Oxidation of D-glucose by N-bromophthalimide in the presence of chlorocomplex of iridium(III): A kinetic and mechanistic study

Article abstract of DOI:10.1007/s11164-011-0367-y

The kinetics of Iridium (III) catalyzed oxidation of D-glucose [D-glu] has been studied by N-bromophthalimide [NBP] in the acidic medium (pH 1.3) at 303 K. The reaction followed first-order kinetics with respect to [NBP] from 0.5×10^-4 to 4.0×10^-4 mol dm^-3. The results indicate the first-order kinetics in [iridium(III)] chloride at lower concentrations from 0.75×10^-6 to 5.8×10^-6 mol dm^-3 tends towards a zero order at its higher concentrations (up to 13.36×10^-6 mol dm^-3). Zero-order kinetics with respect to [D-glu] from 2.0×10^-3 to 20.0×10^-3 mol dm^-3 was observed throughout its variation. A positive effect on the oxidation rate was observed for [Cl-] from 0.2×10^-5 to 2.4×10^-5 mol dm^-3 and the rate of reaction decreased with increase in dielectric constant (decreasing acetic acid from 35 to 15%) of the medium. A negative effect was observed on the reaction rate for [H ⁺] from 1.5×10^-2 to 12.0×10^-2 mol dm^-3. Addition of reduced product of oxidant, i.e., phthalimide from 1.0×10^-4 to 8.0×10^-4 mol dm^-3 did not show a significant effect on the oxidation velocity. Rate of reaction increased with increase in ionic strength from 0.2×10-1 to 4.0×10^-1 mol dm^-3 of the medium, i.e., increase in [KNO₃]; the rate of reaction was increased. From the linear Arrhenius plot of log k₁ versus 1/T, the activation energy (Ea = 50.91 kJ mol^-1) was calculated. With the help of the energy of activation, parameters such as enthalpy of activation -(Delta;H^# = 48.39 kJ mol^-1), entropy of activation -(Delta;S^# = -321.63 JK mol^-1), Gibbs free energy of activation -(Delta;G^# = 145.84 kJ mol^-1) and frequency factor (Log A = -3.97) were also calculated. A suitable mechanism in conformity with kinetic observations was proposed to explain reaction stoichiometry and product analysis. Springer Science+Business Media B.V. 2011.

Full text of DOI:10.1007/s11164-011-0367-y

Res Chem Intermed (2012) 38:507–521
DOI 10.1007/s11164-011-0367-y
Oxidation of D-glucose by N-bromophthalimide
in the presence of chlorocomplex of iridium(III):
a kinetic and mechanistic study
Ajaya Kumar Singh
Alpa Shrivastava Bhawana Jain
•
•
Neerja Sachdev
•
•
Yokraj Katre
Received: 27 May 2011 / Accepted: 17 August 2011 / Published online: 8 September 2011
Ó Springer Science+Business Media B.V. 2011
Abstract The kinetics of Iridium (III) catalyzed oxidation of D-glucose [D-glu] has
been studied by N-bromophthalimide [NBP] in the acidic medium (pH 1.3) at 303 K.
-
The reaction followed first-order kinetics with respect to [NBP] from 0.5 9 10 to
4
-
4
-3
.0 9 10 mol dm . The results indicate the first-order kinetics in [iridium(III)]
4
chloride at lower concentrations from 0.75 9 10 to 5.8 9 10 mol dm tends
-
6
-6
-3
-
towards a zero order at its higher concentrations (up to 13.36 9 10 mol dm ).
6
-3
-
3
-3
Zero-order kinetics with respect to [D-glu] from 2.0 9 10
-
to 20.0 9 10
mol dm was observed throughout its variation. A positive effect on the oxidation
3
-
-5
-5
-3
rate was observed for [Cl ] from 0.2 9 10 to 2.4 9 10 mol dm and the rate of
reaction decreased with increase in dielectric constant (decreasing acetic acid from 35
?
to 15%) of the medium. A negative effect was observed on the reaction rate for [H ]
-
2
-2
from 1.5 9 10 to 12.0 9 10 mol dm . Addition of reduced product of oxidant,
-3
-
i.e., phthalimide from 1.0 9 10 to 8.0 9 10 mol dm did not show a significant
4
-4
-3
effect on the oxidation velocity. Rate of reaction increased with increase in ionic
-3
-
1
-1
strength from 0.2 9 10 to 4.0 9 10 mol dm of the medium, i.e., increase in
KNO ]; the rate of reaction was increased. From the linear Arrhenius plot of log k
[
versus 1/T, the activation energy (Ea = 50.91 kJ mol ) was calculated. With the
3
1
-
1
help of the energy of activation, parameters such as enthalpy of activation
A. K. Singh (&) ꢀ B. Jain
Department of Chemistry, Government V.Y.T.PG. Autonomous College, Durg, Chhattisgarh, India
e-mail: ajayaksingh_au@yahoo.co.in
N. Sachdev
Department of Applied Chemistry, Bhilai School of Engineering, Durg, India
A. Shrivastava
Department of Applied Chemistry, Shri Shankaracharya College of Engineering & Technology,
Bhilai, India
Y. Katre
Department of Chemistry, Kalyan Mahavidyalaya, Bhilai Nagar Sect-7, Bhilai, India
123

