Behavior of IrCl3 as a Homogeneous Catalyst on the Oxidation of N-Acetylglucosamine in Acid Medium and Uncatalyzed Reaction in Alkaline Medium with Bromamine-B: Exploration of Kinetic, Mechanistic and Catalytic Chemistry

Article abstract of DOI:10.1007/s10562-017-2244-9

Abstract: The experimental rate laws for the oxidation of N-acetylglucosamine with bromamine- B are: ? d[BAB]/dt = k^/ [BAB]¹ [GlcNAc]^0.69 [HClO₄]^?0.76 [IrCl₃]^0.48 [BSA]^?0.33 in acid medium and –d[BAB]/dt = k^/?[BAB]¹ [GlcNAc]¹ [NaOH]^0.79 in alkaline medium. The IrCl₃ catalyzed reaction is thirteen fold faster than uncatalyzed reaction. Appropriate mechanisms and rate laws were deduced. Graphical Abstract: The reaction of N-acetylglucosamine with Bromamine-B in acid and alkaline medium is [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-017-2244-9

Catalysis Letters
https://doi.org/10.1007/s10562-017-2244-9
Behavior of IrCl3 as a Homogeneous Catalyst on the Oxidation
of N-Acetylglucosamine in Acid Medium and Uncatalyzed Reaction
in Alkaline Medium with Bromamine-B: Exploration of Kinetic,
Mechanistic and Catalytic Chemistry
Dakshayani Shankarlingaiah1 · Puttaswamy1
Received: 20 May 2017 / Accepted: 4 November 2017
© Springer Science+Business Media, LLC, part of Springer Nature 2017
Abstract
The experimental rate laws for the oxidation of N-acetylglucosamine with bromamine- B are: − d[BAB]/dt=k/ [BAB]1
[GlcNAc]0.69 [HClO4]−0.76 [IrCl3]0.48 [BSA]−0.33 in acid medium and –d[BAB]/dt=k/ [BAB]1 [GlcNAc]1 [NaOH]0.79 in
alkaline medium. The IrCl3 catalyzed reaction is thirteen fold faster than uncatalyzed reaction. Appropriate mechanisms
and rate laws were deduced.
Graphical Abstract
The reaction of N-acetylglucosamine with Bromamine-B in acid and alkaline medium is
OH
COOH
O
OH-/ H+(IrCl3)
HO
HO
C
H
3
HO
H3COCHN
+
4PhSO2NBrNa
(BAB)
4PhSO NH
+ 4Na+ + 4Br-
+
2
2
-CH3COOH
-NH3
-CO2
CH2OH
OH
aldonic acid
(GlcNAc)
Keywords IrCl3 catalysis · N-Acetylglucosamine · Bromamine-B · Acid and alkaline media · Kinetics-mechanism
1 Introduction
supplement similar to the usage of glucosamine. From the
literature survey, it is evident that a lot of attention has been
paid on the determination of this substrate, however negli-
gible attention has been drawn on the oxidation of GlcNAc
from the standpoint of its kinetic and mechanistic chemis-
try [2–5]. Additionally, the impact of platinum group metal
ions in the catalytic transformation of this substrate has not
yet been investigated. In biochemical reactions, kinetic as
well as catalytic knowledge is very important to optimize a
particular redox system. Hence, current research knowledge
would be very beneficial to the kineticists who are working
on the oxidative-mechanistic chemistry of this substrate in
biological systems.
N-Acetylglucosamine (GlcNAc) is chemically known as
2-(acetylamino)-2-deoxy-d-glucose, which is a monosac-
charide derivative of glucose [1]. This is a monomeric unit
of chitin being the second most abundant natural polymer,
next to cellulose. Chitin is a polymer present in exoskeleton
of insects and crustaceans. GlcNAc is also used as a dietary
* Puttaswamy
pswamy_chem@yahoo.com
1
Department of Post Graduate Studies in Chemistry,
Organic N-haloamines have attracted much attention in
the past few years owing to their diverse behavior to act as
Bangalore University, Central College Campus,
Bangalore 560001, India
Vol.:(0123456789)
1 3

