Chemistry Reference
In-Depth Information
lifetime of the DEPMPO adduct, it has been suggested to freeze the solution to
liquid nitrogen temperature for detection (Dambrova et al. 2000).
8.12
The 'Haber-Weiss Reaction'
In their brilliant papers, Haber and Weiss (1932, 1934) studied the iron-cata-
lyzed decomposition of H
2
O
2
and proposed the reaction of HO
2
•
with H
2
O
2
as a
single step [reaction (100)]. This has since been shown to proceed in two steps
[reactions (101) and (102); Czapski and Ilan 1978; Bielski 1985; Rush and Bielski
1985].
HO
2
•
+ H
2
O
2
→
H
2
O + O
2
+
•
OH
(100)
HO
2
•
+ Fe
3+
O
2
+ Fe
2+
+ H
+
→
(101)
Fe
2+
+ H
2
O
2
Fe
3+
+
•
OH + OH
−
→
(102)
The latter is well known as 'Fenton reaction' (Fenton and Jackson 1899). Over
the years, the 'Haber-Weiss reaction' [reaction (100)] has often been assumed to
be the source of
•
OH. The much greater likelihood of transition metal catalysis,
which is also possible in biological systems, has often been overlooked. Traces of
transition metal ions may be present in otherwise pure model systems (Fridov-
ich and Porter 1981), and may even be leached out of the glassware (Ulanski et
al. 1996c). It is now clear that the uncatalyzed reaction (100) is slow [
k
= 0.2 dm
3
mol
−
1
s
−
1
; Ferradini and Seide 1969;
k
= 0.5 dm
3
mol
−
1
s
−
1
,
k
(O
2
•
−
+ H
2
O
2
) = 0.13
10
9
dm
3
mol
−
1
s
−
1
; Weinstein and Bielski 1979] and can be neglected for the ma-
jority of experimental conditions. The thermodynamics of the uncatalyzed and
catalyzed 'Haber-Weiss reaction' has been discussed in some detail by Koppenol
(1983).
In contrast to the Haber-Weiss reaction, the reaction of O
2
•
−
with HOCl [reac-
tion (103)] proceeds without transition metal catalysis (Candeias et al. 1993).
×
HOCl + O
2
•
−
•
OH + Cl
−
+ O
2
→
(103)
This reaction is considered to be an important step in the phagocytic killing of
microorganisms by free-radical processes (Saran et al. 1999). In this reaction,
also Cl
•
was considered to play a role (for the complexities of
•
OH/Cl
−
reactions
in aqueous solution, see Yu and Barker 2003a,b; Yu et al. 2004).
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