Chemistry Reference
In-Depth Information
The analogous reactions with N
2
O (Dainton and Peterson 1962) are commonly
used to convert e
aq
−
into
•
OH [reaction (10);
k
= 9.1
10
9
dm
3
mol
−
1
s
−
1
(Janata
and Schuler 1982)], since N
2
O is largely inert against free-radical attack. Some
much slower reactions, often leading to chain reactions, are given by other
strongly reducing radicals (Cheek and Swinnerton 1964; Sherman 1967a-c; Ryan
and Freeman 1977; Ryan et al. 1978; Al-Sheikhly et al. 1985; Wang et al. 1996).
×
e
aq
−
+ N
2
O
N
2
+ O
•
−
(O
•
−
+ H
2
O
•
OH + OH
−
)
→
→
(10)
10
10
dm
3
mol
−
1
s
−
1
] instead
of N
2
O may lead to complications, since H
2
O
2
also reacts with
•
OH,
•
H and many
other radicals (Chap. 6).
The alternative of using H
2
O
2
[reaction (11);
k
= 1.3
×
e
aq
−
+ H
2
O
2
•
OH + OH
−
→
(11)
In their reactions with tertiary butylhydroperoxide,
•
H and e
aq
−
show a differ-
ent selectivity (Phulkar et al. 1990). While
•
H undergoes reactions (12) and (13)
with about equal probability, i.e., both
t
BuO
•
and
•
OH are formed, i.e.,
aq
−
yields
only
t
BuO
•
[reaction (14)]. This preference in splitting the peroxidic bond is due
to the much higher solvation energy of the hydroxide compared to the tertiary
butoxide ion. For a detailed study on the reaction of
•
H with H
2
O
2
see Mezyk and
Bartels (1995).
t
BuOOH + H
•
→
t
BuO
•
+ H
2
O
(12)
t
BuOOH + H
•
→
t
BuOH +
•
OH
(13)
t
BuOOH + e
aq
−
t
BuO
•
+ OH
−
→
(14)
With thiols, e
aq
−
reacts predominantly by dissociative ET [reaction (15)], but also
formation of molecular hydrogen [reaction (16)] is observed (Hoffman and Ha-
yon 1973). As one might expect, there is a considerable drop in the rate constant
when the thiol group is deprotonated (for a systematic study, see Mezyk 1995).
e
aq
−
+ RSH
R
•
+ SH
−
→
(15)
e
aq
−
+ RSH (+ H
+
)
→
RS
•
+ H
2
(16)
In a number of reactions that are written as dissociative electron attachments,
short-lived radical anions are in fact intermediates. A case in point is 5BrUra
(Chap. 10). An interesting behavior is shown by the radical anion of
N
-bromo-
succinimide which does not release a bromide ion but rather fragments into
a bromine atom and a succinimide anion [reactions (17) and (18)] (Lind et al.
1991).
The concerted (one-step) dissociative ET is undergone by
σ
* electrophores;
the (
*) electrophores give rise to two-step processes (Savéant 1993; Schmit-
tel and Ghorai 2001).
σ
* +
π
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