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three concurrent pathways involved base-assisted electron transfer steps.
Pathway 1 involved the formation of an intermediate adduct, (HOCl)(O)O
−
,
which reacted rapidly to yield
ClO
−
and HOClO
2
. The formation of
ClO
−
occurred from the rapid reaction between HOClO
2
and OH
−
. This pathway
displayed first-order kinetics with respect to the concentrations of ClO
2
and
OH
−
. In pathway 2, a different intermediate, (OClOOH)
−
, reacted with ClO
2
to form
ClO
−
and an additional intermediate, OClOOH. The reaction of
OClOOH with OH
−
produced HOClO and HOO
−
. The reaction of the latter
species with ClO
2
could give
ClO
−
and O
2
[152]. An alternative to these steps
was also proposed in which OClOOH reacted with OH
−
to produce OClOH
−
,
which reacted with a second molecule of ClO
2
to generate OClOH and
ClO
−
.
Pathway 2 also gave a first-order rate expression with respect to the concentra-
tion of ClO
2
and was important at low levels of ClO
2
. Finally, pathway 3
involved the formation of a dimer intermediate, Cl
2
O
4
, which reacted with
OH
−
(an electron transfer step). This pathway displayed a second-order rate
expression with respect to the concentration of ClO
2
and was important at high
concentration of ClO
2
.
The catalytic effect of the hypohalite ion, Ox
−
(x = Cl, Br), on the dispro-
portionation of ClO
2
has also been studied [153]. A first-order dependence in
both [ClO
2
] and [Ox
−
] was observed at low concentrations of
ClO
−
. Reactions
became second-order in [ClO
2
] at excess [
ClO
−
], and observed rates were
inversely proportional to the concentration of
ClO
−
. In the first step of the
proposed steps of catalysis of the hypohalite, the reaction between ClO
2
and
Ox
−
involved an electron transfer to form
ClO
−
and Ox (Eqs. 3.19 and 3.20):
ClO OCl
+
−
ClO ClO
−
+
k
=
9 1 1
.
×
0
−
1
/M/s
;
K
=
5 1 1
.
×
0
−
1
0
(3.19)
2
2
19
19
−
−
−
7
.
(3.20)
ClO OBr
+
ClO BrO
+
k
=
2 0
.
/M/s
;
K
=
1 3 1
.
×
0
2
2
2
0
2
0
Values of
K
19
and
K
20
explain the observed suppression by
ClO
−
. The activa-
tion parameters,
∆
H
‡
and Δ
S
‡
, of the first step were 61 kJ/mol and −43 J/mol/K,
respectively, for OCl
−
/ClO
2
and 55 kJ/mol and −49 J/mol/K, respectively, for
OBr
−
/ClO
2
. The positive
∆
H
‡
and negative Δ
S
‡
values indicate that both ClO
2
and Ox
−
come together before electron transfer to form
ClO
−
and Ox. In the
second step, reactions between ClO
2
and xO were fast to form xOClO
2
(Eqs.
3.21 and 3.22):
ClO ClO ClOClO
+
→
k
= ×
7 10
9
/M/s
(3.21)
2
2
21
8
ClO BrO BrOClO
+
→
k
=
1 0 10
.
×
/M/s
.
(3.22)
2
2
22
The hydrolysis of xOClO
2
was also fast to yield
ClO
−
and Ox
−
. The catalysis
of ClO
2
disproportionation by
BrO
−
has also shown to be effective [153].
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