Kinetics of Ir (III)-catalyzed oxidation of D-glucose by potassium iodate in aqueous alkaline medium

Article abstract of DOI:10.1080/07328300902999311

The kinetics of Ir (III) chloride-catalyzed oxidation of D-glucose by iodate in aqueous alkaline medium was investigated at 45°C. The reaction follows first-order kinetics with respect to potassium iodate in its low concentration range but tends to zero o

Full text of DOI:10.1080/07328300902999311

Journal of Carbohydrate Chemistry, 28:278–292, 2009
ꢀC
Copyright Taylor & Francis Group, LLC
ISSN: 0732-8303 print / 1532-2327 online
DOI: 10.1080/07328300902999311
Kinetics of Ir (III)-catalyzed
Oxidation of D-glucose
by Potassium Iodate in
Aqueous Alkaline Medium
Surya Prakash Singh,1 Ashok Kumar Singh,1
and Ajaya Kumar Singh2
1Department of Chemistry, University Of Allahabad, Allahabad, India
2Department of Chemistry, Government V.Y.T.PG. Autonomous College Durg,
Chhattisgarh-491001, India
The kinetics of Ir (III) chloride-catalyzed oxidation of D-glucose by iodate in aqueous
alkaline medium was investigated at 45◦C. The reaction follows first-order kinetics with
respect to potassium iodate in its low concentration range but tends to zero order at its
higher concentration. Zero-order kinetics with respect to [D-glucose] was observed. In
the lower concentration range of Ir (III) chloride, the reaction follows first kinetics,
while the order shifts from first to zero at its higher concentration range. The reaction
–
follows first-order kinetics with respect to [OH ] at its low concentration but tends
–
towards zero order at higher concentration. Variation in [Cl ] and ionic strength of the
medium did not bring about any significant change in the rate of reaction. The first-
order rate constant increased with a decrease in the dielectric constant of the medium.
The values of rate constants observed at four different temperatures were utilized to
calculate the activation parameters. Sodium salt of formic acid and arabinonic acid have
been identified as the main oxidation products of the reaction. A plausible mechanism
from the results of kinetic studies, reaction stoichiometry, and product analysis has
been proposed.
Keywords Kinetics; Oxidation; Ir (III) chloride; D-glucose; Potassium iodate
INTRODUCTION
Glucose is one of the very important carbohydrates in biology. The study of
carbohydrates is one of the most exciting fields of organic chemistry. Vast
Received December 05, 2007; accepted February 25, 2009.
Address correspondence to Ajaya Kumar Singh, Department of Chemistry, Govt.
V.Y.T.PG. Autonomous College Durg (CG). E-mail ajayaksingh au@yahoo.co.in
278

Kinetics of Ir (III)-catalyzed Oxidation of D-glucose
279
literature is available on the kinetics of oxidation of carbohydrates by various
organic and inorganic oxidants. The oxidation of aldoses by chlorine, bromine,
and iodine has been reported in alkaline media.[1] The aldonic acids as primary
products of oxidation of aldoses by bromine have been extensively studied by
Isbell and coworkers,[2] who pointed out that β -aldoses (C-1 equatorial) are ox-
idized much faster than α -aldoses (C-1 axial). The catalyzed and uncatalyzed
oxidation of sugars has been studied in detail by using N-halo compounds.[3–5]
Different metal ion catalysts like chromium (III),[6] ruthenium (III),[7–9] Ir (III)
chloride,[10,11] and Pd (II)[12,13] have been used in the oxidation by different ox-
idizing agents. Among the different metal ions, ruthenium (III) and iridium
(III) are highly efficient. Lack of studies on the oxidation of glucose by iodate
in the presence of Ir (III) chloride in the alkaline medium has encouraged us
to investigate the kinetic behavior of the title reaction in order to continue our
study on metal ion in oxidation reactions by iodate. This study will enable un-
derstanding the complicated biochemical reaction in living bodies and will also
help to understand the catalytic activities of Ir (III) chloride along with oxida-
tive capacity of iodate in alkaline solution. Preliminary experimental results
indicate that the reactions of D-glucose with iodate in alkaline medium with-
out a catalyst were too sluggish to be measured, but the reactions become facile
in the presence of a micro-quantity of Ir (III) chloride catalyst. Therefore, in or-
der to explore the mechanism of D-glucose-iodate reactions in alkaline medium
and also to study the catalytic action of Ir (III) chloride, Ir (III) chloride as a
catalyst was selected in the present work. In the present communication, we
report for the first time the results of the detailed investigation on the kinetic
and mechanistic aspects of Ir (III) chloride-catalyzed oxidation of D-glucose by
potassium iodate in alkaline medium at 45◦C. Objectives of the present study
are (a) to elucidate a plausible mechanism, (b) to deduce an appropriate rate
law, (c) to identify the oxidation products, (d) to ascertain the various reactive
species, (e) to find the catalytic efficiency of Ir (III) chloride, and (f) to deter-
mine the complex formation.
EXPERIMENTAL
Material
Sodium perchlorate, potassium iodate, sodium hydroxide, and D-glucose
(E. Merck) were used as supplied without further purification by preparing
their solutions in doubly distilled water. A stock solution of Ir (III) chloride
(Johnson Metthey) was prepared by dissolving the sample in hydrochloric acid
of known strength. Sodium perchlorate was used to maintain the ionic strength
of the medium. All other reagents were of AnalaR grade and doubly distilled
water was used throughout the work. The reaction vessels were coated from
outside with black paint to avoid any photochemical reaction.

