Kinetic study of oxidation of valine by N-bromophthalimide in presence of iridium (III) chloride as homogenous catalyst

Article abstract of DOI:10.3109/10799890903555681

The mechanistic study of Ir(III) chloride catalyzed oxidation of Val has been studied by by N-bromophthalimide (NBP) in aqueous perchloric acid medium at 303 K. The reaction followed first order kinetics with respect to [NBP] and zero order kinetics with respect to [Val]. At lower concentration range of Ir(III) chloride, the reaction followed first order kinetics while tending to zero order at its higher concentration. A negative effect was observed for [H+] and [NHP] (phthalimide) whereas variation in [Hg(OAc)₂] (mercuric acetate), [Cl-], ionic strength (I) and dielectric constant of the medium did not bring about any significant change on the rate of reaction. The rate constants observed at five different temperatures (298 K-318 K) were used to calculate the activation parameters. A plausible mechanism from the results of kinetic studies, reaction stoichiometry and product analysis has been proposed. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.3109/10799890903555681

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Synthesis and Reactivity in Inorganic, Metal-Organic,
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Kinetic Study of Oxidation of Valine by N-
bromophthalimide in Presence of Iridium (III)
Chloride as Homogenous Catalyst
a
a
a
b
c
Ajaya Kumar Singh , Bhawana Jain , Reena Negi , Yokraj Katre , Surya P. Singh
&
d
Virender K. Sharma
a
Department of Chemistry , Govt. V. Y. T. PG. Autonomous College Durg(C.G.) , India
Department of Chemistry , Kalyan Mahavidyalya , Bhilai Nagar Sect-7, Bhilai, India
b
c
Department of Applied Chemistry , Graduate School of Engineering, Osaka University ,
Suita, Osaka, Japan
d
Chemistry Department , Florida Institute of Technology , Melbourne, Florida, USA
Published online: 16 Feb 2010.
To cite this article: Ajaya Kumar Singh , Bhawana Jain , Reena Negi , Yokraj Katre , Surya P. Singh & Virender K. Sharma
(2010) Kinetic Study of Oxidation of Valine by N-bromophthalimide in Presence of Iridium (III) Chloride as Homogenous
Catalyst, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 40:2, 71-77
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 40:71–77, 2010
Copyright © Taylor & Francis Group, LLC
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.3109/10799890903555681
Kinetic Study of Oxidation of Valine by
N-bromophthalimide in Presence of Iridium (III) Chloride
as Homogenous Catalyst
1
1
1
2
Ajaya Kumar Singh, Bhawana Jain, Reena Negi, Yokraj Katre,
3
4
Surya P. Singh, and Virender K. Sharma
1
Department of Chemistry, Govt. V. Y .T. PG. Autonomous College Durg(C.G.), India
Department of Chemistry, Kalyan Mahavidyalya, Bhilai Nagar Sect-7, Bhilai, India
Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Suita,
2
3
Osaka, Japan
4
Chemistry Department, Florida Institute of Technology, Melbourne, Florida, USA
Val is an essential α-amino acid. However, there are only few
The mechanistic study of Ir(III) chloride catalyzed oxidation of reports available for the oxidation of Val.[11,12] Recently, the use
Val has been studied by by N-bromophthalimide (NBP) in aqueous of platinum group metal ions, such as osmium, ruthenium and
perchloric acid medium at 303 K. The reaction followed first order
kinetics with respect to [NBP] and zero order kinetics with respect
to [Val]. At lower concentration range of Ir(III) chloride, the re-
iridium either alone or binary mixtures, as catalyst in various
[13,14]
redox processes has been attracted considerable interest.
The mechanism of catalysis depends on the nature of sub-
action followed first order kinetics while tending to zero order at
+
[15]
its higher concentration. A negative effect was observed for [H ] strate, the oxidants and experimental conditions.
