Chemistry Reference
In-Depth Information
5.1
General Remarks
Reactive free radicals such as
•
OH or the e
aq
−
react with many inorganic anions
leading to inorganic radicals which have properties not given by the precursor
radicals. For this reason, these inorganic radicals have been used with advan-
tage, also in DNA research, to create and study free-radical intermediates other-
wise either not accessible or only formed in low yields in competition with other
reactions. A typical example is the specific formation of damaged G sites in DNA
by using inorganic radicals as the oxidant (Martin and Anderson 1998; Milligan
et al. 2000, 2002; Chap. 12.3).
5.2
Formation of Inorganic Radicals
and Their Dimeric Radical Anions
Hydroxyl radicals react with many halide (pseudohalide) ions at close to diffu-
sion-controlled rates thereby forming a three-electron-bonded adduct radical
[e.g., reaction (1);
k
= 1.1
10
10
dm
3
mol
−
1
s
−
1
; Zehavi and Rabani 1972]. These
adducts may decompose into OH
−
and the halide (pseudohalide) radical which
then complexes with another halide (pseudohalide) ion yielding the dihalogen
radical anion [reactions (2) and (3);
k
2
= 4.2
×
10
10
dm
3
mol
−
1
s
−
1
; for
resonance Raman spectra of such intermediates, see Tripathi et al. 1985].
10
6
s
−
1
;
k
3
×
≈
•
OH + Br
−
HOBr
•
−
→
(1)
HOBr
•
−
Br
•
+ OH
−
→
(2)
(3)
They are held together via a weak
* three-electron bond. This mechanism
adequately describes the reactions of Br
−
, I
−
, SCN
−
and N
3
−
. Equilibrium con-
stants are compiled in Table 5.1, where it can be seen that even at moderate ha-
lide (pseudohalide) concentrations the equilibrium is shifted to the right [cf.
reaction (3); for a direct determination of the forward reaction, see Nagarajan
and Fessenden 1985].
A number of mixed complexes have also been characterized (Schöneshöfer
and Henglein 1969, 1970; Schöneshöfer 1969, 1973; Ershov et al. 2002). In this
context, it is interesting that Cl
•
also undergoes a weak three-electron bond with
water (Sevilla et al. 1997).
Similar hypervalent iodine radicals (9
σσ
−
I
−
2) are formed in the reaction of al-
kyl radicals with alkyliodides (R
•
+ RI
R
2
I
•
), and as an intramolecular com-
plex they are stable enough that a reaction with O
2
is only low (Miranda et al.
2000). Such 9
→
2 radicals have also been postulated as intermediates in the
reduction of alkylhalides by
−
X
−
α
-hydroxyalkyl radicals (Lemmes and von Sonntag
1982).
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