Kinetic and mechanistic aspects of osmium(VIII) catalyzed oxidation of DLornithine by copper(iii) periodate complex in aqueous alkaline medium

Article abstract of DOI:10.1524/zpch.2010.5521

The oxidation of DL-ornithine monohydrochloride (OMH) by diperiodatocuprate(III) (DPC) has been investigated in the presence of osmium(VIII) catalyst in aqueous alkaline medium at a constant ionic strength of 0.20 mol dm^-3 spectrophotometrically. The reaction exhibits 1:4 stoichiometry i.e., [OMH]: [DPC]. The order of the reaction with respect to [DPC] was unity while the order with respect to [OMH] was less than unity over the concentration range studied. The rate increased with an increase in [OH -] and decreased with an increase in [IO₄^-]. The order with respect to [Os(VIII)] was unity. The reaction rates revealed that Os(VIII) catalyzed reaction was about nine-fold faster than the uncatalyzed reaction. The oxidation products were identified by spectral analysis. Suitable mechanism has been proposed. The reaction constants involved in the different steps of the reaction mechanism were calculated. The catalytic constant (KC) was also calculated at different temperatures. The activation parameters with respect to slow step of the mechanism and also the thermodynamic quantities were determined. Kinetic experiments suggest that [OsO₄(OH) ₂]^2- is the reactive Os(VIII) species and [Cu(H ₂IO₆)(H₂O)₂] is the reactive copper(III) species. by Oldenbourg Wissenschaftsverlag, Muenchen.

Full text of DOI:10.1524/zpch.2010.5521

Z. Phys. Chem. 224 (2010) 865–882 . DOI 10.1524.zpch.2010.5521
© by Oldenbourg Wissenschaftsverlag, München
Kinetic and Mechanistic Aspects of
Osmium(VIII) Catalyzed Oxidation of DL-
ornithine by Copper(III) Periodate Complex in
Aqueous Alkaline Medium
*
By Jyothi C. Abbar, Shweta J. Malode, and Sharanappa T. Nandibewoor
P. G. Department of Studies in Chemistry, Karnatak University, Dharwad 580003, India
(Received October 20, 2009; accepted April 9, 2010)
Ornithine . Diperiodatocuprate(III) . Os(VIII) Catalysis . Oxidation .
Kinetics
The oxidation of DL-ornithine monohydrochloride (OMH) by diperiodatocuprate(III) (DPC)
has been investigated in the presence of osmium(VIII) catalyst in aqueous alkaline medium at
a constant ionic strength of 0.20 mol dmK3 spectrophotometrically. The reaction exhibits 1:4
stoichiometry i.e., [OMH]: [DPC]. The order of the reaction with respect to [DPC] was unity
while the order with respect to [OMH] was less than unity over the concentration range studied.
The rate increased with an increase in [OH –] and decreased with an increase in [IO4K]. The
order with respect to [Os(VIII)] was unity. The reaction rates revealed that Os(VIII) catalyzed
reaction was about nine-fold faster than the uncatalyzed reaction. The oxidation products were
identified by spectral analysis. Suitable mechanism has been proposed. The reaction constants
involved in the different steps of the reaction mechanism were calculated. The catalytic constant
(KC) was also calculated at different temperatures. The activation parameters with respect
to slow step of the mechanism and also the thermodynamic quantities were determined.
Kinetic experiments suggest that [OsO4(OH)2]2K is the reactive Os(VIII) species and
[Cu(H2IO6)(H2O)2] is the reactive copper(III) species.
1. Introduction
Ornithine is a non-protein amino acid [1], derived from the breakdown of argi-
nine during the citric acid cycle. It helps to build muscle and reduce body fat,
especially when combined with the amino acids arginine and carnitine. It is also
needed for the formation of citrulline, proline and glutamic acid. Ornithine helps
to remove toxic ammonia from the liver, and reduces the effects of cirrhosis of
*
Corresponding author. E-mail: stnandibewoor@yahoo.com
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the liver and disorders associated with liver malfunction. Ornithine is the major
source of polyamines in mammalian physiological systems [2]. Increased urinary
polyamine levels have been demonstrated in humans with varied types of can-
cers. The metabolism of DL-[1-14C]ornithine monohydrochloride in rats with
either Walker 256 carcinoma or chemically induced methylcholanthrene tumors
was studied, whose attempts showed to develop ornithine as a biological marker
of cancer [3, 4].
Transition metals in their higher oxidation states can generally be stabilized
by chelation with suitable polydentate ligands. These metal chelates such as
diperiodatocuprate(III) [5], diperiodatoargentate(III)[6] and diperiodatonickel-
ate(IV) [7] are good oxidants in a medium with an appropriate pH value. Diperio-
datocuprate(III) is a versatile one-electron oxidant and the oxidation study of
DPC is scanty in view of its limited solubility and stability in aqueous medium.
