Osmium(VIII) catalyzed oxidation of DL-ornithine monohydrochloride by a new oxidant, diperiodatoargentate(III) in aqueous alkaline medium

Article abstract of DOI:10.1080/15533171003766519

The kinetics of osmium(VIII) (Os(VIII)) catalyzed oxidation of DL-ornithine monohydrochloride (OMH) by diperiodatoargentate(III) (DPA) in alkaline medium at 298 K and a constant ionic strength of 0.10 mol dm^-3 was studied spectrophotometrically. The stoichiometry is, i.e., [OMH]:[DPA] [image omitted] 1:2. The main products were identified by spot tests, IR, 1H NMR, GC-MS spectral studies. A suitable mechanism is proposed. The reaction constants involved in the different steps of the mechanism are calculated. The catalytic constant (K_c) was also calculated for Os(VIII) catalysis at different temperatures. The active species of catalyst and oxidant have been identified. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15533171003766519

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Osmium(VIII) Catalyzed Oxidation of DL-
Ornithine Monohydrochloride by a New Oxidant,
Diperiodatoargentate(III) in Aqueous Alkaline
Medium
a
a
a
Shweta J. Malode , Jyothi C. Abbar & Sharanappa T. Nandibewoor
a
P.G. Department of Studies in Chemistry , Karnatak University , Dharwad, India
Published online: 10 May 2010.
To cite this article: Shweta J. Malode , Jyothi C. Abbar & Sharanappa T. Nandibewoor (2010) Osmium(VIII) Catalyzed
Oxidation of DL-Ornithine Monohydrochloride by a New Oxidant, Diperiodatoargentate(III) in Aqueous Alkaline Medium,
Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 40:4, 246-256
To link to this article: http://dx.doi.org/10.1080/15533171003766519
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 40:246–256, 2010
Copyright © Taylor & Francis Group, LLC
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533171003766519
Osmium(VIII) Catalyzed Oxidation of DL-Ornithine
Monohydrochloride by a New Oxidant,
Diperiodatoargentate(III) in Aqueous Alkaline Medium
Shweta J. Malode, Jyothi C. Abbar, and Sharanappa T. Nandibewoor
P.G. Department of Studies in Chemistry, Karnatak University, Dharwad, India
Diperiodatoargentate(III) (DPA) is a powerful oxidizing
[4]
The kinetics of osmium(VIII) (Os(VIII)) catalyzed oxidation agent in alkaline medium with the reduction potential, 1.74
of DL-ornithine monohydrochloride (OMH) by diperiodatoargen- V. It is widely used as a volumetric reagent for the determina-
tate(III) (DPA) in alkaline medium at 298 K and a constant ionic
strength of 0.10 mol dm was studied spectrophotometrically. The
[5,6]
tion of various organic and inorganic species.
Rao et al.
Jayaprakash
have used DPA as an oxidizing agent for the
kinetics of oxidation of various substrates. They normally found
−3
[7,8]
stoichiometry is, i.e., [OMH]:[DPA] = 1:2. The main products were
1
identified by spot tests, IR, H NMR, GC-MS spectral studies. A
suitable mechanism is proposed. The reaction constants involved in that order with respect to both oxidant and substrate was unity
the different steps of the mechanism are calculated. The catalytic and OH was found to enhance the rate of reaction. It was also
−
C
constant (K ) was also calculated for Os(VIII) catalysis at different
temperatures. The active species of catalyst and oxidant have been
identified.
observed that they did not arrive the possible active species of
DPA in alkali; on the other hand, they proposed mechanisms by
(x+1)−
generalizing the DPA as [Ag(HL)L]
al.
. However, Kumar et
put in an effort to give evidence for the reactive form
[
9–11]
Keywords DL-Ornithine monohydrochloride, diperiodatoargentate
of DPA in the large scale of alkaline pH. When the Ag(III)
species are involved, it would be interesting to know which of
the species is the active oxidant. In the present investigation, we
have obtained the evidence for the reactive species for the DPA
in alkaline medium. The DPA is a metal complex with Ag in
(
III), kinetics, Os(VIII) catalysis, oxidation
1. INTRODUCTION
Amino acids act not only as the building blocks in protein
3+
3+
3
+ oxidation state like Cu in DPC and Fe in hemoglobin.
syntheses but they also play a significant role in metabolism
and have been oxidized by a variety of oxidizing agents.[
The study of the oxidation of amino acids is of interest
because of their biological significance and selectivity towards
Transition metals are known to catalyze many oxidation-
1]
reduction reactions since they involve multiple oxidation states.
In recent years, the use of transition metal ions such as osmium,
ruthenium, palladium, chromium and iridium either alone,as bi-
nary mixtures, or as catalysts in various redox processes has
attracted considerable interest. The role of osmium(VIII) as
a catalyst in some redox reactions has been reviewed.