S. P. Singh et al.
280
Kinetic Procedure
A thermostated water bath was used to maintain the desired tempera-
ture within
0.1◦C. The calculated amount of the reactants [i.e., potassium
iodate, NaOH, Ir (III) chloride, and water] except D-glucose was taken in a
reaction vessel, which was kept in a thermostatic water bath. After allow-
ing sufficient time to attain the temperature of the experiment, a requisite
amount of D-glucose solution, also thermostated at the same temperature, was
rapidly pipetted out and run into the reaction vessel. Measuring the uncon-
sumed amount of potassium iodate iodometrically monitored the progress of
reaction. The rate of reaction (–dc/dt) was determined by the slope of the tan-
–
gent drawn at a fixed [IO3 ] in each kinetic run. The order of reaction in each
reactant was measured with the help of log-log plot of (–dc/dt) versus concen-
tration of the reactants.
Stoichiometry and Product Analysis
The reaction mixture containing the D-glucose, Ir (III) chloride, and NaOH
with excess of potassium iodate was kept for 72 h at 35◦C. The total amount
of iodate consumed by 1 mole of D-glucose for its complete oxidation was de-
termined. Iodometric determination of the unconsumed iodate showed that 2
mole of the oxidant were consumed per mole D-glucose forming sodium salt
of formic acid and arabinonic acid. Accordingly, the following stoichiometric
equation could be formulated:
Ir(III)/OH−
C6H12O6 + 2IO− −−−−−−−−→ HCOO− + C5H9O− + 2HIO2
3
6
Arabinonateion
Sodium salt of formic acid and arabinonic acid were identified as the oxida-
tion product in the oxidation of D-glucose. The sodium salt of formic acid was
analyzed by NUCON gas chromatography using porapak-Q 101 columns and
programmed oven temperature having an FID detector. The major product was
identified as sodium salt of formic acid in the Ir (III) chloride-catalyzed oxida-
tion of D-glucose by comparison of retention time with the retention time of
standard solutions. Sodium formate was also confirmed by spot test,[14] help of
equivalence and kinetic studies, and TLC methods.
RESULTS AND DISCUSSION
Kinetics Study
The kinetics of Ir (III) chloride-catalyzed oxidation of D-glucose by potas-
sium iodate was investigated at several initial concentrations of the reactants
in alkaline medium at 45◦C. The rate (i.e. –dc/dt) of the reaction in each kinetic

