Comparative Study of Kinetic and Mechanistic Study of Oxidation of L-Alanine and L-Proline by Sodium Periodate Catalyzed by Osmium(VIII) in Micromolar Concentrations

Article abstract of DOI:10.1134/S0036024419010096

Abstract: The kinetics of oxidation of two aliphatic α-amino acids (AA), namely, alanine and proline by NaIO₄ has been investigated in alkaline medium in the presence of osmium(VIII) catalyst at a constant ionic strength of 1.0 mol dm^–3 and at 25°C. The reactions were very slow to be measured in the absence of the catalyst. The reactions have a first order with respect both to [Os(VIII)] and [NaIO₄], and fractional order with respect to both [L-alanine] (Ala) and [L-proline](Pro). The reaction show negligible effect of dielectric constant and ionic strength of medium. Increasing [OH^–] concentration was found to decrease the oxidation rates while mercuric acetate acts as scavenger for both the reactions. A plausible oxidation mechanism has been proposed and the rate law expression has been derived. Both spectral and kinetic evidences revealed formation of intermediate complexes between AA and Os(VIII) before the rate-controlling step. Kinetic investigations have revealed that the order of reactivity is Pro > Ala. The complex thus formed reacts with the oxidant [NaIO₄] by an inner-sphere mechanism with formation of the oxidation products of the amino acids which were identified as the corresponding carboxylic compounds, ammonium ion and carbon dioxide. The activation parameters of the first order rate constants were evaluated and discussed.

Full text of DOI:10.1134/S0036024419010096

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2019, Vol. 93, No. 1, pp. 48–58. © Pleiades Publishing, Ltd., 2019.
CHEMICAL KINETICS
AND CATALYSIS
Comparative Study of Kinetic and Mechanistic Study of Oxidation
of L-Alanine and L-Proline by Sodium Periodate Catalyzed
by Osmium(VIII) in Micromolar Concentrations1
Madhu Guptaa, Amrita Srivastavaa,*, and Sheila Srivastavab
aDepartment of Chemistry, University of Lucknow, Lucknow, Uttar Pradesh, India
bChemical Laboratories, Feroze Gandhi College, Raebareli, Uttar Pradesh, India
*e-mail: amrita.sri18@gmail.com
Received March 11, 2018
Abstract—The kinetics of oxidation of two aliphatic α-amino acids (AA), namely, alanine and proline by
NaIO4 has been investigated in alkaline medium in the presence of osmium(VIII) catalyst at a constant ionic
strength of 1.0 mol dm–3 and at 25°C. The reactions were very slow to be measured in the absence of the cat-
alyst. The reactions have a first order with respect both to [Os(VIII)] and [NaIO4], and fractional order with
respect to both [L-alanine] (Ala) and [L-proline](Pro). The reaction show negligible effect of dielectric con-
stant and ionic strength of medium. Increasing [OH–] concentration was found to decrease the oxidation
rates while mercuric acetate acts as scavenger for both the reactions. A plausible oxidation mechanism has
been proposed and the rate law expression has been derived. Both spectral and kinetic evidences revealed for-
mation of intermediate complexes between AA and Os(VIII) before the rate-controlling step. Kinetic inves-
tigations have revealed that the order of reactivity is Pro > Ala. The complex thus formed reacts with the oxi-
dant [NaIO4] by an inner-sphere mechanism with formation of the oxidation products of the amino acids
which were identified as the corresponding carboxylic compounds, ammonium ion and carbon dioxide. The
activation parameters of the first order rate constants were evaluated and discussed.
Keywords: α-amino acids, [NaIO4], kinetics, mechanism, oxidation, osmium(VIII) catalyst
DOI: 10.1134/S0036024419010096
1. INTRODUCTION
involved in removal of pyruvate and glutamate from
the muscles and their transport to the liver. Basically it
occurs in animal protein sources like poultry, beef,
pork and fish. L-Ala levels increase during muscle
contraction in human body. L-Ala has chief role in
transfer of nitrogen from body tissues to liver and is
also involved in glyconeogenesis [7, 8]. Similarly
L-proline, a non-essential amino acid, is an important
component of collagen. According to [9], L-proline
catalyzes aldol condensation of acetone with various
aldehydes with high stereo-specificity. It is the small-
est natural enzyme in the world. L-Proline is oxidized
with the cleavage of pyrrolidine ring at the nearest car-
bon atom from the carboxylic acid group followed by
decarboxylation to 4-aminobutanol or 4-aminobu-
tyric acid, whereas D-proline produces ketoacid. Oxi-
dation of amino acids (AA) is a significant field of
organic chemistry due to the mechanism of amino
acid metabolism.
The transition metal ions (such as osmium, ruthe-
nium and iridium and their mixtures) act as catalysts
for different redox processes [1]. Os(VIII) as a catalyst
plays significant role in some redox reactions [2, 3].
Osmium(VIII) gives stable compounds. OsO4 can be
reduced to +2, +4, and +6 oxidation states in acidic
medium. However, in alkaline medium, it is reduced
to +6 oxidation state only [4]. The unique property
and most favorable reduction potential [5], of
Os(VIII)/Os(VI) is +0.85 V in acidic and +0.30 V in
alkaline media that helps it to act as catalyst with dif-
ferent oxidants in oxidation of relevant substrates
whether they are organic or inorganic [6].
Some aliphatic α-amino acids like alanine and
proline, are involved in the biosynthesis of proteins.
Alanine which is a non-essential amino acid is synthe-
sized in the human body from pyruvate and branched
chain amino acids such as valine, leucine, and isoleu-
cine. Alanine plays a key role in glucose-alanine cycle
between tissues and liver. The glucose-alanine cycle is
There is extensive literature on the kinetics of the
periodate oxidation of amino alcohols [10, 11], dicar-
bonyl compounds [6], and amino acids [10]. Periodate
is a clean and very selective reagent for the oxidation of
1
The article is published in the original.
48

