Os(VIII)/Ru(III) catalysed oxidation of l-valine by Ag(III) periodate complex in aqueous alkaline medium: A comparative kinetic study

Article abstract of DOI:10.1007/s10562-011-0623-1

The kinetics of osmium(VIII) (Os(VIII)) and ruthenium(III) (Ru(III)) catalysed oxidation of l-valine (l-val) by diperiodatoargentate(III) (DPA) in aqueous alkaline medium at 25 °C and a constant ionic strength of 0.006 mol dm^-3 was studied spectrophotometrically. The stoichiometry is the same in both the catalysed reactions, i.e., [l-val]:[DPA] = 1:1. The reaction is of first order in [Os(VIII)], [Ru(III)], and [DPA] and has less than unit order in [l-val] and negative fractional order in [OH^-]. Added periodate had no effect on rate of reaction. The products were identified by spot test and characterized by spectral studies. The catalytic constant (K _C) was also calculated for both catalysed reactions at different temperatures. The activation parameters with respect to slow step of the mechanisms were computed and discussed and thermodynamic quantities were also determined. It has been observed that the catalytic efficiency for the present reaction is in the order of Os(VIII) > Ru(III). The probable active species of catalyst and oxidant have been identified. Graphical Abstract: The kinetics of osmium(VIII) (Os(VIII)) and ruthenium(III) (Ru(III)) catalysed oxidation of l-valine (l-val) by diperiodatoargentate(III) (DPA) in aqueous alkaline medium at 25 °C and a constant ionic strength of 0.006 mol dm^-3 was studied spectrophotometrically. Suitable mechanisms were proposed and active species of DPA, Os(VIII) and Ru(III) were identified. The main products were identified and characterized by spectral studies.

Full text of DOI:10.1007/s10562-011-0623-1

Catal Lett (2011) 141:1526–1540
DOI 10.1007/s10562-011-0623-1
Os(VIII)/Ru(III) Catalysed Oxidation of L-Valine by Ag(III)
Periodate Complex in Aqueous Alkaline Medium: A Comparative
Kinetic Study
•
Shweta J. Malode Nagaraj P. Shetti
•
Sharanappa T. Nandibewoor
Received: 14 March 2011 / Accepted: 7 May 2011 / Published online: 24 May 2011
Ó Springer Science+Business Media, LLC 2011
Abstract The kinetics of osmium(VIII) (Os(VIII)) and
ruthenium(III) (Ru(III)) catalysed oxidation of L-valine
(L-val) by diperiodatoargentate(III) (DPA) in aqueous
alkaline medium at 25 °C and a constant ionic strength of
0.006 mol dm-3 was studied spectrophotometrically. The
stoichiometry is the same in both the catalysed reactions,
i.e., [L-val]:[DPA] = 1:1. The reaction is of first order in
[Os(VIII)], [Ru(III)], and [DPA] and has less than unit
order in [L-val] and negative fractional order in [OH-].
Added periodate had no effect on rate of reaction. The
products were identified by spot test and characterized by
spectral studies. The catalytic constant (KC) was also cal-
culated for both catalysed reactions at different tempera-
tures. The activation parameters with respect to slow step
of the mechanisms were computed and discussed and
thermodynamic quantities were also determined. It has
been observed that the catalytic efficiency for the present
reaction is in the order of Os(VIII) [ Ru(III). The probable
active species of catalyst and oxidant have been identified.
agents [1]. The study of the oxidation of amino acids is of
interest because of their biological significance and selec-
tivity towards the oxidant to yield the different products
[2–4]. L-Valine is an essential amino acid with hydrocarbon
side chains. It is usually found in the interior of proteins. It
has an antagonistic property with structurally similar leu-
cine and isoleucine, and imbalance among these three
items results in suspension of growth. Particular symptoms
of valine deficiency include loss of balance during loco-
motion, changes in the ventral horn and susceptibility to
irritation allergens. Some of valine derivatives have anti-
biotic action.
Diperiodatoargentate(III) (DPA) is a metal complex
with Ag in 3? oxidation state like Cu3? in DPC and Fe3?
in hemoglobin. It is used as a powerful oxidizing agent in
alkaline medium and also as a volumetric reagent for the
determination of various organic and inorganic species
[3–5]. Jayaprakash Rao and other researchers [6, 7] have
studied DPA as an oxidizing agent for the kinetics of
oxidation of various substrates. They normally found that
the order with respect to both oxidant and substrate was
unity and the presence of OH- ions was found to enhance
the rate of reaction. But they did not arrive the possible
active species of DPA in alkali and on the other hand they
proposed mechanisms by generalizing the DPA as
[Ag(HL)L](x?1)-. However, Anil Kumar [8–10] et al. put
an effort to give an evidence for the reactive form of DPA
in the alkaline pH. In the present investigation, we have
obtained the evidence for the reactive species for the DPA
in alkaline medium.
Keywords Kinetics ꢀ Oxidation ꢀ L-Valine ꢀ
Diperiodatoargentate(III) ꢀ Osmium(VIII) catalysis ꢀ
Ruthenium(III) catalysis
1 Introduction
Amino acids act not only as the building blocks in protein
syntheses but they also play a significant role in metabo-
lism and have been oxidized by a variety of oxidizing
Transition metals are known to catalyse many oxida-
tion–reduction reactions since they involve multiple oxi-
dation states. In recent years the use of transition metal ions
such as osmium, ruthenium, palladium, manganese, chro-
mium, iridium either alone or as binary mixtures, as
S. J. Malode ꢀ N. P. Shetti ꢀ S. T. Nandibewoor (&)
P.G. Department of Studies in Chemistry, Karnatak University,
Dharwad 580003, India
e-mail: stnandibewoor@yahoo.com
123

