Chemistry Reference
In-Depth Information
The reactions of
•
OH with
HSO
−
(acidic medium) and
SO
2−
(alkaline
medium) also produce
SO
•−
(Eqs. 5.59, 5.60) [363]:
•
OH HSO
+
−
→
SO
•−
+
H O
k
=
1 7 10
.
×
7
/M/s
(5.59)
5
5
2
59
•
OH SO
+
2
−
→
SO
•−
+
OH
−
k
=
2 1 10
.
×
9
/M/s
.
(5.60)
5
5
60
The absorption band of
SO
•−
is in the UV region (λ
max
= 260−265 nm) with
ε = 1065 ± 80/M/cm [364].
5.3.1.2 Reactivity.
The
•
−
SO
3
radical can act as a reductant and an oxidant.
For example,
•
SO
3
can reduce
Fe O
−
VI
2−
to
Fe O
V
3−
(
k
= (1.9 ± 0.3) × 10
8
/M/s)
4
4
[365]. Reactions of
•
SO
3
with organic compounds involve hydrogen abstrac-
tion or addition to double bonds [352]. The
•
−
SO
3
oxidizes chlorpromazine,
hydroquinine, phenylenediamines, ascorbate, and methoxyphenol with rate
constants varying from 10
6
to 10
9
/M/s [352]. The rate constant of the
•
−
SO
3
radical with Trp at pH 3.2 is ∼8 × 10
4
/M/s, while the reactivity with gly is slug-
gish (
k
< 10
3
/M/s). The
SO
•−
radical is a stronger oxidant than the
•
−
SO
3
radical,
and it oxidizes biological molecules (e.g., ascorbate) more rapidly than
•
−
SO
3
[352]. Comparatively, the
SO
•−
radical is very reactive toward organic com-
pounds in which it is capable of oxidizing by electron transfer, abstracting H
atoms, or adding to double bonds.
In recent years, several studies of the
SO
•−
radical with compounds of
environmental interest have been carried out [366]. The compounds studied
include sulfur-containing compounds (dimethylsulfoxide, dimethylsulfone,
and methane sulfone), pyrimidines, pyridines, gallic acid, fulvic acid, humic
acids, dodecylbenzene sulfonate, iopromide, triclosan, sulfamethoxazole, acet-
aminophen, and microcystin-LR [354, 356, 360, 367-375]. Among the sulfur-
containing compounds, dimethylsulfoxide reacted the fastest at 298 K (Eqs.
5.61-5.63) [373]:
−
CH S O CH SO
(
)
+
•−
→
products
k
=
( .
3 0 0 4
±
. )
×
10
9
/M/s
(5.61)
3
3
4
61
CH O S O CH SO
(
) (
)
+
•−
→
products
k
<
( .
3 9 0 5
±
. )
×
10
6
/M/s
(5.62)
3
3
4
62
CH (O)S O O SO
(
)
−
+
•−
→
products
k
=
( .
1 1 0 3
±
. )
×
10
4
/M/s
.
(5.63)
3
4
63
The activation energies were determined as 12.0 ± 0.4, 11.3 ± 1.3, and
20.7 ± 4.3 kJ/mol for reactions (5.61)-(5.63), respectively. One-electron trans-
fer mechanism for reaction (5.61) has been demonstrated (Eq. 5.64) [376]:
•−
+
2
−
(5.64)
CH S O CH SO
(
)
+
→
CH S O CH SO
(
)
+
.
3
3
4
3
3
gallic acid and the gallate ion reacted with
SO
•−
with bimolecular rate
constants, (6.3 ± 0.7) × 10
8
/M/s and (2.9 ± 0.2) × 10
9
/M/s, respectively [368].
The mechanism involves a formation of the phenoxy radical by the hydrogen
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