Mechanism of Ir(III)-catalysed and Hg(II)-co-catalysed oxidation of reducing sugars by N-bromoacetamide in acidic medium

Article abstract of DOI:10.1002/poc.723

The kinetics of Ir(III)-catalysed and Hg(II)-co-catalysed oxidation of D-glucose (Glu) and D-fructose (Fru) by N-bromoacetamide (NBA) were studied in acidic medium. The reactions follow identical kinetics, being zero order in each sugar concentration. The experimental results show a first-order dependence on NBA and Ir(III) at low concentrations, but tending towards zeroth order at higher concentrations. A negative effect of variation of [H⁺], [Cl^-] and [NHA] was observed whereas the ionic strength (I) of the medium has no influence on oxidation rate. The important feature of the reaction is that it follows a second-order dependence on mercury(II) ion concentration at low concentrations, but it tends towards first order at higher concentrations. Various activation parameters were calculated and recorded. The corresponding acids were identified as the main oxidation products of the reaction. On the basis of the experimental findings, a suitable mechanism consistent with the observed kinetics was proposed. A comparative study was also made between the kinetic results of the present investigation and those of Ru(III)- and Pd(II)-catalysed oxidations of reducing sugars. Copyright

Full text of DOI:10.1002/poc.723

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2004; 17: 249–256
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.723
Mechanism of Ir(III)-catalysed and Hg(II)-co-catalysed
oxidation of reducing sugars by N-bromoacetamide
in acidic medium
Ashok Kumar Singh,1* Shahla Rahmani,1 Bharat Singh,1 Ramesh Kumar Singh2 and Manju Singh2
1Department of Chemistry, University of Allahabad, Allahabad-211002, India
2Department of Chemistry, Kamala Nehru Institute of Physical and Social Sciences, Sultanpur, India
Received 18 August 2002; revised 28 November 2002; accepted 30 September 2003
ABSTRACT: The kinetics of Ir(III)-catalysed and Hg(II)-co-catalysed oxidation of D-glucose (Glu) and D-fructose
(Fru) by N-bromoacetamide (NBA) were studied in acidic medium. The reactions follow identical kinetics, being zero
order in each sugar concentration. The experimental results show a first-order dependence on NBA and Ir(III) at low
concentrations, but tending towards zeroth order at higher concentrations. A negative effect of variation of [Hþ], [Clꢀ]
and [NHA] was observed whereas the ionic strength (I) of the medium has no influence on oxidation rate. The
important feature of the reaction is that it follows a second-order dependence on mercury(II) ion concentration at low
concentrations, but it tends towards first order at higher concentrations. Various activation parameters were calculated
and recorded. The corresponding acids were identified as the main oxidation products of the reaction. On the basis of
the experimental findings, a suitable mechanism consistent with the observed kinetics was proposed. A comparative
study was also made between the kinetic results of the present investigation and those of Ru(III)- and Pd(II)-catalysed
oxidations of reducing sugars. Copyright # 2004 John Wiley & Sons, Ltd.
KEYWORDS: kinetics; oxidation; mechanism; reducing sugars; Ir(III)-catalysis, Hg(II)-co-catalysis; N-bromoacetamide
INTRODUCTION
EXPERIMENTAL
Recently, considerable attention has been focused on the
diverse nature of the chemistry of N-halo compounds1–5
owing to their ability to act as a source of halogenonium
cations, hypohalite species and nitrogen anions which act
as both bases and nucleophiles. N-Bromoacetamide
(NBA) has been widely used as an oxidizing and
halogenating agent in preparative organic chemistry.
Oxidation with NBA as an oxidant with some primary
alcohols,6 dimethyl sulphoxide,7 some aliphatic
ketones8,9 and cyclic ketones,10 and as a substrate in
acidic–alkaline media have been reported. Recent studies
in oxidation of sugar by chloramine-T,11,12 N-bromosuc-
cinimide13 and NBA14 prompted us to undertake the
present work on the investigation of the kinetics and
mechanism of Ir(III)-catalysed oxidation of reducing
sugars [D-glucose (Glu) and D-fructose (Fru)] by NBA
in acidic medium.
All reagents were of the highest purity available. NBA
(Fluka A-G grade) solution was always freshly prepared
and its strength was checked iodimetrically. Solutions of
sugars (Glaxo and Merck) were freshly prepared by
weighing the samples. Mercuric acetate, acetamide,
KCl, perchloric acid and sodium perchlorate solutions
were prepared from samples obtained from Merck. A
solution of the catalyst IrCl3 was prepared by dissolving
the sample in HCl of known strength.