508
A. K. Singh et al.
#
-1
#
-1
(
DH = 48.39 kJ mol ), entropy of activation (DS = -321.63 JK mol ),
# -1
Gibbs free energy of activation (DG = 145.84 kJ mol ) and frequency factor (Log
A = -3.97) were also calculated. A suitable mechanism in conformity with kinetic
observations was proposed to explain reaction stoichiometry and product analysis.
Keywords Kinetics ꢀ Oxidation ꢀ N-bromophthalimide ꢀ D-glucose ꢀ
Iridium(III) chloride
Introduction
Recently, much attention has been paid to N-halo compounds due to their ability to
act as a source of halonium ions, hypohalite species, and nitrogen anion, which act
as a base and nucleophile [1–3]. N-halo compounds have been extensively used as
an oxidizing agent for catalyzed and uncatalyzed reactions [4–8]. Additionally,
N-halo compounds have also been widely used as halogenating reagents for organic
compounds [9–12]. In the recent years, transition metal ions, such as ions of
osmium, ruthenium, and iridium either alone or as binary mixture as catalyst, have
been applied in the oxidation of several redox processes [13, 14]. The mechanism of
catalysis is quite complicated due to the formation of different intermediate
complexes, free radicals, and different oxidizing states of catalyst [15, 16]. Iridium
(
III) chloride is an important platinum group metal ion and has been widely used as
catalyst in various redox reactions [17, 18]. The potential of iridium(III) chloride to
act as a homogenous catalyst in the oxidation process was also recognized for the
oxidation of ethanol [19], methanol [20], formic acid [21] reducing sugar [22], and
glycine [23] by different oxidant in acidic medium.
Glucose is the most important carbohydrates in biological processes. The study
of carbohydrates is one of the most exciting fields of organic chemistry. Vast
literature is available on the kinetics of oxidation of carbohydrates by various
organic and inorganic oxidants [24–27].
This study reports for the first time the kinetics investigation of iridium(III)
catalyzed oxidation of D-glu by NBP in acidic medium at 303 K. The objectives
of the present study are to ascertain real reactive species of catalyst and oxidant,
to deduce the rate law consistent with kinetic results, to calculate activation
parameters, and to elucidate the plausible reaction mechanism.
Experimental
Materials and methods
Analytical-grade chemicals and double-distilled water were used throughout
the investigation. N-bromophthalimide (Lancaster, 98%), was used as received.
A solution of NBP was prepared in 80% acetic acid and stored in a black-coated
flask in order to prevent photochemical deterioration. An aqueous solution of D-glu
(
AR grade) was always freshly prepared. Iridium(III) chloride solution was prepared
1
23