though the mechanism of catalysis depends on the nature of the
substrate, oxidant and on experimental conditions, it has been
that metal ions act as catalysts by one of these dif-
ferent paths such as the formation of complexes with reactants
[2]
the oxidant to yield the different products. DL-Ornithine
monohydrochloride [(± )-2, 5-Diaminopentanoic acid monohy-
drochloride] (OMH) is the most potent amino acid studied for
stimulating the production of release of growth hormone in the
body from the pituitary gland, which in turn helps with fat
metabolism. It is further required for a properly functioning im-
mune system and liver, and assists in ammonia detoxification
and liver rejuvenation. It is also of use in healing and repairing
skin and tissue, and is found in both these body parts. Ornithine
has the ability to regenerate the thymus gland, liver, and heart
tissue, enhance muscle growth, and increase immune system
function.[
[12]
[13]
Al-
[14]
shown
or oxidation of the substrate itself or through the formation of
[15]
free radicals. In an earlier report,
it has been observed that
Os(VIII) forms a complex substrate, which gets oxidized by the
oxidant to form a Os(VII) intermediate followed by the rapid
reaction of Os(VII) with one more mole of oxidant to regenerate
Os(VIII). In another report,[ it has been observed that oxidant-
substrate complex reacts with Os(VIII) to form Os(VI), which
reacts with oxidant in a fast step to regenerate Os(VIII). In some
other reports, it is observed that Os(VIII) forms complex with
substrate, which gets oxidized by the oxidant with the regen-
eration of Os(VIII). Hence, understanding the role of Os(VIII)
3]
16]
Received 8 December 2009; accepted 1 February 2010.
Address correspondence to S.T. Nandibewoor, P.G. Department of
Studies in Chemistry, Karnatak University, Dharwad 580 003, India.
E-mail: stnandibewoor@yahoo.com
[17]
246

OSMIUM(VIII) CATALYZED OXIDATION
247
in catalyzed reactions is important. Osmium(VIII) catalysis in medium porosity fritted glass filter and 40 g of NaOH was added
redox reactions involves several complexes, different oxidation slowly to the filtrate, whereupon a voluminous orange precipi-
states of osmium, etc. We have observed that osmium(VIII) tate agglomerates. The precipitate is filtered and washed three
catalyzes the oxidation of DL-ornithine monohydrochloride by to four times with cold water. The pure crystals were dissolved
3
◦
DPA in alkaline medium in micro amounts.
in 50 cm water and warmed to 80 C with constant stirring
Literature survey reveals that there is no report on os- thereby some solid was dissolved to give a red solution. The
mium(VIII) catalyzed oxidation of DL-ornithine monohy- resulting solution was filtered when it was hot and on cooling at
drochloride by DPA in alkaline medium. In earlier reports of room temperature, the orange crystals separated out and were
DPA oxidation,[ the order in [OH ] was found to be less than recrystallized from water.
18]
−
unity and periodate had a retarding effect in most of the react-
The complex was characterized from its UV spectrum, which
ions, and monoperiodatoargentate(III) (MPA) was considered exhibited three peaks at 216, 255 and 362 nm. These spectral
to be active species. However, in the present study, we have features were identical to those reported earlier for DPA.[ The
observed entirely different kinetic observations, and diperioda- magnetic moment study revealed that the complex is diamag-
toargentate(III) (DPA) itself is found to be an active form of netic. The compound prepared was analyzed [ for silver and
oxidant. In order to understand the active species of oxidant and periodate by acidifying a solution of the material with HCl, re-
20]
21]
catalyst, to compute the activity of the catalyst and to propose covering and weighing the AgCl for Ag and titrating the iodine
−
the appropriate mechanism, the title reaction is investigated in liberated when excess of KI was added to the filtrate for IO .
4
detail. An understanding of the mechanism allows the chemistry The stock solution of DPA was used for the required [DPA]
to be interpreted, understood and predicted.
solution in the reaction mixture.
2. EXPERIMENTAL
2
.3. Instruments Used
(
i) For kinetic measurements, a Peltier accessory (temperature
control) attached to Varian CARY 50 Bio UV-visible spec-
trophotometer (Varian, Victoria-3170, Australia) was used.
2
.1. Materials and Reagents
All reagents used were of analytical reagent grade, and mil-
lipore water was used throughout the work. A solution of DL-
ornithine monohydrochloride (HiMedia Laboratories) was pre-
pared by dissolving an appropriate amount of recrystallised
sample in millipore water. The purity of DL-ornithine mono-
hydrochloride sample was checked by comparing its melting
(ii) For product analysis, a QP-2010S Shimadzu gas chromato-
graph mass spectrometer, Nicolet 5700-FT-IR spectrometer
1
(
(
Thermo, U.S.A), 300 MHz H NMR spectrophotometer
Bruker, Switzerland) were used.
◦
(iii) For pH measurements ELICO pH meter model LI 120 was
used.
point, 232 C with literature data [literature melting point =
◦
233 C]. The required concentration of OMH was obtained from
its stock solution. The osmium(VIII) solution was prepared by
−
3
dissolving OsO4 (Johnson Matthey) in 0.50 mol dm NaOH.
[19]
The concentration was ascertained
by determining the un-
reacted [Fe(CN)6] with standard Ce(IV) solution in an acidic
medium. A stock standard solution of IO4 was prepared by
2.4. Kinetic Measurements
4−
−
The kinetic measurements were performed on a Varian
CARY 50 Bio UV-visible spectrophotometer. The kinetics was
dissolving a known weight of KIO4 (S.D. fine) in hot water,
and used after keeping for 24 h to complete the equilibrium. Its
followed under pseudo first-order condition where [OMH] >
◦
[20]
[DPA] at 25 ± 0.1 C, unless specified. The reaction was initi-
concentration was ascertained by iodometrically
at neutral
ated by mixing the DPA to DL-ornithine monohydrochloride so-
lution, which also contained required concentrations of KNO3,
KOH, catalyst Os(VIII) and KIO4. The progress of reaction was
followed spectrophotometrically at 360 nm by monitoring de-
crease in absorbance due to DPA with the molar absorbancy
pH maintained using phosphate buffer. The pH of the medium
in the solution was measured by Elico model (LI 120) pH me-
ter. KNO3 (AR) and KOH (BDH) were used to maintain ionic
strength and alkalinity of the reaction, respectively. Aqueous
solution of AgNO3 was used to study the product effect, Ag(I).
t-Butyl alcohol (S.D. Fine Chem.) was used to study the dielec-
tric constant of the reaction medium.