The reactants, i.e. NBA, Hg(OAc)2, HClO4, KCl and
Ir(III), were mixed in a black-coated conical flask and
thermostatted at 308 K for thermal equilibrium. A mea-
sured amount of sugar solution, also equilibrated at the
same temperature, was rapidly added to the reaction
mixture. The progress of the reaction was monitored by
measuring the unconsumed NBA iodometrically.
STOICHIOMETRY AND PRODUCT ANALYSIS
Various sets of reactions were performed with different
[NBA]:[sugar] ratios under kinetic conditions for 48 h.
Determination of unconsumed NBA revealed that for the
oxidation of each mole of sugar 2 mol of NBA were
required. The stoichiometric equations are as follows:
*Correspondence to: A. K. Singh, Department of Chemistry,
University of Allahabad, Allahabad-211002, India.
E-mail: ashokeks@rediffmail.com
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 249–256

250
A. K. SINGH ET AL.
2MeCONHBrþ C6H12O6 þ
C5H10O6
2MeCONH2 þ 2HBr þ HCOOH
þ
2H2O ꢀ!
2H2O ꢀ!
ðNBAÞ
ðGluÞ
ðarabinonic acidÞ
ðNHAÞ
C6H12O6 þ
ðFruÞ
C5H10O6
2MeCONHBr þ
þ 2MeCONH2 þ 2HBr þ HCOOH
ðarabinonic acidÞ
The products were found to be identical under both
stoichiometric and kinetic conditions and also by thin-
layer chromatography. Identification of major reaction
product i.e. arabinonic acid, was further verified by paper
chromatography.15
becomes
ꢀ
rate ¼ J½NBAꢁ
ð2Þ
where J, the apparent rate constant, ¼ k[sugar]ꢁ[Hþ]ꢂ
[Ir(III)]ꢃ[Hg(II)]ꢄ
The results obtained with NBA variation are summar-
ized in Table 1. It is clear that almost constant values of
the apparent rate constant (J ) are observed up to
1.00 ꢂ 10ꢀ3 mol dmꢀ3 concentration of NBA with a
slight decrease at higher concentrations. This indicates
that the order of reaction with respect to NBA is unity up
to its 1.00 ꢂ 10ꢀ3 mol dmꢀ3 and thereafter it tends to-
wards zeroth order. Since in the determination of the
order of reaction with respect to all other reactants the
RESULTS
The kinetics of the oxidation of sugars was investigated at
several initial concentrations of the reactants. Figure 1
shows the nature of the reaction. The initial rate of
reaction in each run was determined by the slope of the
tangent drawn at a fixed concentration of NBA, except for
studies of NBA variation, where the slope of the tangent
was drawn at fixed time. Considering NBA, reducing
sugar, hydrogen ions, iridium(III) chloride and Hg(II) as
the main reactants, the general form of rate equation for
the reaction can be written as
experiments were performed with
a
1.00 ꢂ
10ꢀ3 mol dmꢀ3 concentration of NBA, throughout the
study the order of reaction with respect to NBA has
been taken as unity. Under pseudo-first-order conditions,
the concentration of sugar was varied from 1.00 ꢂ 10ꢀ2 to
10.00 ꢂ 10ꢀ2 mol dmꢀ3 (Table 1). Zero-order kinetics
with respect to [sugar] are evident from the very similar
values of J. A linear increase in the apparent first-order
rate constant was observed with increase in Ir(III) chlor-
ide concentration throughout its variation, except at very
high concentrations where a slight deviation from first-
order kinetics was noted. This result was further verified
by the plot of log J vs log[Ir(III)] (Fig. 2), where slope of
the plot gives less than unity order in [Ir(III)] for both the
reducing sugars [0.88 (Glu) and 0.80 (Fru)]. The order of
reaction with respect to hydrogen ions (obtained from
ꢀ
ꢁ
ꢂ
ꢃ
ꢄ
rate ¼ k½NBAꢁ ½sugarꢁ ½Hþꢁ ½IrðIIIÞꢁ ½HgðIIÞꢁ
ð1Þ
In order to determine the experimental rate law, a series
of experiments with varying initial concentrations of
NBA were performed at constant concentration of all
other reactants and at constant temperature. Throughout
the variation of NBA, the concentration of NBA was kept
much lower than that of sugar and H:þ ions. Under this
condition, since the concentrations of other reactants
except NBA will essentially be constant, the rate law (1)
Table 1. Effect of [reactants] on the rate constant at 308 Ka
J ꢂ 104 (sꢀ1
)
[NBA] ꢂ 103
[Sugar] ꢂ 102
(mol dmꢀ3
)
(mol dmꢀ3
)
Glu
Fru
0.50(a)
0.75(a)
1.00(a)
1.50(a)
2.00(a)
2.50(a)
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
12.00(b)
2.00(b)
4.00(b)
6.00(b)
8.00(b)
10.00(b)
5.33
5.08
5.16
4.56
3.80
3.56
2.03
2.07
2.03
1.86
1.96
1.83
5.25
4.93
4.76
4.21
3.86
3.46
3.95
3.44
3.85
3.84
3.72
3.62
Figure 1. Plot between remaining [NBA] and time at 308 K.
