Manganese(III) oxidation of L-serine in aqueous sulfuric acid medium: Kinetics and mechanism

Article abstract of DOI:10.1002/(SICI)1097-4601(1999)31:8<525::AID-KIN1>3.0.CO;2-C

Kinetics and mechanism of oxidation of L-serine by manganese(III) ions have been studied in aqueous sulfuric acid medium at 323 K. Manganese(III) sulfate was prepared by an electrolytic oxidation of manganous sulfate in aqueous sulfuric acid. The dependencies of the reaction rate are: an unusual one and a half-order on [Mn(III)], first-order on [ser], an inverse first-order on [H⁺], and an inverse fractional-order on [Mn(II)]. Effects of complexing agents and varying solvent composition were studied. Solvent isotope studies in D₂O medium were made. The dependence of the reaction rate on temperature was studied and activation parameters were computed from Arrhenius-Eyring plots. A mechanism consistent with the observed kinetic data has been proposed and discussed.

Full text of DOI:10.1002/(SICI)1097-4601(1999)31:8<525::AID-KIN1>3.0.CO;2-C

Manganese(III) Oxidation
of L-Serine in Aqueous
Sulfuric Acid Medium:
Kinetics and Mechanism
S. CHANDRAJU,1 K. S. RANGAPPA,1 N. M. MADE GOWDA2
1Department of Studies in Chemistry, University of Mysore, Manasagangotri, Mysore-570 006, India
2Department of Chemistry, Western Illinois University, 1 University Circle, Macomb, Illinois 61455
Received 29 April 1998; accepted 11 March 1999
ABSTRACT: Kinetics and mechanism of oxidation of L-serine by manganese(III) ions have been
studied in aqueous sulfuric acid medium at 323 K. Manganese(III) sulfate was prepared by an
electrolytic oxidation of manganous sulfate in aqueous sulfuric acid. The dependencies of the
reaction rate are: an unusual one and a half-order on [Mn(III)], first-order on [ser], an inverse
first-order on [Hϩ], and an inverse fractional-order on [Mn(II)]. Effects of complexing agents
and varying solvent composition were studied. Solvent isotope studies in D2O medium were
made. The dependence of the reaction rate on temperature was studied and activation param-
eters were computed from Arrhenius-Eyring plots. A mechanism consistent with the observed
kinetic data has been proposed and discussed. ᭧ 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31:
525–530, 1999
INTRODUCTION
EXPERIMENTAL
Oxidations of organic substrates by manganese(III)are
of special importance due to their biological relevance
[1,2]. The reports on the kinetics of oxidation of var-
ious substrates by manganese(III) [3–12] include re-
actions in perchlorate, sulfate, acetate, and pyrophos-
phate media.
Amino acids serve an important function in our bio-
logical systems and play a significant role in metabo-
lism. In continuation of our interest in oxidation of
organic substrates by Mn(III) [13–18], we have un-
dertaken the present work. We here report the kinetics
and mechanism of oxidation of L-serine by manga-
nese(III) in aqueous sulfuric acid medium.
Solution of manganese(III) sulfate was prepared by the
electrolytic oxidation of 0.200 M manganese(II) sul-
fate in 1.50 M H2SO4 in an electrochemical cell [8].
Only a fraction of manganese(II) could be converted
into manganese(III) owing to low solubility of man-
ganese(III). Furthermore, manganese(III) is always as-
sociated with a large excess of manganese(II) to sup-
press the disproportionation as shown below.
2Mn(III)(aq) EF Mn(II)(aq) ϩ Mn(IV)(aq) (1)
Chromatographically pure sample of L-serine
(Sisco Research Laboratories, India) was further as-
sayed for amino acid content by standard methods
[19]. Freshly prepared aqueous solutions were used
throughout the experiments. Ionic strength of the me-
dium was adjusted by NaClO4. Triply distilled water
was employed for preparing experimental solutions.
Correspondence to: K. S. Rangappa or N. M. Made Gowda
᭧ 1999 John Wiley & Sons, Inc.
CCC 0538-8066/99/080525-06

