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10
6
10
5
10
4
Serine
Glycine
Phenylalanine
Methionine
Gluconic acid
10
3
10
2
4
6
8
10
12
14
pH
Figure 6.39.
Second-order rate constants (
k
, /M/s) as a function of pH at 22°C for the
oxidation of carboxylic acids by ferrate(V). See color insert.
TABLE 6.10. p
K
a
and Rate Constants (/M/s) for the Reaction of Ferrate(V) Species
with Amino Acids in 0.025 M Phosphate at 23°C
Gly
Ser
Met
Phe
p
K
a
9.60
9.06
9.05
9.15
≤3.0 × 10
4
≤3.0 × 10
4
3.0 × 10
4
≤3.0 × 10
4
1.6 × 10
5
3.0 × 10
5
2.0 × 10
5
2.1 × 10
5
Estimated to be unimportant to the overall
reaction
4.5 × 10
6
k
138
(
H FeO RCH NH COO
−
+
(
+
)
−
)
2
4
3
k
139
(
HFeO
2
−
+
RCH NH COO
(
+
)
−
)
4
3
k
140
(
FeO
3
−
+
RCH NH COO
(
+
)
−
)
4
3
k
141
(
HFeO
2
−
+
RCH NH COO
(
)
−
)
8.0 × 10
6
2.8 × 10
6
2.6 × 10
6
4
2
k
142
(
FeO
3
−
+
RCH NH COO
(
)
−
)
<10
3
<10
3
<10
3
<10
3
4
2
Data taken from Rush and Bielski [382].
of the ferrate(V) species was
H FeO HFeO
< >
(Table 6.10). The
higher reactivity of
HFeO
2−
than
FeO
3−
could be explained by considering (1)
the oxygen atoms of
HFeO
2−
have a strong free radical character and (2)
HFeO
2−
is substitutionally more labile than
FeO
3−
, allowing
HFeO
2−
to expand
its coordination sphere. The latter may occur when the amine group attack on
the oxide ligand of ferrate(V) results in a ferrate(V)-AA complex. The
complex formation may account for the two-electron oxidation of amino acids
−
2
−
FeO
3
−
2
4
4
4
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