Potential
and [NHP] (phthalimide) whereas variation in [Hg(OAc)
2
] (mer- of Ir(III) chloride to act as homogenous catalyst in oxidation
−
curic acetate), [Cl ], ionic strength (I) and dielectric constant of the
medium did not bring about any significant change on the rate of
reaction. The rate constants observed at five different temperatures
[16−20]
Preliminary experimental results
process was recognized.
revealed that the reaction did not proceed in the absence of cat-
alyst, but the reaction became facile in the presence of micro
(
298 K–318 K) were used to calculate the activation parameters. A
plausible mechanism from the results of kinetic studies, reaction quantity of Ir(III) chloride. In the present investigation, we re-
stoichiometry and product analysis has been proposed.
port for the first time the results of detailed investigation on the
kinetic and mechanistic aspects of Ir(III) catalyzed oxidation
of Val by NBP in acidic medium at constant temperature. This
study will enable us to understand the complicated biological
reaction in living systems and will also help to understand the
catalytic activity of Ir(III) chloride. Objectives of the present
Keywords Ir(III) chloride, kinetics, NBP, oxidation, valine.
INTRODUCTION
The versatile nature of N-halogeno compounds is due to study are (i) to ascertain the reactive species of catalyst and oxi-
their ability to act as sources of halonium cations, hypo- dant, (ii) to identify the oxidation products, (iii) to elucidate the
halite species and nitrogen anions, which act as both bases plausible reaction mechanism and (iv) to deduce rate law consis-
and nucleophiles. N-halocompounds have been used widely tent with kinetic results and to calculate activation parameters.
as oxidizing and halogenating reagent in variety of organic
compounds.[
1−5]
N-bromophthalimide (NBP) has recently been
Materials
Analytical reagent grade chemicals and triple distilled water
were used throughout the investigation. N-bromophthalimide
Lancaster, 98%) was used as oxidant and melting point of the
developed as an oxidimetric titrant for the determination of cer-
[
6,7]
tain pharmaceuticals.
Literature survey revealed that there is
[
8−10]
limited work done on NBP.
(
sample was found to be 481K. Solution of NBP was prepared
in 80% distilled acetic acid and stored in a black-coated flask to
Received 24 November 2009; accepted 8 December 2009.
One of us (A. K. Singh) is thankful to UGC, Regional office, Bhopal,
M.P., India for Research Project grant.
Address correspondence to Dr. Ajaya Kumar Singh, Chemistry,
MIG-C/125, Vaishali Nagar Bhilai, Durg, Chhattisgarh, 490023, India.
E-mail: ajayaksingh au@yahoo.co.in
prevent photochemical deterioration. The prepared solution of
[21]
NBP was standardized iodometrically
against the standard
solution of sodium thiosulphate using starch as indicator. Stock
solution of Val was prepared by dissolving appropriate amount
of sample in triple distilled water. The Ir(III) chloride solution
71

7
2
A. K SINGH ET AL.
1.6
.4
was prepared by dissolving a known weight of Ir(III) chloride
S.D. fine) in HCl of known strength and stored in a black-
(
1
coated bottle to prevent photochemical decomposition. Standard
solution of KCl, NaClO4 and phthalimide were prepared with
triple distilled water. Hg(OAc)2 (Loba chem., Mumbai, India)
was acidified with 20% acetic acid and perchloric acid (GR) was
diluted with triple distilled water for the present investigation.
