Chemistry Reference
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integration method (Eq. 6.72) in determining the correct values of noncatalytic
(
k
1
) and autocatalytic (
k
2
) rate constants, and in understanding the oxidation
of amino acids by permanganate in aqueous solutions.
Studies on the effects of buffers and colloidal MnO
2
have also been studied
on other α-amino acids [248]. Similar to Gly, both noncatalytic and autocata-
lytic pathways contributed to rate constants for the oxidation of Ala, Glu, Leu,
Ile, and Val by permanganate. Activation parameters of these two pathways
helped to propose mechanisms, which agreed with experimental results.
Recently, the kinetics of the
Tyr-MnO
−
system in the presence of cetyltri-
methylammonium bromide (CTAB) under acidic conditions has been carried
out to learn the effect of the pre- and postcritical micellar concentration
(CMC) of the CTAB on the reaction rates [249]. The rates of the reaction
decreased if the concentration of CTAB was below the CMC. However, if the
concentration was above the CMC, the rates increased. The -OH group in Tyr
was suggested to be responsible for the reactivity of Tyr with
MnO
−
. The tyro-
sine radical and Mn(VI) compound as intermediates and dityrosine as the
product of the reaction were suggested. This is similar to the oxidation of Tyr
by
MnO
−
and H
2
O
2
[250, 251]. Recently, a similar study has also been carried
out for the
MnO Val
−
−
system [252]. This study demonstrated the role of the
hydrophobic chain in the oxidation mechanism in the presence of CTAB.
The kinetics and mechanistic study on the oxidation of L-Met by
enneamolybdomangnate(IV) in perchloric acid has also been carried out
[253]. The orders of the reaction were first order with respect to each reactant.
An increase in [H
+
] increased the reaction rate, which was related to the pro-
tonation of the oxidant. The product of the reaction was methionine sulfoxide.
The postulated mechanism of the reaction involved a direct two-electron
(2 − e
−
) transfer step without the formation of a free radical.
Some reactions involving the oxidation of amino acids by permanganate
have also been performed in alkaline solutions. An example is the oxidation
of L-Phe by Mn(VII) in an alkaline medium [254]. The major oxidation prod-
ucts were identified as ammonia and aldehyde, and Mn(VI) was the reduced
product of Mn(VII). This is in contrast to the reaction studied under neutral
conditions in which colloidal MnO
2
was the reduced species (Eq. 6.78). This
indicates that Mn(VI), which formed initially in the alkaline medium (e.g., Eq.
6.85), did not further react with the reactant and products of the reaction. The
stoichiometry of the reaction was expressed in Equation (6.85):
4
−
−
2
MnO C H CH -CH NH COOH OH
MnO
+
(
)
+
2
4
6
5
2
2
(6.85)
2
−
→
2
+
NH CO C H CH CHO
+
+
+ H O
2
.
3
2
6
5
2
6.2.3.2 Aminopolycarboxylates (APCs).
Oxidation of APCs by permanga-
nate in high alkaline solutions (pH 12-14) has been performed in detail due
to the importance of the reactions in the oxidation of wastes containing chelat-
ing agents such as nitriloacetate (NTA) and ethylenediaminetetraacetate
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