Kinetics and mechanism of oxidation of glutamic acid by n-bromophthalimide in aqueous acidic medium

Article abstract of DOI:10.1155/2011/572603

The kinetics of oxidation of glutamic acid (Glu) with N-bromophthalimide (NBP) was studied in perchloric acid medium at 30 °C by potentiometric method. The reaction is first order each in NBP and glutamic acid and is negative fractional order in [H+]. Addition of KBr or the reaction product, phthalimide had no effect on the rate. Similarly variation of ionic strength of the medium did not affect the rate of the reaction. Also the rate increased with decrease in dielectric constant of the reaction medium. The thermodynamic parameters were computed from Arrhenius and Eyring plots. A suitable mechanism consistent with the kinetic results has been proposed. Copyright E-Journal of Chemistry 2004-2011.

Full text of DOI:10.1155/2011/572603

ISSN: 0973-4945; CODEN ECJHAO
E-Journal of Chemistry
2011, 8(4), 1472-1477
http://www.e-journals.net
Kinetics and Mechanism of Oxidation of Glutamic Acid
by N-Bromophthalimide in Aqueous Acidic Medium
*
N.M.I. ALHAJI and S. SOFIYA LAWRENCE MARY
P.G. and Research Department of Chemistry
Khadir Mohideen College, Adirampattinam - 614 701, India
alhajinmi@yahoo.in
Received 20 April 2011; Accepted 25 June 2011
Abstract: The kinetics of oxidation of glutamic acid (Glu) with
o
N-bromophthalimide (NBP) was studied in perchloric acid medium at 30 C by
potentiometric method. The reaction is first order each in NBP and glutamic
+
acid and is negative fractional order in [H ]. Addition of KBr or the reaction
product, phthalimide had no effect on the rate. Similarly variation of ionic
strength of the medium did not affect the rate of the reaction. Also the rate
increased with decrease in dielectric constant of the reaction medium. The
thermodynamic parameters were computed from Arrhenius and Eyring plots.
A suitable mechanism consistent with the kinetic results has been proposed.
Keywords: Potentiometry, N-Bromophthalimide, Glutamic acid, Oxidation, Mechanism
Introduction
The studies of oxidation of amino acids are of utmost interest in order to understand the
metabolism of the proteins in biological systems. The amino and carboxyl functional groups
present in amino acids RCH(NH )COOH is more reactive compared to the hydrocarbon (R)
2
moiety and hence only these functional groups undergo chemical transformation. The N-halo
compounds interact with a wide range of functional groups as they are capable of existing as
1
halonium cations, hypohalite species and N-anions in aqueous solution . There are many
reports on the kinetics of oxidation of amino acids with N-halo compounds, such as
3-5
2
chloramine-B , chloramine-T
6
bromamine-T , bromamine-B , N-bromoactamide ,
7,8
9
,
N-chloronicotinimide , N-bromonicotinimide , N-chlorosaccharin and N-bromophthalimide
10
11
12
13
etc. Here we report the results of the kinetic studies of the oxidation of glutamic acid with
N-bromophthalimide in aqueous perchloric acid medium.
Experimental
N-Bromophthalimide (Aldrich, purity 99%) was used as received. The standard solution in
distilled water was prepared afresh every week and standardized against thio.
Chromatographically pure L-glutamic acid (SRL, India) was further assayed by acetous
14
perchloric method . All other chemicals were of Analar grade.