D. Shankarlingaiah, Puttaswamy
halonium cations, hypohalites and N-anions [6]. They can
perform both as electrophiles and as nucleophiles depend-
ing on the reaction conditions. These compounds contain
positive halogen ions and are well known mild oxidants.
As a result, they react with a wide range of functional
groups affecting an array of molecular transformations.
Sodium N-chloro-p-toluenesulfonamide or chloramine-T
(CAT) and sodium N-chloro-benzenesulfonamide or chlo-
ramine-B (CAB) are the prominent members of this group
of compounds. These reagents have been employed as oxi-
dizing/chlorinating agents in the kinetic and mechanistic
studies for oxidation of different functional groups [7–9].
The bromine analogues of CAT and CAB are bromamine-
T (C6H4SO2NBrNa·3H2O or BAT) and bromamine-B
(C6H5SO2NBrNa 1.5H2O or BAB). These compounds can
be easily prepared by the bromination of CAT and CAB
and were found to be superior oxidizing agents than the
chloro compounds. Existing literature reveals that rela-
tively less attention [10, 11] was paid towards the kinetics
and mechanism of oxidation of organic substrates involving
BAT and BAB as oxidizing agents. Our preliminary kinetic
runs began with the examination of oxidation of GLcNAc
with both CAT and CAB in acid and alkaline media under
different experimental conditions. Both the reactions were
too sluggish to be measured kinetically. Then we thought of
using BAT and BAB as oxidants in both acid and alkaline
media and found that the reactions to be feasible. However,
the reaction was found to be more brisk with BAB in com-
parison with BAT, under similar experimental conditions.
Hence, we have opted BAB as an oxidant for the oxidation
of GlcNAc in both acid and alkaline media. However, the
oxidation reaction is still sluggish to be measured kinetically
in acid medium and hence it requires some catalyst.
measurable speed by enhancing the reaction rate by 13-fold
faster in comparison with uncatalyzed reaction. For these
reasons, IrCl3 was chosen as a catalyst in acid medium.
In view of these varied observations, the title investiga-
tions were undertaken. In this paper, for the first time, we
have taken up a systematic kinetic study of the oxidation
of GlcNAc–BAB in acid medium catalyzed by IrCl3 and
uncatalyzed reaction in alkaline medium to probe the mecha-
nistic and catalytic chemistry of this redox system in both
media. Efforts were made to arrive at the related rate laws
and to judge the relative reactivity of the reaction in acid
and alkaline media.
2 Experimental and Methods
2.1 Materials
Bromamine-B was obtained by the method of Ahmed and
Mahadevappa [15]. The purity of BAB was verified iodo-
metrically via its halogen content. An aqueous solution
of BAB was prepared afresh, standardized by iodometric
method and preserved in brown bottles to prevent its photo-
chemical deterioration. Analytical grade GlcNAc was pur-
chased from SRL and was used without further purification.
Aqueous solution of GlcNAc was freshly prepared whenever
required. A solution of IrCl3 (s.d.fine) in 0.1 mol dm−3 HCl
was employed as catalyst in acid medium. Allowance was
made for the amount of HCl present in the catalyst solution
while preparing solutions for kinetic runs. All other chemi-
cals used were of analytical grade. Double distilled water
was used throughout the work.
Catalysis by platinum group metal ions plays a vital role
in understanding the mechanistic chemistry of a particular
redox system. The mechanistic chemistry of these catalytic
reactions are quite complex due to the formation of different
intermediate complexes, free radicals and different oxida-
tion states of these catalysts. They are of immense practical
importance as they deal with control of chemical processes
leading to the better understanding of reactions involving
such metal complexes and to reveal mechanistic details of
a particular redox reaction through kinetic study. Amongst
all platinum group metal ions, ruthenium trichloride (RuCl3)
and osmium tetroxide (OsO4) have been extensively used
as homogeneous catalysts for a number of redox reactions
[12–14]. By contrast, little attention has been paid towards
iridium trichloride (IrCl3) catalyst in comparison with other
platinum group metal ions. GlcNAc–BAB redox reactions
were found to be sluggish in acid medium. Alternatively,
we found that IrCl3 catalyst even at trace concentrations
(ca. 5.0×10−5 mol dm−3) can potentially catalyze the Glc-
NAc–BAB reaction and oxidation underwent at a kinetically
2.2 Kinetic Procedure
All the kinetic experiments were performed under
pseudo-first-order conditions by taking a known excess of
[GlcNAc]0 over [BAB]0. Raagaa Ultra Cold Chamber with
digital temperature control (Chennai, India) was used to
control the temperature with an accuracy of 0.1 °C. The
kinetic procedure followed to study the progress of the
reaction was according to a literature procedure [16]. For
each kinetic run, requisite amount of solutions of GlcNAc,
NaOH/HClO4, IrCl3 (in acid medium) and water (to main-
tain the total volume constant for all runs) were taken in
glass stoppered Pyrex boiling tubes which are coated black
from outside. The reaction mixture was kept in thermostat
at 303 K for about 30 min to attain thermal equilibrium. A
measured amount of BAB solution, which was also ther-
mostated at the same temperature, was rapidly added with
stirring to the mixture in the tube. The course of the reaction
was monitored by the iodometric determination of unreacted
BAB in 5 ml aliquots of the reaction mixture withdrawn at
1 3

Behavior of IrCl3 as a Homogeneous Catalyst on the Oxidation of N-Acetylglucosamine in A…
different intervals of time. The pseudo-first-order rate con-
stants (k′ s−1) were calculated from linear plot of log [BAB]
versus time. Kinetic runs were repeated twice and had<5%
standard deviation. All regression coefficients (R2) calcula-
tions were performed with the fx-100z scientific calculator.
3.3 Kinetic Data
At comparable experimental conditions, the kinetics of oxi-
dation of GlcNAc by BAB was investigated at several ini-
tial concentrations of reactants in acid medium catalyzed by
IrCl3 and uncatalyzed reaction in alkaline medium at 303 K.
3 Results
3.3.1 Effect of Varying Concentrations of GlcNAc and BAB
on the Rate of Reaction
3.1 Reaction Stoichiometry
Under pseudo-first-order conditions of [GlcNAc]0 ≫[BAB]0,
at constant [GlcNAc]0, [NaOH]/[HClO4], (also IrCl3 in acid
medium) and temperature, plots log [BAB] versus time
were linear (R2 >0.9918), indicating a first-order depend-
ence of rate on [BAB]0 in both the media. The pseudo-first-
order rate constants (k′, s−1) calculated are independent of
[BAB]0, confirming the first-order dependence of rate on
[BAB]0 (Tables 1, 2). Values of k′ increased with increase in
Reaction mixtures containing various compositions of BAB
to GlcNAc in the presence of 8.0×10−3 mol dm−3 NaOH/
HClO4 (also 5.0 × 10−5 mol dm−3 IrCl3 in acid medium)
were equilibrated at 303 K for 24 h. Iodometric titrations of
unreacted BAB showed that one mole of GlcNAc consumed
four moles of BAB in both the media. Accordingly, the fol-
lowing stoichiometric equation was formulated:
OH
O
COOH
OH-/ H+(IrCl3)
HO
HO
C
H
3
HO
H3COCHN
4PhSO NH + 4Na+ + 4Br-
+
4PhSO2NBrNa
(BAB)
+
2
2
-CH3COOH
-NH3
-CO2
CH2OH
OH
aldonic acid
(GlcNAc)
(1)
3.2 Product Characterization
[GlcNAc]0 (Tables 1, 2) in both media. A plot of log k′ ver-
sus log [GlcNAc]0 was linear (R2 =0.9989) with a unit slope,
suggesting a first-order dependence of rate on [GlcNAc]0 in
alkaline medium. Whereas the order of reaction with respect
to [GlcNAc]0 in acid medium is fractional (Table 1). Fur-
thermore, a plot of k′ versus [GlcNAc]0 were linear in both
cases (R2 >0.9905). Such a plot is passing through the origin
in alkaline medium and with a y-intercept in acid medium,
clearly confirms the order of the reaction is first-order in
alkaline medium and fractional-order in acid medium.
The GlcNAc–BAB reaction was allowed to progress for
24 h at 303 K under stirred conditions in the presence of
NaOH/HClO4 (also IrCl3 in acid medium) separately. After
completion of the reaction (monitored by TLC), the organic
products were subjected to spot tests and chromatographic
analysis (TLC technique). Aldonic acid was identified as the
oxidation product of GlcNac and was confirmed by GC-MS
analysis. The mass spectrum showed a molecular ion peak
at 166 (M+1) amu, confirming aldonic acid (Fig. 1). Acetic
acid was identified by spot test [16]. Ammonia was detected
by Nessler’s reagent test according to the method of Vogel
[17]. The evolving carbon dioxide was detected by conven-
tional lime water test. It was also noted that there was no
further oxidation of aldonic acid under the present kinetic
conditions. Benzensulfonamide (BSA), the reduced prod-
uct of BAB, was extracted with ethyl acetate and identified
[18] by thin layer chromatography using petroleum ether:
chloroform: 1-butanol (2:2:1 v/v) as the solvent system and
iodine as the spray reagent (Rf = 0.88) The determined Rf
value agrees with the value reported in the literature [18].
3.3.2 Effect of Varying Concentrations of Media and IrCl3
on the Rate of Reaction
It is seen from the data in Tables 1 and 2 that values of
k′ decreases with increase in [HClO4] and increases with
increase in [NaOH]. Further, log–log plots of k′ and
[HClO4]/[NaOH] were linear (R2 >0.9946) with slopes of
−0.76 and 0.79 in HClO4 and NaOH medium respectively,
suggesting a negative-fractional-order dependence of rate
on [HClO4] and fractional-order with respect to [NaOH].
However, the rate increased with an increase in IrCl3 in acid
medium (Table 1). The slope of log k′ versus log [IrCl3]
1 3