Oxidation of D-glucose by N-bromophthalimide
509
by dissolving a known weight of iridium(III) chloride (Johnson–Matthey) in HCl of
known strength, and stored in a black-coated bottle. A standard solution of KCl,
HClO , and phthalimide was prepared and mercuric acetate (Loba Chem., Mumbai,
4
India) was acidified (pH 1.3) with 20% acetic acid.
Kinetic procedure
All the kinetic measurements were carried out in a black-coated vessel at 303 K
and performed under pseudo first-order condition with [D-glu]ꢁ[NBP]. The
reaction was initiated by the rapid addition of known amounts of D-glu to a reaction
mixture containing the required amount of the NBP, HClO , iridium(III) chloride,
4
mercuric acetate, acetic acid, and water in glass-stoppered Pyrex boiling tubes and
thermostated at 303 K (pH 1.3). The progress of reaction was monitored by
iodometric determination of unconsumed [NBP] in known aliquots of the reaction
mixtures at different time intervals.
Each kinetic run was studied for 75% reaction. The rate constant in each kinetic
run was determined by plotting a graph between remaining log [NBP] versus time.
Each kinetic run was studied for two half-lives of the reaction. The observed rates of
reaction were within ± 5% in the replicate kinetic run.
Stoichiometry and product analysis
Different ratios of NBP to D-glu were equilibrated at 303 K in the presence of a
requisite amount of all reactants, i.e., perchloric acid, mercuric acetate and acetic
acid under the condition of [NBP]ꢁ[D-glu] for 72 h. Determination of unconsumed
NBP revealed that 2 mol of NBP were required for the oxidation of each mole of
D-glu. Accordingly, the following stoichiometric equation could be formulated as:
o
C
OH
H
o
OH
H
H O
HO
HO
H O
OH
C
H
+ 2
N
Br + H O
HO
HO
H
2
+ 2
N
H
OH
OH
C
H
H
C
O
H
o
NBP
o
NHP
D-Glu
D-glucono-1,5-lactone
ð1Þ
D-glucono-1,5-lactone was confirmed by GC-MS. For GC-MS analysis, the
reaction mixture was extracted with diethyl ether and the ether layer was
concentrated by slow evaporation. The product was analyzed by GC-MS, JEOL-
JMS (Mate-MS system, Japan).
Test for free radicals
To test the presence of free radicals in the reaction, the reaction mixture with acryl
amide was placed in an inert atmosphere for 24 h. When the reaction mixture was
123

510
A. K. Singh et al.
diluted with methanol, it was found that there was no precipitate formed. This
clearly showed free radicals were not formed in the redox reaction under
investigation.
Kinetic measurements and results
Initially, the kinetics of the oxidation of D-glu by NBP in the presence of iridium(III)
chloride under acidic conditions was studied at several initial concentrations of all
the reactants at 303 K. Considering NBP, D-glu, perchloric acid and iridium(III)
chloride as the main reactants, the general form of rate equation for the reaction can
be written as
a
b
c
þ d
Rate ¼ k1½NBPꢂ ½D-gluꢂ ½iridiumðIIIÞꢂ ½H ꢂ
Uniform pseudo-first-order rate constant (k ) values for the variation of [NBP] clearly
ð2Þ
1
indicate that the order with respect to [NBP] is unity. This is also obvious from the
plot of log remaining [NBP] versus time (Fig. 1). The values of rate (-dc/dt) at
-
4
-4
-3
various concentration of [NBP] (from 0.50 9 10
clearly showed first-order kinetics with respect to [NBP] (Fig. 2). Reactions have
to 4.00 9 10 mol dm )
-
3
-3
-3
been studied for [D-glu] (from 2.00 9 10 to 20.00 9 10 mol dm ) at constant
concentration of all reactants at constant temperature. The rate constant values were
found almost constant clearly indicates zero-order kinetics with respect to D-glu
(
Table 1). The reaction followed first-order to zero-order kinetics with respect to
-3
-
6
-6
to 13.36 9 10 mol dm
[
Table 2). With increasing concentration of HClO from 1.5 9 10 to 12.00 9
iridium(III)] chloride from 0.75 9 10
(Fig. 3;
-
2
4
-
2
-3
-5
mol dm , the rate constant decreased from 16.11 9 10 to 4.52 9 10
-5
1
0
-
1
-
-5
-5
(Table 2). The variation of [Cl ] from 0.20 9 10 to 2.40 9 10 mol dm
-3
s
had a positive effect from 2.90 9 10 to 24.70 9 10
-
5
-5 -1
s on the reaction rate
Table 2). Rate of reaction decreased with increase in dielectric constant (D)
(
-
3
-3
?
Fig. 1 Plot between log [NBP] versus time at T = 303 K. [D-glu] = 2.00 9 10 mol dm , [H ] =
-2
-3
-6
[iridium(III)] = 2.90 9 10 mol dm
-3
-5 -3
[KCl] = 1.00 9 10 mol dm ,
5
.00 9 10 mol dm
,
Hg(OAc) ] = 3.00 9 10 mol dm and CH COOH = 20%
,
-4
-3
[
2
3
1
23