Os(VIII)/Ru(III) Catalysed Oxidation of L-Valine
1527
2.2 Instruments Used
catalysts in various redox processes has attracted consid-
erable interest [11]. Although the mechanism of catalysis
depends on the nature of the substrate, oxidant and
experimental conditions, it has been shown [12] 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) (Os(VIII)) and ruthenium(III) (Ru(III))
catalysis in redox reactions involves different degrees of
complexity, due to the formation of intermediate com-
plexes and different states of osmium/ruthenium, etc. The
uncatalysed reaction of oxidation of L-val by DPA has been
studied [13]. We have observed that Os(VIII) and Ru(III)
catalyses the oxidation of L-val by DPA in alkaline medium
in micro amounts. In order to understand the active species
of oxidant and catalyst, to compute the activity of the
catalyst and to propose the appropriate mechanism, the title
reaction is investigated in detail. An understanding of the
mechanism allows the chemistry to be interpreted, under-
stood and predicted.
(i) For kinetic measurements, a Peltier Accessory (tem-
perature control) attached to Varian CARY 50 Bio
UV–Visible spectrophotometer (Varian, Victoria-
3170, Australia) was used.
(ii) For product analysis, Nicolet 5700-FT-IR spectrom-
1
eter (Thermo, USA), 300 MHz H NMR spectropho-
tometer (Bruker, Switzerland) were used.
(iii) For pH measurements ELICO pH meter model LI
120 was used.
2.3 Preparation of DPA
DPA was prepared by oxidizing Ag(I) in presence of KIO4
as described elsewhere [17]: the mixture of 28 g of KOH
and 23 g of KIO4 in 100 cm3 of water along with 8.5 g
AgNO3 was heated just to boiling and 20 g of K2S2O8 was
added in several lots with stirring and then allowed to cool.
It was filtered through a medium porosity fritted glass filter
and 40 g of NaOH was added slowly to the filtrate,
whereupon a voluminous orange precipitate agglomerates.
The precipitate is filtered as above and washed three to four
times with cold water. The pure crystals were dissolved in
50 cm3 water and heated to 80 °C with constant stirring
thereby some solid was dissolved to give a red solution.
The resulting solution was filtered when it was hot and on
cooling at room temperature, the orange crystals separated
out and were recrystallised from water.
2 Experimental and Methods
2.1 Materials and Reagents
All chemicals used were of reagent grade and Millipore
water was used throughout the work. A solution of L-valine
(S. D. Fine Chem.) was prepared by dissolving an appro-
priate amount of recrystallised sample in Millipore water.
The purity of L-valine was checked by comparing its
melting point 294 °C with the literature data [Lit. m.p.
296 °C]. The required concentration of L-valine was
obtained from its stock solution. The Os(VIII) solution was
prepared by dissolving OsO4 (Johnson Matthey) in
0.50 mol dm-3 NaOH. The concentration was ascertained
[14] by determining the unreacted [Fe(CN)6]4- with stan-
dard Ce(IV) solution in an acidic medium. A standard stock
solution of Ru(III) was prepared by dissolving RuCl3 (S.D.
Fine Chem.) in 0.20 mol dm-3 HCl. The concentration
was determined [15] by EDTA titration. KNO3 and KOH
(BDH) were used to maintain ionic strength and alkalinity
of the reaction, respectively. An aqueous solution of
AgNO3 was used to study the product effect, Ag(I). A stock
The complex was characterized from its UV spectrum,
which exhibited three peaks at 216, 255, and 362 nm.
These spectral features were identical to those reported
earlier for DPA [12]. The magnetic moment study revealed
that the complex is diamagnetic. The compound prepared
was analysed for silver and periodate by acidifying a
solution of the material with HCl [18], recovering and
weighing the AgCl for Ag and titrating the iodine liberated
when excess of KI was added to the filtrate for IO4-. The
stock solution of DPA was used for the required [DPA]
solution in the reaction mixture.
2.4 Kinetic Measurements
The kinetic measurements were performed on a Varian
CARY 50 Bio UV–Visible spectrophotometer. The kinet-
ics was followed under pseudo-first-order condition where
[L-val] ꢁ [DPA] at 25 0.1 °C, unless specified. The
reaction was initiated by mixing the DPA to L-valine
solution which also contained required concentrations of
Os(VIII) or Ru(III), KNO3, KOH and KIO4. The progress
of reaction was followed spectrophotometrically at 360 nm
by monitoring decrease in absorbance due to DPA with the
-
standard solution of IO4 was prepared by dissolving a
known weight of KIO4 (Riedel-de-Haen) in hot water and
used after keeping for 24 h to attain the equilibrium. Its
concentration was ascertained iodometrically [16] at neu-
tral pH maintained using phosphate buffer. The pH of the
medium in the solution was measured by ELICO (LI 120)
pH meter. t-Butyl alcohol (S.D. Fine Chem.) was used to
study the dielectric constant of the reaction medium.
123

1528
S. J. Malode et al.
molar absorbancy index, ‘e’ to be 13900 100
dm3 mol-1 cm-1. The spectral changes during the chemi-
cal reaction for the standard condition at 298 K are shown
in Fig. 1. It is evident from the figure that the concentration
of DPA decreases at 360 nm. It was verified that there is a
negligible interference from other species present in the
reaction mixture at this wavelength.
of the reaction vessel surface on the kinetics. Use of
polythene/acrylic wares and quartz or polyacrylate cells
gave the same results, indicating that the surface did not
have any significant effect on the reaction rates.
In view of the ubiquitous contamination of carbonate in
the basic medium, the effect of carbonate was also studied.
Added carbonate had no effect on the reaction rates.
However, fresh solutions were nevertheless used for car-
rying out each kinetic only. Regression analysis of exper-
imental 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.
The pseudo-first-order rate constants, ‘kC’, were deter-
mined from the log (absorbance) versus time plots. The plots
were linear up to 85% completion of reaction under the range
of [OH-] used. The orders for various species were deter-
mined from the slopes of plots of log kC versus respective
concentration of species except for [DPA] in which non-
variation of ‘kC’ was observed as expected to the reaction
condition. During the kinetics a constant concentration, viz.
5.0 9 10-5 mol dm-3 of KIO4 was used throughout the
study unless otherwise stated. Since periodate is present in
excess in DPA, the possibility of oxidation of L-valine by
periodate in alkaline medium at 25 °C was tested. The pro-
gress of the reaction was followed iodometrically. However,
it was found that there was no significant reaction under the
experimental conditions employed compared to the DPA
oxidation of L-valine. The total periodate concentration was
calculated by considering the periodate present in the DPA
solution and that additionally added.
3 Results
3.1 Stoichiometry and Product Analysis
Different sets of reaction mixtures containing varying
ratios of DPA to L-val in the presence constant amount of
OH-, KIO4, KNO3 and Os(VIII)/Ru(III) catalyst were kept
for 4 h in a closed vessel under nitrogen atmosphere. The
remaining concentration of DPA was estimated by spec-
trophotometrically at 360 nm. The results indicated 1:1
stoichiometry for both the catalysed reactions as given in
Eq. 1.
In view of the modest concentration of alkali used in the
reaction medium, attention was also directed to the effect
O
CH3
Ru(III)/Os(VIII)
CH3
CH3
2-
CO- [Ag(H IO ) ]- + 2OH-
+ Ag(I) + 2H3IO6
+
CH CHO
CH
CH
+
ð1Þ
3
6 2
CH3
-
HCO3 + NH3
NH2
The main reaction product was identified as isobutyr-
aldehyde by spot test [19]. The nature of aldehyde was
confirmed by its IR spectrum, which showed a carbonyl
stretch at 1719 cm-1 and a band at 2938 cm-1 due to
1
aldehydic C–H stretch and characterized by its H NMR
spectrum (singlet at d 9.2 ppm due to 1H of –CHO group,
multiplet at d 1.8 ppm due to 1H of –CH, a doublet at d 1.0
ppm due to 6H of two equivalent –CH3 group), thus con-
firming the presence of isobutyraldehyde. It was further
observed that the aldehyde does not undergo further oxi-
dation under the present kinetic conditions. A test for
corresponding acid proved negative.
The by-products were identified as ammonia by Ness-
ler’s reagent [20], the CO2 was qualitatively detected by
bubbling nitrogen gas through the acidified reaction mix-
ture and passing the liberated gas through tube containing
Fig. 1 Spectroscopic changes occurring in the oxidation of L-valine by
alkaline DPA at 25 °C, [DPA] = 5.0 9 10-5, [L-val] = 5.0 9 10-4
,
[OH-] = 0.002, [IO4-] = 5.0 9 10-5 and I = 0.006 mol dm-3 with
scanning time of: 1 1.0, 2 2.0, 3 3.0, 4 4.0, 5 5.0 and 6 6.0 min
123