3
−1
−1
index, ‘ε’ to be 13900 ± 100 dm mol cm . The spectral
changes during the chemical reaction for the standard condition
at 298 K are shown in Figure 1.
It is evident from the figure that the concentration of DPA
decreases at 360 nm. It was verified that there is a negligible
2.2. Preparation of DPA
DPA was prepared by oxidizing Ag(I) in presence of KIO4 as interference from other species present in the reaction mixture
[21]
described elsewhere. The mixture of 28 g of KOH and 23 g of at this wavelength.
3
KIO4 in 100 cm of water along with 8.5 g AgNO3 was heated
The pseudo first-order rate constants, ‘kC’, were determined
just to boiling and 20 g of K2S2O8 was added in several lots from the log (absorbance) versus time plots. The plots were
with stirring and then allowed to cool. It was filtered through a linear up to 85% completion of reaction under the range of

248
S. J. MALODE ET AL.
sion coefficient ‘r’ and the standard deviation ‘S’, of points
from the regression line, was performed with the Microsoft of-
fice Excel—2003 program.
3. RESULTS
3.1. Stoichiometry and Product Analysis
Different sets of reaction mixtures containing varying ra-
tios of DPA to DL-ornithine monohydrochloride in presence
−
of constant amount of OH , Os(VIII) and KNO3, were kept
for 3 hours in a closed vessel under nitrogen atmosphere. The
remaining concentration of DPA was estimated spectrophoto-
metrically at 360 nm. The results indicated 1:2 stoichiometry
(OMH:DPA), as given in Scheme 1.
The main oxidation product was identified as 4-aminobutyric
FIG. 1. Spectroscopic changes occurring in the oxidation of DL-ornithine
acid. The product was extracted with ether and recrystallized
from aqueous alcohol. It was characterized by FT-IR, GC-MS
◦
−5
monohydrochloride by alkaline DPA at 25 C, [DPA] = 5.0 × 10 , [OMH] =
−
4
−
−
−5
−6
5
0
.0 × 10 , [OH ] = 0.08, [IO ] = 5.0 × 10 , [Os] = 5.0 × 10 and I =
4
1
−
3
and H NMR spectral studies.
.10 mol dm with scanning time of: (1) 1.0, (2) 2.0, (3) 3.0, (4) 4.0, (5) 5.0
and (6) 6.0 min
The presence of carboxylic acid was confirmed by IR
spectroscopy,[ which showed >C=O stretching of carboxylic
22]
−
−1
[
OH ] used. The orders for various species were determined acid at 1708 cm indicating the presence of acidic C=O
−1
from the slopes of plots of log kC versus respective concentra- group, O-H stretching of carboxylic acid at 2848 cm indi-
tion of species except for [DPA] in which non-variation of ‘kC’ cating the presence of acidic -OH group and N-H stretching at
−1
was observed as expected to the reaction condition. During the 3427 cm indicating the presence of -NH2 group in the prod-
−5
−3
1
kinetics a constant concentration, viz. 5.0 × 10 mol dm of uct. 4-aminobutyric acid was further characterized by H NMR
KIO4 was used throughout the study unless otherwise stated. spectrum (CDCl3) two triplet at 2.31 δ (a) and 2.69 δ (c) and
Since periodate is present in excess in DPA, the possibility of multiplet at 1.84 δ (due to (b) CH2), 5.44 δ (s, 2H due to –NH2)
oxidation of DL-ornithine monohydrochloride by periodate in and 11.6 δ (s, H due to –COOH), –NH2 and –OH were vanished
◦
alkaline medium at 25 C was tested. The progress of the react- on adding D2O.
ion was followed iodometrically. However, it was found that
Further, the product was subjected to GC-MS spectral anal-
there was no significant reaction under the experimental condi- ysis. GC-MS data was obtained on a QP-2010S Shimadzu gas
tions employed compared to the DPA oxidation of DL-ornithine chromatograph mass spectrometer. The mass spectrum showed
monohydrochloride. The total periodate concentration was cal- a molecular ion peak at 103 amu confirming 4-aminobutyric
culated by considering the periodate present in the DPA solution acid product (Figure 2).
and that was additionally added.
All other peaks observed in GC-MS can be interpreted in
In view of the modest concentration of alkali used in the accordance with the observed structure of the 4-aminobutyric
reaction medium, attention was also directed to the effect of the acid.
reaction vessel surface on the kinetics. Use of polythene/acrylic
The by-products were identified as ammonia by Nessler’s
wares and quartz or polyacrylate cells gave the same results, reagent [ and the CO2 was qualitatively detected by bubbling
23]
indicating that the surface did not have any significant effect on nitrogen gas through the acidified reaction mixture and pass-
the reaction rates.
ing the liberated gas through the tube containing limewater.