1.2
1
0.8
0
0
.6
.4
Kinetic Measurements
All the kinetic measurements were carried out in a black-
coated vessel at a constant temperature, 303 K and performed
under pseudo first order condition with [Val] ꢀ [NBP]. The
reaction was initiated by the rapid addition of known amounts
of oxidant to reaction mixture containing the required amount of
Val, perchloric acid, Ir(III) chloride, Hg(OAc)2, acetic acid and
water in glass stoppered pyrex boiling tubes, and thermosttated
at the same temperature. The progress of reaction was monitored
by iodometric determination of unconsumed [NBP] in known
0.2
0
0
50
100
150
200
Time (min)
FIG. 1. Plot between unconsumed [NBP] versus Time at T = 303 K; [NBP]
−
4
−3
+
−3
−3
−4
=
2.00 × 10 mol dm , [H ] = 0.50 mol dm ; [Val] = 2.00 × 10 mol
−3
−
−5
−3
dm , [Ir(III)] = 1.00 × 10 mol dm ; and [Hg(OAc)2 ] = 3.00 × 10 mol
3
aliquots of the reaction mixtures at different time intervals. The dm and CH COOH = 20%.
3
rate of reaction (-dc/dt) in each kinetic run was determined by
the slope of tangent drawn at fixed [NBP]. Each kinetic run was
studied for two half lives of the reaction. The observed rates of
reaction were reproducible within ± 5% in replicate kinetic run.
The order of reaction in each reactant was determined with the
help of log-log plot of (-dc/dt) versus concentration of reactant.
KINETIC RESULTS
The kinetics of the oxidation of Val by NBP in an aqueous
acidic medium in the presence of Ir(III) chloride was investi-
gated at different concentration of all the reactants. The value
of the initial rate in each kinetic run was calculated from the
slope of the tangent of plot of unconsumed [NBP] versus time
∗
Stoichiometry and Product Analysis
at fixed [NBP] (written as [NBP] ) except in variation of [NBP]
Different ratios of NBP to Val were equilibrated at 303 K in when a tangent was drawn at a fixed time. Plots of log [NBP]
the presence of requisite amount of perchloric acid, mercuric versus time under different initial concentrations of [NBP] are
acetate and acetic acid under the condition of [NBP] ꢀ [Val] shown in Figure 1. The pseudo first order rate constant k1 was
for 48 h. Determination of unconsumed NBP revealed that more calculated as
than two mole of NBP were required for the oxidation of each
mole of Val as in the following equation.
−
(dc/dt)
k1 =
∗
The products were identified as a mixture of nitrile, alde-
hyde and trace of ammonia. Nitrile was identified by the color
[NBP]
reaction[22] and was later confirmed by FT-IR and GC-MS tech-
Each kinetic run was studied for two half lives of the reaction.
The observed rates of reaction were reproducible within ± 5%
in replicate kinetic run. The order of reaction in each reactant
was determined with the help of log-log plot of (-dc/dt) versus
concentration of reactant. For the determination of the order of
reaction with respect to each reactant, the help of Ostwald’s
isolation methods in conjuction with Van’t Hoff’s differntial
method has been taken. Considering NBP, Val, H , Ir(III) chlo-
ride as the main reactants, the general form of the rate equation
for the reaction can be written as
,
niques. Presence of aldehyde was also confirmed by Schiff s
,
reagent. Ammonia was detected by Nessler s reagent. The con-
sumption of more than two moles of NBP per mole of amino acid
may be due to the reaction of ammonia with NBP and may also
be due to the further oxidation of aldehyde. In the spot method,
one drop of liquid sample was placed in a test tube to which
+
3
−3
3
2
cm of 1 mol dm hydroxyl ammonium chloride in propy-
lene glycol and 1 cm of potassium hydroxide in propylene were
added. The mixture was heated at its boiling point for 2 min. and
allowed to cool, followed by addition of 0.5 to 1 cm of ferric
3
α
β
γ
δ
chloride solution. A red to violet color in the solution mixture
was seen, indicating the presence of nitrile. In the FT-IR anal-
Rate = k[NBP] [Val] [catalyst] [H+]
−1
ysis, a sharp peak around 2250 cm also suggested the nitrile.