D. Shankarlingaiah, Puttaswamy
Fig. 1 Mass spectrum of aldonic acid with molecular ion peak at 166 (M+1) amu
(R2 =0.9881) was found to be 0.48, establishing a fractional-
order dependence of rate on [IrCl3].
that oxidation of MeOH with BAB under the present experi-
mental conditions are negligible.
3.3.3 Effect of Varying Dielectric Constant of the Media
on the Rate of Reaction
3.3.4 Effect of Varying Concentration of BSA and Ionic
Strength of Media on the Rate of Reaction
Rate studies were made in H2O–MeOH mixtures at differ-
ent compositions (0–20% v/v) in order to study the effect of
dielectric constant (D) of the solvent medium, with all other
experimental conditions being held constant. The rate was
found to decrease with increase in MeOH content (Table 3)
in both the media. Further, plots of log k′ versus 1/D were
linear (R2 >0.9951, Fig. 2) with negative slopes. The dielec-
tric constant values of MeOH–H2O mixtures reported in the
literature [19] were employed. Blank experiments showed
Addition of benzenesulfonamide (BSA) in the range of
0.3 × 10−4 to 5.0 × 10−4 mol dm−3 to the reaction mixture
retarded the rate in acid medium, whereas negligible effect
was noticed in alkaline medium (Table 4). This indicates that
BSA is involved in the pre-equilibrium step in acid medium
but the same does not in alkaline medium. Further, a plot of
log k′ versus log [BSA] was linear (R2 =0.9945, Fig. 3) with
a negative slope (−0.33), confirming a negative-fractional-
order dependence of rate on [BSA] in acid medium. The
influence of ionic strength (I) of the medium on the reaction
1 3