Kinetics of Ir (III)-catalyzed Oxidation of D-glucose
281
run was determined by the slope of the tangent drawn at fixed concentration of
–
oxidant, potassium iodate. In the variation of oxidant (IO3 ), tangent has been
drawn at a fixed time. The first-order rate constant (k1) was calculated as:
k1 = (−dc/dt)/[IO−]
3
–
The first-order dependence of reaction on IO3 at its lower concentrations tends
to zero order at its higher concentrations. This is also obvious from the plot of
–
(–dc/dt) versus [IO3 ] (Table 1, Fig. 1), indicating first-order kinetics at lower
concentrations and tending toward zero-order kinetics in its higher concentra-
tions. The rates of the reaction (–dc/dt), were calculated at different concen-
trations of D-glucose. This was found to be the same for [D-glucose], showing
zero-order kinetics with respect to [D-glucose] (Table 1). Since order of reac-
tion with respect to D-glucose is zero, in each kinetic run, the rate of reaction
(–dc/dt) will always be equal to the standard zero-order rate constant. The
plot of rate constant k1 versus [Ir (III) chloride] was linear passing through
the origin, suggesting first-order dependence of the rate of reaction on the [Ir
(III) chloride]. At the same time, it also shows that the reaction does not pro-
ceed with measurable velocity in the absence of [Ir (III) chloride] (Table 1, Fig.
2). Kinetics of catalyzed oxidation of D-glucose indicates that on increasing
[NaOH], the value of (–dc/dt) increases at low concentrations and shifts to zero
order at its higher concentrations of NaOH, which is also evident from the plot
Table 1: Effect of variation of [KIO3], glucose, and [Ir (III)] on the rate of Ir
(III)-catalyzed oxidation of D-glucose by iodate in alkaline medium at 45◦C
[KIO3]
[D-Glu]
[Ir (III)]
(–dc/dt) × 108
k1× 105
× 104 M
× 102 M
× 105 M
M sec−1
(sec−1
)
4.0
8.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
1.0
2.0
3.0
4.0
6.0
7.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.68
2.68
2.68
2.68
2.68
2.68
2.68
2.68
2.68
2.68
2.68
2.68
2.68
0.67
1.34
2.68
4.02
6.7
5.09
10.71
13.09
15.14
20.83
23.10
24.20
13.85
13.33
13.85
13.33
13.33
13.85
3.35
16.41
14.28
13.63
10.81
5.34
5.20
5.01
10.0
15.0
40.0
50.0
60.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
13.85
13.33
13.85
13.33
13.33
13.85
3.35
6.18
6.18
13.13
19.25
24.62
26.12
26.92
13.13
19.25
24.62
26.12
26.92
8.04
9.38
Conditions: [NaOH] = 0.20 M, [KCl] = 1.80 × 10−3 M.

S. P. Singh et al.
282
30
25
20
15
10
5
0
0
10
20
30
40
50
60
70
[IO3] x 104 M
–
Figure 1: Dependence of reaction rate on [IO3 ] from data and conditions given in Table 1.
–
of (–dc/dt) versus [NaOH] (Table 2, Fig. 3). This shows a positive effect of [OH ]
on the rate of oxidation of D-glucose. Variation of ionic strength of the medium
–
and [Cl ] did not bring about any significant change in k1 values under the con-
stant experimental conditions (Table 3). The rate of reaction increased with a
decrease in dielectric constant of the medium (by increasing the percentage
of ethanol by volume) (Table 2). Control experiments performed showed that
ethanol was not oxidized by potassium iodate under the experimental condi-
tions. The reaction was studied at different temperatures ranging from 308
to 323 K. (Table 4). From the linear Arrhenius plots of log k1 versus 1/T, the
30
25
20
15
10
5
0
0
2
4
6
8
10
[Ir(III)] x 105 M
Figure 2: Dependence of rate constant (k) on [Ir (III)] from data and conditions given in
Table 1.

