Chemistry Reference
In-Depth Information
Simple olefins do not react with e
aq
−
at an appreciable rate, but compounds with
an extended
-system such as butadiene can also accommodate an additional
electron
(
k
= 8
π
10
9
dm
3
mol
−
1
s
−
1
; Hart et al. 1964). However, as in the case
of benzene, the rate is often below diffusion controlled [reaction (23);
k
= 7.2
×
10
6
dm
3
mol;
1
s;
1
(Gordon et al. 1977); in THF, the reaction of the solvated elec-
tron with benzene is even reversible (Marasas et al. 2003)], and the resulting
radical anion is rapidly protonated by water [reaction (24)].
×
A rapid protonation by water of the electron adducts of spin traps such as DMPO
or 2-methyl-2-nitroso-propane yields the same species as are expected for the
reaction of H
•
(Sargent and Gardy 1975). This prevents a distinction between e
aq
−
and H
•
by using this technique.
H
•
readily adds to C
−
C double bonds. Like
•
OH, it is a pronounced electro-
philic radical (
0.45; Neta 1972) and thus shows a high regioselectivity in its
addition reactions. With e
aq
−
, it shares a fast reaction with O
2
[reaction (25);
k
=
1.2
ρ
=
−
10
10
dm
3
mol
−
1
s
−
1
].
×
H
•
+ O
2
HO
2
•
→
(25)
4.5
H-Abstraction Reactions
H
•
also undergoes H-abstraction reactions, albeit with much lower rates than
•
OH. This is also ref lected in a higher H/D isotope effect [e.g., with 2-PrOH/2-
PrOH-d
2
k
H
/
k
D
7.5, reactions (26) and (27) (Anbar and Meyerstein 1964); see
also Vacek and von Sonntag (1969), vs.
k
H
/
k
D
= 1.5 for
•
OH (Anbar et al. 1966)].
≈
•
H + HC(CH
3
)
2
OH
→
H
2
+
•
C(CH
3
)
2
OH
(26)
•
H + DC(CH
3
)
2
OH
→
HD +
•
C(CH
3
)
2
OH
(27)
If there is competition between addition and H-abstraction, addition is always
preferred. As a consequence, H-abstraction from the sugar moiety is a very mi-
nor process in DNA and related compounds (Das et al. 1985).
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