Uniform pseudo first order rate constant (k ) values of [NBP]
1
−4
−3
−4
−3
)
For GC-MS analysis, the oxidized product was extracted with (from 0.50 × 10 mol dm to 3.00 × 10 mol dm
diethyl ether and the ether layer was concentrated by slow evap- clearly show that the order with respect to [NBP] is unity. This
oration. The concentrated sample was then analyzed by GCMS is also obvious from the plot of (-dc/dt) versus [NBP] (Figure 2,
using a JEOL-JMS (Mate-MS System, Japan) instrument.
Table 1). The reactions have been studied ten fold variation in

OXIDATION OF VALINE
73
5
-3
[
NBP] x 10 (mol dm )
TABLE 2
+
3
.5
3
2.5
2
1.5
1
0.5
0
Effect of varying [NHP], [H ] and [Ir(III)] on the rate of
oxidation of valine at 303 K
6
5
4
3
2
1
0
0
0
1
1
2
2
3
3
4
.5
.5
.5
.5
4
+
5
4
−1
4
−1
[NHP] × 10 [H ] mol [Ir(III)] × 10 k × 10 s k × 10 s
1
1
−3
−3
−3
mol dm
mol dm
dm
(obs.)
(cal.)
1
3
.00
.00
0.50
0.50
0.50
0.50
0.50
0.10
0.20
0.30
0.50
0.60
0.80
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.39 ± 0.19
1.18 ± 0.41
0.66 ± 0.25
0.45 ± 0.20
0.27 ± 0.09
3.95 ± 0.19
3.27 ± 0.12
2.27 ± 0.07
1.40 ± 0.24
1.06 ± 0.20
0.90 ± 0.16
0.65 ± 0.29
0.50 ± 0.24
1.06 ± 0.29
1.69 ± 0.19
2.22 ± 0.27
2.85 ± 0.31
2.94 ± 0.11
1.20
1.10
0.52
0.37
0.21
4.21
2.98
1.94
1.21
1.05
0.81
0.62
0.41
0.78
1.38
1.71
1.86
2.61
5.00
7.00
10.00
—
—
—
—
—
—
—
0
0.5
1
1.5
2
2.5
5
-3
[
Ir(III)] x 10 (mol dm )
FIG. 2. Plot between rate of reaction (-dc/dt) versus [NBP] and [Ir(III)] at T =
+
−3
−3
−3
3
=
03 K; [H ] = 0.50 mol dm , [Val] = 2 × 10 mol dm ; and [Hg(OAc)2 ]
−
4
−3
3 × 10 mol dm and CH3COOH = 20%.
−3
0.30
−3
−3
[
Val] (from 0.50 × 10−3 mol dm to 5.00 × 10 mol dm ) at
—
—
—
—
—
—
—
—
—
—
0.60
1.00
1.30
1.60
2.30
constant concentration of all reactants at constant temperature.
The rate of reaction was found to be nearly the same for all con-
centrations of Val, exhibiting zero order kinetics with respect
TABLE 1
−4
−3
Solution conditions: [NBP] = 2.00 × 10 mol dm , [Val] =
Effect of variation [NBP], [Val] and Acetonitrile on the rate of
oxidation of valine at 303K
−3
−3
−4
−3
2
.00 × 10 mol dm , [Hg(OAc)
2
] = 3.00 × 10 mol dm and
CH
3
COOH = 20%.
4
3
4
−1
[
NBP] × 10
[Val] × 10
Acetonitrile
% by volume
k1 × 10 s
−
3
−3
to Val (Table 1). The reaction follows first order kinetics with
respect to [Ir(III)] at lower concentration but tends to zero order
at higher concentration. This result was further verified by the
plot of -dc/dt versus [Ir(III)] (Figure 2, Table 2). The values of
rate of reaction decreased with increase in proton concentration
Figure 3, Table 1). This may be related to protonation equilib-
rium of Val, which has two forms in the acidic region of the
study (Eq. 2).
mol dm
mol dm
(obs.)