526
CHANDRAJU, RANGAPPA, AND MADE GOWDA
3
dmϪ ), sulfuric acid, sodium perchlorate (to maintain
the total ionic strength constant), manganese(II) and
water (to keep the total volume constant) were taken
in stoppered boiling tubes. Each mixture was ther-
mally equilibrated in a water bath. To this solution was
added an aliquot of pre-equilibrated manganese(III)
sulfate stock solution to give a known overall concen-
tration. The progress of the reaction was monitored for
two half-lives by withdrawing aliquots at regular time
intervals and iodometrically determining the unreacted
manganese(III) concentration in each aliquot using
starch indicator near the end point. The rate constants,
kobs, calculated by the graphical method were repro-
ducible within Ϯ3% error.
Reaction Stoichiometry and Product
Analysis
Ϫ
3
Ϫ3
Figure 1 Absorption spectra of 4.00 ϫ 10 mol dm
The following stoichiometry was observed, with
manganese(III) in H2SO4. [H ](mol dm 3):1.20(a), 1.62(b),
ϩ
Ϫ
[Mn(III)] Ͼ [ser]:
2.04(c), 2.50(d).
2 Mn(III) ϩ RCH(NH2)COOH ϩ H2O !:
ϩ
2 Mn(II) ϩ RCHO ϩ NH4 ϩ CO2 ϩ Hϩ
Regression analyses of experimental data were carried
out on a scientific calculator SHARP EL-506G DAL.
where R ϭ ϪCH2OH. (2)
The aldehyde was quantified through its 2,4-dini-
trophenyl hydrazone derivative. The I.R. spectral data
of the hydrazone derivative supported its characteriza-
tion. Ammonium ion and CO2 were detected by the
conventional tests.
Preliminary Studies
The absorption spectra of manganese(III) sulfate so-
lution were recorded on a Beckman DU-6 UV-visible
spectrophotometer. The maximum absorption (␭
)
max
occurs at 490 nm. The molar absorptivity (␧) at
190 nm depended on pH of the solution (␧ ranged
Test for Free Radicals
1
1
from 131–111 L molϪ cmϪ at [Hϩ] ϭ 1.28–2.98
3
mol dmϪ , Fig. 1).
The reaction mixture initiated polymerization of ole-
finic monomers, acrylonitrile and methyl methacry-
late, indicating an in situ formation of free radicals.
Proper control experiments were also performed.
The formal redox potential (EЈo) of Mn(III)-Mn(II)
couple is a measure of the oxidizing power of the ox-
idant. Different but known amounts of manganese(III)
were anodically generated in a given volume of aque-
ous sulfuric acid and the electrode potential was mea-
sured each time.
RESULTS AND DISCUSSION
The potential measurements were also made at var-
ious concentrations of sulfuric acid and in the presence
Dependence of the Rate on [Mn(III)] and
[ser]
Ϫ
Ϫ
of added anions, HSO4 and ClO4 , and complexing
agents, P2O7 Ϫ FϪ, and ClϪ. A plot of the potential vs.
4
log[Mn(III)]/[Mn(II)] gave a straight line with a slope
of 0.0593 V for one-electron transfer systems at
298 K.
Kinetic runs were performed by varying concentra-
tions of Mn(III) at constant temperature, [Mn(II)],
[ser], [Hϩ], and ionic strength. Plots of log [Mn(III)]
vs. time, [Mn(III)]1/2 vs time, and [Mn(III)] vs time
were all nonlinear, while 1/[Mn(III)]1/2 vs time plots
were linear even beyond 75% of the reaction indicat-
ing that one-and-a-half order with respect to [Mn(III)]
exists (Fig. 2). Further, the values of kobs were repro-
Kinetics Measurements
Mixtures of solutions containing excess (8.00 ϫ
2
3
10Ϫ mol dmϪ ) of serine over Mn(III) (4.00 mol

