiary alkoxyl radicals) is also strongly enhanced in water as compared to the gas phase and organic solvents. If different substituents can be cleaved off, it is the more highly-substituted one (weaker C-C bond) that is broken preferentially (Riichardt 1987). Thus in the case of secondary alkoxyl radicals, substitution in p-position also decides the ratio of 1,2-H-shift and -fragmentation (Schuchmann and von Sonntag 1982). Because of the fast 1,2-H-shift and p-fragmentation reactions in water, intermolecular H-abstraction reactions of alkoxyl radicals [reaction (61)] are usually inefficient, but intramolecular H-abstraction may occur quite readily if an H atom is in a favorable distance (e.g., six-membered transition state). 

Because of the rapid 1,2-H-shift [reaction (51)] and the ready conversion of the ensuing a-hydroxyalkyl radical into H02702 by 02 [cf. reactions (8) and (10)/(11)], primary and secondary peroxyl are often the precursor of 02 in theses systems. Furthermore, the P-fragmentation reaction (60) creates a new radical and hence a new peroxyl radical, a situation which makes the elucidation of mechanistic details often very difficult if not impossible. The peroxyl radical systems that have been investigated in detail thus far have been discussed by von Sonntag and Schuchmann (1997). 

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