[Sugar] ¼ 1.00 ꢂ 10ꢀ2, [Ir(III)] ¼ 3.34 ꢂ 10ꢀ5, [Hg(OAc)2] ¼
mol dmꢀ3. (A) Glu; (B) Fru
1.00
5.00 ꢂ 10ꢀ3
,
[HClO4] ¼ 2.50ꢂ 10ꢀ2
,
[KCl] ¼ 2.00 ꢂ 10ꢀ3
a [Ir(III)] ¼ 3.34 ꢂ 10ꢀ5
,
[HClO4] ¼ 2.50 ꢂ 10ꢀ2
,
[KCl] ¼ 2.00 ꢂ 10ꢀ3
,
[Hg(OAc)2] ¼ (a) 5.00 ꢂ 10ꢀ3 and (b) 2.00 ꢂ 10ꢀ3 mol dmꢀ3
.
Copyright # 2004 John Wiley & Sons, Ltd.
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CATALYTIC OXIDATION OF REDUCING SUGARS BY NBA
251
Table 2. Effect of addition of [Hg(OAc)2], [NHA] and [Clꢀ]
on the rate constant at 308 Ka
J ꢂ 104 (sꢀ1
)
[Hg(OAc)2] ꢂ 103 [NHA] ꢂ 104 [Clꢀ] ꢂ 103
(mol dmꢀ3
)
(mol dmꢀ3
)
(mol dmꢀ3
)
Glu
Fru
1.00(a)
2.00(a)
2.50(a)
3.00(a)
4.00(a)
5.00(a)
5.00
5.00
5.00
5.00
5.00
—
—
—
—
—
—
2(b)
4(b)
10(b)
15(b)
20(b)
—
—
—
—
—
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00(a)
4.00(a)
6.00(a)
8.00(a)
10.00(a)
0.88
2.65
5.09
6.85
9.54
11.55
3.41
3.03
2.24
1.98
1.77
2.65
1.50
1.25
1.00
0.80
0.95
3.38
5.72
6.24
9.04
—
2.57
2.42
1.59
1.42
1.31
—
Figure 2. Plot of log J. versus log [Ir(III)] at 308 K.
[NBA] ¼ 1.00 ꢂ 10ꢀ3, [sugar] ¼ 1.00 ꢂ 10ꢀ2, [Hg(OAc)2] ¼
3.00 ꢂ 10ꢀ3 (A) and 4.00 ꢂ 10ꢀ3 (B), [HClO4] ¼ 2.50 ꢂ
10ꢀ2, [KCl] ¼ 1.00 ꢂ 10ꢀ3 mol dmꢀ3. (A) Glu; (B) Fru
2.00
2.00
2.00
2.00
1.42
1.15
1.05
0.99
2.00
a [NBA] ¼ 1.00 ꢂ 10ꢀ3, [sugar] ¼ 1.00 ꢂ 10ꢀ2, [Ir(III)] ¼ (a) 3.34 ꢂ 10ꢀ5
and (b) 2.00 ꢂ 10ꢀ5, [HClO4] ¼ (a, b) 2.50 ꢂ 10ꢀ2 mol dmꢀ3
.
The results clearly indicate that with increase in concen-
tration of NHA and Clꢀ ions, there is a decrease in the
apparent first-order rate constant. The order with respect
to NHA was found to be ꢀ0.28 (Glu) and ꢀ0.32 (Fru)
from the slope of the plot of log J vs log[NHA]. Similarly,
the order with respect to chloride ions was ꢀ0.62 (Glu)
and ꢀ0.48 (Fru). Variations in the ionic strength of the
medium had an insignificant effect on the apparent first-
order rate constant.