COMPARATIVE STUDY OF KINETIC AND MECHANISTIC STUDY OF OXIDATION
49
O
O
I
Proline
or
Glutamic acid
or
O
+ OH−+ NH3
O−
+ 2 O
I
O− + H2O
+ 2
O
+
Os(VIII)
−
O−
alanine
acetaldehyde
O
O
Carbonate
Iodate
ion
Periodate
ion
Scheme 1. Scheme showing chemical reaction and stoichiometry of oxidation of amino acids by alkaline sodium periodate.
organic compounds containing hydroxy, oxo, amino, The initial rates were calculated from the slope of con-
or carboxyl groups. Oxidation with periodate is also
considered to cause protein denaturation and enzyme
inactivation. On the basis of these findings we have
studied the action of periodate on amino acids. Perio-
date with a redox potential of 0.70 V in alkaline
medium is a more appropriate reagent for oxidation of
organic as well as inorganic substrates [9]. To reveal
the active species of oxidant and catalyst and for deter-
mining the suitable mechanism of both substrates in
comparative way, the title reaction is investigated thor-
oughly.
centration vs. time graph in the initial stages of the
reactions by using plane mirror method. The rate con-
stants were reproducible within 5%. The rate of reac-
tion (dc/dt) in each kinetic run was determined by the
slope of the tangent drawn at fixed concentrations of
NaIO4 in the plots of unconsumed NaIO4 versus time.
The order of the reaction with respect to each reactant
was determined with the help of (dc/dt) values calcu-
lated for different concentrations of each reactant.
3. RESULT AND DISCUSSION
2. EXPERIMENTAL
2.1. Material and Reagents
3.1. Stoichiometry and Product Analysis
To determine the stoichiometry of the reaction,
different sets of experiments with various ratios of
[NaIO4] : [Amino Acids] were performed at 35°C for
48 h and keeping concentration of all other reactants
constant under conditions [NaIO4] ≫ [Amino Acids].
The unconsumed amount of periodate estimated in
different sets shows that 2 mol of NaIO4 was con-
sumed to oxidize 1 mol of amino acid. The following
stoichiometric equation can be written (Scheme 1).
All the chemicals used were of reagent grade; dou-
ble distilled water was used during the experiment.
Solutions of amino acids were prepared in double dis-
tilled water. The purity of samples was confirmed by its
melting point. The solutions of amino acids with nec-
essary concentration were prepared from its stock
solution. The osmium solution was made by dissolving
OsO4 (Johnson Matthey) in 0.50 mol dm–3 NaOH.
For maintaining ionic strength and alkalinity of reac-
tion, KClO3 and NaOH were used, respectively. The
standard solution of NaIO4 was prepared by dissolving
its weighed amount in double distilled water. This
solution was standardized by iodometric titration with
sodium thiosulfate solution which was in turn stan-
dardized against copper sulfate solution. The reaction
kinetics was monitored also by iodometric titration.
The reaction vessels were black coated from exterior to
avoid photochemical effects.
The main products of L-proline and L-alanine
oxidation were identified as L-glutamic acid and acet-
aldehyde, respectively. The products were separated
from reaction mixture by ether extraction, followed by
concentration by evaporation and mixing with con-
centrated hydrochloric acid (2 mL). The residue was
then evaporated several times with water (cal. 5 mL
portions) to remove the excess of hydrochloric acid
and finally with methanol (10 mL). The white needles
produced were collected and dried. The product of
L-Ala oxidation was confirmed by its FT-IR spectrum
showing a carbonyl stretching at 1729 cm–1 and –CH