Kinetics of Ir (III)-catalyzed Oxidation of D-glucose
283
–
Table 2: Comparison of observed rates in the variation of [OH ] with the calculated
ratesa for the Ir (III)-catalyzed oxidation of D-glucose by iodate in alkaline medium
Rate × 108 M sec−1
[NaOH] × 102 M
Observed
Calculatedb
Predictedc
05.00
06.66
08.00
10.00
12.00
15.00
20.00
22.22
25.00
5.12
7.29
8.33
10.00
12.12
13.33
14.85
15.15
15.32
5.67
7.12
6.15
7.17
8.16
7.97
9.55
9.00
10.78
12.36
14.48
15.27
16.16
9.95
11.24
13.16
13.87
14.87
–
aConditions: [IO3 ]= 10.00 × 10−4 M, [IrCl3] = 2.68 × 10−5 M, [D-glucose] = 2.00 × 10−2 M,
[NaOH] = 0.20 M, [KCl] = 1.80 × 10−3 M, T = 45◦C.
bCalculated on the basis of rate law (2).
cCalculated with the help of multiple regression analysis.
activation energy Ea was calculated. With the help of the rate constant kr,
values of the other activation parameters such as enthalpy of activation (ꢀ
H#), entropy of activation (ꢀ S#), Gibbs free energy of activation (ꢀ G#), and
Arrhenius factor (A) were calculated (Table 4). The following form of rate law,
valid for the conditions under which temperature dependence has been mea-
sured, was used to calculate the activation parameters in the Ir (III) chloride-
catalyzed oxidation of D-glucose by potassium iodate in alkaline medium:
ꢀ
ꢁ
d IO−3
ꢀ
ꢁ ꢀ
ꢁ ꢀ
ꢁ
−
= kr IO− Ir(III) OH−
3
dt
18
16
14
12
10
8
6
4
2
0
0
5
10
15
20
25
30
[NaOH] x 102 M
Figure 3: Dependence of reaction rate on [NaOH] from data and conditions given in
Table 1.

S. P. Singh et al.
284
Table 3: Effect of variation of [KCl], ionic strength, and dielectric constant of the
medium on the rate of Ir (III) chloride-catalyzed oxidation of D-glucose by iodate in
alkaline medium at 45◦C
KCl × 103 M
µ × 102 M
% Ethanol
k1× 105 sec−1
1.8
25.00
25.00
25.00
25.00
25.00
25.00
25.00
25.00
25.00
25.00
25.00
4.04
—
—
—
—
—
—
—
5
10
15
20
—
—
—
—
—
—
—
9.72
10.00
9.72
2.8
3.8
4.8
9.70
5.8
9.78
6.8
10.00
9.52
9.8
1.80
1.80
1.80
1.80
1.80
1.80
1.80
1.80
1.80
1.80
1.80
15.85
18.18
25.00
34.27
26.00
26.19
25.92
25.00
26.26
26.19
26.26
7.6
11.16
14.72
18.28
28.96
36.08
–
Conditions: [IO3 ]= 10.00 × 10−4 M, [IrCl3] = 2.68 × 10−5 M, [D-glucose] = 2.00 × 10−2 M,
[NaOH] = 0.20 M.
Test for Free Radical
The addition of the reaction mixtures to aqueous acryl amide monomer so-
lutions, in the dark, did not initiate polymerization, indicating the absence of
Table 4: Values of rate constant at different temperatures and activation
parameters of Ir (III)-catalyzed oxidation of D-glucose by iodate in alkaline medium
Temp.(K)
k1× 105 (sec−1
)
308
05.83
09.09
13.33
16.66
53.00
5.40
313
318
323
Ea(kJ)
kr× 102
–2
–1
(mol sec
)
)
74.22
ꢀ S#
(J)
50.33
ꢀ H#
(kJ)
26.54
5.48
ꢀ G#
(kJ)
A × 102
–2
–1
(mol sec
–
Conditions: [IO3 ]= 10.00 × 10−4 M, [IrCl3] = 2.68 × 10−5 M, [D-glucose] = 2.00 × 10−2 M, [NaOH]
= 0.20 M, [KCl] = 1.80 × 10−3 M.