Os(VIII)/Ru(III) Catalysed Oxidation of L-Valine
1529
-
3.5 Effect of [OH-] and [IO4
]
lime water. The formation of free Ag? in solution was
detected by adding KCl solution to the reaction mixture,
which produced white turbidity due to the formation of AgCl.
It was observed that isobutyraldehyde does not undergo fur-
ther oxidation under the present kinetic conditions.
The effect of alkali was studied in the range from
0.2 9 10-3 to 6.0 9 10-3 mol dm-3 at 25 °C and constant
concentrations of DPA, L-val, KNO3, IO4-, Os(VIII)/
Ru(III) catalysts. The rate constants decreased with
increase in [OH-] and the order was found to be less than
unity in both the cases (Table 1 Os(VIII) catalysed)
(Table 2 Ru(III) catalysed).
Similarly, the effect of [IO4-] was studied in the range
from 1.0 9 10-5 to 1.0 9 10-4 mol dm-3 at constant
concentrations of DPA, L-valine, OH-, KNO3 and
Os(VIII)/Ru(III) catalysts. It was observed that there was
no effect of periodate on the rates of both the reactions
(Tables 1, 2).
3.2 Reaction Orders
As the DPA oxidation of L-valine in alkaline medium pro-
ceeds with a measurable rate in the absence of Os(VIII) or
Ru(III), the catalysed reaction is understood to occur in
parallel paths with contributions from both the catalysed and
uncatalysed paths. Thus, the total rate constant (kT) is equal
to the sum of the rate constants of the catalysed (kC) and
uncatalysed (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-
trations of L-val, OH- and catalysts (Os(VIII) and Ru(III)), in
turn while keeping other concentrations and conditions
constant. The uncatalysed reaction was followed under the
3.6 Effect of [Os(VIII)] or [Ru(III)]
The Os(VIII) concentration was varied from 1.0 9 10-7 to
1.0 9 10-6 mol dm-3 and [Ru(III)] from 1.0 9 10-6 to
1.0 9 10-5 mol dm-3 range, at constant concentrations of
DPA(III), L-valine, OH- and ionic strength. The order in
[Os(VIII)] or [Ru(III)] was found to be unity from the
linearity of the plots of log kC versus log [Os(VIII)] or log
[Ru(III)] (Fig. 3; Tables 1, 2).
condition [L-valine] = 5.0 9 10-4; [DPA] = 5.0 9 10-5
;
[OH] = 0.002; I = 0.006 mol dm-3. The rate constant of
uncatalysed reaction (kU) was obtained by the plot of log
(absorbance) versus time by following the progress of the
reaction spectrophotometrically at 360 nm.
3.7 Effect of Initially Added Products
3.3 Effect of [DPA]
The externally added products, Ag(I) and isobutyraldehyde
did not have any significant effect on the rates of both
catalysed reactions.
The DPA concentration was varied in the range from
1.0 9 10-5 to 1.0 9 10-4 mol dm-3 at 25 °C for both the
catalysts while keeping other reactant concentrations and
conditions constant. The fairly constant pseudo-first-order
rate constants, kC, indicate that the order with respect to
[DPA] was unity (Tables 1, 2). This was also confirmed by
linearity of the plots of log (absorbance) versus time up to
80% completion of the reaction.
3.8 Effect of Ionic Strength (I) and Dielectric Constant
of the Medium (D)
The addition of KNO3, to increase the ionic strength of the
reaction, increased the rate of reaction at constant [DPA],
[L-val], [OH-], [IO4-] and [catalyst]. The plot of log kC
versus HI was found to be linear with positive slope (Fig. 4).
Dielectric constant of the medium, ‘D’ was varied by
varying the t-butyl alcohol and water percentage. The
decrease in dielectric constant of the reaction medium
decreased the rate and the plot of log kC versus 1/D was
linear with negative slope (Fig. 4). The possibility of oxi-
dation of t-butyl alcohol–water media is also checked and
found that there was no significant effect under the
experimental conditions.
3.4 Effect of [L-Valine]
The L-valine concentration was varied in the range from
1.0 9 10-4 to 1.0 9 10-3 mol dm-3 at 25 °C while keep-
ing other reactant concentrations and conditions constant in
the presence of both the catalysts. The kC values increased
with increase in [L-val]. The order with respect to [L-val] was
less than unity (Os(VIII) catalysed Table 1 and Ru(III) cat-
alysed Table 2). This was also confirmed by the plots of kC
versus [L-val]0.75 (Os(VIII) catalysed) and kC versus [L-
val]0.75 (Ru(III) catalysed) which were linear rather than the
direct plot of kC versus [L-val] (Os(VIII) catalysed) and kC
versus [L-val] (Ru(III) catalysed) (Fig. 2) (r C 0.994,
S B 0.003 for Os(VIII) catalysed, r C 0.997, S B 0.004 for
Ru(III) catalysed). However, the order with respect to [L-val]
changes from first order to zero order as the [L-val] increases.
3.9 Polymerization Study
The intervention of free radicals was examined as follows:
the reaction mixture, to which a known quantity of acry-
lonitrile scavenger has been added initially, was kept in an
123

1530
S. J. Malode et al.
Table 1 Effect of variation of [DPA], [L-val], [OH-], [IO4-] and [Os(VIII)] on the Os(VIII) catalysed oxidation of L-valine by DPA in aqueous
alkaline medium at 25 °C and I = 0.006 mol dm-3
[DPA] 9 105
[L-val] 9 104
[OH-] 9 103
[IO4-] 9 105
[Os(VIII)] 9 107
kT 9 102 kU 9 103 kC 9 102 (s-1
)
(mol dm-3
)
(mol dm-3
)
(mol dm-3
)
(mol dm-3
)
(mol dm-3
)
(s-1
)
(s-1
)
Found Calculated
1.0
3.0
5.0
7.0
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
5.0
1.0
3.0
5.0
7.0
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
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
0.2
0.5
1.0
2.0
4.0
6.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
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
1.0
3.0
5.0
7.0
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
5.0
1.0
3.0
5.0
8.0
10.0
3.84
3.53
3.32
3.46
3.45
0.88
2.14
3.32
4.12
4.69
7.98
6.53
5.02
3.32
1.83
1.35
3.35
3.55
3.32
3.67
3.49
0.92
2.02
3.32
5.10
6.33
4.24
3.80
3.63
3.75
3.44
0.89
2.27
3.63
4.39
5.15
8.39
6.96
5.35
3.63
1.99
1.44
3.63
3.79
3.63
3.48
3.33
3.63
3.63
3.63
3.63
3.63
3.42
3.15
2.96
3.08
3.11
0.79
1.91
2.96
3.68
4.17
7.14
5.84
4.49
2.96
1.63
1.21
2.99
3.17
2.96
3.32
3.16
0.56
1.66
2.96
4.74
5.97
2.87
2.87
2.87
2.87
2.87
0.79
1.99
2.87
3.53
4.26
7.51
5.91
4.36
2.87
1.69
1.21
2.87
2.87
2.87
2.87
2.87
0.57
1.72
2.87
4.58
5.73
inert atmosphere for 2 h. Upon diluting the reaction mix-
ture with methanol, precipitate resulted, suggesting there is
participation of free radicals in the reaction for both
Os(VIII) and Ru(III) catalysis. The blank experiments of
either DPA or L-val alone with acrylonitrile did not induce
any polymerization under the same condition as those
induced for the reaction mixture. Initially added acryloni-
trile decreases the rate of reaction indicating free radical
intervention, which is the case in earlier work [21].
4. The activation parameters for the rate determining step
were obtained by the least square method of plot of log k1
and log k2 versus 1/T and are presented in Tables 3 and 4.
3.11 Catalytic Activity
It has been pointed out by Moelwyn-Hughes [22] that in
presence of the catalyst, the uncatalysed and catalysed
reactions proceed simultaneously, so that
x
kT ¼ kU þ KC½Catalystꢂ
ð2Þ
3.10 Effect of Temperature (T)
Here ‘kT’ is the total rate constant, ‘kU’ the pseudo-first-
order rate constant for the uncatalysed reaction, ‘KC’ the
catalytic constant and ‘x’ the order of the reaction with
respect to [Os(VIII)]/[Ru(III)]. In the present investigations,
x values for the standard run were found to be unity for
Os(VIII) and Ru(III). Then the value of KC is calculated
using the equation
The influences of temperature on the rates of reaction were
studied at 15, 25, 35 and 45 °C. The rate constants (k1 and
k2) of the slow step of Schemes 1 and 2 were obtained from
the slopes and the intercepts of the plots of [Catalyst]/kC
versus 1/[L-val], and [Catalyst]/kC versus [OH-] at four
different temperatures. The values are given in Tables 3 and
123