Its use as an analytical reagent is now well recognized [8]. Copper complexes
have a major role in oxidation chemistry due to their abundance and relevance in
biological chemistry [9–11]. Copper(III) is involved in many biological electron
transfer reactions [12]. When Copper(III) periodate complex is the oxidant and
multiple equilibria between different copper(III) species are involved, it would
be interesting to know which of the species is the active oxidant.
Transition metals are known to catalyze many oxidation - reduction reactions
since they involve multiple oxidation states. In recent years the use of transition
metal ions such as osmium, ruthenium, palladium, manganese, chromium, irid-
ium, either alone or as binary mixtures, as catalysts in various redox processes
have attracted considerable interest [13]. The role of osmium(VIII) as a catalyst
in some redox reactions has been reviewed [14]. Although the mechanism of
catalysis depends on the nature of the substrate, the oxidant and experimental
conditions, it has been shown that metal ions act as catalysts by one of these
different paths such as the formation of complexes with reactants or oxidation
of the substrate itself or through the formation of free radicals. Osmium(VIII)
catalysis in redox reactions involves different degrees of complexity, due to the
formation of different intermediate complexes and different oxidation states of
osmium.
The uncatalysed reaction of oxidation of DL-ornithine monohydrochloride
[( )-2,5-Diaminopentanoic acid monohydrochloride] has been studied [15]. Lit-
erature survey revealed that the role of osmium(VIII) catalyst on the oxidation
of this ubiquitous amino acid has not been examined. We have observed that
osmium(VIII) in micro amounts catalyzes the oxidation of OMH by DPC in
alkaline medium. Such studies are of much significance in understanding the
mechanistic profile of ornithine in redox reactions and provide an insight into
the interaction of metal ions with the substrate and its mode of action in biologi-
cal systems. Also to know the active species of Cu(III) and catalyst Os(VIII), to
compute the activity of the catalyst and the complexity of the reaction, a detailed
study of the title reaction becomes important. Hence, the present investigation
is aimed at checking the reactivity of OMH towards DPC in the presence of
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osmium(VIII) catalyst and to arrive at the plausible mechanism on the basis of
kinetic and spectral results..
2. Experimental
2.1 Chemicals and solutions
All reagents were of analytical reagent grade and Millipore water was used
throughout the work. A solution of DL-ornithine monohydrochloride (HiMedia
Laboratories) was prepared by dissolving an appropriate amount of recrystallized
sample in Millipore water. The purity of OMH was checked by comparing its
IR spectrum with literature data and with its m.p. 232–2340C. The required
concentration of OMH was obtained from its stock solution. A standard stock
solution of Os(VIII) was prepared by dissolving OsO4 (Johnson Matthey) in
0.50 mol dmK3 NaOH. The concentration of Os(VIII) was ascertained [16] by
determining the unreacted ferrocyanide solution with standard cerium(IV) solu-
tion in an acidic medium. The copper(III) periodate complex was prepared [17,
18] and standardized by a standard procedure [19]. The UV-vis spectrum with
maximum absorption at 415 nm verified the existence of copper(III) complex.
The ionic strength was maintained by adding KNO3 (AR) solution and the pH
value was regulated with KOH (BDH) solution. A stock solution of IO4K was
prepared by dissolving a known weight of KIO4 (Riedel-de-Hean) in hot water
and used after keeping for 24 h to attain the equilibrium. Its concentration was
ascertained iodometrically [20], at neutral pH maintained using phosphate buffer.
The pH of the medium in the solution was measured by ELICO (LI120) pH
meter. Solutions of OMH and DPC were always freshly prepared before use.
2.2 Instruments used
(i) For kinetics measurements, a CARY 50 Bio UV–vis Spectrophotometer (Var-
ian, Victoria-3170, Australia) was used.
(ii) For product analysis, a QP-2010S Shimadzu gas chromatograph mass
1
spectrometer, Nicolet 5700-FT-IR spectrometer (Thermo, U.S.A.), 300 MHz H
NMR spectrophotometer (Bruker, Switzerland) were used and for pH measure-
ments, an Elico pH meter model LI120 was used.
2.3 Kinetic measurements
The kinetics was followed under pseudo-first-order condition where [OMH] >
[DPC] at 25.0 0.1ºC, unless otherwise specified. The reaction was initiated by
mixing DPC with the OMH solution which also contained the required concentra-
tion of KNO3, KOH, KIO4,and Os(VIII) catalyst. The progress of the reaction was
monitored spectrophotometrically at 415 nm (i.e., decrease in absorbance due to
DPC with the molar absorbency index, ‘ε’ to be 6231 100 dm3 molK1 cmK1 (Liter-
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Jyothi C. Abbar et al.