1
473
N.M.I. ALHAJI et al.
Kinetic measurements
The kinetic runs were carried out in aqueous perchloric acid medium. The ionic strength of
the reaction medium was maintained by addition of NaClO . The kinetics were followed
4
15
potentiometrically in a manner as described earlier . The emf of the cell was measured
periodically using an Equip-Tronics potentiometer while the reaction mixture was
magnetically stirred continuously. The temperature of the reaction mixture was maintained
o
at the desired value to an accuracy of ±0.1 C by circulating thermostated water in the
reaction vessel. The pseudo first-order rate constants (kobs) were calculated from the slopes
of ln (E – E ) versus time plots (r > 0.990) and the results were reproducible to an accuracy
t
∞
of ±5%. The second-order rate constants (k ) were obtained as kobs/[Glu].
2
Stoichiometry and product analysis
o
Varying ratios of NBP to glutamic acid were equilibrated at 30 C in presence of 0.20 M
HClO for 24 hours. Estimation of the unreacted NBP established a1:2 stoichiometry.
4
O
H NCH(R)COOH +
2
Br
N
O
+ 2H O
2
2
O
+
+ 2H + 2Br
−
HOOCCH CH COOH + NH
+ CO
+
2
HN
2
2
3
2
O
(
R = -CH CH COOH)
2 2
16
Ammonia was detected by Nessler’s reagent. Succinic acid was detected by spot test . The
liberated CO was confirmed by lime-water test.
2
Results and Discussion
Kinetics of oxidation of glutamic acid by N-bromophthalimide was investigated at several
initial concentrations of the reactants in HClO medium under pseudo-first order conditions
4
of [substrate] >> [NBP]. At constant [Glu] , [HClO ], [NaClO ] and temperature, plots of
o
4
4
ln(E - E ) vs. time were linear (r > 0.992) indicating first-order dependence of rate on
t
∞
[
confirming first-order dependence on [NBP] . Also, the rate data in Table 1 point out that
NBP] . Furthermore, the rate constants did not change with change in [NBP] (Table 1),
o o
o
kobs value increases linearly in a first-order fashion with increase in the initial concentration
of Glu. A plot of log kobs vs. log [Glu] was linear (r = 0.991) with slope value almost equal
o
to one. Also the plot of kobs vs. [Glu] passed through the origin (Figure 1; r = 0.992),
o
establishing the first order dependence of rate on [Glu] . The rate decreased with increase in
o
+
HClO ] (Table 1) and the plot of log k vs. log [H ] was found to be linear (r = 0.994)
[
showing a negative first-order dependence on [H ] . Further, the plot of k vs. 1/[H ] was
4
obs
o
+
+
o
obs
o
linear and passed through the origin (Figure 2), indicating that the oxidation occurred only
through the acid dependent path under these conditions.
The rate of the reaction is not significantly affected by the change in the ionic strength
of the medium (Table 2) brought about by the addition of sodium perchlorate, pointing out
the participation of a neutral species as a reactant in the rate-determining step. Addition of
phthalimide caused no effect on the rate of reaction (Table 2) suggesting that the step in
which phthalimide is formed as one of the products is not reversible.

Kinetics and Mechanism of Oxidation of Glutamic Acid
1474
Table 1. Pseudo first-order and second-order rate constants for the oxidation of glutamic
a,b
acid by NBP in aqueous perchloric acid at 303 K
3
0 [Glu] , M
4
10 [NBP] , M
2
+
10 [H ] , M
4
c
10 k , s
-1
2
d
10 k , M s
-1 -1
1
o
o
o
obs
2
2
4
8
.0
.0
.0
2.0
2.0
2.0
2.0
2.0
0.5
1.0
3.0
2.0
2.0
2.0
2.0
4.0
4.0
4.0
4.0
4.0
0.4
4.0
4.0
2.0
8.0
12.0
20.0
2.25±0.25
4.12±0.39
8.05±0.71
12.9±1.14
21.9±1.95
4.29±0.38
4.21±0.39
4.18±0.36
8.36±0.81
2.16±0.20
1.11±0.10
0.82±0.07
11.2±1.3
10.3±1.0
10.6±0.9
10.7±1.0
10.9±1.0
10.7±1.0
10.5±1.0
10.5±0.91
20.9±2.0
5.39±0.50
2.79±0.24
2.05±0.19
1
2
2.0
0.0
4
.0
.0
.0
.0
.0
.0
.0
4
4
4
4
4
4
a
As determined by a potentiometric technique following the disappearance of NBP; the error quoted in
1
k values is the 95% confidence limit of ‘Student t test’ . General conditions: [I] = 0.75 M; Estimated
d
from pseudo first-order plots over 60% reaction. Individual k values estimated as kobs/[Glu]o.
7
b
c
2
[
o
Glu] , M
Figure 1. Direct plot of kobs versus [amino acid] for the oxidation of glutamic acid with NBP

