Chemistry Reference
In-Depth Information
ably good reductant, while the phenoxyl radical formed upon water elimination
(Chap. 3.4) has oxidizing properties. Similar reactions play an important role in
purine free-radical chemistry (Chap. 10.3).
6.7
Reactions with Hydrogen Peroxide
Hydrogen peroxide is rather unique in so far as it can act as an oxidant as well as
an H-donor. This is exemplified its reaction with the hydroxymethyl radical. In
this case, ET [reaction (22);
k
= 6
10
4
dm
3
mol
−
1
s
−
1
] is faster than H-abstraction
×
10
3
dm
3
mol
−
1
s
−
1
(Ulanski and von Sonntag 1999)].
[reaction (32);
k
= 2.75
×
CH
2
O + H
+
+ OH
−
+
•
OH
•
CH
2
OH + H
2
O
2
→
(22)
•
CH
2
OH + H
2
O
2
→
CH
3
OH + HO
2
•
(23)
While the former induces a chain reaction, the latter terminates the chain. The
basic form of the hydroxymethyl radical, CH
2
O
•
−
[p
K
a
(
•
CH
2
OH) = 10.8], is a
stronger reducing agent [reaction (24);
k
= 4
10
5
dm
3
mol
−
1
s
−
1
], and the chain
×
length increases with increasing pH.
CH
2
O
•
−
+ H
2
O
2
CH
2
O + OH
−
+
•
OH
→
(24)
At very high pH, H
2
O
2
also dissociates [p
K
a
(H
2
O
2
) = 11.6]. Although the hy-
droxymethyl radical anion undergoes the H-abstraction even faster [reaction
(25),
k
= 2.9
10
4
dm
3
mol
−
1
s
−
1
] the chain length comes to a halt, because the
anion of H
2
O
2
is no longer a good electron acceptor.
×
CH
2
O
•
−
+ HO
2
−
CH
3
O
−
+ O
2
•
−
→
(25)
The methyl radicals can only undergo the H-abstraction reaction, and since the
C
H BDE is higher by ca. 38 kJ mol
−
1
in methane than in MeOH (McMillen and
Golden 1982; Golden et al. 1990; Berkowitz et al. 1994), the rate of reaction (26)
is higher (
k
= 2.7
−
10
4
dm
3
mol
−
1
s
−
1
; Ulanski et al. 1999) than that of reaction
×
(23).
•
CH
3
+ H
2
O
2
→
CH
4
+ HO
2
•
(26)
10
7
dm
3
mol
−
1
s
−
1
; Tsang and
Hampson 1986), the reaction is three orders of magnitude slower. A similar situ-
ation is found for
•
OH [reaction (27);
k
(in water) = 2.7
However, compared to the gas phase (
k
26
= 3.3
×
10
7
dm
3
mol
−
1
s
−
1
(Buxton
×
10
9
dm
3
mol
−
1
s
−
1
(Baulch et al. 1984)]. In water, a
low rate constant has also been found for the reaction of
•
OH with tertiary butyl-
hydroperoxide [reaction (28);
k
= 10
7
dm
3
mol
−
1
s
−
1
; (Phulkar et al. 1990)].
et al. 1988);
k
(gas phase) = 1
×
•
OH + H
2
O
2
H
2
O + HO
2
•
(27)
→
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