+
In view of the ubiquitous contamination of carbonate in the The formation of free Ag in solution was detected by adding
basic medium, the effect of carbonate was also studied. Added KCl solution to the reaction mixture, which produced white
carbonate had no effect on the reaction rates. However, fresh turbidity due to the formation of AgCl. It was observed that
solutions were nevertheless used for carrying out each kinetic 4-aminobutyric acid does not undergo further oxidation under
only. Regression analysis of experimental data to obtain regres- the present kinetic conditions.
SCH. 1. Stoichiometry of Os(VIII) catalyzed oxidation of DL-ornithine monohydrochloride by alkaline diperiodatoargentate(III).

OSMIUM(VIII) CATALYZED OXIDATION
249
FIG. 2. GC-mass spectrum of 4-aminobutyric acid with its molecular ion peak at 103 m/z.
3.2. Reaction Orders
(r ≥ 0.9996, S ≤ 0.009) under the concentrations studied (Ta-
As the diperiodatoargentate(III) oxidation of DL-ornithine ble 1). This is also confirmed in the plots of kC versus [OMH]0
.79
monohydrochloride in alkaline medium proceeds with a mea- which is linear rather than the direct plot of kC versus [OMH]
surable rate in the absence of Os(VIII), the catalyzed reaction (Figure 3).
is understood to occur in parallel paths with contributions from
both the catalyzed and uncatalyzed paths. Thus, the total rate 3.5. Effect of [Alkali]
constant (kT) is equal to the sum of the rate constants of the
catalyzed (kC) and uncatalyzed (kU) reactions, so kC = kT − kU.
Hence, the reaction orders have been determined from the slopes
of log kC versus log (concentration) plots by varying the concen-
The effect of alkali on the reaction has been studied in the
−3
range of 0.01 to 0.10 mol dm at constant concentrations of
OMH, DPA, Os(VIII) and a constant ionic strength of 0.10 mol
−3
dm . The rate constants decreased with increasing [alkali] and
−
−
trations of OMH, IO , OH and Os(VIII), in turn, while keeping
4
the order was found to be negative fractional order i.e., −0.33
others constant. The uncatalyzed reaction was followed under
(
Table 1) (r ≥ 0.9948, S ≤ 0.007).
−
−5
5
−4
,
the condition, [DPA] = 5.0 × 10 , [OMH] = 5.0 × 10
−
−
−3
[
OH ] = 0.08, [IO ] = 5.0 × 10 , I = 0.10 / mol dm . The
0.79
X 103 (mol dm−3)
4
[OMH]
rate constant of uncatalyzed reaction (kU) was obtained by the
plot of log (absorbance) versus time by following the progress
of the reaction spectrophotometrically at 360 nm.
4
.0
3.0
2.0
1.0
0.0
2
1
0
0
.4
.6
.8
.0
0.0
0.6
1.2
3
.3. Effect of [diperiodatoargentate(III)]
−5
The DPA concentration was varied in the range of 1.0 × 10
−4
−3
to 1.0 × 10 mol dm and the linearity of the plots of log
(absorbance) versus time up to 85% completion of the reaction
(r ≥ 0.9858, S ≤ 0.016) indicates a reaction order of unity in
[DPA]. This is also confirmed by varying of [DPA], which did
not result in any change in the pseudo first-order rate constants,
kC (Table 1).
1.8
3.4. Effect of [DL-ornithine monohydrochloride]
The DL-ornithine monohydrochloride concentration was
2.4
10.0
−
5
−4
−3
varied in the range of 7.0 × 10 to 7.0 × 10 mol dm
0
.0
2.5
5.0
7.5
◦
at 25 C while keeping other reactant concentrations and con-
OMH] X 10 (mol dm−3)
4
[
ditions constant. The kC values increased with the increase
in concentration of DL-ornithine monohydrochloride indicat- FIG. 3. Plot of kC versus [OMH]0.79 and kC versus [OMH] (conditions as in
ing an apparent less than unit order dependence on [OMH] Table 1).

250
S. J. MALODE ET AL.
TABLE 1
−
−
4
Effect of variation of [DPA], [OMH], [OH ], [IO ] and [Os(VIII)] on the osmium(VIII) catalyzed oxidation of DL-ornithine
◦
−3
monohydrochloride by diperiodatoargentate(III) in aqueous alkaline medium at 25 C and I = 0.10 mol dm
5
4
−
1
−
5
6
2
3
2
−1
)
[
DPA] × 10 [OMH] × 10 [OH ] × 10 [IO ] × 10 [Os(VIII)] × 10 kT × 10 kU × 10
kC × 10 (s
Found Calculated
4
−3
−3
−3
−3
−3
−1
−1
(s )
(mol dm
)
(mol dm
)
(mol dm
)
(mol dm
)
(mol dm
)
(s
)
1
3
5
7
.0
.0
.0
.0
5.0
5.0
5.0
5.0
5.0
0.7
1.0
3.0
5.0
7.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.1
0.2
0.4
0.8
1.0
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
1.0
3.0
5.0
7.0
10.0
5.0
5.0
5.0
5.0
5.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
1.0
3.0
5.0
8.0
10.0
1.85
1.82
2.06
1.99
1.94
0.39
0.54
1.36
2.06
2.29
3.86
3.36
2.65
2.06
1.83
2.09
1.83
2.06
2.09
2.06
0.56
1.23
2.06
3.15
3.89
2.04
1.97
2.19
2.23
2.16
0.41
0.57
1.49
2.19
2.57
4.11
3.58
2.82
2.19
1.99
2.25
1.94
2.19
2.22
2.12
2.19
2.19
2.19
2.19
2.19
1.65
1.63
1.84
1.77
1.72
0.35
0.48
1.22
1.84
2.04
3.45
3.00
2.37
1.84
1.68
1.87
1.63
1.84
1.86
1.85
0.34
1.01
1.84
2.93
3.67
1.75
1.75
1.75
1.75
1.75
0.35
0.48
1.22
1.75
2.15
3.27
2.91
2.38
1.75
1.54
1.75
1.75
1.75
1.75
1.75
0.35
1.05
1.75
2.79
3.49
1
0.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
3
.6. Effect of [Periodate]
−5
Periodate concentration was varied from 1.0 × 10 to 1.0 ×
4
3
.0
.2
− −
4
10
at constant [DPA], [OMH], [OH ], [Os(VIII)] and ionic
strength. It was observed that the added periodate had no effect
on the reaction (Table 1).