Kinetics of Ir (III)-catalyzed Oxidation of D-glucose
285
formation of free radical species in the reaction sequence. The control experi-
ments were also performed under the same reaction conditions.
Reactive Species of Iodates
There is only one report[15] available in the literature that oxidation of or-
ganic or inorganic compounds by sodium or potassium iodate in the presence of
–
iridium has been studied in alkaline medium and IO3 has been assumed as the
reactive species of potassium iodate in alkaline medium. Kinetic data collected
for Ir (III) chloride-catalyzed oxidation of D-glucose by potassium iodate clearly
–
show that IO3 is the reactive species of potassium iodate in alkaline medium.
Reactive Species of Iridium (III) Chloride in Alkaline Medium
–
[IrCl3(H2O)3], [IrCl4(H2O)2] , and [IrCl5(H2O)]2– are reported to be pre-
dominant species in concentration range of 0.1 M to 8 M HCl. In view of the
observed kinetic data and the reported literature, it is reasonable to assume
that the reactive species of Ir (III) chloride is [IrCl3 (H2O)3]. Since the study for
the Ir (III) chloride-catalyzed oxidation of D-glucose has been made in alkaline
medium, a decision can be made about reactive species of Ir (III) chloride after
taking into account the effect of NaOH concentration. On the basis of observed
kinetic data related to first-order kinetics at low concentrations of NaOH in
the oxidation of D-glucose and an increase in absorbance from 1.68 to 2.08 and
2.44 of Ir (III) chloride solution with two different concentrations of NaOH, it
can be concluded that the following equilibrium is established:
ꢀ
ꢂ
ꢃ
ꢁ
ꢀ
ꢂ
ꢃ
ꢁ
Ir H2O Cl3 + OH− ꢂ IrCl3 H2O 2 OH − + H2O
ꢁ
3
–
Out of these two species (i.e., [IrCl3(H2O)3] and [IrCl3(H2O)2OH] ), the
–
species [IrCl3(H2O)2OH] can be safely assumed the reactive species of iridium
(III) chloride in the present investigation.
Reactive Form of Sugar in Alkaline Medium
It is reported[16] that in the presence of alkali, reducing sugars undergo
a tautomeric change resulting in the formation of an enediol and an enediol
anion. The formation of the enediol and an enediol anion in alkaline medium
is also supported by the work of Isbell and coworkers.[17] The observed reaction
–
rate with respect to [OH ] for the Ir (III) chloride-catalyzed oxidation of D-
glucose has led us to assume that enediol form of sugar which is actually taking
part in the reactions under investigation.
The possibility of the formation of a complex in oxidation of D-glucose oc-
–
curs between reactive species of Ir (III) chloride (i.e., [IrCl3 (H2O)2OH] ) and
–
reactive species of iodate (i.e., IO3 ) in alkaline medium. In order to verify the

S. P. Singh et al.
286
existence of an above complex, spectra for the solution of Ir (III) chloride and
–
–
OH with two different concentrations of IO3 have been collected. From the
spectra, it is clear that with the addition of potassium iodate solution, there
is an increase in absorbance from 2.08 to 2.54 and 2.62, with a slight shift
in λmax value toward longer wavelength (i.e., from 216 to 217 and 220 nm).
–
This increase in absorbance with the increase in [IO3 ] can be considered as
an indication of formation of complex between the reactive species of Ir (III)
chloride and reactive species of iodate in alkaline medium (Sch. 1). The shift
in λmax value toward longer wavelength is due to the fact that the combina-
–
–
tion of a chromophore, IO3 , and an auxochrome, OH , gives rise to another
chromophore
.
K1
OH-
[IrCl3(H2O)2OH]-
(C2)
(i)
+
[Ir(H2O)3Cl3]
(C1)
H2O
2-
+
O
O
K2
-
[IrCl3(H2O)2OH]- + IO3
+ H2O (ii)
I
O
IrCl3(H2O)OH
(C2)
(C3)
2-
2-
O
O
k
(iii)
O
I
H O
+
IrCl3(OH)
2
O
O
I
O
IrCl3(H2O)OH
slow and rate
determining step
O
(C3)
O
C
O-
C
2-
OH
H
C
+2OH-
+
+
[IrCl3(H2O)(OH)2)]2-
O
O
fast
-
R
IO2
I
IrCl3(OH)
+
OH
C
R
HO
H
(iv)
wher
e R stands for C3H7O3
O
C
O-
C
-
Ir(III)
+ IO3 + OH-
+ H2O
-
HCOO-
RCOO-
+
IO2
(v)
+
R
HO
H
Reaction Scheme 1. Plausible mechanism for the stepwise Ir (III)-catalyzed oxidation of
D-glucose.

