Peroxyl Radicals - Free-Radical-Induced DNA Damage and Its Repair - page 165

8.9
Alkoxyl Radicals in Peroxyl Radical Systems

In most peroxyl radical systems investigated so-far alkoxyl radicals play a cer-
tain, 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 10 6 s − 1 ) 1,2-H-shift [reaction (51)]. In competition, ≈

β -fragmentation also occurs [reaction (60)].

R 3 C − O • → R • + R 2 C=O (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-substi-
tuted one (weaker C

C bond) that is broken preferentially (Rüchardt 1987). Thus
in the case of secondary alkoxyl radicals, substitution in

−

β -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

β

-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).

β

R 3 CO • + RH → R 3 COH + R • (61)

Because of the rapid 1,2-H-shift [reaction (51)] and the ready conversion of
the ensuing -hydroxyalkyl radical into HO 2 • /O 2 • − by O 2 [cf. reactions (8) and

(10)/(11)], primary and secondary peroxyl are often the precursor of O 2 • − in the-
ses systems. Furthermore, the

α

-fragmentation reaction (60) creates a new radi-
cal 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).

β

8.10
Oxygen Uptake

Oxygen uptake measurements can give considerable mechanistic information.
In normal peroxyl radical reactions the G (O 2 -uptake) will range between 3 10 − 7 ×

10 − 7 mol J − 1 (Table 8.2). The lower value will be found when half of the
O 2 is reformed (for example, the formate system), the higher value when all O 2
is consumed during the decay of the peroxyl radicals. As soon as G (O 2 -uptake)
exceeds the upper limit of 6

and 6 ×

10 − 7 mol J − 1 , a chain reaction must prevail [exam-
ples are some polymers including poly(U)]. However, there is also the interesting
situation that G (O 2 -uptake) is below 3

×

10 − 7 mol J − 1 . This means that some of the
radicals do not react with O 2 . A case in point are the • OH-induced reactions of
purines (Chap. 10.3).

×