1
475
N.M.I. ALHAJI et al.
+
/[H ], M
-1
1
+
Figure 2. Plot of k vs. 1/[H ] for the oxidation of glutamic acid with NBP
2
Table 2. Influence of ionic strength and phthalimide (NHP) on reaction rate for the
a
oxidation of glutamic acid with NBP at 303 K
4
10 kobs, s
-1
4
10 [NHP], M
4
10 kobs, s
-1
[
I], M
0
.30
.50
.75
.00
4.08±0.41
4.07±0.39
4.12±0.39
4.18±0.40
0
4.12± 0.39
4.06±0.39
4.18±0.41
0
0
1
1.0
3.0
a
+
General conditions: [NBP] = 0.0002 M; [Glu] = 0.004 M; [H ] = 0.04 M; [I] = 0.75 M unless
otherwise stated
The involvement of free-radical intermediates during the reaction can be excluded as
the rate constant is not affected by the addition of acrylonitrile (Table 3). The added KBr has
no effect on the rate of oxidation (Table 3), establishing that the course of the oxidation does
+
not involve Br ion or Br as active species. Addition of acetonitrile to the reaction mixture
increased the rate (Table 3) and a plot of log kobs vs. 1/D is linear with a positive slope.
2
Table 3. Influence of added acrylonitrile (AN), KBr and solvent polarity on reaction rate for
a
the oxidation of glutamic acid with NBP at 303 K
-1
4
AN], M 10 kobs, s
4
10 kobs, s
-1
b
4
10 kobs, s
-1
[
[KBr], M
0
% CH CN (v/v)
3
0
4.12±0.39
4.16±0.40
4.18±0.40
4.12 ± 0.39
4.09 ± 0.39
4.08 ± 0.39
4.09 ± 0.40
0
4.12±0.39
5.28±0.51
7.36±0.75
10.1±0.98
0
0
.003
.006
0.001
0.002
10
20
0
.004
40
a
+
General conditions: [NBP] = 0.0004 M; [Glu] = 0.004 M; [H ] = 0.04 M; [I] = 0.75 M
The reaction was studied at three other temperatures (298, 308 and 318 K) keeping
other experimental conditions constant. From the arrehenius and eyring plots, the
thermodynamic parameters for the oxidation of glutamic acid were found to be E = 48.69
a
≠
≠
≠
1
1
kJ/mol, ∆H = 44.69 kJ/mol, ∆G = 79.12 kJ/mol and ∆S =114.84 JK mol .

Kinetics and Mechanism of Oxidation of Glutamic Acid
1476
Mechanism
+
The possible oxidation species in acidified NBP solution are NBP, NBPH , HOBr and
+
H OBr . In the present study the involvement of NBPH can be ruled out on the basis that
+
2
+
the reaction shows a negative dependence on [H ]. Molecular bromine or bromonium ion the
-
reactive species may not be the reactive species here because the added Br has no effect on
+
the rate of the reaction in present study. Another possibility of the HOBr or H OBr being
2
the reactive species can be excluded on the observation that the reaction rate is not affected
by the addition of phthalimide. Therefore the only choice left is to assume NBP as the
oxidizing species in the present study. Variation of the ionic strength of the medium does not
alter the rate indicating that no non-ionic species are involved in the rate determining step.
The dielectric effect is found to be positive and hence it can be inferred that the transition
state formed is less polar and there is charge disposal under these conditions. The amino acid
+
−
S ( NH CH(R)COO ) under the strongly acidic condition as in the present study exits as a
o
3
+
cation SH ( NH CH(R)COOH). The negative first-order dependence of reaction rate on
+
3
+
[
H ] indicates that the amino acid takes part in the reaction in its zwitter ionic form S .
o
Based on these findings, the following mechanism has been proposed for the oxidation of
glutamic acid by NBP (Scheme1).
K
1
(1)
H
3
NCH(R)COOH
H
NCH(R)COO
+
3
+
H
R = -CH CH
2
2
COOH
O
R
O
K
2
NH
3
Br
N
+
H
3
NCH(R)COO
C
slow
H
COO Br
N
O
O
fast
(2)
O
R
−
NH
3
+
N
C
H
COO Br
O
R
C
H
N
2
k
3
H
O
slowest
rds
H
R
CH NH2
+
−
(3)
(4)
C
O
Br
+ CO
2
+ H + Br
2
H O
RCHO +
NH +
4
R
CH NH
O
2
fast
O
−
H+
fast
N
+
NH
(5)
O
O
O
O
fast
RCHO + Br
N
+
RCOOH + H + Br
−
+
H
O
2
+
NH
(6)
O
O
Scheme 1

1
477
N.M.I. ALHAJI et al.
and the rate law is derived as
The proposed mechanism is also supported by the moderate value of energy of
≠
activation and other thermodynamic parameters. The large negative ∆S indicates the
formation of an arranged complex in the rate determining step.
Conclusion
The NBP oxidation of glutamic acid follows a S 2 type mechanism, in which NBP itself is
N
the oxidizing species. This mechanism is supported by the experimental data such as the
reaction stoichiometry, the oxidation products and the activation parameters.
Acknowledgment
The authors thank the college management for providing the necessary facilities.
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