MANGANESE(III) OXIDATION OF L-SERINE
527
Table II Effect of [Hϩ] on the Rate of Oxidation of
Serine by Manganese(III) Sulfate at 323 K
3
[Mn(III)]O ϭ 4.00 ϫ 10Ϫ3 mol dmϪ3; I ϭ 6.24 mol dmϪ
2
[Ser]O ϭ 8.00 ϫ 10Ϫ mol dmϪ3; [Mn(II)]O ϭ 0.126 mol
3
dmϪ
ϩ
[H2SO4]
(mol dm
[H ]
103 kobs
Ϫ3
Ϫ3
Ϫ1/2
Ϫ1
)
(mol dm
)
(dm3/2 mol
s )
1.50
2.00
2.50
3.00
3.50
4.00
1.62
2.08
2.50
2.96
3.50
4.00
6.56
5.25
4.17
3.63
3.14
2.70
Figure 2 Plots of 1/[Mn(III)]1/2 vs time: [Ser]o ϭ 8.00 ϫ
Ϫ
2
Ϫ
Ϫ
10 mol dm 3; [Mn(II)] ϭ 0.126 mol dm 3; [H2SO4] ϭ
؉
Dependence of the Rate on [H ]
1.50 mol dm 3; Temp. ϭ 323 K, and 103 [Mn(III)]O (mol
Ϫ
Ϫ
dm 3) ϭ 1.00(a), 2.00(b), 4.00(c), 6.00(d), 8.00(e).
Kinetic measurements were made in H2SO4-NaHSO4
solutions of different [Hϩ]. The effect [Hϩ] was eval-
uated using a standard curve [20] of [H2SO4] vs
[Hϩ]. The rate decreased with an increase in [Hϩ] (Ta-
ble II) and a plot of log kobs vs log [Hϩ] was linear
with a negative slope (Fig. 3) indicating an inverse
first-order dependence on [Hϩ].
ducible within Ϯ0.1% error (Table I). Reactions were
carried out keeping all experimental conditions con-
stant and by varying [ser]. Plot of log k vs log [ser]
gave a straight line with unit slope indicating the first
order with respect to serine.
Dependence of the Rate on [Mn(II)] and
Added Salts
Table I Pseudo One-and-a-Half-Order Rate Constants
(kobs) for the Oxidation of Serine by Manganese(III)
Sulfate at 323 K, [H2SO4] ϭ 1.50 mol dmϪ3, I ϭ 6.24
An increase in [Mn(II)]O, decreased the rate (Table I).
A plot of log kobs vs log [Mn(II)]O gave a straight line
with a negative slope representing an inverse-frac-
3
mol dmϪ
103kobs
103 [Mn(III)]O
102[ser]O
(mol dm
10[Mn(II)]O
(dm3/2
Ϫ3
Ϫ3
Ϫ3
Ϫ
Ϫ1
(mol dm
)
)
(mol dm
)
mol 1/2 s
)
1.00
2.00
4.00
6.00
8.00
10.0
12.0
4.00
4.00
4.00
4.00
4.00
4.00
4.00
4.00
4.00
4.00
4.00
4.00
4.00
4.00
8.00
8.00
8.00
8.00
8.00
8.00
8.00
4.00
6.00
8.00
10.0
12.0
14.0
16.0
8.00
8.00
8.00
8.00
8.00
8.00
8.00
1.26
1.26
1.26
1.26
1.26
1.26
1.26
1.26
1.26
1.26
1.26
1.26
1.26
1.26
0.520
0.820
1.26
1.72
2.26
2.78
3.34
6.60
6.56
6.56
6.52
6.50
6.54
6.52
3.07
4.68
6.56
8.71
10.23
12.30
15.49
13.18
9.55
6.56
5.13
4.27
3.51
3.09
ϩ
Figure 3 Plot of log k(obs) vs log [H ]: [Mn(III)]O ϭ
4.00 ϫ 10 mol dm 3; [Ser]O ϭ 8.00 ϫ 10 mol dmϪ3;
[Mn(II)]O ϭ 0.126 mol dm 3; I ϭ 6.24 mol dm 3; T ϭ
Ϫ3
Ϫ
Ϫ2
Ϫ
Ϫ
323 K.