On the basis of the observed values of ꢀ, ꢁ, ꢂ, ꢃ and ꢄ
in [NBA]ꢀ, [sugar]ꢁ, [Hþ]ꢂ, [Ir(III)]ꢃ and [Hg(II)]ꢄ, the
experimental rate law for the oxidation of both reducing
sugars, Glu and Fru, can be written as Eqns (3) and (4),
respectively:
Figure 3. Plot of log J versus log[Hþ] at 308 K. [NBA]ꢀ¼5
1.00 ꢂ 10ꢀ3, [sugar] ¼ 1.00 ꢂ 10ꢀ2, [Ir(III)] ¼ 3.34 ꢂ 10
[Hg(OAc)2] ¼ 2.00 ꢂ 10ꢀ3, [KCl] ¼ 2.00 ꢂ 10ꢀ3 mol dmꢀ3
(A) Glu; (B) Fru
,
.
1:40
rate ¼ k½NBAꢁ½Hþꢁꢀ0:35½IrðIIIÞꢁ ½HgðIIÞꢁ
0:88
ð3Þ
ð4Þ
0:80
1:36
rate ¼ k½NBAꢁ½Hþꢁꢀ0:36½IrðIIIÞꢁ ½HgðIIÞꢁ
perchloric acid) was determined as ꢀ0.35 (Glu) and
ꢀ0.35 (Fru) from the slope of the plot of log J vs log[H]þ
(Fig. 3).
When the effects of additions of NHA and Clꢀ on the
rate of reaction are also taken into consideration, the rate
laws (3) and (4) will become
Successive addition of Hg(OAc)2 to the reaction mix-
ture increases the J (Table 2), which is observed to be
directly proportional to [Hg(II)]2 at low concentrations,
indicating a second-order reaction with respect to
[Hg(II)]. On moving from low to higher concentration
of Hg(II), a possible shift in order from two to one was
observed. The possible change in order with respect to
Hg(II) from two to one is also supported by the plot of
log J vs log[Hg(II)], where the order in the case of
glucose and fructose was found to be 1.40 and 1.36,
respectively. Since the clarification of the role of acet-
amide (NHA) and Clꢀ ions is significant in ascertaining
the reactive species of NBA and Ir(III)chloride, the
effects of variations in NHA and Clꢀ concentrations on
the apparent rate constant were also studied (Table 2).
1:40
rate ¼ k½NBAꢁ½Hþꢁꢀ0:35½IrðIIÞꢁ ½HgðIIÞꢁ
0:88
ꢂ ½NHAꢁꢀ0:28½Clꢀꢁꢀ0:62
ð5Þ
ð6Þ
0:80
1:36
rate ¼ k½NBAꢁ½Hþꢁꢀ0:36½IrðIIÞꢁ ½HgðIIÞꢁ
ꢂ ½NHAꢁꢀ0:32½Clꢀꢁꢀ0:48
The reactions were studied in the temperature range
303–318 K and the values of activation parameters such
¼
as energy of activation (Ea), entropy of activation (ÁS )
and enthalpy of activation (ÁH ) at 308 K are given in
Table 3. The identical kinetic results for the oxidation of
¼
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 249–256

252
A. K. SINGH ET AL.
Table 3. Rate constants and activation parameters at 308 K
¼
¼
k
Ea
ÁH
(kcal molꢀ1 a
ÁS
(e.u.)
Arrhenius factor, A
(molꢀ1 dm3 sꢀ1
Sugar
(molꢀ1 dm3 sꢀ1
)
(kcal molꢀ1 a
)
)
)
Glucose
Fructose
33.7
30.3
28.7
27.1
29.3
27.7
38.0
32.7
3.61 ꢂ 1021
0.25 ꢂ 1021
a kcal ¼ 4.184 kJ.
glucose and fructose suggest that the reactions follow a
common mechanism.