Fig. 1. UV-vis. spectral changes during the oxidation of DL-ornithine by alkaline diperiodatocu-
prate(III) at 298K, [DPC] = 5.0!10K5, [OMH] = 5.0!10K4, [OHK] = 0.08, [Os(VIII)] =
8.0!10K7 and I = 0.20 mol dmK3 with scanning time of: (1) 0.5, (2) 1.0, (3) 1.5, (4) 2.0, (5)
2.5 and (6) 3.0 min.
ature ε = 6230 [21]) which is the maximum absorption wavelength of DPC. The
spectral changes during the chemical reaction for the standard condition at 298 K
are shown in Fig. 1. It is evident from the figure that the concentration of DPC de-
creases at 415 nm. It was verified that there was almost no interference from other
species in the reaction mixture at this wavelength.
The pseudo-first order rate constants, ‘kT’, were determined from the log
(absorbance) vs time plots. The plots were linear up to 85% completion of reac-
tion under the range of [OHK] used. During the kinetics, a constant concentration
viz. 1.0!10K5 mol dmK3 of KIO4 was used throughout the study unless other-
wise stated. Since excess of periodate is present in DPC, the possibility of oxida-
tion of OMH by periodate in alkaline medium at 25ºC was tested and found that
there was no significant interference due to KIO4 under experimental conditions.
The total concentrations of periodate and OHK was calculated by considering
the amount present in DPC solution and that additionally added. Kinetic runs
were also carried out in N2 atmosphere in order to understand the effect of
dissolved oxygen on the rate of the reaction. No significant difference in the
results was obtained under a N2 atmosphere and in the presence of air. In view
of the ubiquitous contamination of carbonate in the basic medium, the effect of
carbonate was also studied. The added carbonate had no effect on the reaction
rates.
Regression analysis of experimental data to obtain regression coefficient r
and the standard deviation S, of points from the regression line, was performed
with the Microsoft office Excel 2003 program.
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Scheme 1.
3. Results
3.1 Stoichiometry and product analysis
Different sets of reaction mixtures containing varying ratios of DPC to OMH in
presence of constant amount of OHK, KIO4, KNO3 and Os(VIII) were kept for
6 h in a closed vessel under nitrogen atmosphere. The remaining concentration
of DPC was assayed by measuring the absorbance at 415 nm. The results indi-
cated 1:4 stoichiometry as given in Scheme 1.
After completion of reaction, the reaction mixture was acidified, concentrated
and extracted with ether. The reaction product was further recrystallized from
aqueous alcohol. The main reaction product was identified as 4-aminobutyric
acid. This was the only organic product obtained in the oxidation which was
confirmed by a single spot on thin layer chromatography and was characterized
1
by FT-IR, GC-MS and H NMR spectral studies.
The IR spectroscopy showed a >C = O stretching of carboxylic acid at
1708 cmK1 indicating the presence of acidic C = O group, O-H stretching of
carboxylic acid at 2848 cmK1 indicating the presence of acidic – OH group, and
also N-H stretching at 3427 cmK1 indicating the presence of -NH2 group in 4-
aminobutyric acid. GC-MS data was obtained on a QP-2010S Shimadzu gas
chromatograph mass spectrometer. The mass spectral data showed a molecular
ion peak at 103 m.z confirming the presence of 4-aminobutyric acid (Fig. 2).
All other peaks observed in GC-MS can be interpreted in accordance with the
1
observed structure of 4-aminobutyric acid. It was also subjected to H NMR
spectrum (CDCl3), two triplet at 2.31 δ (a) and 2.69 δ (c) and a multiplet at 1.84
δ (due to (b) CH2), 5.44 δ (s, 2H due to - NH2) and 11.6 δ (s, H due to –COOH).
-NH2 and –OH disappeared on D2O exchange.
The byproducts were identified as ammonia by Nessler’s reagent [19] and
CO2 was qualitatively detected by bubbling nitrogen gas through the acidified
reaction mixture and passing the liberated gas through the tube containing lime-
water. Finally copper(II) was identified by UV-vis spectra.
3.2 Reaction orders
As the diperiodatocuprate(III) oxidation of DL-ornithine in alkaline medium pro-
ceeds with a measurable rate in the absence of Os(VIII), the catalyzed reaction
is understood to occur in parallel paths with contributions from both the cata-
lyzed and uncatalyzed paths. Thus the total rate constant (kT) is equal to the sum
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Jyothi C. Abbar et al.