stretching at 2730 cm–1 and –CH bending at 1370 cm–1
(Fig. 1). The product of L-Pro oxidation was con-
firmed by IR spectra with stretching frequencies of
‒NH2, –COOH and carbonyl appeared to be 3430,
2.2. Kinetic Measurements
The kinetic experiments were carried out at 35
0.1°C by mixing the previously thermostatted solution
of sodium periodate to the thermally equilibrated
reaction mixture containing required volume of solu-
tion of amino acids, Os(VIII), NaOH and all other
reagents. Aliquots (5 mL) of the reaction mixture were
taken out at definite intervals of time and poured into
a volumetric flask containing definite volume of
diluted H2SO4 and 4% KI solution. The quantity of
iodine liberated is correspondent to unconsumed oxi-
dant which was estimated with standard sodium thio-
3060, and 1684 cm–1, respectively and C–N vibration
frequencies at 1124 cm–1 (Fig. 2). These products are
confirmed by spot test [12], chromatography (TLC),
2,4-dinitrophenylhydrazine
(DNPH)
derivative
(Brady’s test). Carbon dioxide was qualitatively
detected by bubbling N2 through the acidified reaction
sulfate solution titration using starch as an indicator. mixture and passing the liberated gas through a tube
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50
MADHU GUPTA et al.
Product of alanine
100
50
0
4000
3000
2000
1500
1000
500
Wavenumber, cm−1
Fig. 1. Experimental FT-IR spectrum of product acetaldehyde.
Product of proline
0.8
0.6
0.4
0.2
3430
3060
1124
1684
3000
2000
1000
Wavenumber, cm−1
Fig. 2. Experimental FT-IR spectrum of product glutamic acid.
containing calcium hydroxide. Formation of ammonia of reaction becomes linear indicates a unit reaction
order in periodate. The first order dependence was
also proved from the plot of log [NaIO4] vs. log dc/dt
(Figs. 3, 4).
was determined using Nessler reagent [13]. The product
didn’t undergo oxidation further under these kinetic
conditions.
The effect of AA on the rate of reaction at constant
concentrations of alkali, Os(VIII) and periodate at a
constant ionic strength of 0.005 mol dm–3 were also
determined. The AA concentration is changing in-
3.2. Reaction Orders
The reaction order was calculated from the slope of
log [conc.] vs. log dc/dt plots by changing the concen- between 0.13 × 10–2 to 2.0 × 10–2 mol dm–3 at 35°C
while keeping other reactant concentrations and con-
ditions constant (Figs. 5, 6). The value of k1 rises on
increasing the concentration of AA, which indicates a
trations of AA, alkali, periodate, Os(VIII), and mer-
curic acetate in turn while maintaining all other con-
centrations and conditions constant. The reaction has
first order with respect to periodate between the con- less than unity order with respect to AA. The effect of
alkali in the range of 0.83 × 10–3 to 5.0 × 10–3 mol dm–3
at fixed concentrations of Os(VIII), AA, periodate and
centration range of 0.83 × 10–3 to 5.0 × 10–3 mol dm–3
at fixed concentrations of Os(VIII), NaOH, and
KClO3. The plots of dc/dt vs. time till 80% completion Hg(OAc)2 at constant ionic strength 0.005 mol dm–3
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COMPARATIVE STUDY OF KINETIC AND MECHANISTIC STUDY OF OXIDATION
4 + log[NaIO4]
51
0.8
1.0
1.2
1.4
1.6
1.8
0.643
0.60
0.642
0.641
0.640
0.639
0.638
0.637
0.636
0.635
0.634
0.55
0.50
0.45
0.40
0.35
0.30
log[OH−]
log[NaIO4]
0.8
1.0
1.2
1.4
1.6
1.8
4 + log[OH−]
–
Fig. 3. Plot of log [OH ] vs. log(dc/dt) and log[NaIO ] vs. log(dc/dt) for oxidation of L-Ala at 308 K.
4
4 + log[NaIO4]
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
0.8
0.30
0.25