Oxidation of D-glucose by N-bromophthalimide
511
-
3
-3
Fig. 2 Plot between -dc/dt versus [NBP] at T = 303 K. [D-glu] = 2.00 9 10 mol dm
[
,
,
?
-2
-3
-6
-3
-5
H ] = 5.00 9 10 mol dm , [iridium(III)] = 2.90 9 10 mol dm , [KCl] = 1.00 9 10 mol dm
-3
-4
-3
2 3
[Hg(OAc) ] = 3.00 9 10 mol dm and CH COOH = 20%
Table 1 Effect of variation
NBP] and [D-glu] on the rate of
oxidation of D-glu at 303 K
4
NBP] 9 10 mol dm
-3
3
[D-glu] 9 10 mol dm
-3
5
-1
[
1
k 9 10 s
[
0
0
1
1
2
3
4
2
2
2
2
2
.50
.80
.00
.50
.00
.00
.00
.00
.00
.00
.00
.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
3.00
4.00
6.00
8.00
10.00
15.00
20.00
10.22
10.30
10.71
10.00
10.11
10.11
10.10
10.50
10.43
10.56
10.27
10.34
10.51
10.98
10.44
The solution conditions were
-
4
[
2
Hg(OAc) ] = 3.00 9 10 mol
-3
-5
dm , [KCl] = 1.00 9 10 mol
2.00
2.00
-3
-5
dm , [KNO
3
] = 0.20 9 10
mol dm and CH COOH =
0%
-
3
3
2
.00
2
(decreasing % acetic acid) of the medium. The effect of ionic strength (I) of the
medium on the rate was studied using the KNO solution, keeping the other
3
experimental conditions constant. A positive effect of ionic strength of the medium
was observed for the iridium(III)-catalyzed reaction (Table 3). Increasing [mercuric
acetate] and [NHP] did not affect the reaction rate (Table 3). The effect of
temperature on the reaction rate was determined by keeping constant concentration
-
5
of other constituents of the solution. The values of rate constants (from 7.47 9 10
5 -1
-
to 27.46 9 10
were utilized to calculate the activation parameters. From the linear Arrhenius plot
s ) observed at five different temperatures (from 298 to 318 K)
-
of log k versus 1/T (Fig. 4), the activation energy (Ea = 50.91 kJ mol ) was
1
1
calculated. With the help of the energy of activation, parameters such as enthalpy
123