Fig. 2. Mass spectrum of reaction product, 4-aminobutyric acid with its molecular ion peak at
103 m.z.
of the rate constants of the catalyzed (kC) and uncatalyzed (kU) reactions, so kC =
kT K kU. Hence the reaction orders have been determined from the slopes of log
kC vs log (concentration) plots by varying the concentrations of OMH, IO4K,
OHK, and catalyst Os(VIII), in turn while keeping others constant. The uncata-
lyzed reaction was followed under the condition, [DPC] = 5.0!10K5, [OMH] =
5.0!10K4, [OHK] = 0.08, [IO4K] = 1.0!10K5, I = 0.20 . mol dmK3. The rate
constant of uncatalyzed reaction (kU) was obtained by the plot of log (absorb-
ance) vs time by following the progress of the reaction spectrophotometrically
at 415 nm.
3.3 Evaluation of pseudo first order rate constants
The oxidant [DPC] was varied in the range of 1.0!10K5 to 1.0!10K4 and the
linearity of the plots of log (absorbance) vs time up to 85% completion of the
reaction (r ≥ 0.9549, S ≤ 0.016) indicates a reaction order of unity in [DPC].
This is also confirmed by varying [DPC], which did not result in any change in
the pseudo-first-order rate constants, kC (Table 1).
3.4 The dependence of rate on the concentration of DL-ornithine
The effect of OMH was studied in the range of 6.0!10K5 to 6.0!10K4 mol dmK3
at constant concentrations of DPC, OHK, IO4K and a constant ionic strength of
0.20 mol dmK3 in the presence of Os(VIII) catalyst. At constant temperature, the
kC values increased with increase in [OMH] indicating the order with respect to
[OMH] as less than unity (Table 1) (r ≥ 0.9997, S ≤ 0.009). This was also
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Kinetic and Mechanistic Aspects of Osmium(VIII) Catalyzed Oxidation of …
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0.55
Fig. 3. Plots of kC vs [OMH]
and kC vs [OMH] (conditions as in Table 1).
confirmed by the plot of kC vs [OMH]0.55 which were linear rather than the direct
plot of kC vs [OMH] (Fig.3).
3.5 The dependence of rate on the concentration of OH−
The effect of alkali was studied in the range of 0.02 - 0.20 mol dmK3 at constant
concentrations of DPC, OMH, IO4K and ionic strength in the presence of Os(VIII)
catalyst. The rate constants increased with increase in [alkali]and the order was
found to be less than unity i.e., 0.41 (Table 1) (r ≥ 0.9973, S ≤ 0.007). This was
also confirmed by the plot kC vs [OHK]0.40 which were linear rather than the
direct plot of kC vs [OHK] (Fig 4).
3.6 The dependence of rate on the concentration of IO4−
The effect of periodate was studied in the range of 5.0!10K6 to 5.0!10K5 mol dmK3
at constant concentrations of DPC, OMH, OHK and ionic strength in the presence
of Os(VIII) catalyst. The experimental results indicated that the kC values de-
creased with increase in [IO4K]. The order with respect to IO4K was negative
fractional i.e., K0.45 (Table 1) (r ≥ 0.9957, S ≤ 0.004).
3.7 The dependence of rate on [Os(VIII)]
The [Os(VIII)] concentration was varied from 2.0!10K7 to 2.0!10K6 mol dmK3
range, at constant concentration of DPC, OMH, and alkali and constant ionic
strength. The order in [Os(VIII)] was found to be unity from the linearity of the
plot of kC vs [Os(VIII)] (Table 1) (Fig 5).
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Jyothi C. Abbar et al.
Fig. 4. Plots of kC vs [OHK]0.40 and kC vs [OHK] (conditions as in Table 1).
Fig. 5. Unit order plot of kC vs [Os(VIII)].
3.8 The dependence of rate on ionic strength (I) and dielectric constant (D)
The effect of ionic strength (I) was studied by varying [KNO3]. The dielectric
constant of the medium (D) was studied by varying the t-butyl alcohol and water
percentage. It was found that there was no significant effect of ionic strength
and dielectric constant of the medium on the rate of the reaction.
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Table 1. Effect of variation of [DPC], [OMH], [OHK], [IO4K] and [Os(VIII)] on the os-
mium(VIII) catalysed oxidation of DL-ornithine by diperiodatocuprate(III) in aqueous alkaline
medium at 298 K and I = 0.20 mol dmK3
.