528
CHANDRAJU, RANGAPPA, AND MADE GOWDA
Table III Effect of Complexing Agents on the Rate of Oxidation of Serine by Manganese(III) Sulfate at 323 K (Other
conditions are as in Table II)
Ϫ
Ϫ
4Ϫ
102 [Cl ]
103 kobs
102 [F ]
103 kobs
102[P2O7
(mol dm
]
)
103 kobs
Ϫ
3
Ϫ
Ϫ1
Ϫ
3
Ϫ
Ϫ1
Ϫ3
Ϫ
Ϫ1
(mol dm
)
(dm3/2 mol 1/2 s
)
(mol dm
)
(dm3/2 mol 1/2 s
)
(dm3/2 mol 1/2 s
)
—
0.50
1.0
6.56
6.24
5.98
5.46
—
0.5
1.0
1.5
6.56
6.26
6.02
5.82
—
0.50
1.0
6.56
4.96
4.18
3.46
1.5
1.5
2
Ϫ
Ϫ
[SO4
]
103 kobs
10[HSO4
(mol dm
]
)
103 kobs
Ϫ
3
Ϫ
Ϫ1
Ϫ3
Ϫ
Ϫ1
(mol dm
)
(dm3/2 mol 1/2 s
)
(dm3/2 mol 1/2 s
)
—
0.50
1.0
6.56
6.48
6.39
6.20
—
0.50
1.0
6.56
6.42
6.35
6.21
1.5
1.5
tional order (Ϫ0.78) with respect to [Mn(II)]. The ef-
ionic strength, indicating a negative salt effect (Table
IV).
4
fect of complexing agents such as P2O7 Ϫ, FϪ, ClϪ,
2
SO4 Ϫ and HSO4Ϫ on the rate of oxidation was inves-
tigated (Table III). The rate decreased with increasing
Dependence of the Rate on Temperature
4
concentration of P2O7 Ϫ, FϪ and ClϪ while HSO4Ϫ and
The reaction was studied at different temperatures
(313–328K) (Table V) and the activation parameters
were computed from Arrhenius-Eyring plots (Table
VI).
The Mn(III)-ser reaction stoichiometry of 2 : 1 was
observed as in eq. (2).
Amino acids are known to exist in the following
equilibria depending upon acidity or basicity of the
solution:
2
SO4 Ϫ had a negligible effect on the rate.
Addition of sodium pyrophosphate, sodium fluo-
ride and sodium chloride lower the redox potential of
Mn(III)-Mn(II) couple. This may be attributed to the
2Ϫ
displacement of coordinated SO4 ligand leading to
the establishment of a new Mn(III)-Mn(II) couple with
4
P2O7 Ϫ, FϪ, or ClϪ as ligands.
Dependence of the Rate on Dielectric
Constant of Solvent and Ionic Strength
R9CH9COOH
R9CH9COOϪ ϩ Hϩ
NH2
(SO)
NH2
(SϪ)
The dielectric constant (or permittivity, D) of solvent
medium was altered by the addition of HOAc to the
reaction mixture (0–15% v/v). The rate decreased
with increase in HOAc content of solvent (Table IV)
and a plot of log kobs vs 1/D was linear with a negative
slope (r ϭ 0.998). The reaction was studied at varying
ionic strength. The rate decreased with increase in
ϩHϩ
R9CH9COOϪ
R9CH9COOH
ϪHϩ
NH3
ϩ
(S)
NH3
ϩ
(SHϩ)
where S is a dipolar/zwitter ion of the amino acid.
Table IV Effect of Dielectric Constant of Solvent and
Ionic Strength on the Rate of Oxidation of Serine by
Manganese(III) in Sulfate Medium (Other conditions
are as in Table II)
Table V Pseudo One-and-a-Half-Order Rate
Constants (kobs) for the Oxidation of Serine by
Manganese(III) Sulfate at Different Temperatures
(Other conditions are as in Table II)
103 kobs
103 kobs
% of HOAc
(v/v)
(dm3/2
I
(dm3/2
Ϫ
Ϫ
1
Ϫ3
Ϫ
Ϫ1
mol 1/2 s
)
(mol dm
)
mol 1/2 s
)
T
(K)
103 kobs
(dm3/2 mol
s )
Ϫ1/2
Ϫ1
—
5.0
10.0
15.0
6.56
5.81
5.24
4.94
6.00
6.24
6.50
7.00
7.50
6.97
6.56
6.21
4.82
4.23
313
318
323
328
1.82
3.55
6.56
12.88