512
A. K. Singh et al.
-
4
-3
-3
Fig. 3 Plot between rate constant versus [iridium(III)] at T = 303 K. [NBP] = 2.00 9 10 mol dm
,
,
-3
-3
?
-2
-3
-5
D-glu] = 2.00 9 10 mol dm , [H ] = 5.00 9 10 mol dm , [KCl] = 1.00 9 10 mol dm
[
[
-
4
-3
] = 3.00 9 10 mol dm and CH
-4
] = 3.00 9 10 mol dm
-3
Hg(OAc)
2
3
COOH = 20%. [Hg(OAc)
2
and CH
3
COOH = 20%
#
-1
#
of activation (DH = 48.39 kJ mol ), entropy of activation (DS = -321.63 JK
-
1
#
-1
mol ), Gibbs free energy of activation (DG = 145.84 kJ mol ) and frequency
factor (Log A = -3.97) were also calculated.
Discussion
Reactive species of NBP
It has been reported earlier by several workers [28–32] that NBP is a good oxidizing
and brominating agent because of the large polarity of the [N–Br bond. NBP, like
other similar N-halo imides, may exist in various forms in acidic medium, i.e., free
?
?
NBP, protonated NBP, Br , HOBr (H OBr) , as per the following equilibrium:
2
NBP þ H2O ꢀ HOBr þ NHP
ð3Þ
þ
þ
NBP þ H ꢀ NHP þ Br
ð4Þ
ð5Þ
ð6Þ
þ
þ
NBP þ H ꢀ ðNBPHÞ
þ
þ
HOBr þ H ꢀ ðH2OBrÞ
?
when (H OBr) is taken as the reactive species, the rate law obtained shows first-
2
order kinetics with respect to hydrogen ion concentrations, contrary to our observed
?
negative fractional order in [HClO ]. Therefore, the possibility of (H OBr) and
4
2
?
cationic bromine (Br) as reactive species is ruled out. When HOBr is assumed as
the reactive species, the derived rate law failed to explain the negligible effect of
phthalimide. Hence, neither of these species can be considered as the reactive
species. Thus, the only choice left is NBP, which when considered as the reactive
species, leads to a rate law capable of explaining all the kinetic observations and
other effects. Hence, in the light of kinetic observation, NBP can safely be assumed
to be the main reactive species for the present reaction.
1
23

Oxidation of D-glucose by N-bromophthalimide
513
123

514
A. K. Singh et al.
Table 3 Effect of varying [NHP] and dielectric constant of the medium on the rate of oxidation of D-glu
at 303 K
-
3
4
4
5 -1
k 9 10 s
1
l 9 10 mol dm
[NHP] 9 10
Acetic acid %
by volume
[Hg(OAc) ] 9 10
2
-3
mol dm
-
3
mol dm
1
2
3
4
6
8
.00
.00
.00
.00
.00
.00
–
–
–
–
–
–
3.00
3.00
3.00
3.00
3.00
3.00
3.00
3.00
3.00
3.00
3.00
3.00
4.00
6.00
8.00
3.00
3.00
3.00
3.00
10.30
10.20
10.20
10.10
10.10
10.20
5.44
1
5
–
–
–
–
–
–
–
–
–
–
–
–
20
25
30
35
–
10.49
11.61
12.63
13.41
10.62
10.57
10.82
10.99
9.91
–
–
–
0
0
2
4
.20
.80
.40
.00
20
20
20
20
-
11.4
14.1
17.2
4
-3
-3
-3
?
Solution conditions were [NBP] = 2.00 9 10 mol dm , [D-glu] = 2.00 9 10 mol dm , [H ] =
-3
.00 9 10 mol dm , [Ir(III)] chloride = 2.90 9 10 mol dm
-2
-6
-3
5
-4
-3
1
Fig. 4 Plot between log k versus 1/T at T = 303 K. [NBP] = 2.00 9 10 mol dm , [D-glu] = 2.00 9
-6 -3 -4 -3
-
3
-3
1
[
0
mol dm
,
KCl] = 1.00 9 10 mol dm and CH COOH = 20%
[Iridium(III)] = 2.90 9 10 mol dm
-3
,
2
[Hg(OAc) ] = 3.00 9 10 mol dm ,
-5
3
1
23