0.20
0.15
0.10
0.05
0
0.7
0.6
0.5
0.4
0.3
0.2
log[OH−]
log[NaIO4]
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
4 + log[OH−]
–
Fig. 4. Plot of log [OH ] vs. log(dc/dt) and log[NaIO ] vs. log(dc/dt) for oxidation of L-Pro at 308 K.
4
on the reaction have been considered (Figs. 3, 4). The oxidant, alkali, reductant, and catalyst. It is observed
rate constants decreased with increase in alkali con-
centration shows negative fractional order dependence
of rate on alkali concentration.
that both added products had no effect on the reac-
tion rate. To maintain the OH– concentration the
moderately higher concentration of NaOH was used
in the reaction. Hence, the effect of soluble CO2 on
the rate was observed by carrying out the kinetics
experiments in the presence of CO2 and N2. It was
found that there was no variation of rate constants
which indicates that dissolved CO2 had none affect
3.3. Effect of Added Products
−
The effects of added products
and acetalde-
IO3
hyde were studied in concentration range 8.0 × 10–4
to 5.0 × 10–3 mol dm–3 at fixed concentrations of on rate of reaction.
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52
MADHU GUPTA et al.
3 + log[L-Ala]
0
0.2
0.4
0.6
0.8
1.0
1.2
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.65
0.60
0.55
0.50
0.45
0.40
0.35
Os(VIII)
L-Ala
0
0.2
0.4
0.6
0.8
1.0
1.2
6 + log[Os(VIII)]
Fig. 5. Plot of reaction rate log[Os(VIII)] vs. log(dc/dt) and log[L-Ala] vs. log(dc/dt) for oxidation of L-Ala at 308 K.
3 + log[L-Pro]
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0.75
0.25
0.20
0.15
0.10
0.05
0
0.70
0.65
0.60
0.55
0.50
Os(VIII)
L-Pro
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
6 + log[Os(VIII)]
Fig. 6. Plot of reaction rate log[Os(VIII)] vs. log(dc/dt) and log[L-Pro] vs. log(dc/dt) for oxidation of L-Pro at 308 K.
3.4. Effect of Ionic Strength
and Dielectric Constant
tures. The values specified in Table 1 were used to cal-
culate approximately the activation parameters. The
energy of activation was evaluated from the Arrhenius
plot of log k vs. 1/T (Fig. 7) and other activation
parameters obtained are tabulated in Table 1. It was
observed that the rate of oxidation of L-proline is
faster than that of oxidation of L-alanine (Fig. 7).
The effect of ionic strength was determined by
using KClO3 in concentration range between 0.1 to
0.9 mol dm–3 at constant concentrations of alkali, oxi-
dant, reductant and catalyst at 35°C. Similarly, dielec-
tric constant of the medium was also varied using
CH3COOH and H2O in different ratios. As result of
our experiments, we can conclude that both had no
significant effects on the rate of reaction.
3.6. Effect of [Os(VIII)]
The Os(VIII) concentration was varied in the range
from 1.31 × 10–6 to 7.88 × 10–6 mol dm–3, at constant
concentrations of NaOH, AA, periodate, Hg(OAc)2,
and ionic strength. The reaction order with respect to
[Os(VIII)] was established to be unity from the linear-
ity of plots of log[Os(VIII)] vs. log(dc/dt) (Figs. 5, 6).
3.5. Effect of Temperature
The effect of temperature on Os(VIII) catalyzed
oxidation was studied at 30, 35, 40, and 45°C. The rate
constants, (k2), of the slow step of Scheme 1 were cal-
culated from the slopes and the intercepts of the plots
of [Os(VIII)]/k1 vs. 1/[AA] at four different tempera-
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53
Table 1. Thermodynamic activation parameters for the oxidation of L-alanine by sodium periodate catalyzed by Os(VIII)
in alkaline medium
Reaction for oxidation of L-Ala and L-Pro
Thermodynamic activation parameters
L-Ala
