Chemistry Reference
In-Depth Information
TABLE 6.6. Second-Order Rate Constants,
k
(/M/s) of
Amines at pH 7.0 and 25°C
Compound
k
(/M/s)
Reference
MMA
4.0 × 10
1
[390]
DMA
3.3 × 10
2
[390]
DMA
8.9 × 10
0
[305]
1.6 × 10
1
TMA
[390]
DMAP
9.5 × 10
−1
[305]
DMEA
6.4 × 10
0
[305]
DMFA
5.5 × 10
−1
[305]
DMDC
>1.5 × 10
6
[305]
NDMA
9.8 × 10
−1
[305]
Aniline
6.6 × 10
3
[306]
4.4 × 10
5
DMAB
[305]
DMAI
2.8 × 10
2
[305]
DMAP
1.6 × 10
5
[305]
proposed based on the EPR signal in the reaction between ferrate(VI) and
aniline to produce azobenzene. Recently, a study under similar experimental
conditions did not confirm the free radical-based mechanism, and the forma-
tion of an imidoiron(VI) intermediate before the formation of
cis-
azobenzene
was suggested (Eq. 6.125) [376]:
C H NH
milliseconds-seconds
FeO
2
−
→
imidoiron VI inte
(
)
rmediate
6
5
2
4
(6.125)
C H NH
seconds-minutes
→
cis
-azobenzene Fe II
+
(
).
6
5
2
Similar steps have also been proposed in the oxidation of
para
-substituted
anilines [376]. Significantly, the oxidation of
ortho
-substituted anilines pro-
ceeds via a one-electron step, involving a free radical mechanism [376].
6.3.2.2 Amino Acids.
The oxidation of amino acids by ferrate(VI) and
ferrate(V) under anaerobic and alkaline conditions has been studied [377].
The rates of the reactions were determined as a function of pH and ranged
from 1.0 × 10
1
/M/s to 1.5 × 10
3
/M/s and 1.0 × 10
5
/M/s to 1.5 × 10
7
/M/s for the
oxidation by ferrate(VI) and ferrate(V), respectively. Linear Taft type plots of
the rate constants versus log(
K
/
K
0
) were constructed (Fig. 6.34) [377]. In the
plots,
K
0
is the ionization constant for the unsubstituted amino acid Gly
(
, and
K
's are the corresponding dissociation constants
of amino acids, in which the α-H atom has been replaced by different R groups
(
H CH NH COO
−
(
+
)
−
)
3
. The plots suggest ferrate(VI) and ferrate(V) react pref-
erentially with an
α−
R CH NH COO
−
(
+
)
−
)
3
(
3
group of the amino acids. The attack of
ferrate(V) on either nitrogen or the α-carbon atoms of the amino acids was
further explored by performing the reactivity of ferrate(V) with a methyl
CH NH
+
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