Os(VIII)/Ru(III) Catalysed Oxidation of L-Valine
1531
Table 2 Effect of variation of [DPA], [L-val], [OH-], [IO4-] and [Ru(III)] on the Ru(III) catalysed oxidation of L-valine by DPA in aqueous
alkaline medium at 25 °C and I = 0.006 mol dm-3
[DPA] 9 105
[L-val] 9 104
[OH-] 9 103
[IO4-] 9 105
[Ru(III)] 9 106
kT 9 102
(s-1
kU 9 103
(s-1
kC 9 102 (s-1
)
(mol dm-3
)
(mol dm-3
)
(mol dm-3
)
(mol dm-3
)
(mol dm-3
)
)
)
Found
Calculated
1.0
3.0
5.0
7.0
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
5.0
1.0
3.0
5.0
7.0
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
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
0.2
0.5
1.0
2.0
4.0
6.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
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
1.0
3.0
5.0
7.0
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
5.0
1.0
3.0
5.0
8.0
10.0
2.99
2.77
2.59
2.71
2.76
0.66
1.68
2.59
3.24
3.56
6.20
5.14
3.95
2.59
1.43
1.06
2.57
2.79
2.59
2.97
2.83
0.86
1.55
2.59
4.28
5.20
4.24
3.80
3.63
3.75
3.44
0.89
2.26
3.63
4.39
5.15
8.39
6.96
5.35
3.63
1.99
1.44
3.56
3.79
3.63
3.48
3.33
3.63
3.63
3.63
3.63
3.63
2.57
2.39
2.23
2.33
2.42
0.57
1.46
2.23
2.81
3.04
5.36
4.45
3.42
2.23
1.23
0.92
2.22
2.41
2.23
2.63
2.49
0.49
1.19
2.23
3.92
4.84
2.15
2.15
2.15
2.15
2.15
0.57
1.48
2.15
2.68
3.28
5.65
4.44
3.28
2.15
1.28
0.91
2.15
2.15
2.15
2.15
2.15
0.43
1.29
2.15
3.45
4.31
been studied by Ag(III) species which may be due to its
strong versatile nature of two electrons oxidant. Among the
various species of Ag(III), Ag(OH)-4 , DPA and ethylenebis
(biguanide), (EBS), silver(III) are of maximum attention to
the researchers due to their relative stability [23]. The
stability of Ag(OH)-4 is very sensitive towards traces of
dissolved oxygen and other impurities in the reaction
medium whereupon it had not drawn much attention.
However, the other two forms of Ag(III) [24] are consid-
erably stable; the DPA is used in highly alkaline medium
and EBS is used in highly acidic medium.
kT ꢃ kU
kC
KC ¼
¼
ðwhere kT ꢃ kU ¼ kCÞ
x
x
½Catalystꢂ
½Catalystꢂ
ð3Þ
The values of KC were evaluated for both the catalysts 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 catalysts were
computed. These results are summarized in Table 5. The
value of KC for Os(VIII) is 5.9 9 104 and Ru(III) is
4.5 9 103 at 298 K. The values of KC indicate that
Os(VIII) is more efficient catalyst compared to Ru(III) in
the oxidation of L-valine by DPA in alkaline medium.
The literature survey [17] reveals that the water soluble
DPA has a formula [Ag(IO6)2]7- with dsp2 configuration of
square planar structure, similar to diperiodatocopper(III)
complex with two bidentate ligands, periodate to form a
planar molecule. When the same molecule is used in
alkaline medium, it is unlikely to exist as [Ag(IO6)2]7- as
periodate is known to be in various protonated forms [25]
depending on pH of the solution as given in following
multiple equilibria (4)–(6).
4 Discussion
In the later period of twentieth century the kinetics of
oxidation of various organic and inorganic substrates have
123

1532
S. J. Malode et al.
-3
[L-val]0.75 103 mol dm
8.0
6.0
4.0
2.0
0.0
(A)
8.0
6.0
4.0
2.0
0.0
0.0
2.0
4.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
(A)
0.0
0.5
1.0
1.5
-3
2.0
0.0
3.0
6.0
9.0
12.0
[L-val] 103 mol dm
-3
-3
[Os(VIII)] 107 mol dm
[L-val]0.75 103 mol dm
6.0
4.0
2.0
0.0
6.0
4.0
2.0
0.0
0.0
3.5
2.8
2.1
1.4
0.7
0.0
(B)
(B)
1.0
2.0
3.0
4.0
0.0
0.5
1.0
1.5
[L-val] 103 mol dm-3
0.0
3.0
6.0
9.0
12.0
-3
[Ru(III)] 106 mol dm
Fig. 2 Plots of a kC versus [L-val]0.75 and kC versus [L-val] (Os(VIII)
catalysed, conditions as in Table 1), b kC versus [L-val]0.75 and kC
versus [L-val] (Ru(III) catalysed, conditions as in Table 2)
Fig. 3 Unit order plots of a kC versus [Os(VIII)], b kC versus
[Ru(III)]
DPA, may be depicted as [Ag(H3IO6)2]-. The similar
speciation of periodate in alkali was proposed [26] for
diperiodatonickelate(IV).
H5IO6 ꢀ H4IO6ꢃ þ Hþ
ð4Þ
ð5Þ
ð6Þ
H4IO6 ꢀ H3IO62ꢃ þ Hþ
ꢃ
It is known that L-valine exists in zwitterionic form in
aqueous medium [27]. In highly acidic medium, it exists in
the protonated form, whereas in highly basic medium it is
in the fully deprotonated form [27].
H3IO6 ꢀ H2IO63ꢃ þ Hþ
2ꢃ
Periodic acid exists as H5IO6 in acid medium and as
-
H4IO6 near pH 7. Thus, under the present alkaline
2-
conditions, the main species are expected to be H3IO6
and H2IO63-. At higher concentrations, periodate also
tends to dimerise [5]. However, formation of this species is
negligible under conditions employed for kinetic study. On
contrary, the authors [8] in their recent studies have
proposed the DPA species as [Ag(HL)2]x- in which ‘L’ is a
periodate with uncertain number of protons and ‘HL’ is a
protonated periodate of uncertain number of protons. This
can be ruled out by considering the alternative form [25] of
4.1 Mechanism of Os(VIII) Catalysis
Os(VIII) is known to form different complexes at dif-
ferent OH- [28] concentrations, [OsO4(OH)2]2- and
[OsO5(OH)]3-
.
At higher concentration of OH-,
[OsO5(OH)]3- is significant. At lower concentrations of
OH-, as employed in the present study, and since the rate
of oxidation decreased with increase in [OH-], it is rea-
sonable that [OsO4(OH)2]2- was operative and its forma-
tion is important in the reaction. In earlier report [29], it has
been observed that in Os(VIII) catalysed reaction, in view
2-
3-
IO4- at pH [ 7 which is in the form H3IO6 or H2IO6
.
Hence, DPA could be as [Ag(H3IO6)2]- or [Ag(H2IO6)2]3-
in alkaline medium. Therefore, under the present condition,
123