3
.7. Effect of [Os(VIII)]
2.4
1.6
−6
The [Os(VIII)] concentration was varied from 1.0 × 10
−5
−3
to 1.0 × 10 mol dm range, at constant concentrations of
diperiodatoargentate(III), DL-ornithine monohydrochloride, al-
kali and ionic strength. The order in [Os(VIII)] was found to
be unity from the linearity of the plot of kC versus [Os(VIII)]
0
.8
(Table 1) (Figure 4) (r ≥ 0.9994, S ≤ 0.019).
3.8. Effect of Ionic Strength (I ) and Dielectric Constant
0.0
of the Medium (D)
0.0
3.0
6.0
6
9.0
12.0
−3
The addition of KNO3 at constant [DPA], [Os(VIII)], [OMH],
[
Os(VIII)] X 10 (mol dm
)
−
−
[
OH ] and [IO ] was found that increasing ionic strength had
4
FIG. 4. Unit order plot of [Os(VIII)] versus kC.
no significant effect on the rate of the reaction.

OSMIUM(VIII) CATALYZED OXIDATION
251
K1
2
-
-
-
[
Ag(H IO )(H IO )] + H O
[Ag(H IO ) ] + [OH ]
3 6 2
2
6
3
6
2
K2
NH2
COO-
2-
Complex(C)
+
[OsO (OH) ]
4
2
NH2
k
2-
Complex(C) + [Ag(H
IO
)
]-
NH2
+ Ag(II) + CO + 2H IO
6
2
3
3
6
2
slow
.
2
-
+
[OsO (OH) ]
4 2
NH2
fast
OH
NH2
NH2
+ Ag(II)
+ NH + Ag(I)
3
.
-
CHO
NH2
fast
OH-
NH2
2-
NH2
-
3
+ Ag(I) + 2H IO6 + H O
2
+ [Ag(H IO ) ]
3
6 2
2
COOH
CHO
SCH. 2. Detailed scheme for the Os(VIII) catalyzed oxidation of DL-ornithine monohydrochloride by alkaline diperiodatoargentate(III)
Varying the t-butyl alcohol and water percentage varied di- polymerization under the same condition as those induced for
electric constant of the medium, ‘D’. The D values were cal- the reaction mixture. Initially added acrylonitrile decreases the
culated from the equation D = DwVw + DBVB, where Dw rate of reaction indicating free radical intervention, which is the
and DB are dielectric constants of pure water and t-butyl alco- case in earlier work.[24]
hol, respectively, and Vw and VB are the volume fractions of
components water and t-butyl alcohol, respectively, in the total
mixture. There was no effect of dielectric constant on the rate of
reaction.
3
.11. Effect of Temperature (T)
The influence of temperature on the rate of reaction was stud-
◦
ied at 15, 25, 35 and 45 C. The rate constants (k), of the slow
step of Scheme 2 were obtained from the slopes and the intercept
of the plots of [Os(VIII)]/kC versus 1/[OMH] and [Os(VIII)]/kC
3.9. Effect of Initially Added Products
−
Initially added products, Ag(I), and 4-aminobutyric acid did versus [OH ] at four different temperatures. The values are
not have any significant effect on the rate of reaction.
given in Table 2. The activation parameters for the rate deter-
mining step were obtained by the least square method of plot of
log k versus 1/T and are presented in Table 2.
Thus, from the observed experimental results-
The rate law for Os(VIII) catalyzed reaction is given as:
1.0
0.79
− −0.33
[OH ]
1.0
[Os(VIII)]
Rate = kC [DPA]
[OMH]
3
.12. Catalytic Activity
It has been pointed out by Moelwyn- Hughes [25] that in
presence of the catalyst, the uncatalyzed and catalyzed reactions
proceed simultaneously, so that
3.10. Test for Free Radicals (Polymerization Study)
The intervention of free radicals was examined as follows,
the reaction mixture, to which a known quantity of acryloni-
trile scavenger has been added initially, was kept in an inert
atmosphere for 1 hour. Upon diluting the reaction mixture with
kT = kU + KC [catalyst]×
[1]
Here kT is the observed pseudo first-order rate constant in
methanol, precipitate resulted, suggesting there is participation the presence Os(VIII) catalyst, kU the pseudo first-order rate
of free radicals in the reaction. The blank experiments of either constant for the uncatalyzed reaction, KC the catalytic constant
DPA or DL-ornithine alone with acrylonitrile did not induce any and ‘x’ the order of the reaction with respect to [Os(VIII)]. In the

