Peroxyl Radicals - Free-Radical-Induced DNA Damage and Its Repair - page 162

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

The rate constant increases with increasing electron-donating capacity of the
C

C double bond (Shoute et al. 1994) and oxidative power of the peroxyl radi-
cal (Sawaki and Ogata 1984), and the addition of acylperoxyl radicals has been
found to be about 10 5 times faster than that of alkylperoxyl radicals. In the gas
phase, the propyl-2-peroxyl radical adds to 2,3-dimethylbut-2-ene with a rate
constant of

−

6 dm 3 mol − 1 s − 1 (Sway and Waddington 1982). With rate constants
as low as this, the addition reaction (44) must be the rate determining step of
epoxide formation, since the rate of the subsequent step (45) ranges from 10 2 s − 1
(Phulkar et al. 1990) to 10 6 s − 1 (Bloodworth et al. 1984).

≈

8.8
Bimolecular Decay of Peroxyl Radicals

Peroxyl radicals which do not decay by one of the unimolecular processes dis-
cussed 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 O 2 • − ; cf. reaction (7)].,

(46)

The tetroxide intermediate is a well-established in organic solvents at low tem-
peratures (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 mol − 1 (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. Vari-
ous decay processes of the tetroxide limits its steady-state concentration: the re-
verse 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 10 9 dm 3 mol − 1 s − 1 (Neta et al. 1990).

−

R 2 CH-O 4 -CHR 2 R 2 C=O + R 2 CHOH + O 2 (47)

→

R 2 CH-O 4 -CHR 2 → 2 R 2 C=O + H 2 O 2 (48)