Os(VIII)/Ru(III) Catalysed Oxidation of L-Valine
1533
1/D 102
of less than unit order in substrate, unit order in Os(VIII)
and oxidant, Os(VIII) is regenerated through formation of
Os(VII). In another case [30] Os(VIII) is regenerated by
Os(VI) intervention in view of unit order each in osmium,
substrate and oxidant. In some other reports [28] it is
observed that Os(VIII) forms a complex with substrate,
which is oxidised by the oxidant with regeneration of the
catalyst. Hence the study of behavior of Os(VIII) in cata-
lysed reaction becomes significant. To explain all the
observed orders, Scheme 1 is proposed for Os(VIII) cata-
lysed reaction.
1.7
1.6
1.5
1.4
1.3
1.2
1.2
1.3
1.4
1.5
(A)
1.9
1.7
1.5
1.3
In the prior equilibrium step 1, the hydroxyl ion con-
centration with fractional order in OH- concentration, the
main oxidant species is likely to be [Ag(H3IO6)2]- and its
formation by the above equilibrium is important in the
present study. The less than unit order in [L-val] presum-
ably results from the formation of a complex (C1) between
the Os(VIII) species and L-valine. This complex (C1) reacts
with one mole of DPA in a slow step to give the free
radical species of L-val, Ag(II) with the regeneration of
catalyst, Os(VIII). Further this free radical species of L-val
reacts with Ag(II) in a fast step to form the products iso-
butyraldehyde, Ag(I) as given in Scheme 1.
0.3
1.7
0.6
0.9
1.2
1.5
1.8
1.3
I1/2 10
1/D 102
1.5
1.6
1.4
2.0
1.8
1.6
1.4
1.2
1.1
1.2
1.3
1.4
(B)
Spectroscopic evidence for the complex formation
between Os(VIII) and L-val was obtained from UV–Vis
spectra of L-val (5.0 9 10-4), Os(VIII) (5.0 9 10-7
,
[OH-] = 0.002 mol dm-3) and a mixture of both. A
hypsochromic shift of 5 nm from 246 to 241 nm in the
spectra of Os(VIII) to the mixture of Os(VIII) and L-val
was observed. Attempts to separate and isolate the complex
were not successful. The Michaelis–Menten plot proved
the complex formation between catalyst and substrate,
which explains less than unit order in [L-val]. Such a
0.3
0.6
0.9
1.2
1.5
1.8
I1/2 10
Fig. 4 Effect of ionic strength and dielectric constant of the medium
for a Os(VIII) catalysed, b Ru(III) catalysed, oxidation of L-valine by
DPA at 25 °C
Scheme 1 Detailed scheme for
the Os(VIII) catalysed oxidation
of L-valine by DPA
K1
[Ag(H2IO6)(H3IO6)]2-
[Ag(H3IO6)2]-
OH-
+
H O
+
2
K2
CH3
COO-
+ [OsO4(OH)2]2-
CH
CH
Complex
(C1)
CH3
NH2
CH3
CH3
.
k1 / slow
CH CH + HCO3- + Ag(OH)+ + 2 H3IO6
2-
[Ag(H IO ) ]-
Complex (C1)
+
3
6 2
2OH-
+ [OsO4(OH)2]2-
NH2
CH3
CH3
.
fast
+ Ag(OH)+
CHO
Ag(I)
+
CH
NH
+
CH
CH
3
CH3
CH3
NH2
123

1534
S. J. Malode et al.
Scheme 2 Detailed scheme for
the Ru(III) catalysed oxidation
of L-valine by DPA
K3
K4
[Ag(H2IO6)(H3IO6)]2-
[Ag(H3IO6)2]-
OH-
+
H O
+
2
CH3
CH3
COO-
+ [Ru(H2O)5OH]2+
CH
Complex
(C2)
CH
NH2
CH3
CH3
.
k2 / slow
CH CH + HCO3- + Ag(OH)+ + 2 H3IO6
NH2
2-
Complex (C2) [Ag(H IO ) ]-
+
3
6 2
2OH-
+ [Ru(H2O)5OH]2+
CH3
CH3
CH3
.
fast
+ Ag(OH)+
+
CHO
Ag(I)
+
CH
NH
3
CH
CH
CH3
NH2
complex between a catalyst and substrate has also been
observed in other studies [28]. The rate law (8) for the
Scheme 1 could be derived as
[Os(VIII)]/kC versus 1/[L-val] (r C 0.9996, S B 0.00133),
[Os(VIII)]/kC versus [OH-] (r C 0.9948, S B 0.00142)
should be linear as shown in Fig. 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., log K1 versus 1/T (r C 0.9213,
S B 0.1034)] and the values of the enthalpy of reaction
DH, entropy of reaction DS and free energy of reaction DG,
were calculated. These values are also given in Table 3. A
comparison of DH value (21.1 0.4 kJ mol-1) from K2
with that of DH# (43.4 2.0 kJ mol-1) of rate determin-
ing step supports that the second step of Scheme 1 is fairly
fast since it involves low activation energy [31]. In the
same manner, K2 values were calculated at different tem-
peratures and the corresponding values of thermodynamic
quantities are given in Table 3.
ꢃd½DPAꢂ
Rate ¼
dt
k1K1K2½lꢃvalꢂ½OsðVIIIÞꢂ½DPAꢂ
¼
ð7Þ
ð8Þ
K1 þ K1K2½l-valꢂ þ ½OHꢃꢂ þ K2½l-valꢂ½OHꢃꢂ
Rate
¼ kC ¼ kT ꢃ kU
½DPAꢂ
k1K1K2½lꢃvalꢂ½OsðVIIIÞꢂ
¼
K1 þ K1K2½l-valꢂ þ ½OHꢃꢂ þ K2½l-valꢂ½OHꢃꢂ
This explains all the observed kinetic orders of different
species. The rate law (8) can be rearranged to be Eq. 9,
which is suitable for verification.
The effect of increasing ionic strength on the rate
explains qualitatively the reaction between two negatively
charged ions, as seen in Schemes 1 and 2. Amis has shown
that a plot of log kC versus 1/D is linear with a negative
slope for a reaction between a negative ion and a dipole or
two dipoles, and with a positive slope for a positive ion–
dipole interaction [32].
½OsðVIIIÞꢂ
½OHꢃꢂ
½OHꢃꢂ
1
1
¼
þ
þ
þ
kC
k1K1K2½l-valꢂ
k1K1
K1K2½l-valꢂ k1
ð9Þ
According to Eq. 9, other conditions being constant, the
plots of [Os(VIII)]/kC versus [OH-] and [Os(VIII)]/kC
versus 1/[L-val] should be linear and found to be so
(Fig. 5). From the intercepts and slopes of such plots, the
reaction constants K1, K2 and k1 were calculated as
(9.1 0.1) 9 10-4 mol dm-3
(1.05 0.03) 9 103 dm3 mol-1
(5.3 0.2) 9 105 dm3 mol-1 s-1
However, in the present study, an increase in the content
of t-butyl alcohol in the reaction medium leads to the
decrease in the reaction rate (Fig. 4a Os(VIII) catalysed),
which is in agreement with Amis theory [32].
The moderate values of DH# and DS# were both favorable
for electron transfer processes. The favorable enthalpy was
due to release of energy on solutions changes in the transition
state. The negative value of DS# suggests that the interme-
diate complex is more ordered than the reactants [33]. The
observed modest enthalpy of activation and a higher rate
constant for the slow step indicates that the oxidation pre-
sumably occurs via an inner-sphere mechanism. This con-
clusion is supported by earlier observations [34].
,
,
,
respectively. These
constants 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 the
Scheme 1.
The thermodynamic quantities for the different equi-
librium steps, in Scheme 1 can be evaluated as follows.
The L-valine and hydroxide ion concentrations (Table 1)
were varied at different temperatures. The plots of
123