0
1
1
2
3
.50
.00
.50
.00
.00
2.00
2.00
2.00
2.00
2.00
—
—
—
—
—
1.21 ± 0.21
1.23 ± 0.17
1.20 ± 0.32
1.26 ± 0.21
1.22 ± 0.09
(
2
2
2
2
2
2
2
.00
.00
.00
.00
.00
.00
.00
0.50
1.00
1.50
2.00
3.00
4.00
5.00
1.33± 0.11
1.42 ± 0.26
1.31 ± 0.17
1.35 ± 0.24
1.61 ± 0.22
1.57 ± 0.17
1.29 ± 0.31
+
+
−
RCHN H3COOH ꢀꢁ H + RCHNH2COO pH = 2.39 [1]
—
—
—
—
—
—
The equilibrium of Eq. (1) suggests that the positive species of
Val increases with increase in the proton concentration while
the neutral species of Val decreases. If positive species of Val
involved in the reaction, the rates were expected to increase with
increase in proton concentration. This is contrary to the results of
Figure 3. Hence the neutral species of Val is possibly involved in
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
0
5
1.41 ± 0.81
1.23 ± 0.15
1.20 ± 0.24
1.29 ± 0.22
1.39 ± 0.25
1.25 ± 0.15
1.22 ± 0.21
−
the oxidation by NBP. Variation of [Cl ] ion, dielectric constant
and ionic strength to the reaction mixture did not affect the rate
significantly, indicating chloride ion is not involved in any step
prior to the rate determining step in the scheme (1) proposed and
effect of ionic strength suggesting involvement of a nonionic
species in the rate determining step. Subsequently the ionic
strength of the reaction mixture was not kept constant for kinetic
runs. There was no significant effect of increase in concentration
10
15
20
30
40
−4
−3
,
Solution conditions: [Hg(OAc)
2
−3
] = 3.00 × 10 mol dm
Ir(III)] = 1.00 × 10 mol dm , [H ] = 0.50 mol dm and
CH
COOH = 20%.
−5
+
−3
[
of Hg(OAc) on rate of reaction. But as the concentration of
NHP increased rate of reaction decreased, indicating negligible
2
3

74
A. K SINGH ET AL.
9
8
7
6
5
4
3
2
1
0
TABLE 3
Effect of temperature and corresponding activation parameter
on the rate of oxidation of Val
4
−1
Temperature K
k1 × 10 s (obs.)
298
303
1.00 ± 0.36
1.38 ± 0.11
2.11 ± 0.18
3.42 ± 0.20
4.52 ± 0.24
51.69
49.17 ± 0.06
119.23 ± 1.16
13.67 ± 0.68
5.10 ± 0.4
2.50
3
3
3
08
13
18
−
1)
Ea (kJ mol
ꢂH (kJ mol
ꢂS (JK mol
ꢂG (kJ mol
0
0.2
0.4
0.6
H ] (mol dm )
0.8
1
1.2
+
-3
#
−1
−1
−1
[
)
)
)
#
#
+
FIG. 3. Plot between rate of reaction (-dc/dt) versus [H ] at T = 303 K;
−
4
−3
−5
−3
−3
[
1
NBP] = 2 × 10 mol dm , [Ir(III)] = 1.00 × 10 mol dm ; [Val] = 2 × log A
−
3
−3
−4
−1 −1
s
0
mol dm , [Hg(OAc)2 ] = 3 × 10 mol dm and CH3COOH = 20%. kr × 10
−
−3
4
−3
Solution conditions: [NBP] = 2.00 × 10 mol dm , [Val] =
effect of NHP on the rate of reaction. The rate of oxidation
−3
−3
+
2
.00 × 10 mol dm , [H ] = 0.50 mol dm , [Hg(OAc)
2
] = 3.00 ×
−
4
−3
−5
−3
depends on the polarity of medium. When the polarity of the 10 mol dm [Ir(III)] = 1.00 × 10 mol dm and CH COOH =
3
solvent was increased, the rate of reaction was unaffected. To 20%.