L-Pro
L-Ala
11.73
L-Pro
14.45
temperature (K)
log A
10k (s–1)
10k (s–1)
303.0
308.0
313.0
318.0
1.84
2.49
3.46
4.42
1.96
2.44
2.97
3.56
Ea/(kJ mol–1)
41.58
40.18
–5.79
38.39
82.93
80.35
–6.61
71.83
∆H#/(kJ mol–1)
∆S #/(J K–1 mol–1)
∆G#/(kJ mol–1)
3.7. Reactive Species of Osmium(VIII) Tetraoxide
in Alkaline Medium
3.8. Reactive Species of Sodium Periodate
in Alkaline Medium
In aqueous alkaline medium and at high tempera-
Os(VIII) exists as different complexes at different
OH– [14] concentrations, such as [OsO4(OH)2]2– and
ture, periodate is present in form of HIO6 [8, 15]. Peri-
−
[OsO5(OH)]3–. As OH– concentration increases, the odic acid exists as H IO and as H
in neutral
4IO6
6
medium. Therefore, a5t pH > 7, the main species are
role of [OsO5(OH)]3– becomes significant. As in case
of present work, the concentration of OH– decreases
the rate of reaction, it is logical that [OsO5(OH)2]2– is
the active species (Scheme 1).
−, H 2−, and H
(Scheme 2). As the concen-
3−
IO4 3IO6
2IO6
tration rises, periodate forms a dimer [15, 16]. Com-
monly [17], in oxidation processes, periodate
decreases and OH– increases the rate of reaction. But
in present study, completely different results have been
obtained, since OH– retards the reaction (Table 2).
Among the various species of periodate in alkaline
0.70
0.65
0.60
0.55
0.50
0.45
0.40
medium, − is designated as the active species for the
IO4
title reaction.
3.9. Reactive Species of Amino Acid in Alkaline Medium
0.35
L-Ala
L-Proline has a cyclic structure with an imino [18]
group attached at one end by –CH2 and at other end
by >CH–COOH. Some reports about the oxidation of
L-proline claimed that the ring cleavage took place
between the N and C of, by retaining the –NH2 group
with the main moiety without liberating ammonia,
and the decarboxylation was proposed as a mechanism
for the oxidation [19]
0.30
0.25
0.20
L-Pro
31.2 31.4 31.6 31.8 32.0 32.2 32.4 32.6 32.8 33.0 33.2
1/T × 10−4
Fig. 7. Effect of temperature on rate constant for oxidation
of amino acids (AA).
H
N C COOH
H
−
+
⎧
⎫
H5IO5 ꢀ H4IO6 + H
⎪
⎪
⎨
⎪
⎪
⎬
H IO− ꢀ IO− + 2H O
4
6
4
2
H4IO6− ꢀ H3IO62− + H+
L-Ala exists in form of zwitterions [20, 21] in aqueous
medium. Particularly, in acidic medium it is in pro-
tonated form, while in alkaline medium it exist as
anionic form according the equilibrium given in
Scheme 3.
⎪
⎪
H IO ꢀ H3IO62− + H+
2−
6
⎪
⎪
⎩
⎭
3
Scheme 2. Various forms of periodate in different medium.
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MADHU GUPTA et al.
Table 2. Effect of variation of [NaIO ], [L-Ala], [OH–], [ −], and [Os(VIII)] on the osmium(VIII) catalyzed oxidation
IO4
4
of L-alanine by sodium periodate in aqueous alkaline medium at 298 K and i = 0.005 mol dm–3
7
7
K1 × 102
(observed) (observed)
L-Ala
K1 × 102
[NaIO4]
× 103,
mol dm–3
[Hg(OAc)2] (–dc/dt) × 10 , (–dc/dt) × 10 ,
Os(VIII)
× 106,
[NaOH]
× 103,
[S] × 102,
mol dm–3
×103,
mol dm–3 s–1 mol dm–3 s–1
mol dm–3 mol dm–3
L-Ala
L-Pro
mol dm–3
L-Pro
0.83
1.00
1.25
1.67
2.50
5.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
0.13
0.17
0.25
0.50
2.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
2.6
2.6
2.6
2.6
2.6
2.6
1.31
3.94
5.25
6.56
7.88
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
2.6
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
0.83
1.25
1.67
2.50
5.00
1.00
1.00
1.00
1.00
1.00
1.25
1.25
1.25
1.25
1.25
1.25
1.25
1.25
1.25
1.25
1.25
1.25
1.25
1.25
1.25
1.25
1.25
1.25
1.25
1.25
1.25
0.83
1.00
1.67
2.50
5.00
2.34
2.49
3.00
4.34
4.65
5.00
2.28
2.67
2.73
3.00
3.30
1.64
1.71
2.24
2.40
3.43
2.50
2.20
2.00
1.78