Os(VIII)/Ru(III) Catalysed Oxidation of L-Valine
1535
Table 3 Activation parameters and thermodynamic quantities for the
Os(VIII) catalysed oxidation of L-valine by DPA in aqueous alkaline
medium with respect to the slow step of Scheme 1: (A) effect of
temperature, (B) activation parameters (Scheme 1), (C) effect of
temperature to calculate K1 and K2 for the Os(VIII) catalysed oxi-
dation of L-valine by DPA in alkaline medium, (D) thermodynamic
quantities using K1 and K2
Table 4 Activation parameters and thermodynamic quantities for the
Ru(III) catalysed oxidation of L-valine by DPA 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 K3 and K4 for the Ru(III) catalysed oxidation
of L-valine by DPA in alkaline medium, (D) thermodynamic quanti-
ties using K3 and K4
(A)
(A)
Temperature (K)
k1 9 10-6 (dm3 mol-1 s-1
)
Temperature (K)
k2 9 10-5 (dm3 mol-1 s-1
)
288
298
308
318
0.36
0.53
1.05
2.13
288
298
308
318
0.28
0.44
0.70
1.49
(B)
(B)
Parameters
Values
Parameters
Values
Ea (kJ mol-1
DH# (kJ mol-1
DS# (J K-1 mol-1
)
45.9
43.4
10.5
40.3
13.8
Ea (kJ mol-1
DH# (kJ mol-1
DS# (J K-1 mol-1
)
41.8
39.4
)
)
)
)
-23.8
46.5
DG# (kJ mol-1
)
DG# (kJ mol-1
)
log A
log A
11.9
(C)
(C)
Temperature (K) K1 x 103 (mol dm-3
)
K2 9 10-3 (dm3 mol-1
)
Temperature (K) K3 9 103 (mol dm-3
)
K4 9 10-3 (dm3 mol-1
)
288
298
308
318
1.01
0.91
0.79
0.68
0.65
1.05
1.22
1.55
288
298
308
318
1.06
0.91
0.76
0.70
0.61
0.91
1.43
1.83
(D)
(D)
Thermodynamic quantities
DH (kJ mol-1
DS (J K-1 mol-1
DG298 (kJ mol-1
Values from K1
Values from K2
Thermodynamic quantities
DH (kJ mol-1
DS (J K-1 mol-1
DG298 (kJ mol-1
[DPA] = 5.0 9 10-5
Values from K3
Values from K4
)
-9.9
-91.6
17.4
21.1
)
-10.7
-94.2
17.4
28.7
)
129
)
153
)
-17.2
)
-16.9
[DPA] = 5.0 9 10-5, [L-val] = 5.0 9 10-4, [OH-] = 0.002, [IO4-] =
5.0 9 10-5, [Os(VIII)] = 5.0 9 10-7, I = 0.006/mol dm-3
,
[L-val] = 5.0 9 10-4
,
[OH-] = 0.002,
[IO4-] = 5.0 9 10-5, [Ru(III)] = 5.0 9 10-6, I = 0.006/mol dm-3
The equilibrium step 1 and the stoichiometry were
same as in case of Os(VIII) catalysis. In most of the
works on DPA oxidation, periodate shows retarding effect
and OH- had an increasing effect on the rate of reaction.
However, in the present kinetic study, entirely different
kinetic observations have been obtained, OH- retarded
the rate of the reaction and periodate showed no effect.
Hence, the active species of oxidant is found to be
[Ag(H3IO6)2]-. The less than unit order in [L-valine]
indicates the formation of a complex (C2) between the
Ru(III) species and L-valine. This complex (C2) reacts
with one mole of DPA in a slow step to give the free
radical species of L-val, Ag(II) with the regeneration of
4.2 Mechanism of Ru(III) Catalysis
It is interesting to identify the probable Ru(III) chloride
species in alkaline media. Electronic spectral studies have
confirmed that ruthenium chloride exists in hydrated form
as [Ru(H2O)5OH]2?. In the present study, it is quite
probable that the [Ru(III)(OH)x]3-x, the x-value would
always be less than six because there are no definite reports
of any hexahydroxy ruthenium species. The remainder of
the coordination sphere would be filled by water molecules.
Hence, under the conditions employed, e.g.
[OH-] ꢁ [Ru(III)], Ru(III) is mostly present as the
hydroxylated species, [Ru(H2O)5OH]2? [35].
123