determine the activation parameters, the reaction was performed
at five different temperatures (298–313 K). Arrhenius plot of log
Stoichiometry of the Reaction
k1 versus 1/T (Figure 4) was used to calculate activation energy
Stoichiometric experiments were conducted by mixing dif-
(Ea). On the basis of Ea, and with the help of specific constant
ferent amounts of Val and NBP at a molar ratio of 1 : 10 ([NBP] :
[Val]). The results are reported in Table 4. The results clearly
demonstrate the consumption of more than two moles of NBP
per mole of Val. This stoichiometric reactant ratio and the forma-
tion of NHP, nitrile, aldehyde, ammonia and CO2 in the product
analysis indicate the following stochiometric equation.
(
kr ), values of the other activation parameters such as enthalpy
#
#
of activation (ꢂH ), entropy of activation (ꢂS ), Gibb’s free of
activation (ꢂG ) and frequency factor (A) were evaluated and
#
these values are given in (Table 3).
Test for Free Radicals
To test the presence of free radicals in the reaction, the reac-
tion mixture with acryl amide was placed in an inert atmosphere
for 24 h. When the reaction mixture was diluted with methanol,
it was found that there is no precipitate in the reaction mixture.
This clearly showed that there was no formation of free radical
in the redox reaction under investigation.
Val + 2.6 NBP → Products
[2]
DISCUSSION
Reactive Species of NBP
It has been reported earlier by several workers[
23−29]
that
NBP is good oxidizing and brominating agent because of large
1.8
1.6
1.4
1.2
1
TABLE 4
Consumption of NBP in the oxidation of Val with NBP at
3
03 K
0.8
0.6
0.4
0.2
0
4
4
4
[
NBP] × 10 [NBP] × 10 [NBP] × 10
4
−3
−3
−3
mol dm
consumed
[
Val] × 10 mol dm
mol dm
final
[NBP] :
[Val]
−3
mol dm
initial
3
.1
3.15
3.2
3.25
3.3
3.35
3.4
2
.00
4.00
.00
20.00
40.00
60.00
14.66
29.00
43.20
5.34
11.00
16.80
1:2.67
1:2.75
1:2.80
3
-1
1
0 /T (K )
6
−
+
4
−3
FIG. 4. Plot between log k1 versus 1/T; [NBP] = 2 × 10 mol dm , [Ir(III)]
−
4
−3
−
5
−3
−3
−3
−3
Solution conditions: [Hg(OAc) ] = 3 × 10 mol dm , [Ir(III)] =
=
1.00 × 10 mol dm , [Val] = 2 × 10 mol dm , [H ] = 0.50 mol dm
;
2
−
5
−3
+
−3
−4
−3
1.00 × 10 mol dm , [H ] = 0.50 mol dm and CH COOH = 20%.
and [Hg(OAc)2 ] = 3 × 10 mol dm and CH3COOH = 20%.
3

OXIDATION OF VALINE
75
polarity of >N-Br bond. NBP, like other similar N-halo imides, On the basis of steps (I) to (V), Eqs (9) can be obtained in the
may exist in various forms in acidic medium, i.e., free NBP, following equations, respectively:
+
+
protonated NBP, Br , HOBr, (H2OBr) , as per the following
equilibria:
2
k K1K2[Ir(III)] [NBP]
rate =
[9]
+
[
H ] [NHP]
NBP + H2O ꢀꢁ HOBr + NHP
[3]
[4]
[5]
[6]
At any time in the reaction, the total concentration of NBP, i.e.