Kinetics of Ir (III)-catalyzed Oxidation of D-glucose
287
Suggested Mechanism
–
On the basis of observed kinetic results and taking [IrCl3 (H2O)2OH] and
–
IO3 as the most active species of Ir (III) chloride and potassium iodate, respec-
tively, the following reaction steps (Sch. 1) have been proposed in the oxidation
of glucose by iodate ion in the presence of Ir (III) chloride.
On the basis of all the steps in Scheme 1 for the oxidation of D-glucose
and stoichiometry of the reaction, the rate of the reaction in terms of loss of
concentration of iodate ions can be written as equation (1):
ꢀ
ꢁ ꢀ
ꢁ ꢀ
ꢁ
2kK1K2 OH− IO−3
Ir(III)
T
1 + K1 OH− + K1K2 OH− IO3− T+ K1K2 OH− Ir III
Rate =
ꢀ
ꢁ
ꢀ
ꢁ ꢀ
ꢁ
ꢀ
ꢁ
(1)
–
[
(
)]f
Equation (1) is the rate law that is valid for all the concentrations of IO3
and OH and Ir (III) chloride on the rate oxidation of glucose.
–
Since, at any time t, in the reaction, C3 will always be less than C1, the
right-hand side of equation (1) will be less than 1 and as a result the inequality
–
–
K1K2 [OH ] [IO3 ] << 1 can be assumed to be a valid one. Further, since [Ir (III)
–
chloride]T is in the order of 10−5, the inequality K1K2 [OH ] [Ir (III)]T << 1
can also be taken as a valid one. Now, with these inequalities being in existence
–
–
and with first order in low [IO3 ] at which the experiments were performed for
the study of the effect of [OH ] and [D-glucose] on the rate of reaction and
first order in [Ir (III) chloride]T [except at very high concentration of Ir (III)
–
–
chloride], it is reasonable to assume that 1 +K1K2 [OH ] >> K1K2 [OH ] [Ir
–
–
(III)]T + K1K2[OH ][IO3 ] and under this condition equation (1) will be reduced
to equation (2):
ꢀ
ꢁ ꢀ
ꢁ ꢀ
ꢁ
2kK1K2 OH− IO−3
Ir(III)
T
T
Rate =
ꢀ
ꢁ
(2)
1 + K1 OH−
–
Equation (2) is the rate law valid for low concentration of [IO3 ] and Ir (III)
chloride and for all the concentrations of [OH ]. Equation (2) can also be
–
written as:
1
Rate
1
1
=
ꢀ
ꢁ ꢀ
ꢁ ꢀ
ꢁ +
ꢀ
ꢁ ꢀ
ꢁ
(3)
2kK1K2 OH− IO−3
Ir(III)
2kK2 IO−3
Ir(III)
T
T
T
T
d [IO−3 ]
where Rate = −
dt
1
rate
–
According to equation (3), if a plot is made between
and 1/[OH ], a
straight line having an intercept on y-axis will be obtained. When a plot was
–
made between 1/Rate and 1/[OH ], a straight line having intercept on 1/Rate
axis was obtained (Fig. 4). This proves the validity of the rate law (2) and
hence the proposed reaction Scheme 1. From the slope and intercept of the
straight line, the values of K1 and kK2 have been calculated and found to be
4.66 and 5.60, respectively. Utilizing K1 and kK2 values, the reaction rates for

S. P. Singh et al.
288
0.25
0.2
0.15
0.1
0.05
0
0
0.05
0.1
0.15
0.2
0.25
-2
-1
1
0 /[NaOH] M
–
Figure 4. Dependence of 1/Rate on 1/[OH] from data and conditions given in Table 1.
–
the variation of [OH ]in the Ir (III) chloride-catalyzed oxidation of D-glucose
have been calculated by the help of rate law (2) and found to be very close to
the rates observed experimentally (Table 2). The close resemblance between
the calculated and the observed rates further proves the validity of the rate
law (2) and hence the proposed reaction Scheme 1.
Multiple Regression Analysis
With the help of multivariate regression analysis, a relationship in the
case of D-glucose between observed pseudo first-order rate constant, k1, and
–
concentrations of all the reactants of the reaction except [IO3 ] was found as :
k1 = k[OH−]0.55[Ir(III)]0.50
(4)
where k = 6.07 × 10−2
With the help of equation (4), the reaction rate predicted for the variation of
hydroxyl ion concentrations in the oxidation of D-glucose was found to be very
close to the calculated and the observed rates (Table 2). The close similarly be-
tween the three rates (i.e., observed, calculated, and predicted) clearly proves
the validity of the rate law (2) and hence the proposed reaction mechanism.