1536
S. J. Malode et al.
Spectroscopic evidence for the complex formation
between Ru(III) and L-val was obtained from UV–Vis
Table 5 Values of catalytic constant (KC) at different temperatures
and activation parameters calculated using KC values for Os(VIII) and
Ru(III) catalysed reactions
spectra of L-val (5.0 9 10-4), Ru(III) (5 9 10-6
,
[OH-] = 0.002 mol dm-3) and a mixture of both. A
bathochromic shift of 9 nm from 354 to 365 nm in the
spectra of Ru(III) to the mixture of Ru(III) and L-val was
observed. Attempts to separate and isolate the complex
were not successful. The Michaelis–Menten plot proved
the complex formation between catalyst and substrate,
which explains less than unit order in [L-val]. Such a
complex between a catalyst and substrate has also been
observed in other studies [36]. The rate law (11) for the
Scheme 2 could be derived as
Temperature (K)
KC 9 10-4
KC 9 10-4
288
298
308
318
3.12
5.91
12.4
24.7
52.8
50.3
15.3
45.7
14.0
0.24
0.45
0.95
1.89
52.7
50.3
-6.0
52.1
12.9
Ea (kJ mol-1
DH# (kJ mol-1
DS# (J K-1 mol-1
)
)
)
DG# (kJ mol-1
)
log A
ꢃd½DPAꢂ
[DPA] = 5.0 9 10-5
,
,
[L-val] = 5.0 9 10-4
,
[OH-] = 0.002,
Rate ¼
dt
[IO4-] = 5.0 9 10-5
[Os(VIII)] = 5.0 9 10-7
,
k2K3K4½lꢃvalꢂ½RuðIIIÞꢂ½DPAꢂ
[Ru(III)] = 5.0 9 10-6, I = 0.006/mol dm-3
¼
K3 þ K3K4½l-valꢂ þ ½OHꢃꢂ þ K4½l-valꢂ½OHꢃꢂ
ð10Þ
2.0
(A)
Rate
¼ kC ¼ kT ꢃ kU
½DPAꢂ
1.5
1.0
0.5
0.0
288 K
298 K
k2K3K4½lꢃvalꢂ½RuðIIIÞꢂ
¼
K3 þ K3K4½l-valꢂ þ ½OHꢃꢂ þ K4½l-valꢂ½OHꢃꢂ
ð11Þ
This explains all the observed kinetic orders of different
species. The rate law (11) can be rearranged to be Eq. 12,
which is suitable for verification.
308 K
318 K
½RuðIIIÞꢂ
½OHꢃꢂ
½OHꢃꢂ
1
1
¼
þ
þ
þ
0.0
0.3
0.6
0.9
1.2
kC
k2K3K4½l-valꢂ
k2K3
K3K4½l-valꢂ k2
-1
1/[L-val] 10-4 dm3 mol
ð12Þ
1.2
0.8
0.4
0.0
According to Eq. 12, other conditions being constant, the
plots of [Ru(III)]/kC versus [OH-] and [Ru(III)]/kC versus
1/[L-val] should be linear and found to be so (Fig. 6). From
the intercepts and slopes of such plots, the reaction constants
K3, K4 and k2 were calculated as (9.1 0.4) 9
10-4 mol dm-3, (9.1 0.3) 9 102 dm3 mol-1, (4.4 0.2) 9
104 dm3 mol-1 s-1, respectively. These constants 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 the Scheme 2.
The thermodynamic quantities for the different equi-
librium steps, in Scheme 2 can be evaluated as follows.
The L-valine and hydroxide ion concentrations (Table 2)
were varied at different temperatures. The plots of
[Ru(III)]/kC versus 1/[L-val] (r C 0.9982, S B 0.00132),
[Ru(III)]/kC versus [OH-] (r C 0.9873, S B 0.00145)
should be linear as shown in Fig. 6. From the slopes and
intercepts, the values of K3 are calculated at different
temperatures. A vant Hoff’s plot was made for the variation
of K3 with temperature [i.e., log K3 versus 1/T (r C 0.961,
S B 0.1035)] and the values of the enthalpy of reaction
(B)
288 K
298 K
308 K
318 K
0.0
2.0
4.0
6.0
8.0
[OH-] 103 mol dm-3
Fig. 5 Verification of rate law (9) for the Os(VIII) catalysed
oxidation of L-valine by DPA. Plots of a [Os(VIII)]/kC versus 1/[L-
val], b [Os(VIII)]/kC versus [OH-], at six different temperatures
(conditions as given in Table 1)
catalyst, Ru(III). Further this free radical species of L-val
reacts with Ag(II) in a fast step to form the products
isobutyraldehyde, Ag(I) as given in Scheme 2.
123

Os(VIII)/Ru(III) Catalysed Oxidation of L-Valine
1537
25.0
interaction [32]. However, in the present study, an increase
in the content of t-butyl alcohol in the reaction medium
leads to the decrease in the reaction rate (Fig. 4b for
Ru(III) catalysed), which is in agreement with Amis theory
[32].
(A)
20.0
15.0
10.0
5.0
288 K
The values of DH#, DS#, DG# and rate constants (k1 and
k2) indicate that the order of reactivity of the catalysts is
Os(VIII) [ Ru(III) for the oxidation of L-valine by DPA.
The Os(VIII) catalysed reaction, however, is reasonably
fast in view of readiness of Os(VIII) to act across the –
COO group and the Ru(III) catalysed reaction is slower,
probably owing to the less ability of the Ru(III) to act
across the –COO group. The activation parameters evalu-
ated for the catalysed and uncatalysed reactions explain the
catalytic effect on the reaction. The catalyst Os(VIII) or
Ru(III) forms the complex with substrate, which enhances
the reducing property of the substrate over that without
catalyst (Os(VIII) or Ru(III)). Further, the catalyst Os(VIII)
or Ru(III) modifies the reaction path by lowering the
energy of activation.
298 K
308 K
318 K
0.0
0.0
0.3
0.6
0.9
1.2
1/[L-val] 10-4 dm3 mol-1
20.0
15.0
10.0
5.0
(B)
288 K
298 K
The values of activation parameters for the Os(VIII) and
Ru(III) catalysed oxidation of some amino acids by DPA
are summarized in Tables 6 (for Os(VIII)) and 7 (for
Ru(III)). The entropy of activation for the title reaction
falls within the observed range. Variation in the rate within
the reaction series may be caused by change in the enthalpy
or entropy of activation. Changes in the rate are caused by
changes in both DH# and DS#, but these quantities vary
extensively in a parallel fashion. A plot of DH# versus DS#
is linear according to equation
308 K
318 K
0.0
0.0
2.0
4.0
6.0
8.0
[OH-] 103 mol dm
-3
Fig. 6 Verification of rate law (12) for the Ru(III) catalysed
oxidation of L-valine by DPA. Plots of a [Ru(III)]/kC versus 1/[L-
val], b [Ru(III)]/kC versus [OH-], at six different temperatures
(conditions as given in Table 2)
DH# ¼ bDS# þ constant
b is called the isokinetic temperature. It has been asserted
that apparently linear correlation of DH# with DS# are
sometimes misleading and the evaluation of b by means of
the above equation lacks statistical validity [37]. Exner
[38] advocates an alternative method for the treatment of
experimental data. If the rates of several reactions in a
series have been measured at two temperatures and log k10
(at T2) is linearly related to log k1 (at T1), i.e., log k10 ¼
a þ b log k1; he proposes that b can be evaluated from the
equation
DH, entropy of reaction DS and free energy of reaction DG,
were calculated. These values are also given in Table 4. A
comparison of the DH value of second step
(28.7 kJ mol-1
)
of Scheme 2 with that of DH#
(39.4 kJ mol-1) 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 fast and involves low activation
energy [31]. In the same manner, K4 values were calculated
at different temperatures and the corresponding values of
thermodynamic quantities are given in Table 4. The neg-
ative value of DS# suggests that intermediate complex is
more ordered than the reactants [33].
b ¼ T1T2ðb ꢃ 1Þ=ðT2b ꢃ T1Þ
We have calculated the isokinetic temperature as
157.9 K for Os(VIII) catalysed reaction by plotting log k10
at 303 K versus log k1 at 298 K (r C 0.9893, s B 0.003) as
in Fig. 7 and for Ru(III) catalysed reaction, b value as
84.3 K by plotting log k20 at 303 K versus log k2 at 298 K
(r C 0.9969, s B 0.005) as in Fig. 8. The values of b for
both Os(VIII) (157.9 K) and Ru(III) (84.3 K) catalysed
The effect of increasing ionic strength on the rate
explains qualitatively the reaction between two negatively
charged ions, as seen in Scheme 2. Amis has shown that a
plot of log kC versus 1/D is linear with a negative slope for
a reaction between a negative ion and a dipole or two
dipoles, and with a positive slope for a positive ion–dipole
123