Behavior of IrCl3 as a Homogeneous Catalyst on the Oxidation of N-Acetylglucosamine in A…
Table 1 Effect of varying BAB, GlcNAc, HClO4 and IrCl3 concentra-
Table 3 Effect of varying the dielectric constant of the medium on
the reaction rate at 303 K
tions on the reaction rate at 303 K
104 [BAB]0 103
103 [HClO4] 105 [IrCl3] 104 k′ (s−1
)
% MeOH (v/v)
D
104 k′ (s−1
)
(mol dm−3
)
[GlcNAc]0
(mol dm−3
)
(mol dm−3
)
Acid (IrCl3 cata-
lyzed)
Alkaline
(mol dm−3
)
0.3
0.5
1.0
2.0
3.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
2.0
2.0
2.0
2.0
2.0
0.5
1.0
2.0
4.0
8.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
2.0
4.0
8.0
12.0
16.0
8.0
8.0
8.0
8.0
8.0
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
9.0
5.42
5.38
5.44
5.46
5.41
3.39 (1.84)
4.41 (2.56)
5.44 (3.26)
7.05 (4.16)
8.66 (4.60)
15.5
10.1
5.44
4.09
3.22
2.75
4.38
5.44
6.91
7.95
0
5
10
20
76.73
74.50
72.37
67.48
5.44
4.89
4.56
3.45
4.36
4.10
3.89
3.50
Experimental
conditions:
[BAB]0 =1.0×10−4
mol
dm−3
;
[GlcNAc]0 =2.0×10−3
mol
dm−3
;
[NaOH/
HClO4]=8.0×10−3 mol dm−3; [IrCl3]=5.0×10−5 mol dm−3 (in acid
medium); I=0.1 mol dm−3(in alkaline medium); T=303 K
Alkaline
0.75
0.70
0.65
0.60
0.55
0.50
Acid(IrCl3catalyzed)
Values in parenthesis refer to rate constants obtained for varying Glc-
NAc concentrations at constant [PhSO2NH2]=5.0×10−4 mol dm−3
0.01280.01300.01320.01340.01360.01380.01400.01420.01440.01460.01480.0150
1/D
Table 2 Effect of varying BAB, GlcNAc and NaOH concentrations
on the reaction rate at 303 K
104 [BAB]0
103 [GlcNAc]0
103 [NaOH]
104 k′ (s−1
)
Fig. 2 Plots of log k′ versus 1/D
(mol dm−3
)
(mol dm−3
)
(mol dm−3
)
Table 4 Effect of varying the benzenesulfonamide concentrations on
0.3
0.5
1.0
2.0
3.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
2.0
2.0
2.0
2.0
2.0
0.5
1.0
2.0
4.0
8.0
2.0
2.0
2.0
2.0
2.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
2.0
4.0
8.0
12.0
16.0
4.40
4.31
4.36
4.29
4.30
1.19
2.22
4.36
8.77
17.2
1.42
2.43
4.36
5.30
7.16
the reaction rate at 303 K
104 [BSA] (mol dm−3
)
104 k′ (s−1
)
Acid (IrCl3 catalyzed)
Alkaline
0.3
0.5
1.0
3.0
5.0
7.02
6.39
5.44
3.98
3.22
4.33
4.28
4.36
4.44
4.41
Experimental
conditions:
[BAB]0 =1.0×10−4
mol
dm−3
;
[GlcNAc]0 =2.0×10−3
mol
dm−3
;
[NaOH/
HClO4]=8.0×10−3 mol dm−3; [IrCl3]=5.0×10−5 mol dm−3 (in acid
medium); I=0.1 mol dm−3(in alkaline medium); T=303 K
kept constant. We have noticed salient results in this regard
(Table 5). The ionic strength showed negligible influence
on the reaction rate in acid medium, whereas in alkaline
medium the rate of reaction increases with increase in ionic
strength. A plot of log k′ versus I1/2 was linear (R2 =0.9994)
Experimental conditions: I=0.1 mol dm−3
rate was carried out in the range of 0.05–0.3 mol dm−3
using NaClO4 solution, with other experimental conditions
1 3

D. Shankarlingaiah, Puttaswamy
0.8
0.7
0.6
Acid(IrCl3catalyzed)
Alkaline
0.85
0.80
0.75
0.70
0.65
0.60
0.55
0.50
Alkaline
Acid(IrCl3catalyzed)
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
5+log[BSA]
I1/2
Fig. 3 Plots of log k′ versus log [BSA]
Fig. 4 Plots of log k′ versus I1/2
Table 5 Effect of varying ionic strength of the medium on the reac-
other experimental conditions constant. From the linear
Arrhenius plots of log k′ versus 1/T (R2 >0.9988), activa-
tion parameters for the overall reaction were computed for
acid and alkaline media. All these results are summarized
in Table 6.
tion rate at 303 K
[NaClO4] (mol dm−3
)
104 k′ (s−1
)
Acid (IrCl3 catalyzed)
Alkaline
0.05
0.1
0.2
0.3
5.41
5.44
5.39
5.46
3.82
4.36
5.14
5.95
3.3.7 Test for Free Radicals
No induced polymerization was observed when acrylamide
was added to the reaction mixture ruling out a free radical
mechanism.
Experimental
conditions:
[BAB]0 =1.0×10−4
mol
dm−3
;
[GlcNAc]0 =2.0×10−3
mol
dm−3
;
[NaOH]/
[HClO4]=8.0×10−3 mol dm−3; [IrCl3]=5.0×10−5 mol dm−3 (in acid
medium); I=0.1 mol dm−3 (in alkaline medium); T=303 K
4 Discussion
with positive slope in alkaline medium (Fig. 4). For this rea-
son, the ionic strength of the reaction system was maintained
at constant concentration of 0.1 mol dm−3 for kinetic runs in
alkaline medium in order to swamp the reaction.
4.1 Reactive Species of BAB
As organic N-haloamines have similar chemical properties, it
is expected that similar equilibria exists in solutions of these
compounds [20]. Bromamine-B acts as an oxidizing agent in
both acidic and alkaline media. In general BAB undergoes a
two-electron change in its reactions. The oxidation potential
of BAB/sulfonamide couple is pH dependent and decreases
with increase in the pH of the medium [20]. Bromamine-
B like its chlorine analogues CAT and CAB behaves as a
strong electrolyte in aqueous solutions [21–23] forming dif-
ferent species as shown in Eqs. (2–8):
3.3.5 Effect of Varying Concentration of Halide ions
and on the Rate of Reaction
The effects of Cl− or Br− were studied by adding
2.0×10−2 mol dm−3 NaCl or NaBr solution to the reaction
mixture. There was no significant effect of these halide ions
on the rate of the reaction, suggesting that no interhalogen
was formed in the reaction.
PhSO2NBrNa ⇌ PhSO2BNBr− + Na+
(2)
3.3.6 Effect of Varying Temperature on the Rate of Reaction
PhSO2NBr− + H+ ⇌ PhSO2NHBr
(3)
The effect of temperature on the rate of the reaction was
studied in the temperature range of 293–313 K, keeping
1 3