Kinetics of Ir (III)-catalyzed Oxidation of D-glucose
289
For the oxidation of D-glucose, a reaction path has been proposed where
the most reactive activated complex has been formed by the interaction of a
–
–
similarly charged species [IrCl3(H2O)2OH] and IO3 . In this case the tran-
sition state will have the same charge, but it will be dispersed over a larger
volume, as a result of which the transition state will be less polar than the
reactants. This would lead to an increase in entropy. The observed positive
entropy of activation in the oxidation of glucose by iodate in the presence of
Ir (III) chloride is certainly evidence for step (ii) of Scheme 1. In step (ii) the
complex in the transition state has the same charge dispersed over a larger
volume than the charged species. The proposed mechanism is also supported
by the moderate values of energy of activation and other parameters.
Effect of Dielectric Constant and Calculation of the Size of the
Activated Complex
In order to find out the effect of dielectric constant of the medium on the
reaction rate, the reaction has been studied with different dielectric constant
(D) of the medium at constant concentration of all other reactants at constant
temperature. It is clear from the Table 3 that –dc/dt and k (rate constant) val-
ues are increased with the decrease in dielectric constant of the medium. The
dependence of the rate constant on the dielectric constant of the medium is
given by the following equation:
2
˜
ZAZBe N
1
D
log = log k0 −
k1
×
2.303 (4π ∈0) dAB RT
The decrease in first-order rate constant with the increase in dielectric
constant of the medium is also evident form plots of log k1 versus 1/D (Fig. 5).
The plot of log k1 versus 1/D was linear, having a negative slope. This clearly
supports the involvement of similar charges in the rate-limiting step in the
proposed mechanism. The value of dAB has been evaluated with the help of the
slope of the straight line and found to be 1.60A0.
Comparative Studies
An effort has also been made to compare our experimental find-
ings with the results earlier reported for oxidation of reducing sugars by
N-bromoacetamide[18] in the presence of Ir (III) chloride and also Ru (III)
chloride oxidation of indigo carmine[19] by iodate in acidic medium. The present
paper shows similarity with Ir (III) chloride-catalyzed oxidation of reducing
sugars being first to zero order with respect to oxidant and catalyst and zero
order with respect to reducing sugars. Since the present study has been per-
formed in alkaline medium, the reducing sugar molecule participates in the
reaction in the enediol form, whereas in the reported Ir (III) chloride-catalyzed

S. P. Singh et al.
290
1.60
1.55
1.50
1.45
1.40
1.35
1.30
1.25
1.20
1.15
1.34
1.36
1.38
1.40
1.42
1.44
1.46
1.48
1.50
102/D
Figure 5. Dependence of log k1 on 1/D from data and conditions given in Table 2.
oxidation, it participates in the reaction as such. When the present study with
respect to the role of iodate was compared with the Ru (III)-catalyzed oxidation
–
of indigo carmine, it was found that the species IO3 was the reactive species
of potassium iodate in acidic as well as in alkaline medium. As far as the ki-
–
netic order with respect to IO3 is concerned, it is first order throughout the
–
variation of [IO3 ] in the reported Ru (III)-catalyzed oxidation and first to zero
order in the present study of the Ir (III)-catalyzed oxidation of reducing sug-
ars. On the basis of the observed kinetic data and spectral information, it is
–
concluded that [IrCl3 (H2O)2OH] is the reactive species of Ir (III) chloride
in the present study of the oxidation of glucose in alkaline medium, whereas
in the reported[20] Ir (III) chloride-catalyzed oxidation of reducing sugars, the
reactive species of Ir (III) chloride was found to be [IrCl6].3–
CONCLUSION
Oxidation of glucose by iodate in alkaline medium is very sluggish, but
–
it becomes facile in the presence of Ir (III) chloride catalyst. IO3 and
–
[IrCl3(H2O)2OH] have been assumed as the reactive species of potassium io-
date and Ir (III) chloride in alkaline medium, respectively. The rate of reaction
of glucose is unaffected by the ionic strength of the medium. Oxidation prod-
ucts have been identified. Activation parameters were evaluated. The observed
results have been explained by a plausible mechanism and the related rate law
has been deduced. It can be concluded that Ir (III) chloride acts as an efficient
catalyst for the oxidation of D-glucose by iodate in alkaline medium.

Kinetics of Ir (III)-catalyzed Oxidation of D-glucose
291
ACKNOWLEDGEMENT
We wish to thank two anonymous reviewers and the editor for useful comments
to refining the manuscript a lot.
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