105 [DPC] 104[OMH] 101[OHK
]
105 [IO4K
]
107
[Os(VIII)]
mol dmK3
102kT
sK1
103kU
sK1
102kC
mol dmK3 mol dmK3 mol dmK3 mol dmK3
sK1
Found
Calculated
1.0
3.0
5.0
8.0
10.0
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
5.0
5.0
5.0
5.0
5.0
0.6
1.0
2.0
5.0
6.0
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
0.2
0.4
0.8
1.0
2.0
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5
0.8
1.0
3.0
5.0
1.0
1.0
1.0
1.0
1.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
2.0
5.0
8.0
10.0
20.0
2.61
2.67
2.74
2.56
2.57
0.80
1.20
1.90
2.74
3.01
1.40
2.01
2.74
3.00
3.52
3.51
2.82
2.74
1.70
1.19
0.90
1.83
2.74
3.37
6.45
2.90
3.00
2.88
2.53
2.30
0.85
1.28
2.08
2.88
3.38
1.48
2.15
2.88
3.36
3.74
3.80
3.30
2.88
1.85
1.33
2.88
2.88
2.88
2.88
2.88
2.31
2.37
2.45
2.30
2.34
0.72
1.07
1.69
2.45
2.67
1.25
1.79
2.45
2.66
3.15
3.12
2.49
2.45
1.52
1.05
0.61
1.55
2.45
3.09
6.17
2.46
2.46
2.46
2.46
2.46
0.70
1.03
1.62
2.46
2.62
1.23
1.85
2.46
2.64
3.07
3.00
2.64
2.46
1.48
1.06
0.61
1.54
2.46
3.08
6.16
3.9. Effect of initially added products
Initially added products, copper(II) (CuSO4) and 4-aminobutyric acid did not
have any significant effect on the rate of reaction.
Thus, from the observed experimental results, the rate law for Os(VIII) cata-
lyzed reaction is given as:
3.10 Polymerization study
The possibility of free radicals was detected as follows: the reaction mixture, to
which a known quantity of acrylonitrile (scavenger) had been added initially,
was kept for 2 h in an inert atmosphere. On diluting the reaction mixture with
methanol, a white precipitate was formed, indicating the intervention of free
radicals in the reactions. The blank experiments of either DPC or DL-ornithine
alone with acrylonitrile did not induce any polymerization under the same condi-
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tion as those induced for the reaction mixture. Initially added acrylonitrile de-
creases the rate of reaction indicating free radical intervention, which is the case
in earlier work [22].
3.11 Effect of temperature
The kinetics was studied at four different temperatures 288 K, 298 K, 308 K and
318 K under varying concentrations of DL-ornithine, alkali and periodate, and
osmium(VIII), keeping other conditions constant. The rate constants, (k), of the
slow step of Scheme 3 were obtained from the slopes and the intercepts of the
plots of [Os(VIII)].kC vs 1.[OMH] at four different temperatures. The values
are given in Table 2. The energy of activation for the rate determining step was
obtained by the least - squares method of plot of log k vs 1.T and other activation
parameters calculated for the reaction are presented in Table 2.
3.12 Catalytic activity
It has been pointed out by Moelwyn-Hughes [23] that in the presence of catalyst,
the uncatalyzed and catalyzed reactions proceed simultaneously, so that,
(1)
Here kT is the observed pseudo first-order rate constant in the presence of
Os(VIII) catalyst ; kU, the pseudo-first-order rate constant for uncatalyzed; KC,
the catalytic constant and ‘x’ the order of the reaction with respect to Os(VIII).
In the present investigations; x values for the standard run were found to be
unity.
Then, the value of KC is calculated using the equation,
(2)
The values of KC were evaluated for Os(VIII) catalyst at different tempera-
tures and were found to vary at different temperatures. Further, plots of log KC
vs 1.T were linear and the values of energy of activation and other activation
parameters with reference to catalyst were computed. These results are summa-
rized in Table 3. The value of KC is 3.07!104 at 298 K.
4. Discussion
The water-soluble copper(III) periodate complex is reported [24]to be
[Cu(HIO6)2(OH)2]7K. However, in an aqueous alkaline medium and at a high
pH range as employed in the study, periodate is unlikely to exist as HIO64K (as
present in the complex) as is evident from its involvement in the multiple equilib-
ria [25] (3)–(5) depending on the pH of the solution.
(3)
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Table 2. Activation parameters and thermodynamic quantities for the osmium(VIII) catalysed
oxidation of OMH by diperiodatocuprate(III) in aqueous alkaline medium with respect to the
slow step of Scheme 3: (A) effect of temperature, (B) activation parameters (Scheme 3), (C)
effect of temperature on K1, K2 and K3 for the osmium(VIII) catalysed oxidation of OMH by
diperiodatocuprate(III) in aqueous alkaline medium, (D) thermodynamic quantities using K1, K2
and K3.
