Chemistry Reference
In-Depth Information
8.3
Geometries, Spin Densities, Oxidative Power
and p
K
a
Values of Peroxyl Radicals
Equilibrium geometries, harmonic vibrational frequencies, dipole moments and
hyperfine couplings for a series of peroxyl radicals have been calculated by an
ab initio method (Besler et al. 1986). The spin density of
17
O-enriched peroxyl
radicals correlates well with the Taft substituent parameter
* and the ET rate
with strong reductants (Sevilla et al. 1990b; for a theoretical study see Raiti and
Sevilla 1999; for further studies on the reduction of peroxyl radicals see Packer
et al. 1980; Alfassi et al. 1987; Asmus et al. 1988; Schuchmann and von Sonntag
1988; Neta et al. 1989; El-Agamey and McGarvey 2002). The redox potential of
simple alkylperoxyl radicals is E
7
= 0.77 V; it is substantially increased by elec-
tron-withdrawing substituents [E
7
(CCl
3
OO
•
) = 1.15 V, E
7
(RC(O)OO
•
) = 1.6 V;
Merényi et al. 1994; for arylperoxyl radicals, see Alfassi et al. 1995]. The highly
chlorinated peroxyl radicals oxidize some nucleobase anions (at high pH) quite
effectively (Kapoor and Gopinathan 1992), but their reduction potential is too
low to oxidize even Gua at substantial rates in neutral solutions.
A similar gradation is observed, when O
2
•
−
is the reductant. The rate of the re-
action of the most powerful peroxyl radical, the acetylperoxyl radical, with O
2
•
−
is close to diffusion-controlled [reaction (7);
k
σ
10
9
dm
3
mol
−
1
s
−
1
; Schuchmann
≈
-hydroxyethylperoxyl radical reacts with O
2
•
−
merely with a rate constant near 10
7
dm
3
mol
−
1
s
−
1
(Bothe et al. 1983).
and von Sonntag 1988], while the
α
CH
3
C(O)OO
•
+ O
2
•
−
CH
3
C(O)OO
−
+ O
2
→
(7)
The peroxyl radical group is among the most strongly electron-withdrawing
substituents (Schuchmann et al. 1989), and in a plot of the pK
a
values of substi-
tuted acetic acids vs. the Taft
* constant its value of 3.7 falls in between those
of the cyano and nitro groups. This strong electron withdrawing property of the
peroxyl radical function strongly lowers, of course, the p
K
a
value of the peroxyl
radical compared to that of the parent compound (acetic acid: Schuchmann et al.
1989; malonic acid: Schuchmann et al. 2000; formamide: Muñoz et al. 2000). p
K
a
values of peroxyl radicals can be predicted (Muñoz et al. 2000) using the above
Ta f t
σ
σ
* value and the compiled Taft parameters (Perrin et al. 1981).
8.4
HO
2
•
/O
2
•
−
-Elimination Reactions
Peroxyl radicals undergo a number of unimolecular processes. The most ubiq-
uitous one is the elimination of HO
2
•
/O
2
•
−
. They govern the peroxyl free-radical
chemistry of carbohydrates (von Sonntag 1980) and prevent their autoxidation
in aqueous solution (Schuchmann and von Sonntag 1978). The driving force of
the HO
2
•
elimination is due to the formation of a double bond [e.g
.
, reactions (8)
and (9);
k
8
= 650 s
−
1
;
k
9
= 800 s
−
1
; Bothe et al. 1977, 1983; Pan and von Sonntag
1990; Wang et al. 1993; Pan et al. 1993; Fang et al. 1995a]. It has been suggested
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