Chemistry Reference
In-Depth Information
Table 5.1.
Compilation of equilibrium constants of some dimeric radical anions
Equilibrium constant/dm
3
mol
−1
Dimeric radical
anion
Reference
Cl
2
•
−
6.0
×
10
4
1.4
Buxton et al. (1998);
Yu and Barker (2003)
10
5
×
Br
2
•
−
3.9
×
10
5
Liu et al. (2002)
I
2
•
−
1.1
×
10
5
Baxendale et al. (1968);
Schwarz and Bielski (1986)
(SCN)
2
•
−
2
×
10
5
Baxendale et al. (1968);
Buxton and Stuart (1995)
N
6
•
−
0.33
Alfassi et al. (1986)
In basic and neutral solutions, Cl
•
is a stronger oxidant than
•
OH (cf. Ta-
ble 5.2), and the formation of Cl
2
•
−
only proceeds in acid solution [reactions (4)
and (5); Anbar and Thomas 1964]. Details of this very complex situation and the
involvement of equilibrium (6) have been redetermined (Buxton et al. 1998). It is
evident that the even more strongly oxidizing f fluorine atom cannot be produced
this way.
(4)
HOCl
•
−
+ H
+
→
H
2
O + Cl
•
(5)
(6)
An exception is the reaction of
•
OH with the cyanide ion. Its
•
OH adduct rapidly
protonates even at high pH, but in this reaction the cyanide radical is not formed
because of its very high reduction potential (Wardman 1989). It rather under-
goes an enol
keto tautomerization [overall reaction (7); Behar and Fessenden
1972; Behar 1974; Büchler et al. 1976; Bielski and Allen 1977; Muñoz et al. 2000].
→
•
OH + CN
−
+ H
2
O
→
•
C(O)NH
2
(7)
Since bicarbonate/carbonate are omnipresent, it is of special interest that
•
OH
also reacts with these ions, yielding the oxidizing CO
3
•
−
radical [reactions (8 and
9);
k
8
= 3.9
10
6
dm
3
mol
−
1
s
−
1
; Buxton and Elliot
1986], i.e. bicarbonate is about 40 times less reactive.
10
8
dm
3
mol
−
1
s
−
1
,
k
9
= 8.5
×
×
•
OH + CO
3
2
−
OH
−
+ CO
3
•
−
→
(8)
Search WWH ::
Custom Search