Temperature . K
(A)
10K5 k . dm3 molK1sK1
288
0.23
0.47
0.80
1.58
298
308
318
Parameters
(B)
Values
47
Ea . kJ molK1
ΔH# . kJ molK1
ΔS# . JKK1 molK1
ΔG# . kJ molK1
log A
2
44.8 0.5
K5.01 1.0
46
12.9 0.2
101K1 . dm3 molK1 104K2 . moldmK3
5
Temperature . K
10K4K3 .
dm3 molK1
(C)
288
0.28
0.51
0.91
1.48
2.59
1.36
0.68
0.37
6.51
298
7.06
7.88
8.75
308
318
Thermodynamic quantities
(D)
Values from K1
Values from K2
Values from K3
ΔH . kJ molK1
ΔS . JKK1 molK1
ΔG298 . kJ molK1
42
K49.1
K239
22
7.56
118
116
7.36
K27.6
(4)
(5)
Periodic acid exists as H5IO6 in an acid medium and as H4IOK around pH
6
7. Thus, under the conditions employed in alkaline medium, the main species
are expected to be H3IO62Kand H2IO63K. At higher concentrations, periodate also
tends to dimerise. However, formation of this species is negligible under condi-
tions employed for kinetic study. Hence, at the pH employed in this study, the
soluble copper(III) periodate complex exists as diperiodatocuprate(III),
[Cu(H2IO6)(H3IO6)]2K, a conclusion also supported by earlier work [26, 27].
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Table 3. Values of catalytic constant (KC) at different temperatures and activation parameters
calculated using KC values.
Temperature . K
288
10K5KC
0.15
0.30
0.61
1.22
52
298
308
318
Ea . kJ molK1
ΔH# . kJ molK1
ΔS# . JKK1 molK1
ΔG# . kJ molK1
log A
50.3
9.68
47.4
13.7
Scheme 2.
It is known that DL-ornithine exists in the form of Zwitterion [28] 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 [28] (Scheme 2).
Osmium(VIII) is known to form different complexes at different OHK con-
centrations, [OsO4(OH)2]2K and [OsO5(OH)]3K. At higher concentration of
OHK, [OsO5(OH)]3K is significant. At lower concentrations of OHK, as em-
ployed in the present study and since the rate of oxidation increased with increase
in [OHK], it is reasonable that [OsO4(OH)2]2K was operative and its formation
is important in the reaction.
Lister [29] proposed the copper(III) periodate in alkaline medium into three
forms as diperiodatocuprate(III) (DPC), monoperiodatocuprate(III) (MPC) and
tetrahydroxocuprate(III). The latter is ruled out as its equilibrium constant is
8.0!10K11 at 40oC. Hence, in the present study, in view of the negative less
than unit order in periodate on rate of reaction, monoperiodatocuprate(III) MPC
is considered to be the active species of copper(III) periodate complex. The
results of increase in the rate with increase in alkali concentration and decrease
in the rate with increase in periodate concentration suggest that equilibria of
different copper(III) periodate complexes are possible as in Eq. (6) and (7).
(6)
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(7)
The inverse fractional order in [IO4K] might also be due to this reason. The
reaction between DPC and OMH in alkaline medium presents [DPC].[OMH] =
1:4 stoichiometry. Since the reaction was enhanced by [OHK], added periodate
retarded the rate and first order dependency in [DPC] and [Os(VIII)] and frac-
tional order in [OMH] and [OHK], a plausible reaction mechanism has been
proposed which also explains all other experimental observations as shown in
Scheme 3.
Anionic species of OMH reacts with osmium(VIII) active species to form a
complex (C) which further reacts with one mole of MPC in a slow step to give
the free radical species of OMH, Cu(II) with regeneration of catalyst, os-
mium(VIII). Further this free radical species of OMH reacts with one more mole-
cule of MPC species in a fast step to form 4-aminobutaraldehyde intermediate.
This 4-aminobutaraldehyde then reacts with two more moles of MPC in a further
fast step to form products such as 4-aminobutyric acid, Cu(II) and periodate. All
these results may be interpreted in the form of Scheme 3. The probable structure
of the complex (C) is given below.
Spectroscopic evidence for the complex formation between Os(VIII) and
OMH was obtained from UV - vis spectra of OMH (5.0!10K4), Os(VIII)
(8.0!10K7, [OHK] = 0.08 mol dmK3) and a mixture of both. A bathochromic
shift of about 4 nm from 364 to 368 nm in the spectra of Os(VIII) to the mixture
of Os(VIII) and OMH was observed. The Michaelis-Menten plot proved the
complex formation between catalyst and substrate, which explains less than unit
order in [OMH]. Such a complex between a catalyst and substrate has also been
observed in other studies[30].