+
NBP + H ꢀꢁ NHP + Br
[NBP]T can be represented as
+
+
NBP + H ꢀꢁ (NBPH)
+
+
[NBP] = [NBP] + [HOBr] + [C ]
[10]
HOBr + H ꢀꢁ (H2OBr)
T
2
Substitution of [HOBr] and [C2] into Eq. 10 and rearrange it
+
When (H2OBr) is taken as reactive species, the rate law
obtained shows first order kinetics with respect to hydrogen
ion concentrations, contrary to our observed negative frac-
+
[
NBP] [H ] [NHP]
T
[NBP] = [
[11]
+
+
H ] [NHP] + K1[H ] + K1K2[Ir(III)]
+
tional order in [HClO4]. Therefore, the possibility of (H2OBr)
and cationic bromine (Br) as reactive species is ruled out.
+
Substitution of Eq. (11) into Eq. (9) gives Eq. (12) as
When HOBr is assumed as the reactive species, the derived
rate is lawfully capable to explain negative effect of phthal-
imide. Thus, the only choice left is HOBr, which when con-
sidered as the reactive species, leads to a rate law capable
of explaining all the kinetic observations and other effects.
2kK K [Ir(III)] [NBP]
T
1
2
rate = [
[12]
+
+
H ] [NHP] + K1[H ] + K1K2[Ir(III)]
The rate expression obtained in equation (12) can be re-
Hence, in the light of kinetic observation, HOBr can safely written as
be assumed to be the main reactive species for the present
reaction.
+
+
[
NBP]T
[H ] [NHP]
[H ]
1
=
+
+
[13]
rate
2kK1K2[Ir(III)]
2kK2[Ir(III)] 2k
Reactive Species of Ir(III) Chloride
T
According to the Eq. (13), the plot between [NBP] /rate and
+
A spectrophotometric study of the kinetics of the hydration [H ] or 1/[Ir(III)] or [NHP] would be a straight line having an
3
−
−
2−
of IrCl and of the addition of Cl to [Ir(H2O)Cl5] in 1.0–2.5 intercept on the y-axis. A reasonably linear relationship was
6
◦
[30]
M HClO4 (or HCl) at 50 C was reported.
obtained in this plot (Figure 5). The value of k, K1 and K2
calculated from the slope and intercept of the plot were 1.07 ×
−
3 −1
−6
−3
3
k1
10
s
, 3.82 × 10 mol dm and 8.18 × 10 , respectively.
3
−
2
−
IrC1 + H2O
ꢀ
ꢁ
[IrC15(H O)] + C1
[7] Utilizing these values of the constants, the reaction rate were
6
2
k−1
+
calculated for the variations of [Ir(III)], [H ] and [NHP]. Almost
the same values of two rates, i.e., the calculated and observed,
further proves the validity of the rate law (5) and hence the
proposed reaction scheme.
Visible and ultraviolet absorption spectra of the new Ir(III)
complexes [IrCl4(H2O)2] and [IrCl3(H2O)3], together with the
−
3
−
2−
spectra of [IrCl6] and [IrCl5H2O] in 2.5F HClO4 −1.2F Na-
[31]
ClO4 were also reported
and found in reasonable agreement
-3
-1
3
1
0 /[Ir(III)] (mol dm )
with those reported by Poulsen and Garner[ and Jorgensen.
30]
[32]
3
5
30
25
20
15
10
5
0
3−
When Ir(III) chloride dissolved in 0.1 M HCl solution, [IrCl6]
2
1
0
5
0
5
1
and [IrCl5H2O]2 species were formed. On the basis of negli-
gible effect of chloride ion on the rate of reaction and also
assuming the existence of the above equilibrium in the reaction,
−
0
3−
[
IrCl6] is taken as the reactive species of Ir(III) chloride in the
10
5
acidic medium.
15
Considering the reactive species of Ir(III) chloride and NBP
and with the help of above experimental findings, the probable
reaction mechanism is proposed and considering the fact that 1
mole of Val is oxidized by 2 moles of NBP.