252
S. J. MALODE ET AL.
TABLE 2
Activation parameters and thermodynamic quantities for the osmium(VIII) catalyzed oxidation of DL-ornithine
monohydrochloride by diperiodatoargentate(III) in aqueous alkaline medium with respect to the slow step of Scheme 2: (A)
Effect of temperature, (B) Activation parameters (Scheme 2), (C) Effect of temperature to calculate K1 and K2 for the Os(VIII)
catalyzed oxidation of DL-ornithine monohydrochloride by diperiodatoargentate(III) in alkaline medium, (D) Thermodynamic
quantities using K1 and K2
k × 10− (dm mol
4
3
−1 −1
s
)
(
(
A) Temperature (K)
288
298
308
318
1.33
2.18
3.98
4.95
B) Parameters
Ea(k J mol−1)
Values
34.6
32.2
–53.8
48.2
#
−1
−1
ꢀ
ꢀ
ꢀ
H (k J mol
)
#
−1
S (J K mol
)
#
−1
G (k J mol
)
log A
10.4
1
−3
K2 × 10−3(dm mol
3
−1
)
(
(
C) Temperature (K)
K1x 10 (mol dm
)
288
298
308
318
1.25
0.68
0.51
0.45
0.59
1.07
1.49
2.36
D) Thermodynamic quantities
Values from K1
Values from K2
−
1
ꢀ
ꢀ
ꢀ
H (k J mol
)
–25.6
–108.4
6.6
33.9
171.6
–17.3
−
1
−1
)
S (J K mol
G298 (k J mol
−
1
)
DPA] = 5.0 × 10−5, [OMH] = 5.0 × 10−4, [OH ] = 0.08, [IO ] = 5.0 × 10−5, [Os(VIII)] = 5.0 × 10−6, I = 0.10 / mol dm−3
−
−
.
[
4
present investigations, × values for the standard run were found
to be unity for Os(VIII). Then the value of KC is calculated
using the equation,
TABLE 3
Values of catalytic constant (KC) at different temperatures and
activation parameters calculated using KC values
kT − kU
kC
KC =
=
(where kT − kU = kC) [2]
×
[Catalyst]
×
[
Catalyst]
4
Temperature (K)
KC × 10−
The values of KC were evaluated at different temperatures
and found to vary at different temperatures. Further, plots of log
KC versus 1/T were linear and the values of energy of activation
and other activation parameters with reference to catalyst were
computed. These results are summarized in Table 3. The value
2
2
3
3
88
98
08
18
0.19
0.37
0.69
1.03
43.2
40.7
–39.9
52.6
11.1
Ea(k J mol−1)
◦
3
of KC at 25 C is 3.7 × 10 .
#
−1
−1
ꢀ
ꢀ
ꢀ
H (k J mol
)
#
−1
S (J K mol
)
#
−1
G (k J mol
)
4
. DISCUSSION
Inthelater periodofthe20thcentury, thekineticsofoxidation
of some organic and inorganic substrates have been studied
by Ag(III) species, which may be due to its strong versatile
log A
−
5
−4
−
−
[DPA] = 5.0 × 10 , [OMH] = 5.0 × 10 , [OH ] = 0.08, [IO ]
4
−
5
−6
−3
.
= 5.0 × 10 , [Os(VIII)] = 5.0 × 10 , I = 0.10 / mol dm

OSMIUM(VIII) CATALYZED OXIDATION
253
3
−
−
−
3−
nature of two electrons oxidant. Among the various species [OsO (OH)]. At higher concentration of OH , [OsO (OH)]
5
5
−
of Ag(III), Ag(OH) , diperiodatoargentate(III) and ethylenebis is significant. At lower concentrations of OH , as employed in
4
(
biguanide), (EBS), silver(III) are of maximum attention to the the present study, and since the rate of oxidation decreased with
[26]
−
The stability of increase in [OH ], it is reasonable that [OsO (OH) ] was op-
4
2−
researchers due to their relative stability.
2
−
Ag(OH) is very sensitive towards traces of dissolved oxygen erative and its formation is important in the reaction. In earlier
4
and other impurities in the reaction medium whereupon it had reports,[ it has been observed that in Os(VIII) catalyzed re-
15]
not drawn much attention. However, the other two forms of action, in view of less than unit order in substrate, unit order in
Ag(III) [ are considerably stable; the DPA is used in highly Os(VIII) and oxidant, Os(VIII) is regenerated through forma-
27]
tion of Os(VII). In another case,[ Os(VIII) is regenerated by
16]
alkaline medium and EBS is used in highly acidic medium.
[21]
The literature survey
reveals that the water soluble diperi- Os(VI) intervention in view of unit order each in osmium, sub-
odatoargentate(III) (DPA) has a formula [Ag(IO6)2]7 with dsp
−
2
[17]
strate and oxidant. In some other reports, it is observed that
configuration of square planar structure, similar to diperiodato- Os(VIII) forms a complex with substrate, which is oxidized by
copper(III) complex with two bidentate ligands, periodate to the oxidant with regeneration of the catalyst. Hence, the study of
form a planar molecule. When the same molecule is used in behavior of Os(VIII) in catalyzed reaction becomes significant.
7−
alkaline medium, it is unlikely to exist as [Ag(IO6)2] as peri- To explain all the observed orders, Scheme 2 is proposed for
[28]
odate is known to be in various protonated forms
on pH of the solution as given in following multiple equilibria
3)–(5).
depending osmium(VIII) catalyzed reaction.