1.25
2.75
2.40
2.50
3.20
2.80
0.40
0.48
0.53
0.78
1.00
1.84
0.30
0.34
0.40
0.43
0.45
0.20
0.21
0.23
0.24
0.26
0.28
0.23
0.21
0.20
0.19
0.33
0.29
0.27
0.28
0.30
4.32
4.33
4.34
4.35
4.37
4.39
2.39
4.45
4.70
5.36
6.37
2.39
3.08
3.11
3.93
4.40
3.62
3.13
2.70
2.42
2.31
4.23
3.83
4.17
5.16
5.26
1.87
2.10
2.91
3.64
3.30
6.10
3.12
3.95
3.98
4.64
5.27
1.04
1.08
1.26
1.40
1.69
1.85
1.73
1.86
1.10
1.03
1.42
1.34
1.19
1.03
0.84
3.10. Various Complexes Formed during
the Studied Reactions
Ionic structure of AA reacts with Os(VIII) active spe-
cies to give a complex, which further react with one
mole of periodate in slow step with regeneration of
catalyst. After this it further react with another mole of
periodate in fast step with formation of products as
shown in Scheme 1. The probable structure of the
complex [C3] is given below:
The catalyst Os(VIII) forms the complex (C3) with
AA. Further, the catalyst Os(VIII) alters the reaction
pathway by lowering the energy of activation. The
Os(VIII) catalyzed reaction, however, is fast in view of
quickness of Os(VIII) to act across the ‒COO bond.
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COMPARATIVE STUDY OF KINETIC AND MECHANISTIC STUDY OF OXIDATION
55
2−
O
3−
O
O
O
O
OH
O
OH
OH
O
OH
O
Os
O
Os
O
NH
HO
O
NH2
Structure of the L-Ala complex
with Os(VIII)
Structure of the L-Pro complex
with Os(VIII)
The spectroscopic evidence for formation of complex
between catalyst and substrate was obtained from UV–
Vis spectra of AA (1.0 × 10–2 mol dm–3), Os(VIII)
(2.6 × 10–6 mol dm–3), [OH–] (1.0 × 10–3 mol dm–3),
and mixture of both. A hypsochromic shift between
amino acid spectrum and that of the mixture of
Os(VIII) and L-Ala was observed. The Lineweaver–
Burk plot proved the complex formation between
Os(VIII) and AA, which explains fractional order with
respect to [AA]. The rate law for Scheme 4 is derived as,
DISCUSSION
A Vant Hoff’s plot was prepared for the variation of
k with temperature (i.e., log k versus 1/T) (Fig. 1) and
the values of the enthalpy of reaction ∆H, entropy of
reaction ∆S, and free energy of reaction ∆G were cal-
culated. These values are also specified in Table 1.
Insignificant effect of ionic strength and dielectric
constant in catalyzed reaction might be due to involve-
ment of neutral species in the reaction. The negative
value of ∆S# shows that the intermediate complex is
more ordered than the reactants [22]. The observed
enthalpy of activation and an elevated rate constant for
the slow step indicates the fact that the oxidation most
possibly occurs by means of an inner sphere mecha-
nism. This conclusion is supported by a previous study
[23]. Deviation in the rate within the reaction series
may be caused by alteration in the enthalpy or entropy
of activation.
The mechanism as in Scheme 4 and rate law are
demonstrated by plotting the graphs of dc/dt versus
1/[OH–] and dc/dt versus 1/[AA] which should be lin-
ear (Figs. 3–6). From the slopes and intercepts of such
plots, the values of k, K1, and K2 are calculated. The K1
found in this study is in close conformity with the
reported value [15]. This proves the formation of
[OsO4(OH)2]2–. A negative value of ΔS* suggests that
the two ionic species combine in rate determining step
to give a single intermediate complex which is more
ordered than the reactants [15, 17, 21]. The low value
of rate constant for slow step of the mechanism con-
firms that the oxidation apparently occurs through an
inner-sphere mechanism. This conclusion was sup-
ported by previous literature [22–25].
d[IO4−]
dt
R a t e =
,
(1)
−