MANGANESE(III) OXIDATION OF L-SERINE
529
Table VI Comparison of Kinetic Data and Activation Parameters for the Oxidation of Serine by Manganese(III) in
Acetate, Pyrophosphate and Sulfate Media
Medium
Studied
Observed order
[Mn(II)]
ϩ
[Mn(III)]
[ser]
[H ]
Ref
HOAc
Pyrophosphate
H2SO4
0.5
1.0
1.5
1
1
1
0
Ϫ0.46
0.90
Ϫ1.07
13
13
Ϫ0.77
Ϫ1.74
This work
Ea
(kJ mol
⌬H‡
(kJ mol
⌬S‡
⌬G‡
(kJ mol
Ϫ1
Ϫ1
Ϫ
Ϫ1
Ϫ1
logA
)
)
(JK 1 mol
)
)
Ref
HOAc
Pyrophosphate
H2SO4
7.9
4.4
15.1
81.2
59.8
107.2
61.1
60.9
103.4
Ϫ156.2
Ϫ158.0
Ϫ24.7
111.7
112.7
111.4
13
13
This work
It has been assumed that Mn(III) species present
Based on the slow step in Scheme I, the overall rate
of oxidation of serine is given by
ϩ
in aqueous sulfuric acid medium are Mn3 (aq),
ϩ
Mn(OH)2 (aq), and MnSO4ϩ(aq), due to the existence
ϩ
rate ϭ Ϫd[Mn(III)]/dt ϭ k3[Mn3 ][Y]
(5)
of the following equilibrium:
ϩ
Ϫ
MnOH2 (aq) ϩ HSO4 (aq) EF
Substitution for [X], from equilibrium (i) in the steady
state equation for [Y] leads to eq. (6).
ϩ
MnSO4
ϩ H2O (4)
(aq)
Ϫ
Since the effect of HSO4 on the reaction is neg-
ligible, Mn3 (aq) and Mn(OH)2ϩ species become
ϩ
ϩ
ϩ
[Y] ϭ K1k2[Mn3 ][SHϩ]/[Hϩ]{kϪ [Mn2 ]
(aq)
2
ϩ k3[Mn3 ]} (6)
important, as reported for many Mn(III) reactions [6–
10,12]. In view of this, the proposed mechanism is
ϩ
shown in Scheme I.
MnOH2ϩ ϩ R9CH9COOH
Substitution for [Y] in Equation (5) leads to the fol-
lowing rate law:
K1
NH3
ϩ
rate ϭ K1k2k3[Mn(III)]2[ser]/[Hϩ]{kϪ [Mn(II)
2
ϩ k3[Mn(III)]} (7)
(SHϩ)
R9CH9COH 3ϩ ϩ H2O
fast (i)
The rate law is consistent with the observed kinetic
data: first-order in [ser], one-and-a-half-order in
[Mn(III)], a negative first-order in [Hϩ], and a negative
fractional-order in [Mn(II)].
NH2
O
Mn
R9CH9COH ϩ Mn2ϩ
NH2
The reaction rate in D2O medium usually decreases
in acid inhibiting reactions. It is well known that
D3Oϩ ion [16] is a stronger acid than H3Oϩ ion by a
factor of 2–3. The present observations substantiate
this view and also indicate a fast pre-equilibrium pro-
ton transfer [21]. The rate constant kH2O was 6.56 ϫ
(X)
k2
X
fast (ii)
kϪ2
O
.
ϩ
(Y)
1
1
10Ϫ3 sϪ while kD2O was 4.24 ϫ 10Ϫ3 sϪ leading to
the solvent isotope effect, kH2O/kD2O ϭ 1.55 and jus-
tifying the proposed mechanism.
k3
Mn3ϩ ϩ Y
.
ϩ
R9CH ϩ Mn2ϩ ϩ Hϩ ϩ CO2
slow (iii)
fast (iv)
It is observed that the addition of complexing ani-
4
ons such as P2O7 Ϫ, ClϪ, and FϪ decreased the rate of
NH2
(Z)
reaction by decreasing the redox potential. This may
be attributed to the displacement of the coordinated
ligand by the anion leading to the establishment of a
ϩ
R9CHO ϩ NH4
Z ϩ H2O
Ϫ
2Ϫ
new Mn(III)-Mn(II) couple. The HSO4 and SO4
Scheme I
ions had a negligible effect on the redox potential and
on the reaction rate.
It is seen from Table VI, highly negative values of
⌬S‡ in the case of acetate and pyrophosphate media
Here, R ϭ ϪCH2OH, SHϩ is protonated serine, X is
a metal-serine intermediate and Y and Z are transient
free radical cationic species.

530
CHANDRAJU, RANGAPPA, AND MADE GOWDA
indicated the formation of a more rigid transition state
while the relatively higher ⌬S‡ value in sulfuric acid
medium indicated a moderately rigid transition state.
BIBLIOGRAPHY
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The UV-Vis spectrum of pure manganese(III) has ␭
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at 490 nm. When serine is mixed with manganese(III),
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CONCLUSIONS
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The kinetic data of the oxidation of L-serine by
Mn(III) species in different media are summarized in
Table VI. It is found that the kinetic orders with re-
spect to [Mn(III)] vary: an half-order in acetate, a first-
order in pyrophosphate, and one-and-a-half-order in
sulfate media, respectively. This indicates that differ-
Ϫ
Ϫ
ent species of Mn(III), Mn(OAc)4 , Mn(H2P2O7)33
and MnOH2ϩ are the species present in acetate, pyro-
phosphate, and sulfate media, respectively. The order
in [ser] is generally one in all cases. The dependencies
on [Mn(II)] are: an inverse fractional order each in
pyrophosphate and sulfate media and a zero-order in
acetate medium. The other features include the varying
dependence on [Hϩ]: inverse fractional-order in ace-
tate, an inverse first-order in sulfate and a fractional-
order in pyrophosphate medium.
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One of the authors (SC) is grateful to the management, Bhar-
athi Education Trust, Bharathi Nagar, for permission to work
in the Department of Chemistry, University of Mysore,
Manasagangotri, Mysore, India.