In the prior equilibrium step 1, the hydroxyl ion concentration
−
(
with fractional order in OH concentration, the main oxidant
−
species is likely to be [Ag(H3IO6)2] and its formation by the
H5IO6 ꢁ— —— ꢂ— H4IO6 +H+
−
[3] above equilibrium is important in the present study. The less than
−
2−
3−
+
unit order in [OMH] presumably results from the formation of
a complex (C) between the Os(VIII) species and DL-ornithine
monohydrochloride. This complex (C) reacts with one mole of
H4IO ꢁ— —— ꢂ— H3IO6
+ H
[4]
6
H3IO62 ꢁ— —— ꢂ— H2IO6
−
+
+ H
[5]
DPA in a slow step to give the free radical species of OMH,
Ag(II) with the regeneration of catalyst, Os(VIII). Further this
free radical species of OMH reacts with Ag(II) in a fast step
to form 4-aminobutyraldehyde intermediate. This intermediate
reacts with one mole of DPA in a further fast step to form the
products such as 4-aminobutyric acid, Ag(I) and periodate as
given in Scheme 2.
−
Periodic acid exists as H5IO6 in acid medium and as H4IO6
near pH 7. Hence, under alkaline conditions as employed in this
study, the main species are expected to be H3IO2 and H2IO6
−
3−
4]
.
6
[
At higher concentrations, periodate also tends to dimerise.
However, formation of this species is negligible under condi-
tions employed for kinetic study. On contrary, the authors[7,8]
in their recent studies have proposed the DPA species as
The probable structure of the complex (C) is given as,
X−
[
Ag(HL)2]
in which ‘L’ is a periodate with uncertain num-
ber of protons and ‘HL’ is a protonated periodate of uncertain
number of protons. This can be ruled out by considering the
3-
[28]
−
alternative form
of IO4 at pH > 7, which is in the form
O
2−
3−
−
O
C
OH
O
H3IO or H2IO . Hence, DPA could be as [Ag(H3IO6)2]
6
6
3−
or [Ag(H2IO6)2] in alkaline medium. Therefore, under the
NH2
O
Os
O
present condition, diperiodatoargentate(III), may be depicted as
O
−
[
Ag(H3IO6)2] . The similar speciation of periodate in alkali
OH
[29]
NH2
was proposed for diperiodatonickelate(IV).
It is known that DL-ornithine exists in the form of zwitterion
in aqueous medium. In highly acidic medium it exists in the
protonated form, where as in highly basic medium it is in the
deprotonated form,[ as
30]
Spectroscopic evidence for the complex formation between
Os(VIII) and OMH was obtained from UV–vis spectra of OMH
−4
−6
−
−3
)
(
5.0 × 10 ), Os(VIII) (5.0 × 10 , [OH ] = 0.08 mol dm
and a mixture of both. A bathochromic shift of 4 nm from
64 nm to 368 nm in the spectra of Os(VIII) to the mixture
NH2
COO-
3
NH2
of Os(VIII) and OMH was observed. Attempts to separate and
isolate the complex were not successful. The Michaelis-Menten
plot proved the complex formation between catalyst and sub-
strate, which explains less than unit order in [OMH]. Such a
4.1. Mechanism
Osmium(VIII) is known to form different complexes complex between a catalyst and substrate has also been ob-
−(17)
2−
[17]
at different OH
concentrations, [OsO4(OH)2]
and served in other studies.
The rate law (7) for the Scheme 2

254
S. J. MALODE ET AL.
could be derived as,
3.6
(A)
–
d[DPA]
dt
288 K
rate =
kK1K2[OMH][Os(VIII)][DPA]
2.4
=
[6]
−
−
K1+K1K2[OMH] + [OH ] + K [OMH][OH ]
2
rate
=
=
kC = kT − kU
[
DPA]
298 K
1
0
.2
.0
kK1K2[OMH][Os(VIII)]
−
−
K1+K1K2[OMH] + [OH ] + K [OMH][OH ]
2
308 K
318 K
[
7]
This explains all the observed kinetic orders of different
0
.0
0.4
0.8
1.2
1.6
species. The rate law (7) can be rearranged to be equation (8),
which is suitable for verification.
-
4
3
-1
1/[OMH] X 10 (dm mol )
−
[
Os(VIII)]
[OH ]
[OH]
1
1
k
0
.9
=
+
+
+
(
B)
kC
kK1K2[OMH]
kK1
K1K2[OMH]
[8]
According to Equation (8), other conditions being constant,
288 K
−
the plots of [Os(VIII)]/kC versus [OH ] and [Os(VIII)]/kC ver-
sus 1/[OMH] should be linear and found to be so (Figure 5).
From the intercepts and slopes of such plots, the reaction con-
0.6
−
2
stants K1, K2 and k were calculated as (6.8 ± 0.1) × 10
298 K
−3
3
3
−1
4
mol dm , (1.07 ± 0.03) × 10 dm mol , (2.2 ± 0.2) × 10
0
0
.3
.0
3
−1 −1
dm mol
s , respectively. The value K1 obtained is in good
308 K
[31]
agreement with previously reported value.
These constants
318 K
were used to calculate the rate constants and compared with the
experimental kC values and found to be in reasonable agreement
with each other, which fortifies Scheme 2.
