Carbon-Centered Radicals - Free-Radical-Induced DNA Damage and Its Repair - page 98

of 2.3 has been chosen here (Table 6.1), has been discussed in the given refer-
ence.
Methyl substitution increases the electron density at the neighboring carbon
due to hyperconjugation effects and thus shifts the p K a of the radical to higher
values as it does with the parent compound (Table 6.1).
As expected, radical cations may have especially low p K a values due to their
positive charge. A good example is phenol (p K a = 10) whose radical cation has a
p K a value of -2 (Dixon and Murphy 1976). Here, the difference with respect to its
parent is as large as 12 p K units [equilibrium (10)].

Similar effects are observed with the nucleobase-derived radical cations
(Chap. 10.2).

6.3
Oxidation of Carbon-Centered Radicals

Reduction potentials of radicals may be determined by pulse radiolysis
(Chap. 13.3) or photomodulated voltammetry (Wayner and Houman 1998; for a
compilation, see Steenken 1985; Wardman 1989).
Carbon-centered radicals which are substituted by electron-donating groups
such as NR 2 are readily oxidized. A convenient one-electron oxidant

is Fe(CN) 6 3 − . For example, it oxidizes

− OR or −

α -hydroxyalkyl or α -alkoxyalkyl radicals

10 9 dm 3 mol − 1 s − 1 ; Adams and
Willson 1969). Substitution by electron-donating groups that are not capable of
rapid deprotonation, such as

at practically diffusion-controlled rates ( k ≈ 2 ×

OCH 3 , stabilizes the resulting carbocation. They
then can have a considerable lifetime in water (Steenken et al. 1986b) while the
lifetime of the parent, C 2 H 4 • + , is only

−

∼ 100 fs (Mohr et al. 2000).

-hydroxyalkyl radicals, the corresponding carbonyl com-
pounds are formed in full yields. In contrast, the oxidation of

In the case of α

-alkoxyalkyl
radicals by Fe(CN) 6 3 − may not always be a straightforward outer-sphere ET reac-
tion (Janik et al. 2000a,b). Details are as yet not fully understood.
In studies on the • OH-induced aromatic hydroxylation, the oxidation of hy-
droxycyclohexadienyl radicals by Fe(CN) 6 3 − has often been used for the determi-
nation of the yield of a given precursor radical (Volkert et al. 1967; Volkert and
Schulte-Frohlinde 1968; Klein et al. 1975). Other oxidants such as Cu 2+ , Ag + , Fe 3+
or Cr 3+ give lower yields, and complications are apparent, since, for example,
the oxidation potential of Ag + (0.8 V) is higher than that of Fe(CN) 6 3 − (0.36 V;
Bhatia and Schuler 1974). The substituent has a strong inf luence on the rate of
oxidation (Table 6.2), and quantitative oxidation to the corresponding phenol
[reaction (11)] is only observed with electron-donating substituents (Buxton et
al. 1986). Even the terephthalate ion • OH-adduct requires the stronger oxidant

α