performed with and without the presence of Hg(II) under
identical conditions and it was observed that the rate of
oxidation of Glu in the absence of Hg(II) was greater
(6.60 ꢂ 10ꢀ8 mol dmꢀ3 sꢀ1) than that measured in the
presence of Hg(II) (10.10 ꢂ 10ꢀ8 mol dmꢀ3 sꢀ1). The
increase in rates in the absence of Hg(II) is due to parallel
oxidation of Br2, which is produced as a result of
interaction between NBA and Brꢀ formed in the reaction
DISCUSSION
In acidic medium NBA6 undergoes in the following
equilibria:
K1
MeCONHBr þ H2O Ð MeCONH2 þ HOBr
MeCONHBr þ H2O Ð MeCONH2 þ HOBr
HOBr þ Hþ Ð ½H2OBrꢁþ
ð7Þ
ð8Þ
ð9Þ
ð12Þ
ð13Þ
ð14Þ
ðNBAÞ ðNHAÞ
K2
MeCONHBr þ Hþ Ð ½MeCONH2Brꢁþ
2ꢀ
½IrCl6ꢁ3ꢀ þ H2O Ð ½IrCl5ðH2OÞꢁ þ Clꢀ
½MeCONH2Brꢁþ þ H2O Ð MeCONH2 þ ½H2OBrꢁþ ð10Þ
ðC1Þ
ðC2Þ
K3
þ
Perusal of the above two sets of equilibria [(7), (8) and
(9), (10)] shows that there can be four possible reactive
species, i.e. NBA itself, HOBr, [MeCONH2Br]þ and
[H2OBr]þ . In the light of the kinetic observations,
especially the order with respect to [NHA], HOBr can
be assumed to be the main reactive species of NBA for
þ
---
HgðIIÞ þ HOBr Ð ½Hg OBrꢁ þ H
ðC3Þ
2
3ꢀ
6
6
6
6
7
7
7
7
K4
Ð
2ꢀ
þ
---
½IrCl5ðH2OÞꢁ
ðC2Þ
½Hg OBrꢁ
---
þ
IrCl5 O Br þH2O
the present investigation.
6
6
4
7
7
5
Reversible interconversion of [IrCl6]3ꢀ and [Ir(OH2)
ðC3Þ
j
Hg
Cl5]2ꢀ via reaction (11) has long been known.16,17
A
kinetic study of hexachloro and aquopentachloro com-
plexes of iridium(III) in aqueous solutions has also been
reported.18
ðC4Þ
ð15Þ
2ꢀ
½IrCl6ꢁ3ꢀ þ H2O Ð ½IrCl5ðH2OÞꢁ þ Clꢀ
ð11Þ
2
3ꢀ
2
3þ
Hg
j
6
6
6
7
7
7
6
6
6
7
7
7
7
5
The observed negative effect of Clꢀ ions on the reaction
rate suggests that equilibrium (11) is favoured to left side
in acidic solution of Ir(III) chloride. On this basis it can
be safely assumed that the reactive species of Ir(III)
chloride is [IrCl5(H2O)]2ꢀ. This assumption also finds
support from earlier workers.18–20
k5
---
--- ---
IrCl5 O Br þHgðIIÞ ꢀ! Br O IrCl4
6
7
6
slow
4
5
4
j
"
---
Cl Hg
Hg
(C4)
(C5)
ð16Þ
Several experiments were performed with mercuric
acetate in the absence of oxidant i.e. NBA, and it was
observed that the reaction does not proceed. This shows
that mercuric acetate does not play any role as an oxidant.
However, only when experiments were performed with
NBA, Hg(II) and Ir(III) as catalyst, it was observed that
the reaction proceeds with measurable velocity. On the
basis of these observations and observed order (1.40 for
Glu and 1.36 for Fru) with respect to [Hg(II)], it can be
concluded that the zero velocity of uncatalysed reaction
is increased in the presence of Ir(III) as catalyst and
Hg(II) as co-catalyst, which is used in the reaction as a
scavenger for bromide ions. The experiments were also
k6
C5 þ S ꢀꢀꢀꢀ!HCOOH þ ½IrCl5ðH2OÞꢁ2ꢀ þ RCHO
fast=H2O
þ 2HgðIIÞ þ Brꢀ
ð17Þ
ð18Þ
where S stands for reducing sugar.
RCHO IꢀrðꢀIIꢀIÞꢀþꢀNꢀBꢀAꢀꢀþꢀHꢀg!ðIIÞ RCOOH
where R stands for C4H9O4.
Scheme 1
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 249–256

CATALYTIC OXIDATION OF REDUCING SUGARS BY NBA
253
as product. In order to eliminate the possibility of parallel
Br2 oxidation and to ensure pure NBA oxidation, Hg(II)
was used throughout the investigation by maintaining its
concentration either equal to or higher than that of NBA.
Considering the reactive species of Ir(III) and NBA and
with the help of above experimental findings, the prob-
able mechanism shown in Scheme 1 is proposed.