Behavior of IrCl3 as a Homogeneous Catalyst on the Oxidation of N-Acetylglucosamine in A…
Table 6 Effect of varying temperature on the rate of reaction and acti-
vation parameters for the oxidation of GlcNAc with BAB in acid and
alkaline media
reaction (9) is quite favorable. Furthermore, the rate retarded
by the added BSA suggests that Eq. (6) plays a dominant role
and HOBr is the most active oxidizing species in the present
investigations.
Temperature 104 k′ (s−1
)
Kc
(K)
Alkaline
Acid
4.2 Study of Complex Formation of IrCl3
and Reactive Species
IrCl3 Cata-
lyzed
Uncatalyzed
It is known that IrCl3 in hydrochloric acid gives IrCl63− spe-
cies [25]. It has also been reported that iridium (III) and
iridium (I) ions are the stable species of iridium [26]. Fur-
ther, the aqueous solution of IrCl63− gives [IrCl5(H2O)]2−,
[IrCl4(H2O)2]− and [IrCl3(H2O)3] species [27–29] as shown
below:
293
298
303
308
313
2.13
2.83
4.22
5.44
8.52
11.5
44.6
0.22 0.03
0.32 0.04
0.43 0.06
0.66 0.09
0.83 0.12
3.23
4.36
6.61
8.95
Ea (kJ mol−1
ΔH≠
)
51.7
60.5
55.7
49.2 ( 0.01) 42.1 ( 0.02) 57.9 ( 0.02) 53.2
93.7 ( 0.23) 93.0 ( 0.24) 99.6 ( 0.29) 81.3
−146 ( 0.05) −168 ( 0.18) −137 ( 0.22) −92.7
(kJ mol−1
)
[
]
IrCl3.xH2O + 3HCl → IrCl6 3− + xH2O + 3H+
(10)
ΔG≠
(kJ mol−1
ΔS≠
)
[
]
[
(
)]
IrCl6 3− + H2O ⇌ IrCl5 H2O 2− + Cl−
(11)
(JK1 mol1)
Experimental
In the present study, addition of Cl− in the form of NaCl
condition:
[BAB]0 =1.0×10−4
mol
dm−3
[NaOH]/
;
[GlcNAc]0 =2.0×10−3
mol
dm−3
;
at fixed [H+] had no effect on the rate, suggesting that equi-
librium (11) does not play any role in the reaction. Hence,
the complex ion [IrCl5(H2O)]2− is assumed to be the active
catalyst species that interacts with the oxidant species HOBr
to form a complex intermediate.
[HClO4]=8.0×10−3 mol dm−3; [IrCl3]=5.0×10−5 mol dm−3 (in acid
medium); I=0.1 mol dm−3 (in alkaline medium)
2 PhSO2NHBr ⇌ PhSO2NH2 + PhSO2NBr2
(4)
Evidence for the complex formation between oxidant,
substrate and catalyst is acquired from the UV–Visible
spectra of BAB, GlcNAc, IrCl3, and their mixture. Absorp-
tion maxima appear at 218 nm for BAB, 444 nm for IrCl3
in aqueous acidic medium, and 418 nm for (BAB + IrCl3)
mixture. A hypsochromic shift of 26 nm from 444 to 418 nm
of IrCl3 suggests that the complexation occurs between BAB
and IrCl3. Further, the spectra show an absorption maxima
for GlcNAc at 214 nm and for (IrCl3 +BAB+GlcNAc) mix-
ture at 437 nm. A bathochromic shift of 19 nm supports the
formation of complex between oxidant, catalyst and sub-
strate in acid medium (Fig. 5).
PhSO2NBr2 + H2O ⇌ PhSO2NHBr + HOBr
(5)
PhSO2NHBr + H2O ⇌ PhSO2NH2 + HOBr
(6)
HOBr ⇌ H+ + OBr−
(7)
(8)
HOBr + H+ ⇌ H2O+Br
In acidified solution of BAB, the probable oxidizing
species are PhSO2NHBr, PhSO2NBr2, HOBr and possi-
bly H2O+Br. If PhSO2NBr2 was to be the reactive species,
then the rate law predicts the second-order dependence on
[BAB]0 (Eq. 4) which is not true, since the first-order with
respect to [BAB]0 is noticed in the present case. This clearly
rules out the possibility of PhSO2NBr2 being reactive spe-
cies. Further, protonation of monochloramine-B has been
reported [24] as:
4.3 Reaction Scheme and Rate Law in Acid Medium
In view of the above facts and considering all the experi-
mental data, the following mechanism (Scheme 1) may be
formulated for IrCl3 catalyzed oxidation of GlcNAc by BAB
in acid medium.
PhSO2NHBr + H+ ⇌ PhSO2N+H2Br
(9)
Because organic N-haloamines have similar chemical
properties, the same equilibrium can be expected for BAB
also. In the present investigations, the inverse-fractional-
order dependence of rate on [H+] suggests that the reverse of
Scheme 2 depicts electron flow for the oxidation of Glc-
NAc by BAB in acid medium catalyzed by IrCl3 along with
structures of intermediate complexes. In the fast equilibrium
step (step (i)), PhSO2N+H2Br undergoes deprotonation to
give PhSO2NHBr. In the next step (step (ii)), PhSO2NHBr
1 3