From Scheme 3, the rate law (9) can be derived.
(8)
(9)
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Jyothi C. Abbar et al.
Scheme 3. Detailed Scheme for Os(VIII) catalyzed oxidation of DL-ornithine by alkaline diperi-
datocuprate(III).
which explains all the observed kinetic orders of different species.
The rate law (9) can be rearranged into the following form which is suitable
for verification.
(10)
According to Eq. (10), other conditions being constant, plots of [Os(VIII)].
kC vs 1.[OMH], 1.[OHK] and [H3IO62K] should be linear and are found to be
so at different temperatures (Fig. 6). The slopes and intercepts of such plots lead
to the values of K1, K2, K3 and k (Table 2). The value of K1 and K2 are in good
agreement with the literature [31]. Using these constants, the rate constants were
calculated and compared with the experimental kC values. There was a reasonable
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agreement with each other (Table 1), which fortifies the proposed mechanism
(Scheme 3).
The negligible effect of ionic strength and dielectric constant of medium on
the rate explains qualitatively the reaction between neutral and negatively
charged ions, as seen in Scheme 3. The thermodynamic quantities for the differ-
ent equilibrium steps, in Scheme 3 can be evaluated as follows. The [OMH],
[OHK] and [H3IO62K] (Table 1) were varied at four different temperatures. The
plots of [Os(VIII)].kC vs 1.[OMH], [Os(VIII)].kC vs 1.[OHK] and [Os(VIII)].
kC vs [H3IO62K] should be linear and are found to be so. From the slopes and
intercepts, the values of K1, K2 and K3 were calculated at four different tempera-
tures. A vant Hoff’s plot was made for the variation of K1, K2 and K3 with
temperature (log K1 vs 1.T, log K2 vs 1.T and log K3 vs 1.T). The values of
enthalpy of reaction ΔH, entropy of reaction ΔS and free energy of reaction ΔG
were calculated for the first, second and third equilibrium steps. These values
are given in Table 2. A comparison of the ΔH value (42.08 kJ molK1) from K1
with that of ΔH# (47.29 kJ molK1) of rate- limiting step supports that the reaction
before the rate determining step is fairly fast as it involves low activation energy
[32, 33], The negative value of ΔS#(K5.01 JKK1 molK1) suggests that intermedi-
ate complex is more ordered than the reactants [34]. The observed modest en-
thalpy of activation and higher rate constant for the slow step indicate that the
oxidation presumably occurs via an inner-sphere mechanism. This conclusion is
supported by earlier observation [35]. The activation parameters evaluated for
the catalyzed and uncatalyzed reactions explain the catalytic effect on the reac-
tion. The catalyst Os(VIII) forms the complex (C) with substrate, which enhances
the reducing property of substrate than that without catalyst. Further, the catalyst
Os(VIII) modifies the reaction path by lowering the energy of activation.
5. Conclusion
Among the various species of copper(III) in alkaline medium,
[Cu(H2IO6)(H2O)2] is considered to be the active species for the title reaction.
The active species of Os(VIII) is found to be [OsO4(OH)2]2K. The reaction rates
revealed that Os(VIII) catalysed reaction is about nine-fold faster than the uncata-
lysed reaction. . It becomes apparent that, in carrying out this reaction, the role
of reaction medium is crucial. Activation parameters were evaluated for the cata-
lyzed reaction. Catalytic constants and the activation parameters with reference
to catalyst were also computed. The overall sequence described here is consistent
with all the experimental evidences including the product, spectral, mechanistic
and kinetic studies.
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Jyothi C. Abbar et al.
Fig. 6. Verification of rate law (9) for the oxidation of DL-ornithine by diperiodatocuprate(III).
Plots of (A) [Os(VIII)].kC vs 1.[OMH], (B) [Os(VIII)].kC vs 1.[OHK], (C) [Os(VIII)].kC vs
[H3IO62K] at four different temperatures (conditions as in Table 1).
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Appendix
Derivation of rate law
According to Scheme 3,
(11)
(12)
The total concentration of [DPC]T is given by
where T and f refer to total and free concentrations
Similarly
In view of low concentrations of Osmium(VIII) used,
Similarly,
(13)
(14)
In view of low concentration of [DPC] and [H3IO62K] used,
(15)
Substituting equations (12), (13), (14) and (15) in equation (11), and omitting
subscripts, the final form of the equation obtained is,
References
1. A. L. Lehninger, Biochemistry: The Molecular Basis of Cell Structure and Function.
Worth Publishers, New York (1970) p. 446.