Step (III) is a slow step in the proposed scheme (1), hence
the rate can be written as
20
0
25
0
0.2
0.4
0.6
0.8
1
1.2
+
-3
[
H ] (mol dm )
+
−3
FIG. 5. Plot between [NBP]T /rate versus [H ] and 1/[Ir(III)]; [NBP] = 2 ×
−
4
−3
−3
−4
1
0
mol dm , [Val] = 2 × 10 mol dm ; and [Hg(OAc)2] = 3 × 10
−
3
Rate = 2k[C2]
[8] mol dm and CH3COOH = 20%.

76
A. K SINGH ET AL.
TABLE 5
Activation parameters for Val with different catalyst (for isokinetic temperature)
4
−1
4
−1
#
−1
#
−1
#
−1
Val + catalyst
k1× 10 (s
)
k2× 10 (s
)
ꢂH (kJ mol
)
ꢂS (JK mol
)
ꢂG (kJ mol
)
Ref
Val + Pd(II)
Val + Ru(III)
Val + Ir(III)
2.25
1.62
1.38
3.72
2.80
2.11
54.72
57.31
49.17
129.61
126.32
119.23
15.45
19.10
13.67
[28]
[29]
present work
#
#
The values of ꢂS and ꢂH were both favorable for electron
The activation parameters for the oxidation of Val with dif-
transfer processes. The favorable enthalpy was due to release of ferent catalyst by NBP are summarized in Table 5. The entropy
energy on solution changes in the transition state. In the present of activation for the title reaction change within the observed
study of Ir(III) catalyzed oxidation of Val observed positive range.The isokinetic temperature (β) has been evaluated from
#
[33]
entropy of activation (ꢂS ) very well supports the reaction step the Exner equation
A reaction scheme proposed for the oxidation of Val supports
from the order of frequency factor (A) and values of free energy
of activation (ꢂG ) for the redox system.
β = T T (b − 1)/T b − T
1
2
2
1
[14]
#
A plot of log k2 at 303 K versus log k1 at 308 K gave isoki-
netic temperatiure as 392.04. Higher values of β (392.04) than
the experimental temperature (303 K) indicates that the rate is
governed by the enthalpy of activation.[
34]
Activation Parameters
The reaction was studied at five different temperatures (25–
◦
4
5 C), for the catalyzed reaction. From the plot of logk1 versus
1/T (Arrhenius plot), the activation energy (Ea) was evaluated.
The proposed mechanism is also supported by the moderate
values energy of activation and other thermodynamic parame-
ters. The positive values of free energy of activation and of the
enthalpy of activation suggest that the transition state is highly
solvated, while the high positive entropy of activation indicates
that the formation of rigid associated transition states. The order
of frequency factor (A) and values of free energy of activation
#
(
ꢂG ) for the redox system also supports a reaction scheme
proposed for the oxidation of Val by NBP in the presence of
Ir(III) chloride under acidic conditions.
CONCLUSIONS
Oxidation of Val by NBP in perchloric acid media was found
to be very sluggish, but oxidation became facile in the pres-
ence of micro-quantities of Ir(III) chloride catalyst. HOBr and
+
−
RCHNH (COO ) were the reactive species for the NBP and
3
Val, respectively. Among the various species of Ir(III) chloride
3−
in an acidic medium, [IrCl6] was determined to be the cat-
alytic species. In the proposed reaction scheme, the equilibrium
3
−
4−
of [IrCl6] and HOBr gave [IrCl6OBr] , which on dissocia-
3−
tion formed [IrCl5OBr] through a slow rate-determining step.
The species formed was responsible for oxidation of Val. In the
scheme, each molecule of Val consumed more than two moles of
−
NBP to give nitrile, aldehyde, NHP, CO2, and Br in agreement
with the experimentally observed stoichiometry. The deduced
rate law for the reaction scheme was also consistent with the
kinetics of the reaction.
FIG. 6. Reaction scheme: Reaction path for the oxidation of Val by NBP in
the presence of Ir(III) chloride.

OXIDATION OF VALINE
77
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