Oxidation of D-glucose by N-bromophthalimide
515
Reactive species of Ir(III) chloride
3
A spectrophotometric study of the kinetics of the hydration of [IrCl ] and of the
-
6
-
addition of a Cl to [Ir(H O)Cl ] in 1.0–2.5 M HClO (or HCl) at 323 K was
2-
2
5
4
reported [33]. It is known that iridium(III) chloride in hydrochloric acid gives
3
IrCl6] species [34]. It has also been reported that iridium(III) and iridium(I) ions
-
[
are the stable species of iridium [35]. Further, the equation of [IrCl ] gives
3-
6
2
-
-
[
may be shown by the general equation:
IrCl H O] , [IrCl (H O) ] and [IrCl (H O) ] species [36, 37]. This equilibrium
5 2 4 2 2 3 2 3
IrCl6ꢂ þ n-H O ꢀ ½IrCl-ðH OÞꢂ þ Clꢃ
3
ꢃ
3ꢃn
ð7Þ
and
½
when iridium(III) chloride dissolved in 0.1 M HCl solution [IrCl ]
2
2
3
-
6
2
-
[
IrCl (H O)] species were formed. On the basis of the effect of chloride ions
5 2
3
-
on the reaction rate, [IrCl6] have been considered to be the reactive species of
iridium(III) chloride.
Considering the reactive species of Iridium(III) chloride and NBP with the
help of the above experimental findings, the probable reaction mechanism is
proposed, and considering the fact that 1 mol of D-glu is oxidized by 2 mol of
NBP.
2
-
On the basis of the observed kinetics result, [IrCl (H O) (OH) ] as the most
2
2
2
2
active species of iridium(III) chloride and NBP itself as reactive species of NBP
allowed us to propose a reaction Scheme 1. On the basis of steps (i) to (iii) in
Scheme 1 for the oxidation of D-glu and considering the fact that 1 mol of D-glu is
oxidized by 2 mol of NBP, the rate of reaction can be written as:
Rate ¼ k½C3ꢂ
On the basis of equilibrium steps (i) to (vii), Eqs. 9–13 can be written as:
ð8Þ
ꢃ
K½C1ꢂ½Cl ꢂ
½
C ꢂ ¼
ð9Þ
ð10Þ
ð11Þ
2
þ
½
H ꢂ
K2½C2ꢂ½NBPꢂ
½
C ꢂ ¼
3
þ
½
H ꢂ
k K1K2½C1ꢂ½NBPꢂ
Rate ¼
þ
½
H ꢂ
At any moment in the reaction, the total concentration of [NBP], i.e., [NBP] can be
T
shown as:
½
On substituting the value of [C ] from Eq. 11, we get Eq. 13
NSBꢂ ¼ ½NBPꢂ þ ½C ꢂ
ð12Þ
ð13Þ
T
3
3
þ
½
NBPꢂ ½H ꢂ
T
½
NBPꢂ ¼
þ
ꢃ
½
H ꢂ þ K1K2½C1ꢂ½Cl ꢂ
123

516
A. K. Singh et al.
From, Eqs. 8, 11, and 13, we get Eq. 14
k K1K2½C1ꢂ½Cl ꢂ½NBPꢂ
ꢃ
T
Rate ¼
ð14Þ
þ
ꢃ
½
H ꢂ þ K1K2½C1ꢂ½Cl ꢂ
But
½
C ꢂ ¼ ½IrðIIIÞꢂ
1
So Eq. 14 can be written as:
Rate ¼
ꢃ
ꢃ
kK1K2½IrðIIIÞꢂ½NBPꢂ½Cl ꢂ
ð15Þ
þ
½
H ꢂ þ K1K2 ½IrðIIIÞꢂ½Cl ꢂ
Equation 15 is the rate law based on the observed kinetic orders with respect to each
reactants of the reaction can be very easily explained. We can write Eq. 15 as
Eq. 16:
þ
½
NBPꢂ
½H ꢂ
1
¼
þ
ð16Þ
ꢃ
Rate
kK1K2 ½IrðIIIÞꢂ½Cl ꢂ
k
-
?
According to Eq. 16, if a plot is made between [NBP]/rate versus 1/[Cl ] and [H ],
a straight line having an intercept on y-axis will be obtained (Figs. 5, 6). This proves
the validity of the rate law (15) and hence the proposed reaction Scheme 1. From
the slope and intercept of the straight line, the values of k & K K have been
1
2
-
4
-1
8
-1
3
calculated and found to be 2.15 9 10 s , 8.30 9 10 mol dm , respectively.
-
Utilized K K value, the reaction rates for the variation of [Cl ] in the iridium(III)
1
2
chloride catalyzed oxidation of D-glu have been calculated by the help of rate law
16) and found to be very close to the rates observed experimentally.
(
Effect of ionic strength
The ionic strength (I) effect on the reaction rate has been described according to the
theory of Bronsted and Bjerrum, which postulates the reaction through the
formation of an activated complex. According to this theory, the effect of ionic
strength on the rate for a reaction involving two ions is given by the relationship
1=2
log k ¼ log k þ 1:02Z Z I
ð17Þ
here, Z Z are the valency of the ions A and B and k and k are the rate constants in
1
0
A B
A
B
1
0
the presence and absence of the added electrolyte, respectively. A plot of log k
1
1
against I should be linear with a slope of 1.02 Z Z . If Z Z have similar signs,
/2
A
B
A B
the quantity Z Z should be positive, and the rate increases with the ionic strength,
A
B
having a positive slope, while if the ions have dissimilar charges, the quantity ZAZB
should be negative and the rate would decrease with increase in ionic strength,
having a negative slope. In the present study, a primary salt effect is observed as the
rate increases with increase in ionic strength of the medium supporting the
involvement of ions of same sign in the rate-determining step (Scheme 1). Positive
1
/2
slope obtained from the plot of log k versus I clearly supports the step (III) of the
Scheme 1.
1
23