k3K2[IO4 ][Os(VIII)T ][AA]
Rate =
,
.

1 + K1[OH−] + K2[AA]




k3K2[Os(VIII)T ][AA]
Rate
(2)
k =
=

[IO4−]
1 + K [OH−] + K [AA]


1
2
The rate law (2) can be rearranged to Eq. (3), which
is suitable for verification
Os(VIII)T
K1[OH−]
k3K2[AA] k3K2[AA] k3
1
1
(3)
=
+
+
.
k
The plots of [Os(VIII)]/k vs. 1/[AA] and
[Os(VIII)]/k vs. [OH–] were linear (Figs. 8, 9), but the
oxidation mechanisms of both substrates are totally
different. Oxidation of L-Ala have free radical inter-
mediate, while oxidation of L-Pro involves intermedi-
ate compound.
O
O
aqueous
4. CONCLUSION
O−
OH
The deviation of concentration of Os(VIII) with
alkali as shown in Fig. 7 indicates that [OsO4(OH)2]2–
is the reactive species; its concentration depends lin-
early on [OH–] in a wide range of concentrations. The
concentrations of the other two species,
[OsO5(OH)]3– and [OsO3(OH)3]– are either
NH2
NH3
L-Alanine
O
O
basic
OH−
OH
O−
+ H2O
decreased or increased significantly at various [OH–]
and are not varied comparable to the variation of dc/dt
for different [OH–]. Hence, they are not measured as
reactive species. The formation of [OsO4(OH)2]2– is
significant in this study as reported earlier [14, 26].
NH2
NH2
L-Alanine
Scheme 3. Scheme showing zwitterions of L-Ala.
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56
MADHU GUPTA et al.
−
2−
O
Os
O
Os
O
k1
OH−
O
O
OH
O
OH
O
+
+H2O
k−1
H2O
OH
O
[C1]
[C2]
2−
O
Os
O
k2
k−2
O
OH
O
Amino acid
Complex
[C3]
+
OH
[C1]
2−
O
O
O
O
I
OH
O
slow
O−
O−
Adduct
Os
Complex
[C3]
+
+ O=I
O
+
+ CO2
k3
O−
OH
O
O
O
+ O=I
O
O
I
fast
+ OH−+ NH3
Adduct
Product
+ H2O
+
O−
O
CO2 + OH−
HCO3−
Scheme 4. Detailed scheme for the Os(VIII) catalyzed oxidation of AA by alkaline sodium periodate.
[OH−] × 10−3, mol dm−3
0
2
4
6
8
10 12 14 16 18 20 22
0.21
0.20
0.19
0.18
0.17
0.16
0.15
0.14
26
24
22
20
18
16
14
Pro
OH−
0
2
4
6
8
10 12 14 16 18 20 22
1/L-Pro × 10−2, dm3 mol−1
Fig. 8. Verification of rate law (A.4) in form of (A.5) for Os(VIII) catalyzed oxidation of L-Pro by sodium periodate at 25°C.
The fractional order with respect to each AA is an vation parameters were also considered for slow step of
implied fact to support the expectation of the pre-
equilibrium before rate determining step. Oxidation
the reaction. Activation parameters and catalytic con-
stants with respect to catalyst were also calculated.
Without Os(VIII), the reaction between AA and
sodium periodate is slow, whereas the addition of a
small amount of Os(VIII) accelerates the reaction due
to formation of adduct between AA and
products were determined. Among the variety of spe-
−
cies of periodate in alkaline medium,
is consid-
IO4
ered the active species for the title reaction. Os(VIII) is
found to be as [OsO4(OH)2]2− as active species. Acti-
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COMPARATIVE STUDY OF KINETIC AND MECHANISTIC STUDY OF OXIDATION
57
[OH−] × 10−3, mol dm−3
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
11.0
0.17
0.16
0.15
0.14
0.13
0.12
0.11
10.5
10.0
9.5
9.0
8.5
8.0
7.5
7.0
L-Ala
OH−
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
1/L-Ala × 10−2, dm3 mol−1
Fig. 9. Verification of rate law (A.4) in form of (A.5) for Os(VIII) catalyzed oxidation of L-Ala by sodium periodate at 25°C.
−
osmium(VIII). As a catalyst, osmium(VIII) did not
(A.4)
−k1C1[OH ]+ k−1[C2]− k2[C1][AA]+ k−2[C3]=0.
Similarly we have rate of formation of [C2],
undergo reduction to Os(VI), but it catalyzes the forma-
tion of active adduct and is regenerated by reaction with
− in the rate determining step to give a free radical.
IO4
d[C2]
dt
= k C [OH−] − k [C ].
(A.5)
1[ ]
1
−1
2
ACKNOWLEDGMENTS
On applying steady state approximation to the above
equation we get,
The authors are thankful to the Head, Department
of Chemistry, University of Lucknow, Lucknow, for
providing necessary laboratory facilities, spectral, ele-
mental and biological activity data. Authors wish to
thank reviewers for greatly improving the paper.
k1[C1][OH−] − k−1[C2] = 0,
(A.6)
k1[C1][OH−]
(A.7)
[C2] =
.
k−1
From Eqs. (A.4) and (A.6) we get,
CONFLICT OF INTEREST
k−2[C3]
k2[AA]
(A.8)
[C1] =
.
None of the authors of the above manuscript has
declared any conflict of interest which may arise from
being named as an author in the manuscript.
Putting the value of
in Eq. (A.7) we get,
k1k−2[OH−][C3]
k−1 k2[AA]
C
[ ]
1
[C2] =
,
APPENDIX
(A.9)
K1[OH−][C3] 
k1
k−1
k2 
k−2
Derivation of Rate Law for Scheme 1
[C2] =
∵ K =
; K2 =
.