D. Shankarlingaiah, Puttaswamy
If [BAB]t is the total effective concentration of oxidant,
A
3.0
2.5
2.0
1.5
1.0
0.5
0.0
then
B
[
]
[
]
+ [HOBr] + Complex-I + Complex-II
[BAB]t = PhSO2N+H2Br + PhSO2NHBr
[
]
[
]
(12)
From steps (i), (ii), (iii) and (iv) of the Scheme1, we
obtain
[PhSO2NH2][Complex-II]
[PhSO2NHBr] =
(13)
C
D
K2K3K4[IrCl3][GlcNAc][H2O]
E
[PhSO2NH2][Complex-II][H+]
[PhSO2N+H2Br] =
K1K2K3K4[GlcNAc][IrCl3][H2O]
200
300
400
500
600
Wavelength(nm)
(14)
(15)
[Complex-II]
K3K4[IrCl3][GlcNAc]
Fig. 5 UV–Visible spectra of (a) BAB, (b) GlcNAc, (c) IrCl3, (d)
Mixture of (BAB+IrCl3), (e) Mixture of (BAB+IrCl3 +GlcNAc)
[HOBr] =
[Complex-II]
K4[GlcNAc]
undergoes hydrolysis to form HOBr, which is the active oxi-
dizing species, along with the elimination of PhSO2NH2. In
the next fast equilibrium step (step (iii)), HOBr co-ordinates
to catalytic species and forms an intermediate complex-I.
Further, in the step (iv), this intermediate complex-I reacts
[Complex-I] =
(16)
By substituting for [PhSO2NHBr], [PhSO2N+HBr],
[HOBr] and [Complex-I] from Eqs. (13–16) into Eq. (12)
and solving for [Complex-II], we obtain,
ꢀ
}
K1K2K3K4[BAB]t[GlcNAc][IrCl3][H2O]
[PhSO2NH2][H+] + K1[PhSO2NH2] + K1K2[H2O] + K1K2K3[IrCl3][H2O] + K1K2K3K4[GlcNAc][IrCl3][H2O]
[Complex-II] =
(17)
From the slow and rds (step (v)) of Scheme 1,
with substrate to give an another intermediate complex-II. In
the slow and rds step (step (v)), complex-II disproportionates
to form an another intermediate complex-III. Finally, the
complex-III reacts with three more moles of oxidant fol-
lowed by several fast steps, leading to the formation of the
ultimate product, aldonic acid.
[
]
Rate = k5 Complex − II
(18)
By substituting for [Complex-II] from Eq. (17) into
Eq. (18), the following rate law is obtained:
K1
PhSO2N+H2Br
PhSO2NHBr
+ H+
PhSO2NHBr
fast (i)
K2
PhSO2NH2 + HOBr
Complex-I
fast (ii)
+ H2O
K3
fast (iii)
fast (iv)
HOBr + IrCl3
Complex-I + GlcNAc
Complex-II
K4
k5
Complex-II
Complex-III
slow & rds (v)
fast (vi)
k6
Complex-III + 3 HOBr
Products
Scheme 1 A general reaction scheme for IrCl3-catalyzed oxidation of GlcNAc by BAB in acid medium
1 3

Behavior of IrCl3 as a Homogeneous Catalyst on the Oxidation of N-Acetylglucosamine in A…
K1
(fast)
K2
+ H+
PhSO2N+H2Br
PhSO2NHBr
(i)
PhSO NHBr
(ii)
+ H2O
2
HOBr + PhSO2NH2
(fast)
(iii)
HOBr + [IrCl5 (H2O)]2-
HOBr
H2O
+
K3
(fast)
[IrCl5]2-
(Complex-I)
OH
(iv)
HOBr
+
[IrCl5]2-
OH
O
K4
HO
HO
O
Br--OH
[IrCl5]2-
HO
HO
(fast)
H3COCHN
(Complex-I)
O....H
H3COCHN
OH
(Complex-II)
(GlcNAc)
OH
OH
O
OH
O
HO
(v)
HO
HO
Br--OH
[IrCl5]2-
k5
O
H
HO
HO
-[IrCl5(H2O)]2-
(slow&rds)
HO
-HBr
H3COCHN
H3COCHN
O....H
O
Br
O
H3COCHN
(Complex-II)
(Intermediate)
O
OBr
C
OH
C
OH
O
H2O
H+/
O
O
k6
H2O
H
N
2HOBr
HO
HO
(vi)
H
C
C
OBr
CH3
C
(fast)
H
C
N
H
C
CH3
HO
H
H3COCHN
3
O
HO
C
H
3
CH2OH
(Intermediate)
CH2OH
O
OBr
O
Br
C
C
C
H
O
O
C
C
C
H
NH2
OBr
H
NH
H
C
HO
H
CH3
3
HO
3
CH2OH
CH2OH
-CO2
O
H2O
-Br-
CH3COOH
HOBr
+
H3C
OBr
COOH
C
CHO
H
C
NH
HOBr
+ H2O
-NH3
HO
H
HO
C
H
HO
C
H
3
3
3
CH2OH
CH2OH
CH2OH
aldonic acid
Scheme 2 A descriptive mechanistic interpretation for the IrCl3 catalyzed oxidation of GlcNAc by BAB in acid medium
1 3