On the basis of the mechanism in Scheme 1, the rate in
terms of decrease in concentration of NBA can be
expressed as
According to Scheme 1, the total concentration of Ir(III)
can be expressed as
½IrðIIIÞꢁT ¼ ½C1ꢁ þ ½C2ꢁ þ ½C4ꢁ
ð26Þ
On substituting the values of [C2] and [C4] from Eqns
(21) and (24) in Eqn (26), we obtain
½IrðIIIÞꢁT½NHAꢁ½Hþꢁ½Clꢀꢁ
½C1ꢁ ¼
½NHAꢁ½Hþꢁð½Clꢀꢁ þ K2Þ þ K1K2K3K4½NBAꢁ½HgðIIÞꢁ
d½NBAꢁ
ð27Þ
rate ¼ ꢀ
¼ 2k5½C4ꢁ½HgðIIÞꢁ
ð19Þ
dt
From Eqns (25) and (27), the final rate law can be written
as
where 2 indicates the number of moles of NBA consumed
2
d½NBAꢁ
dt
2k5K1K2K3K4½NBAꢁ½IrðIIIÞꢁT½HgðIIÞꢁ
rate ¼ ꢀ
¼
ð28Þ
½NHAꢁ½Hþꢁð½Clꢀꢁ þ K2Þ þ K1K2K3K4½NBAꢁ½HgðIIÞꢁ
per mole of sugar. On applying the law of chemical
equilibrium to steps (12)–(15), we obtain the equations
According to Eqn (28), when a plot was constructed
between [Ir(III)]T ꢂ [Hg(II)]/rate and 1/[Hg(II)] or ½Clꢀꢁ,
a straight line having an intercept on the y-axis was
obtained for both reducing sugars. From the intercepts
and slopes of the straight lines, the value of K2 was
calculated as 2.68 ꢂ 10ꢀ4M (Glu) and 3.02 ꢂ 10ꢀ4M (Fru)
½NHAꢁ½HOBrꢁ
K1 ¼
ð20Þ
ð21Þ
ð22Þ
ð23Þ
½NBAꢁ
½C2ꢁ½Clꢀꢁ
½C1ꢁ
ꢀ
ꢃ
at 35 C. With these values of K2 and ½Cl ꢁ used in our
experiments [(2.00–10.00) ꢂ 10ꢀ3 mol dmꢀ3], the in-
equality ½Clꢀꢁ ꢄK2 can be assumed as valid and hence
Eqn (28) will be reduced to
K2 ¼
½C3ꢁ½Hþꢁ
K3 ¼
½HgðIIÞꢁ ½HOBrꢁ
2
½C4ꢁ
d½NBAꢁ
dt
2k5K1K2K3K4½NBAꢁ½IrðIIIÞꢁT½HgðIIÞꢁ
½NHAꢁ½Hþꢁ½Clꢀꢁ þ K1K2K3K4½NBAꢁ½HgðIIÞꢁ
ð29Þ
K4 ¼
rate ¼ ꢀ
¼
½C2ꢁ ½C3ꢁ
From Eqns (20)–(23), we can obtain
The total concentration of NBA can be expressed as
K1K2K3K4½C1ꢁ½NBAꢁ½HgðIIÞꢁ
½C4ꢁ ¼
ð24Þ
½NHAꢁ½Hþꢁ½Clꢀꢁ
½NBAꢁT ¼ ½NBAꢁ þ ½HOBrꢁ þ ½C3ꢁ þ ½C4ꢁ
ð30Þ
Substituting the value of [C4] from Eqn (24) in Eqn (19),
we have
On substituting the values of [HOBr], [C3] and [C4] from
Eqns (20), (22) and (24) in Eqn (30) and on assuꢀming
the inequalities ½Clꢀꢁ ꢄ K2 and [NHA] ½Hþꢁ½Cl ꢁ ꢄ
K1K2K3K4 [NBA] [Hg(II)], we have
2
d½NBAꢁ 2k5K1K2K3K4½C1ꢁ½NBAꢁ½HgðIIÞꢁ
rate ¼ ꢀ
¼
dt
½NHAꢁ½Hþꢁ½Clꢀꢁ
ð25Þ
½NBAꢁT½NHAꢁ½Hþꢁ½Clꢀꢁ
½NBAꢁ ¼
ð31Þ
½NHAꢁ½Clꢀꢁ½Hþꢁ þ K1½Hþꢁ½Clꢀꢁ þ K1K3½HgðIIÞꢁ½Clꢀꢁ þ K1K2K3K4½IrðIIIÞꢁT½HgðIIÞꢁ
Considering Eqns (29) and (31), the final rate law can be
obtained as
2
d½NBAꢁ
2k5K1K2K3K4½NBAꢁT½IrðIIIÞꢁT½HgðIIÞꢁ
rate ¼ ꢀ
¼
dt
½NHAꢁ½Clꢀꢁ½Hþꢁ þ K1½Hþꢁ½Clꢀꢁ þ K1K3½HgðIIÞꢁ½Clꢀꢁ þ K1K2K3K4½IrðIIIÞꢁT½HgðIIÞꢁ þ K1K2K3K4½NBAꢁT½HgðIIÞꢁ
ð32Þ
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 249–256