Oxidation of D-glucose by N-bromophthalimide
517
K1
Cl-
[IrCl (H O) (OH) ]2-
3
2
2
2
O+
[
IrCl (H O) OH]
+
+ H
3
(i)
2
2
3
C1
C
2
K2
2
-
2-
[
IrCl (H O) (OH) ]
+
NBP
[IrCl (NBP)(H O)(OH) ]
3
2
2
2
3
2
2
(ii)
-
H O
2
C2
C3
k
2
-
2-
[
IrCl (NBP)(H O)(OH) ]
[IrCl (OBr)(H O)(OH)]
3 2
3
2
2
+
NHP
(iii)
(iv)
slow
C3
C4
2-
[
IrCl (OBr)(H O)(OH)]
2-
3
2
+
D-Glu
[
IrCl3(OBr)(D-Glu)OH]
2
H O
+
C4
C
5
O-
H
H O
OH
2-
-
[
IrCl (OBr)(D-Glu)OH]
+ 3 H O
[IrCl
(H
O)
OH]
-
HO
3
2
2
2
3
+
Cl +
HO
H
(v)
H
H
OBr
C5
C6
O-
OH
H
H
H O
OH
H
O
HO
HO
HO
HO
(vi)
H
H
H
OH
O
H
H
OBr
C
6
D-glucono-1,5-lactone
OH
H
H OH
O
H
OH
OH
H
HO
HO
O
HO
HO
H
H
OH
O
H
O
(vii)
H
D-gluconic acid ion
alkaline NH
2
OH
FeCl + phenol
3
FeCl + HCl
3
yellow colour
blue colour
Scheme 1 Reaction path for the oxidation of D-glu by NBP in the presence of Iridium(III) chloride
123

518
A. K. Singh et al.
?
Fig. 5 Plot between [NBP]/rate versus [H ] at T = 303 K. [NBP] = 2.00 9 10 mol dm
-4
-3
,
] = 3.00 9
-
3
-3
-6
[Iridium(III)] = 2.90 9 10 mol dm
-3
[
1
D-glu] = 2.00 9 10 mol dm
-3
,
,
[Hg(OAc)
2
-4
-5
-3
0
3
mol dm , [KCl] = 1.00 9 10 mol dm and CH COOH = 20%
-
-4
Fig. 6 Plot between [NBP]/rate versus 1/[Cl ] at T = 303 K. [NBP] = 2.00 9 10 mol dm , [D-glu]
-3
-3
-3
-6
[Iridium(III)] = 2.90 9 10 mol dm , [H ] = 5.00 9 10 mol dm ,
-3
?
-2
-3
=
2.00 9 10 mol dm
,
-
4
-3
[
2 3
Hg(OAc) ] = 3.00 9 10 mol dm and CH COOH = 20%
Effect of dielectric constant and calculation of the size of an activated complex
dAB) in the oxidation of D-glu
(
In order to determine 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 Table 3 that k (rate constant) values decreased with the decreasing
dielectric constant of the medium, i.e., increasing % of acetic acid. This effect is
given by the following equation:
2
ZAZBe N
1
log k ¼ log k ꢃ
ꢄ
ð18Þ
1
0
2
:303ð4pbÞdABRT
D
1
23