The thermodynamic quantities for the different equilibrium
steps, inScheme2canbeevaluatedas follows. TheDL-ornithine
monohydrochloride and hydroxide ion concentrations (Table 1)
were varied at different temperatures. The plots of [Os(VIII)]/kC
0
.0
0.3
0.6
-
0.9
1.2
1.5
−3
[
OH ] X 10 (mol dm )
FIG. 5. Verification of rate law (7) for the Os(VIII) catalyzed oxidation of
DL-ornithine monohydrochloride by diperiodatoargentate(III). Plots of (A)
versus 1/[OMH] (r ≥ 0.9996, S ≤ 0.00133), [Os(VIII)]/kC [Os(VIII)]/kC vs 1/[OMH], (B) [Os(VIII)/kC vs [OH ], at six different tem-
−
−
peratures (conditions as given in Table 1).
versus [OH ] (r ≥ 0.9948, S ≤ 0.00142) should be linear as
shown in Figure 5. From the slopes and intercepts, the values
of K1 are calculated at different temperatures. A vant Hoff’s
plot was made for the variation of K1 with temperature [i.e.,
#
#
reaction (Scheme 2). The moderate ꢀH and ꢀS values are
log K1 versus 1/T (r ≥ 0.9213, S ≤ 0.1034)] and the values
of the enthalpy of reaction ꢀH, entropy of reaction ꢀS and
free energy of reaction ꢀG, were calculated. These values are
also given in Table 2. A comparison of the ꢀH value of second
#
favorable for electron transfer reaction. The value of ꢀH was
due to energy of solution changes in the transition state. The
#
−1
−1
negative value of ꢀS (−53.8 J K mol ) suggests that the
intermediate complex (C) is more ordered than the reactants.[
The observed modest enthalpy of activation and a higher rate
constant for the slow step indicates that the oxidation presum-
ably occurs via an inner-sphere mechanism. This conclusion
is supported by earlier observations.[ The activation parame-
ters evaluated for the reaction explain the catalytic effect on the
reaction. The catalyst Os(VIII) forms the complex (C) with sub-
strate which enhances the reducing property of the substrate than
that without catalyst. Further, the catalyst Os(VIII) modifies the
reaction path by lowering the energy of activation.
32]
−
1
#
step (33.9 k J mol ) of Scheme 2 with that of ꢀH (32.2 k
−1
J mol ) obtained for the slow step of the reaction shows that
these values mainly refer to the rate limiting step, supporting the
fact that the reaction before rate determining step is fairly slow
33]
[18]
and involves high activation energy.
In the same manner,
K2 values were calculated at different temperatures and the
corresponding values of thermodynamic quantities are given in
Table 2.
The negligible effect of ionic strength and dielectric constant
in the reaction might be due to the presence of various ions in

OSMIUM(VIII) CATALYZED OXIDATION
255
5
. CONCLUSIONS
The Os(VIII) catalyzed oxidation of DL-ornithine mono-
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toargentate(III) itself is considered to be the active species for
1
7. Sirsalmath, K.T., Hiremath, C.V., and Nandibewoor, S.T. Kinetic, mechanis-
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8. Seregar, V., Veeresh, T.M., and Nandibewoor, S. T. Osmium(VIII)
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2
53–260.
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0. Panigrahi, G.P., and Misro, P.K. Kinetics and mechanism of osmium(VIII)-
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APPENDIX
According to Scheme 2
1
4. Singh, A.K., Srivastava, S., Srivastava, J., Srivastava, R., and Singh, P. Stud-
ies in kinetics and mechanism of oxidation of d-glucose and d-fructose by
alkaline solution of potassium iodate in the presence of Ru(III) as homoge-
neous catalyst. J. Mol. Catal. A Chemical 2007, 278, 72–81.
kK1K2[OMH][Os(VIII)][DPA]
1
5. EI-Tantawy, Y.A., Abu-Shady, A.I., and Ezzat, I.T. The mechanism
of Os(VIII)-catalyzed Ce(IV)—As(III) reaction. J. Inorg. Nucl. Chem.
−
rate = k[C][Ag(H IO6) ] =
3
2
−
[
OH ]
1
978,40, 1683–1684.
[1]

256
S. J. MALODE ET AL.
The total concentration of DPA is given by (where T and F
stand for total and free)
Similarly,
−
−
[
OH ]T = [OH ]F
[4]
−
[
DPA] = [DPA] + [Ag(H IO6)2]
T
F
3
ꢀ
ꢁ
−
[
OH ] + K2
=
[DPA]F
−
[
OH ]
[
Os(VIII)]T = [Os(VIII)]F + [C]
Therefore,
= [Os(VIII)] + K [OMH][Os(VIII)]
F
2
F
ꢀ
ꢁ
−
[
DPA] [OH ]
T
−
ꢀ
ꢁ
[
DPA] =
[2]
F
[
Os(VIII)]T
[
OH ] + K1
[Os(VIII)]F
[5]
{
1 + K2[OMH]}
Similarly,
[
OMH]T = [OMH]F + [C]
Substituting equations (2)–(5) in equation (1) and omitting
the subscripts T and F, we get
=
=
[OMH]F + K2[OMH] [Os(VIII)]
F
[OMH] (1 + K2[Os(VIII)])
F
rate
In view of low concentration of Os(VIII) used,
= k = k − k
C
T
U
[
DPA]
kK1K2[OMH][Os(VIII)]
=
−
−
K1 + K1K2[OMH] + [OH ] + K2[OMH][OH ]
[
OMH] = [OMH]F
[3]
T