D. Shankarlingaiah, Puttaswamy
K1K2K3K4k5[BAB]t[GlcNAc][IrCl3][H2O]
[PhSO2NH2][H+] + K1[PhSO2NH2] + K1K2[H2O] + K1K2K3[IrCl3][H2O] + K1K2K3K4[GlcNAc][IrCl3][H2O]
(19)
Rate =
other thermodynamic parameters with respect to IrCl3 cata-
Rate law (19) is in complete agreement with the observed
lyst were computed and recorded in Table 6.
kinetic data wherein a first-order dependence of rate on
[BAB]t fractional-order on both [GlcNAc] and [IrCl3] and
negative fractional-order each on [H+] and [PhSO2NH2] was
observed.
It was thought worthwhile to compare the reactivity of
GlcNAc by BAB in acid medium in the absence of IrCl3
catalyst, under identical experimental conditions. Hence,
the reaction was carried out at different temperatures
(293–313 K) and from the Arrhenius plot of log k′ versus
Since rate=k′ [BAB]t, then Eq. (19) becomes
K1K2K3K4k5[GlcNAc][IrCl3][H2O]
[PhSO2NH2][H+] + K1[PhSO2NH2] + K1K2[H2O] + K1K2K3[IrCl3][H2O] + K1K2K3K4[GlcNAc][IrCl3][H2O]
k� =
(20)
Inversion and transformation of the above equation yields,
1/T (R2 = 0.9988), values of activation parameters for the
uncatalyzed reaction in acid medium were also calculated
ꢀ
}
[PhSO2NH2][H+]
K1K2K3K4k5[GlcNAc][IrCl3][H2O] K2K3K4k5[IrCl3][GlcNAc][H2O] K3K4k5[GlcNAc][IrCl3] K4k5[GlcNAc]
[PhSO2NH2]
1
k�
1
1
1
=
+
+
+
+
k5
(21)
(22)
ꢀ
}
[PhSO2NH2][H+]
[PhSO2NH2]
1
k�
1
1
1
1
=
+
[GlcNAc] K1K2K3K4k5[IrCl3][H2O] K2K3K4k5[IrCl3][H2O] K3K4k5[IrCl3] K4k5
+
+
+
k5
According to Eq. (22), a plot of 1/k′ versus 1/[Glc-
(Table 6). It is seen from Table 6 that IrCl3 catalyzed reac-
tion rate is found to be nearly 13-fold faster than the uncata-
lyzed reaction. Thus, the observed rate of oxidation in the
presence of IrCl3 catalyst justifies the need of IrCl3 catalyst
for a facile oxidation of GlcNAc by BAB in acid medium.
The activation parameters evaluated for the IrCl3 catalyzed
and uncatalyzed reactions explain the catalytic effect on the
reaction. The catalyst IrCl3 forms a complex-I with the oxi-
dizing species (HOBr), which increases the oxidizing prop-
erty of the oxidant than without IrCl3 catalyst. Further, the
catalyst IrCl3 favorably modifies the reaction path by lower-
ing the energy of activation as shown in Table 6.
NAc] should be linear with an intercept equal to 1/k5. The
applicability of Eq. (22) is clearly shown that 1/k′ versus
1/[GlcNAc] plot is linear (R2 = 0.9960) with an intercept
for the standard run at constant [BAB], [HClO4], [IrCl3],
[PhSO2NH2] and temperature as shown in Table 1. The
value of the decomposition constant k5 was found to be
4.34×10−4 s−1.
Moelwyn-Hughes [30] pointed out that the catalyzed and
uncatalyzed reactions proceed simultaneously and the rela-
tionship is
[
]
x
k1 = k0 + KC catalyst
(23)
4.4 Reaction Scheme and Rate Law in Alkaline
Medium
Here k1 is the observed pseudo-first-order rate constant
obtained in the presence of IrCl3 catalyst and k0 is that for
the uncatalyzed reaction, KC is the catalytic constant and x is
the order of the reaction with respect to IrCl3 which is found
to be 0.48 in the present study. The value of KC is calculated
using the equation
In alkaline solutions of CAB, PhSO2NCl2 does not exist
[20]. Further, Bishop and Jennings [20] have shown in alka-
line solutions of CAB, the possible oxidizing species are
PhSO2NHCl, HOCl and the anions PhSO2N−Cl and OCl−.
Similar observations are reported for alkaline BAB solutions
by Hardy and Johnston [22] and the following equilibria
were also established:
[
]
0.48
KC = k1 − k0∕ IrCl3
(24)
The values of KC have been evaluated at different tem-
peratures (293–313 K) and KC was found to vary with tem-
perature. Further, a plot of log KC versus 1/T was linear
(R2 = 0.9988) and the values of energy of activation and
PhSO2N−Br + H2O ⇌ PhSO2NHBr + OH−
(25)
PhSO2NHBr + H2O ⇌ PhSO2NH2 + HOBr
(26)
1 3

Behavior of IrCl3 as a Homogeneous Catalyst on the Oxidation of N-Acetylglucosamine in A…
If HOBr was to be the primary oxidizing species as
indicted in Eq. (26), a first-order retardation of rate by added
BSA would be expected. However, no such effect was noticed
in the present study. If PhSO2NHBr is the reactive species,
retardation of the rate by [OH−] is expected (Eq. 25), which
is also contrary to the experimental observations. Hence, in
the present investigations, the rate of reaction is accelerated
by OH− ions clearly indicating that the anion PhSO2N−Br is
the most likely oxidizing species involved in the oxidation
of GlcNAc by BAB. In view of the above observations and
experimental results, a reaction mechanism (Scheme 3) is
formulated for oxidation of GlcNAc with BAB in alkaline
medium. An anomeric carbon of GlcNAc loses a proton and
gets converted to GlcNAc− in alkaline medium, which is con-
sidered as a reactive substrate species [31].
By substituting for [GlcNAc−] from Eq. (30) into
Eq. (27), the following rate law is obtained.
K7k8[OH−][GlcNAc]t[PhSO2N−Br]
Rate =
(31)
[H2O]+K7[OH−]
The [PhSO2N−Br] is the concentration of the reactive
species of BAB, then [PhSO2N−Br] is [BAB], accordingly
we can write Eq. (31) as Eq. (32):
K7k8[OH−][GlcNAc]t[BAB]
Rate =
(32)
[H2O]+K7[OH−]
Rate law (32) is in accordance with the experimental
findings.
Since rate=k′ [GlcNAc]t, then
K7k8[BAB][OH−]
[H2O] + K7[OH−]
The probable mode of oxidation of GlcNAc by BAB in
alkaline medium is depicted in Scheme 4. In the fast step
(step (i)), GlcNAc undergoes deprotonation in alkaline
medium resulting in the formation of corresponding anion
(GlcNAc−). In the next slow and rds (step (ii)), the anion
reacts with one mole of the oxidant species (PhSO2N−Br)
to form an intermediate complex-IV. In the last successive
fast steps (step (iii)), intermediate complex-IV reacts with
another three more moles of oxidant species yielding the
final product which is the aldonic acid.
k� =
(33)
(34)
[H2O]
1
1∕k� =
+
K7k8[BAB][OH-] k8[BAB]
Based on Eq. (34), a double reciprocal plot of 1/k′ versus
1/[OH−] is a straight line (R2 =0.9983). From the intercept
of such a plot, value of k8 could be derived as 11.1 s−1.
The proposed mechanisms and the derived rate laws in
both the media are supported by the following experimental
findings:
From slow and rds step (ii) of Scheme 3,
[
]
Rate = k8[GlcNAc−] PhSO2N−Br
(27)
The ionic strength (I) effect on the reaction rate has been
described according to the theory of Bronsted and Bjerrum
[32], which postulates the reaction through the formation of
an activated complex. According to this theory, the effect of
ionic strength on the rate of a reaction involving two ions is
given by the relationship:
If [GlcNAc]t is the total effective concentration of the
substrate, then
[GlcNAc]t = [GlcNAc−] + [GlcNAc]
(28)
By step (i) of Scheme 3
[GlcNAc-][H2O]
[GlcNAc] =
log k� = log k0 + 1.018ZAZBI1∕2
(35)
(29)
K7[OH-]
Here ZA and ZB are the valency of the ions A and B, and k
and k0 are the rate constants in the presence and absence of
the added electrolyte, respectively. A plot of log k′ against
I1/2 should be linear with a slope of 1.018 ZAZB. If ZA and
ZB have similar signs, then the slope of such a plot is posi-
tive. Hence, the quantity ZAZB is positive and consequently
the rate increases with increase in the ionic strength of the
By substituting for [GlcNAc] from Eq. (29) into Eq. (28)
and solving for [GlcNAc−], we obtain,
K7[OH−][GlcNAc]t
[H2O]+K7[OH−]
[GlcNAc−] =
(30)
K7
GlcNAc + OH-
GlcNAc- +H2O
fast (i)
k8
GlcNAc- + PhSO2N-Br
slow & rds (ii)
Complex- IV
k9
Complex-IV + 3 PhSO2N-Br
Products
fast (iii)
Scheme 3 A general reaction scheme for the oxidation of GlcNAc by BAB in alkaline medium
1 3