Oxidation of D-glucose by N-bromophthalimide
519
where k is the rate constant in a medium of infinite dielectric constant, Z and Z
B
0
A
are the charges of reacting ions, dAB refers to the size of activated complex, and T is
absolute temperature and D is the dielectric constant of the medium. The decrease in
first-order rate constant with the increase in dielectric constant of the medium is also
evident form plots of log k and 1/D. The plot of log k versus 1/D, was linear,
1
1
having a negative slope was obtained, indicating an interaction between a charged
ion and a dipolar molecule. The value of dAB value was evaluated with the help of
˚
the slope of the straight line and found to be 3.07 A.
Role of entropy of activation and other activation parameters
Entropy of activation plays an important role in the case of reaction between ions or
between an ion and a neutral molecule or a neutral molecule forming ions. When a
reaction takes place between two ions of opposite charge, their union will result in a
lowering of the net charge and, due to this, some frozen solvent molecules will be
released with an increase of entropy. On the other hand, however, when a reaction
takes place between two similarly charged species, the transition state will be a
more highly charged ion, and due to this, more solvent molecules will be required
than for the separate ions, leading to a decrease in entropy. On the basis of this
information, observed negative entropy of activation in the oxidation of D-glu by
NBP in the presence of iridium(III) catalyzed supports the rate-determining step of
the proposed Scheme 1. Values of frequency factor (A) and free energy of
activation also support the reaction Scheme 1 proposed for the oxidation of D-glu.
Multiple regression analysis
With the help of multivariate regression analysis, a relationship between observed
pseudo first-order rate constant (k ) and concentrations of all the reactants of the
reaction was found to be:
1
k ¼ k½Hþꢂ0:58½iridiumðIIIÞꢂꢃ0:60½Cl ꢂ
ꢃ 0:71
ð19Þ
1
-
1
where k = 9.33 9 10
.
With the help of Eq. 19, the reaction rate predicted for the hydrogen ions,
chloride ions and iridium(III) chloride concentrations in the oxidation of D-glu were
found to be very close to the calculated and observed rates (Table 2). The close
similarity among the two rates, i.e., the calculated and predicted rates, clearly proves
the validity of the rate law (15), and hence the proposed mechanism.
Comparative studies
When an effort was made to compare the findings of iridium(III) catalyzed
oxidation of D-glu by NBP Ru(III) [38]—catalyzed oxidation of D-glu, it was found
that free NBP is the reactive species of NBP in each case. Zero-order kinetics in
[D-glu] was observed for both iridium(III) and Ru(III) systems clearly shows that
there is no effect of [D-glu] on the rate of reaction. The present study shows
123

520
A. K. Singh et al.
similarity with [Ru(III)] [38]-catalyzed oxidation of D-glu as for as formation of a
complex between transition metal-catalyst (iridium(III) and Ru(III)) with D-glu is
concerned. Iridium(III) and Ru(III) systems show the same behavior in respect to
?
-
the order in [Hg(II)] and [H ]. The present study differs with respect to [Cl ] and
-
ionic strength of the medium with our earlier study [38]. A negative effect of [Cl ]
-
was observed in Ru(III), whereas a positive effect of [Cl ] on the rate of reaction
was observed in the present study. The rate of reaction was not influenced by the
change in ionic constant (l) of the medium, whereas the rate of reaction increased
with an increase in ionic strength of the medium in the present study.
Conclusions
Iridium(III) catalyzed oxidation of glucose by NBP was studied at 303 K. Oxidation
of D-glu by NBP in acidic medium becomes facile in the presence of iridium(III)
chloride. Oxidation products have been identified. NBP itself and [IrCl
3
2
-
(
H O) (OH) ] were identified as reactive species of NBP and iridium. Various
2 2 2
activation parameters have been evaluated. In the scheme, each molecule of D-glu
consumed 2 mol of NBP to D-glucono-1,5-lactone and phthalimide in agreement
with the experimental observed stoichiometry. The deduced rate law from the
reaction scheme was also consistent with kinetics of the reaction. Future studies on
the iridium(III) chloride catalyzed oxidation of other biological relevant carbohy-
drates by NBP would also be of interest. In conclusion, it can be said that
iridium(III) chloride is an efficient catalyst for the oxidation of D-glu by NBP in
acidic medium.
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