254
A. K. SINGH ET AL.
Significant values of the terms K1K2K3K4[Ir(III)]
the reactants
[Hg(II)] and K1K2K3K4 [Hg(II)][NBA]T at very high con-
centrations of Hg(II), NBA, Ir(III) and ½Clꢀꢁ and at very
low concentrations of Hþ and NHA in the denominator of
rate law (32) clearly demonstrate that order of reaction
with respect to Hg(II) is much less than two, whereas in
NBA and Ir(III) it is less than one. It also explains the
negative fractional order in Clꢀ, Hþ ions and NHA.
Negligible values of K1K2K3K4[Ir(III)][Hg(II)] and
K1K2K3K4[Hg(II)][NBA]T at very low concentrations of
Hg(II), ꢀNBA and Ir(III) and at very high concentrations of
Hþ, Cl and NHA clearly support the nearly second-
order kinetics with respect to Hg(II), first-order kinetics
with respect to both NBA and Ir(III), the negative frac-
tional order in both Hþ and NHA and inverse order in Clꢀ
ions. Since experiments on the variation of chloride ion
concentration were performed at moderate and constant
concentrations of all other reactants, the order with
respect to ½Clꢀꢁ was found to be ꢀ0.62 (Glu) and
ꢀ0.43 (Fru) instead of ꢀ1.00 as indicated by the rate
law (32) at low concentrations of Hg(II), NBA and Ir(III)
and at high concentrations of Hþ and NHA. In this way,
we find that there is great resemblance between the
derived rate law (32) and the experimental rate laws (5)
and (6) based on the kinetic observations.
2
4
3ꢀ
5
:Hg
j
IrðCl5ÞðOBrÞ
and Hg2þ ions. The positive entropy of activation ob-
served in the oxidation of Glu and Fru is due to desolva-
tion of the activated state rather than the reactants. The
same order of the frequency factor for both reducing
sugars also indicates the operation of a single mechanism
in the Ir(III)-catalysed oxidation of the sugars by NBA in
the presence of perchloric acid.
Efforts were also made to compare the findings in this
work with results already reported13,14 for the Pd(II)-
catalysed oxidation of D-arabinose (Ara), D-xylose (Xyl)
and D-galactose (Gal) by NBS and with Ru(III) and
Hg(II) co-catalysed oxidation of D-galactose (Gal) and
D-ribose (Rib) by NBA. The observed second-order
kinetics with respect to Hg(II) at low concentrations
and first-order kinetics at higher concentrations clearly
distinguish the present investigation from the other two
where the order with respect to [Hg(II)] was found to be
either first tending towards zeroth14 or zeroth order13
throughout [Hg(II)] variations. On the basis of zeroth
order in [Hg(II)], it is concluded that the role of Hg(II) is
limited only up to its function as a Brꢀ ion scavenger in
the Pd(II)-catalysed oxidation of reducing sugars. In
contrast to this finding, the reported first-order kinetics
which tend to zeroth order at higher [Hg(II)] in the
Ru(III)-catalysed oxidation led the authors to assume
that the role of Hg(II) is not limited up to its function
as a Brꢀ ion scavenger but that it also takes part in the
reaction prior to the rate-determining step. The proposed
reaction path14 shows the interaction of Hg(II) with the
reactive species of NBA, i.e. HOBr, to form the complex
½Hg OBrꢁþ which ultimately reacts in the form of
Equation (32) can also be written as.
1
½NHAꢁ½Clꢀꢁ½Hþꢁ
¼
2
rate
2k5a½NBAꢁT½IrðIIIÞꢁT½HgðIIÞꢁ
½Hþꢁ½Clꢀꢁ
þ
2
2k5b½NBAꢁT½IrðIIIÞꢁT½HgðIIÞꢁ
½Clꢀꢁ
þ
þ
þ
2k5c½NBAꢁT½IrðIIIÞꢁT½HgðIIÞꢁ
1
2k5½NBAꢁT½HgðIIÞꢁ
1
ð33Þ
2k5½IrðIIIÞꢁT½HgðIIÞꢁ
where a ¼ K1K2K3K4, b ¼ K2K3K4 and c ¼ K2K4.