2. S. S. Cohen, Introduction to the Polyamines. Prentice-Hall, New Jersey (1971).
3. B. G. M. Durie, S. E. Salmon and D. H. Russell, Cancer Res. 37 (1977) 214.
4. D. C. Buffkin, M. M. Webber, W. D. Davidson, L. F. Bassist and R. C. Verma,
Cancer Res. 38 (1978) 3225.
5. K. B. Reddy, B .Sethuram and T. Navaneeth Rao, Indian J. Chem. A 23 (1984) 593.
6. A. Kumar, P. Kumar and P. Ramamurthy, Polyhedron 18 (1999) 773.
Unauthenticated
Download Date | 6/6/16 1:04 PM

882
Jyothi C. Abbar et al.
7. R. S. Shettar and S. T. Nandibewoor, J. Mol. Cat. A. Chem. 234 (2005) 137.
8. B. Sethuram, Some Aspects of Electron Transfer Reactions Involving Organic Mole-
cules. Allied Publishers (P) Ltd., New Delhi (2003) p. 78.
9. J. L. Piere, Chem. Soc. Rev. 29 (2000) 251.
10. E. I. Solomon, P. Chen, M. Metz, S. K. Lee and A. E. Palmer, Angew. Chem. Int.
Ed. Engl. 40 (2001) 4570.
11. M. A. Halcrow, Angew. Chem. Int. Ed. Engl. 40 (2001) 816.
12. J. Peisach, P. Alsen and W. E. Bloomberg, The Biochemistry of Copper. Academic
Press, New York (1966) p. 49.
13. A. K. Das, Coord. Chem. Revs. 213 (2001) 307.
14. G. H. Hugar and S. T. Nandibewoor, Transition Met. Chem. 19 (1994) 215.
15. J. C. Abbar, S. J. Malode and S. T. Nandibewoor, J. Mol. Cat. A. Chem. 313 (2009)
88.
16. O. C. Saxena, Microchem. J. 12 (1967) 609.
17. P. K. Jaiswal and K. L. Yadava, Indian J. Chem. 11 (1973) 837.
18. C. P. Murthy, B. Sethuram and T. N. Rao, Z. Phys. Chem. 262 (1981) 336.
19. G. H. Jeffery, J. Bassett, J. Mendham and R. C. Denney, Vogel’s Textbook of Quanti-
tative Chemical Analysis. Fifth ed., ELBS, Longman, Essex U.K (1996) p.455, p.679.
20. G. P. Panigrahi and P. K. Misro, Indian J. Chem. A 16 (1978) 762.
21. N. P. Shetti and S. T. Nandibewoor, Z. Phys. Chem. 223 (2009) 299.
22. R. V. Jagdeesh and Puttaswamy, J. Phys. Org. Chem. 21 (2008) 844.
23. E. A. Moelwyn-Hughes, Kinetics of Reaction in Solutions. Oxford Univ. Press, Lon-
don (1947) p. 297.
24. K. B. Reddy, B. Sethuram and T. Navaneeth Rao, Z. Phys. Chem. 268 (1987) 706.
25. Jr. J. C. Bailar, H. J. Emeleus, S. R. Nyholm and A. F. Trotman- Dikenson, Compre-
hensive Inorganic Chemistry. Pergamon Press, Oxford. vol. 2 (1975) p.1456.
26. K. B. Reddy, B. Sethuram and T. Navaneeth Rao, Indian J. Chem. A 20 (1981) 395.
27. C. P. Murthy, B. Sethuram, K. B. Reddy and T. Navaneeth Rao, Indian J. Chem. A
23 (1984) 593.
28. R. Chang, Physical Chemistry with Applications to Biological Systems, McMillan,
New York (1981) p. 326.
29. M. W. Lister, Can. J. Chem. 31 (1953) 638.
30. K. N. Vinod, Puttaswamy and Ninge Gowda, J. Mol. Cat. A. Chem. 298 (2009) 60.
31. D. C. Hiremath, T. S. Kiran and S. T. Nandibewoor, Cat. Lett. 122 (2008) 144.
32. K. S. Rangappa, M. P. Raghavendra, D. S. Mahadevappa and D. Channegouda, J.
Org. Chem. 63 (1998) 531.
33. D. C. Bilehal, R. M. Kulkarni and S. T. Nandibewoor, Can. J. Chem. 79 (2001)
1926.
34. A. Weissberger, E. S. Lewis (Eds.), Investigations of Rates and Mechanism of Reac-
tions in Techniques of Chemistry. Vol. 4, Wiley, New York (1974) p. 421.
35. S. A. Farokhi and S. T. Nandibewoor, Tetrahedron 59 (2003) 7595.
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