480
KANNAN AND RAMACHANDRAN
toward the nucleophilic attack [13]. The first step rep-
Eq. (5).
resents the nucleophilic interaction of the peroxide
at the NH3 group. This is equivalent to the re-
ꢆ
ꢇ
+
rc
r
[PMS]0 − [PMS]t
2
obs
action of peroxomonosulfate dianion with the elec-
= k2
ꢀ
ꢁ
(15)
obs
1
obs
k
+ k ([PMS] − [PMS]
0
t
+
trophile NH3 . The second and third steps (Eqs. (11)
and (12)) represent the electrophilic interaction of
the peroxide molecule. The peroxide oxygen is elec-
tronically saturated and less polarizable. Therefore,
it is more difficult for the carboxylate ion to ap-
proach the peroxide oxygen. This repulsion can be re-
rc
r
The ratio ( ) varies within the limits given by Eqs. (16)
and (17).
ꢄ
ꢅ
ꢅ
lim it
rc
r
t → 0
= 0
(16)
(17)
+
duced by the presence of H ion. This means that the
−
COOH group may be more reactive than the COO
ꢄr
ꢄ
ꢅ
k
obs[PMS]0
2
lim it
c
group.
t →∝
=
ꢀ
ꢁ
obs
1
+ k 2o bs[PMS]0
r
k
Perusal of the results in Table II shows that there is
no correlation between the rate constants and the struc-
ture of the amino acids. The oxidation of amino acid
through the carboxylate group (Eq. (12)) is observed
only in alanine. This can be explained as follows.
The breakdown of the complex may proceed through
the nonconcerted process in which carbon–carbon
bond cleavage is to precede carbon–nitrogen bond
formation. Thus the partial positive charge developed
on the ꢀ-carbon in the activated state is stabilized by
the electron-releasing/donating methyl groups thereby
favoring the reaction. If it is so, one would expect that
the reaction should occur in ꢀ-amino butyric acid also.
However, we have not observed the rate constant. This
may be due to the fact that the contributions from the
reactions (10) and (11) (Table II) are significant in
rc
r
Thus ( ) is plotted against time for glycine, N-methyl
glycine, alanine, and ꢀ-amino-n-butyric acid (Fig. 5).
−
3
In all the cases [PMS]0 = 3.8 (± 0.1) × 10 M and
∼75% conversion of the oxidant is followed. The lim-
iting value is 0.96 in glycine and decreases to 0.88 in
N-methyl glycine.This decrease is due to a large in-
obs
crease in k (∼12 times) relative to the increase in
1
obs
k
(∼4 times). This suggests that the substitution of
2
electron-donating methyl group at the nitrogen (reac-
−
tion center) enhances oxidation by both HSO5 and the
intermediate. The observed limits rc/r in alanine and
ꢀ-amino-n-butyric acid are ∼0.62 and ∼0.40. Even
though the substitution of electron-donating groups at
obs
1
the ꢀ-carbon enhances k (∼2 to 3 times), there is a
obs
ꢀ
-amino butyric acid and hence with the limited exper-
large decrease in k2 values (∼8 to 15 times). The oxi-
ꢂ
imental points we could not calculate k by nonlinear
dation products resulting from these amino acids are
aliphatic aldehydes namely formaldehyde, acetalde-
hyde, and propionaldehyde. Thus, +I group substitu-
tion at the carbonyl group results in a large decrease in
the reactivity of the intermediate and this will explain
why the critical concentration [AA]0 increases.
The foregoing discussions suggest that the catalytic
reaction occurs in all the amino acids studied in this
report and the relative importance depends upon the
reactivity of the intermediate resulting from the prod-
uct aldehyde and PMS. Further the substitution at the
amino group is in no way inhibiting the catalytic re-
action and in fact quickens the catalytic process. This
suggests that the formation of Schiff’s base type inter-
mediate between the amino acid and the corresponding
aliphatic aldehyde may not be the probable mechanism
for the autocatalysis.
1
regression.
The oxidation of 2-methyl alanine by PMS was
◦
also carried out at 35 C. The results showed that
the oxidation is a perfect first-order with respect
to each [PMS] and 2-methyl alanine concentration.
The oxidation of 2-methyl alanine differs from ala-
nine only in the final product where aliphatic ke-
tone (acetone) is obtained. Even the added acetone
has no effect on the rate of oxidation of 2-methyl
alanine. Therefore, the methyl substitution at the
ꢀ
-carbon atom of alanine removes the autocataly-
sis. This suggests that the reaction between alde-
hyde and PMS is responsible for the autocatalytic
effect.
The autocatalytic effect is observed in all the
amino acids including N-methyl glycine. In fact the
critical concentration for N-methyl glycine is the low-
est one (Table I). This suggests that the contribution
from the catalytic path is more significant in N-methyl
glycine than in other amino acids. The proportion of
the catalytic pathway with respect to the total reac-
tion can be calculated from the ratio of the rate of
the catalytic pathway (rc) to the total rate (r) from
The formation of relatively stable and water-soluble
hydroperoxides from aliphatic carbonyl compounds
have already been reported [14,15]. The hydroxyl hy-
droperoxides are the main products when equimolar
quantities of aldehyde and hydrogen peroxides are
used, and ꢀ,ꢀ-dihydroxy peroxides are produced when
excess aldehyde is used.