with the complex [RuCl5S]2ꢀ as most reactive oxidizing
species in the rate-determining step. In Scheme 1, Hg(II),
in addition to its role as a Brꢀ ion scavenger, reacts with
HOBr, the reactive species of NBA, to form the complex
---OBrꢁþ
According to Eqn (33), if a plot is made of 1/rate versus
1/[NBA]T, ½Clꢀꢁ or [NHA], then a straight line having
positive intercept on the y-axis should be obtained. When
1/rate values are plotted against 1/[NBA]T, ½Clꢀꢁ and
[NHA], straight lines with positive intercepts on the y-
axis were obtained, which proves the validity of rate Eqn
(32) and hence the proposed mechanism. Linear plots
with positive intercepts on the 1/rate axis clearly demon-
strate the retarding effect of [NBA] and decreasing effect
of ½Clꢀꢁ and [NHA] on the rate. Owing to the rate
equation being complicated, it was not possible to calcu-
late individual constants shown in the mechanistic steps.
In the present study of the oxidation of reducing sugars
by NBA in the presence of Ir(III) as a homogeneous
catalyst, the activated state will be less polar caused by
½Hg
showing a resemblance to the Ru(III)-cata-
lysed oxidation14 but its further involvement in the rate-
determining step to form the transient complex C5, i.e.
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 249–256

CATALYTIC OXIDATION OF REDUCING SUGARS BY NBA
255
clearly distinguishes the present study from the other two
studies reported earlier.13,14
kinetic data which led the authors to assume that HOBr
is the reactive species of NBS. Needless to say, the
oxidation rate is unaffected by the addition of succini-
mide. In this way it can be concluded that HOBr is the
reactive species of both NBA and NBS in the Ru(III)-,
Ir(III)- and Pd(II)-catalysed oxidation of reducing su-
gars. A decreasing effect of ½Hþꢁ was observed in all
three catalysed reactions. The change in order of with
respect to [Ir(III)] from first to zeroth at higher concen-
trations of Ir(III) shows that Ir(III) participants in the
reaction in a different way to Ru(III) and Pd(II), where
first-order kinetics were observed throughout their con-
centration variation.
The positive entropy of activation observed in both
Ru(III)- and Ir(III)-catalysed oxidation of reducing su-
gars by NBA indicates that the activated state will be less
polar than the reactants whereas the negative entropy of
activation in the case of Pd(II) shows that the activated
complex would be more extensively solvated than the
reactant species.
The present study differs entirely from the other two
studies13,14 with respect of the order in [sugar]. The
fractional positive effect of sugar concentration on the
rate in both cases supports the contention that the sugar
molecule with its active participation in the reaction
forms a complex with the catalyst in one case14 and
2ꢀ
interacts with the most reactive species ½PdCl3OBrꢁ in
the other.13
The observed zero effect of sugar concentration can
be viewed in a different way by negating the possibility
of the formation of an active complex between the
reactive species of Ir(III) and the reducing sugar. The
negative effect of ½Clꢀꢁ on the rate constant of the reac-
2ꢀ
tion led us to assume that the species ½IrCl5ðH2OÞꢁ is
the reactive species of Ir(III) in the present investigation,
whereas on the basis of positive effect of the chloride
3ꢀ
ion, ½RuCl6ꢁ is reported to be the true reactive species
of ruthenium(III) chloride in the NBA oxidation of Gal
and Rib. ½PdCl4ꢁ2ꢀ, being the lone species in the solution
of PdCl2 and HCl, has been taken as the reactive species
of PdCl2 in NBS oxidation of reducing sugars.13 The
inverse fractional order in [acetamide] was the basis for
considering HOBr as the reactive species of NBA in
both Ir(III)- and Ru(III)-catalysed oxidation of reducing
sugars, but in Pd(II)-catalysed oxidation it is only the
The reaction scheme for the Ir(III) and Hg(II) co-
catalysed oxidation of reducing sugars by NBA is shown
in Scheme 2. This reaction scheme corresponds to the
oxidation of D-glucose, and earlier steps have already
been given Scheme 1.
DA similar mechanism can be drawn for the oxidation
of fructose.
ð34Þ
ð35Þ
ð36Þ
ð37Þ
Scheme 2
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 249–256

256
A. K. SINGH ET AL.
12. Rangappa KS, Raghavendra MP, Mahadevappa DS, Gowda DC.
Carbohydr. Res. 1998; 306: 57–67.
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J. Phys. Org. Chem. 2004; 17: 249–256