1
d[IO4−]
dt
K2[AA]
= rate = k3[IO4−][Complex],
(A.1)
Therefore from Eqs. (A.2), (A.8), and (A.9), we get
total concentration of catalyst i.e.,
is equal to sum of concentration of,
[Os(VIII)T ]
[C3]
K2[AA]
K1[OH−][C3]
K2[AA]
(A.2)
(A.3)
Os(VIII)T = [C1] + [C2] + [C3],
Os(VIII)T =
+
+ [C3]
d[C1]
= −k1[C1][OH−] + k−1[C2]
[C3]
K2[AA]
K1[OH−][C3]
dt
=
+
+ [C3]
K2[AA]
− k2[C1][AA] + k−2[C3].
(A.10)
−


1 + K1[OH ] + K2[AA]
On applying steady state approximation to Eq. (A.3)
we get,
= [C ]
,



3
K2[AA]

RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 93 No. 1 2019

58
MADHU GUPTA et al.
12. G. H. Jeffery, J. Bassett, J. Mendham, and R. C. Denny,


K2[AA][Os(VIII)T ]
Vogel’s Textbook of Quantitative Chemical Analysis, 5th
[C ] =
.



3
1 +K [OH−] + K [AA]
ed. (ELBS Longman, Essex UK, 1996), p. 455.

1
2
13. A. I. Vogel, A Text Book of Practical Organic Chemistry
Including Quantitative Organic Analysis (ELBS Long-
man, Essex, UK, 1973), p. 332.
Therefore, from Eqs. (A.1) and (A.10) we get,
−




k3K2[IO4 ][Os(VIII)T ][AA]
(A.11)
Rate =
.

14. G. K. Vemulapalli, Physical Chemistry (Prentice-Hall,
1 + K1[OH−] + K2[AA]

Englewood Cliffs, NJ, 1997), p. 734.
The rate law is in agreement with all observed kinetics.
15. B. Sethuram, Some Aspects of Electron Transfer Reac-
tions Involving Organic Molecules (Allied, New Delhi,
2003), p. 78.
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