D. Shankarlingaiah, Puttaswamy
OH
OH
O
K7
O
OH-
HO
HO
+
HO
HO
(i)
-H2O
(fast)
H3COCHN
O
H
H3COCHN
O-
(GlcNAc)
OH
O
OH
OH
O
-PhSO2N-H
k8
(slow&rds)
O
H
PhSO2N-Br
HO
HO
+
HO
HO
(ii)
HO
HO
-PhSO2NH2
-Br-
O
Br
H3COCHN
H3COCHN
O-
H3COCHN
O
(Intermediate)
O
OBr
C
H2O
OH
OH
C
C
C
O
2PhSO2N-Br/H2O
OH-/H2O
O
OBr
HO
HO
H
N
O
H
N
(iii)
H
C
C
H
C
(fast)
-2PhSO2NH2
CH3
H3COCHN
HO
H
CH3
HO
C
H
-H+
O
3
3
CH2OH
(Intermediate)
CH2OH
OBr
H
O
O
C
C
C
O
OBr
O
Br
C
C
C
H
N
C
H
H
NH2
H
CH3
HO
H
3
HO
3
CH2OH
CH2OH
O
H2O
HOBr
CH3COOH
+
H3C
OBr
-CO2
H
C
NH
COOH
C
PhSO2N-Br
CHO
C
+H2O
-NH3
HO
C
H
HO
H
3
HO
H
3
3
-PhSO2NH2
CH2OH
CH2OH
CH2OH
aldonic acid
Scheme 4 A descriptive mechanistic interpretation for the oxidation of GlcNAc by BAB in alkaline medium
medium. If the ions have dissimilar charges, then slope of
such a plot is negative. Hence, quantity ZAZB is negative
and hence the rate would decrease with an increase in ionic
strength of the medium. If one of the ions is neutral, the
slope of such a plot is zero. Hence the quantity of ZAZB is
zero and subsequently the rate of the reaction is independent
1 3

Behavior of IrCl3 as a Homogeneous Catalyst on the Oxidation of N-Acetylglucosamine in A…
of ionic strength of the medium. In the present investiga-
tions, the quantity ZAZB observed is: (i) positive for the reac-
tion carried out in alkaline medium, signifying the participa-
tion of similar charges in rds as can be seen in Scheme 4,
and (ii) zero for IrCl3 catalyzed reaction in acid medium
indicating that one of the interacting species involved in the
rds is non-ionic in nature while the other is ionic as can be
seen in rds of Scheme 2. Hence, the observed ionic strength
effect on the rates of the reaction in acid and alkaline media
is in accordance with Bronsted and Bjerrum concept [32].
Many approaches have been put forward to explain quan-
titatively the effect of dielectric constant of the medium on
the rates of reactions in solutions [33, 34]. For limiting case
of zero angle approach between two dipoles or an ion–dipole
system, Amis [33] has shown that a plot of log k′ versus
1/D gives a straight line, with a positive slope for a reaction
involving a positive ion and a dipole and a negative slope for
a negative ion–dipole or dipole–dipole interactions. In the
present investigations, plots of log k′ versus 1/D were linear
(R2 >0.9951) with negative slopes for reactions in both the
media. The observation in case of IrCl3 catalyzed reaction
confirms the participation of a negative-ion and a dipole in
the rds of Scheme 2.
5 Conclusion
The kinetics of the oxidation of GlcNAc by BAB in
acid medium catalyzed by IrCl3 and alkaline medium
obeys the experimental rate laws: − d[BAB]/dt = k/
[BAB]1[GlcNAc]0.69[HClO4]−0.76[IrCl3]0.48 [BSA]−0.33 and
−d[BAB]/dt=k/ [BAB]1[GlcNAc]1[NaOH]0.79, respectively
(here BSA is benzenesulfonamide). Aldonic acid is iden-
tified as the oxidation product of GlcNAc in both media.
Activation parameters have been evaluated. IrCl3 catalyzed
reaction is about 13-fold faster than the uncatalyzed reaction
in acid medium. Reaction of GlcNAc with BAB is tenfold
faster in alkaline medium in comparison with acid medium
(without IrCl3 catalyst). Based on the kinetic data, suitable
mechanisms and relevant rate laws have been worked out.
Acknowledgements One of the authors (S D) is gratefully acknowl-
edge the UGC, New Delhi for awarding UGC-BSR Meritorious
Scholarship. The Authors are also thankful to Prof. M.A. Pasha of
this department for his valuable suggestions on the proposed reaction
schemes.
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medium cannot be explained by Amis theory, due to the
presence similar charges in the rds. This observation can be
explained by following equation
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