1538
S. J. Malode et al.
Table 6 Activation parameters
for Os(VIII) catalyzed oxidation
of some amino acids by DPA
(for isokinetic temperature)
Amino acids
k1 9 10-4
k10 9 10-4
(dm3 mol-1 s-1
at 303 K
DH#
DS#
Reference
)
(dm3 mol-1 s-1
at 298 K
)
(kJ mol-1
)
(J K-1 mol-1
)
L-Leucine
Tyrosine
L-Lysine
L-Alanine
L-Proline
L-Valine
0.10
0.20
0.46
0.66
1.51
0.22
0.11
0.21
0.47
0.73
1.91
0.33
14
-155
-154
-138
-128
-10
[40]
3.3
5.2
19
[40]
[40]
[40]
25
[40]
32.2
-53.8
Present work
Table 7 Activation parameters
for Ru(III) catalyzed oxidation
of some amino acids by DPA
(for isokinetic temperature)
Amino acids
k2 9 10-4
k20 9 10-4
(dm3 mol-1 s-1
at 303 K
DH#
DS#
Reference
)
(dm3 mol-1 s-1
at 298 K
)
(kJ mol-1
)
(J K-1 mol-1
)
L-Alanine
L-Leucine
L-Lysine
Tyrosine
L-Valine
0.0218
0.10
0.0271
0.108
2.0767
4.12
12
-128
-155
-142
-148
-23.8
[41]
9.3
5.9
2.6
39.4
[41]
1.9768
4.09
[41]
[41]
4.39
7.03
Present work
5.0
4.0
3.0
2.0
3.5
3.0
2.5
(5)
(5)
(4)
(4)
(3)
(6)
(3)
(2)
2.0
1.5
1.0
(1)
(2)
(1)
1.0
0.0
1.5
2.0
2.5
3.0
3.5
1.0
2.0
3.0
4.0
3 + logk 1
3 + logk 2
Fig. 7 Plot of log k10 at 303 K versus log k1 at 298 K for isokinetic
temperature in case of Os(VIII) catalysed reaction (Table 6). 1 L-
leucine, 2 tyrosine; 3 L-lysine, 4 L-alanine, 5 L-proline, 6 L-valine
Fig. 8 Plot of log k20 at 303 K versus log k2 at 298 K for isokinetic
temperature in case of Ru(III) catalysed reaction (Table 7). 1 L-
alanine, 2 L-leucine, 3 L-lysine, 4 tyrosine, 5 L-valine
reactions are lower than experimental temperature
(298 K).This indicates that both the rates of reactions are
governed by entropy of activation [39]. The linearity and
the slope of the plot obtained may confirm that the kinetics
of these reactions follow a similar mechanism, as previ-
ously suggested.
products were identified. Among the various species of
Ag(III) in alkaline medium, in earlier reports the mo-
noperiodatoargentate(III) (MPA) was the active species,
whereas DPA is considered to be the active species for the
title reaction. Active species of Os(VIII) is found to be
[OsO4(OH)2]2- and that for Ru(III) is [Ru(H2O)5OH]2?
.
Activation parameters were evaluated for both the cata-
lysed reactions. Catalytic constants and the activation
parameters with reference to catalyst were also computed.
Catalytic efficiency is Os(VIII) [ Ru(III).
5 Conclusion
The comparative study of Os(VIII) and Ru(III) catalysed
oxidation of L-valine by DPA was studied. Oxidation
123

Os(VIII)/Ru(III) Catalysed Oxidation of L-Valine
Appendix A
1539
Â
Ãꢃ
½DPAꢂT ¼ ½DPAꢂFþ AgðH3IO6Þ2
ꢀ
ꢁ
½OHꢃꢂ þ K4
½OHꢃꢂ
¼ ½DPAꢂF
According to Scheme 1
Â
Ãꢃ
Rate ¼ k1½C1ꢂ AgðH3IO6Þ2
Therefore,
k1K1K2½lꢃvalꢂ½OsðVIIIÞꢂ½DPAꢂ
ꢀ
ꢁ
½DPAꢂT½OHꢂꢃ
½OHꢃꢂ þ K3
¼
ð13Þ
½OHꢃꢂ
½DPAꢂF ¼
ð19Þ
The total concentration of DPA is given by (where T and
F stand for total and free)
Similarly,
Â
Ãꢃ
½DPAꢂT ¼ ½DPAꢂFþ AgðH3IO6Þ2
½l-valꢂT ¼ ½l-valꢂ þ½C2ꢂ ¼ ½l-valꢂFþK4½l-valꢂF½RuðIIIÞꢂ
¼ ½l-valꢂFð1 þ K4½RuðIIIÞꢂÞ
ꢀ
ꢁ
½OHꢃꢂ þ K2
½OHꢃꢂ
F
¼ ½DPAꢂF
In view of low concentration of Ru(III) used,
Therefore,
½l-valꢂT ¼ ½l-valꢂF
Similarly,
ð20Þ
ꢀ
ꢁ
½DPAꢂT½OHꢃꢂ
½OHꢃꢂ þ K1
½DPAꢂF ¼
ð14Þ
½OHꢃꢂ ¼ ½OHꢃꢂF
½RuðIIIÞꢂT ¼ ½RuðIIIÞꢂ þ½C2ꢂ
ð21Þ
ð22Þ
Similarly,
T
¼ ½RuðIIIÞꢂFþK4½l-valꢂ½RuðIIIÞꢂ
½l-valꢂT ¼ ½l-valꢂFþ½C1ꢂ ¼ ½l-valꢂ þK2½l-valꢂF½OsðVIIIÞꢂ
¼ ½l-valꢂFð1 þ K2½OsðVIIIÞꢂÞ
F
F
ꢀ
ꢁ
F
½RuðIIIÞꢂT
½RuðIIIÞꢂF¼
f1 þ K4½l-valꢂg
In view of low concentration of Ru(III) used,
Substituting Eqs. 19–22 in Eq. 18 and omitting the
subscripts T and F, we get
½l-valꢂT ¼ ½l-valꢂF
Similarly,
ð15Þ
Rate
½OHꢃꢂT ¼ ½OHꢃꢂF
½OsðVIIIÞꢂT ¼ ½OsðVIIIÞꢂ þ½C1ꢂ
ð16Þ
¼ kC ¼ kT ꢃ kU
½DPAꢂ
k2K3K4½lꢃvalꢂ½RuðIIIÞꢂ
¼ ½OsðVIIIÞꢂFþK2½l-valꢂ½OsðVIIIÞꢂ ½OsðVIIIÞꢂ
¼
K3 þ K3K4½l-valꢂ þ ½OHꢃꢂ þ K4½l-valꢂ½OHꢃꢂ
F
F
F
ꢀ
ꢁ
½OsðVIIIÞꢂT
¼
f1 þ K2½l-valꢂg
ð17Þ
References
Substituting Eqs. 14–17 in Eq. 13 and omitting the
subscripts T and F, we get
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¼ kC ¼ kT ꢃ kU
½DPAꢂ
k1K1K2½lꢃvalꢂ½OsðVIIIÞꢂ
¼
K1 þ K1K2½l-valꢂ þ ½OHꢃꢂ þ K2½l-valꢂ½OHꢃꢂ
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Â
Ãꢃ
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¼
ð18Þ
½OHꢃꢂ
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