Pd(II)-catalysed and Hg(II)-co-catalysed oxidation of D-glucose and D-fructose by N-bromoacetamide in the presence of perchloric acid: A kinetic and mechanistic study

Article abstract of DOI:10.1016/j.carres.2005.11.012

The kinetics of Pd(II)-catalysed and Hg(II)-co-catalysed oxidation of d-glucose (Glc) and d-fructose (Fru) by N-bromoacetamide (NBA) in the presence of perchloric acid using mercury(II) acetate as a scavenger for Br^- ions have been studied. The results show first-order kinetics with respect to NBA at low concentrations, tending to zero order at high concentrations. First-order kinetics with respect to Pd(II) and inverse fractional order in Cl^- ions throughout their variation have also been noted. The observed direct proportionality between the first-order rate constant (k ₁) and the reducing sugar concentration shows departure from the straight line only at very higher concentration of sugar. Addition of acetamide (NHA) decreases the first-order rate constant while the oxidation rate is not influenced by the change in the ionic strength (μ) of the medium. Variation of [Hg(OAc)₂] shows a positive effect on the rate of reaction. The observed negative effect in H⁺ at lower concentrations tends to an insignificant effect at its higher concentrations. The first-order rate constant decreases with an increase in the dielectric constant of the medium. The various activation parameters have also been evaluated. The products of the reactions were identified as arabinonic acid and formic acid for both the hexoses. A plausible mechanism involving HOBr as the reactive oxidising species, Hg(II) as co-catalyst, and [PdCl₃·S]^-1 as the reactive Pd(II)-sugar complex in the rate-controlling step is proposed.

Full text of DOI:10.1016/j.carres.2005.11.012

Carbohydrate
RESEARCH
Carbohydrate Research 341 (2006) 397–409
Pd(II)-catalysed and Hg(II)-co-catalysed oxidation of D-glucose and
D-fructose by N-bromoacetamide in the presence of perchloric acid:
a kinetic and mechanistic study
Ashok Kumar Singh,* Jaya Srivastava, Shahla Rahmani and Vineeta Singh
Department of Chemistry, University of Allahabad, Allahabad 211002, India
Received 28 June 2005; accepted 4 November 2005
Available online 15 December 2005
Abstract—The kinetics of Pd(II)-catalysed and Hg(II)-co-catalysed oxidation of D-glucose (Glc) and D-fructose (Fru) by N-bromo-
acetamide (NBA) in the presence of perchloric acid using mercury(II) acetate as a scavenger for Brꢀ ions have been studied.
The results show first-order kinetics with respect to NBA at low concentrations, tending to zero order at high concentrations.
First-order kinetics with respect to Pd(II) and inverse fractional order in Clꢀ ions throughout their variation have also been noted.
The observed direct proportionality between the first-order rate constant (k1) and the reducing sugar concentration shows departure
from the straight line only at very higher concentration of sugar. Addition of acetamide (NHA) decreases the first-order rate con-
stant while the oxidation rate is not influenced by the change in the ionic strength (l) of the medium. Variation of [Hg(OAc)2] shows
a positive effect on the rate of reaction. The observed negative effect in H+ at lower concentrations tends to an insignificant effect at
its higher concentrations. The first-order rate constant decreases with an increase in the dielectric constant of the medium. The var-
ious activation parameters have also been evaluated. The products of the reactions were identified as arabinonic acid and formic acid
for both the hexoses. A plausible mechanism involving HOBr as the reactive oxidising species, Hg(II) as co-catalyst, and [PdCl3ÆS]ꢀ1
as the reactive Pd(II)–sugar complex in the rate-controlling step is proposed.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Keywords: Kinetics; Glucose; Fructose; Pd(II)-catalysis; Hg(II)-co-catalysis; N-bromoacetamide; HClO4 medium
1. Introduction
kinetics of redox reactions incorporating certain transi-
tion metal ions, viz., Os(VIII),4,6 Ru(III),5,7 Ru(VIII)8
and Ir(III)9,10 as homogeneous catalysts have been
extensively investigated. Reactions involving Pd(II)11,12
have received limited attention. In view of scant work
on the mode of NBA oxidation with biologically impor-
tant substrates like carbohydrates, there seems to be
much exciting chemistry in further probing the oxidative
capacity of NBA in the oxidation of reducing sugars,
particularly in the presence of Pd(II). Palladium chloride
is the most important Pd(II) salt in the homogeneous
catalytic chemistry of palladium. Because of the com-
mercial importance of the reaction, the kinetics of the
oxidation of ethylene by aqueous Pd(II) chloride were
studied by Henry in 1964.13 Palladium(II) is a well-
known catalyst for several organic transformations.
Some interesting results in the homogeneous catalysis
of reactions of organic compounds, particularly the
N-Haloimides have attained a distinguished place
among oxidising agents for their specific oxidising char-
acter. Amongst them, N-bromoacetamide (NBA) has
been widely used as an oxidising and brominating
reagent1 in preparative organic chemistry, as well as an
analytical reagent to determine the concentration of var-
ious organic and inorganic compounds. Recently, con-
siderable attention has been focused on the diverse
nature of the chemistry of N-halo compounds owing
to their ability to act as a source of halogenonium cat-
ions, hypohalite species and nitrogen anions that act
as both bases and nucleophiles. NBA as an oxidant
has been used for some uncatalysed2,3 processes. The
*
Corresponding author. Tel.: +91 532 2640434; e-mail: ashokeks@
rediffmail.com
0008-6215/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2005.11.012

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A. K. Singh et al. / Carbohydrate Research 341 (2006) 397–409
successful commercial exploitation of the Wacker one-
stage process for the homogeneous catalytic oxidation
of acetaldehyde in the presence of Pd(II) chloride have
contributed to this interest. In the present paper, our
main aim is to ascertain the oxidative mode of NBA
with reducing sugars, the role of reducing sugars in the
redox system, and to explore catalytic potential of Pd(II)
with NBA as an oxidant.
The products were found to be identical under both
stoichiometric and kinetic conditions and also by a spot
test14 and thin-layer chromatography (TLC). Although
arabinonic acid, which is a strong reducing agent, is
formed in the reaction, the possibility of its further oxi-
dation by NBA under our experimental conditions is
completely ruled out.
3. Results and discussion
3.1. Kinetics
2. Experimental
2.1. Materials
A kinetic study of oxidation of Glc and Fru by NBA in
the presence of Pd(II) as catalyst and Hg(OAc)2 as the
Brꢀ ion scavenger under the conditions [sugar] ꢁ [N-
BA] was made in perchloric acid at several reactant con-
centrations. The value of the initial rate in each kinetic
run was calculated from the slope of the tangent of plot
of unconsumed [NBA] versus time at fixed [NBA] except
in [NBA] variation when a tangent has been drawn at a
fixed time. The value of the first-order rate constant (k1)
in each kinetic run was calculated as
An aqueous solution of N-bromoacetamide (NBA) was
prepared afresh each day by direct weighing, and its
strength was checked by iodometric titration of active
bromine. Aqueous solutions of glucose and fructose
were also prepared fresh each day. The solution of
PdCl2 (Qualigen ꢀGlaxoꢁ Chem.) was prepared in HCl
of the known strength. The overall strength of HCl
was maintained at 22.56 · 10ꢀ3 mol dmꢀ3, and the
strength of PdCl2 was 5.64 · 10ꢀ3 mol dmꢀ3. All other
reagents, namely Hg(OAc)2, NaClO4, HClO4, acetamide
(NHA) and KCl were prepared from samples obtained
from E. Merck. All the solutions were prepared with
double-distilled water.
ðꢀdc=dtÞ
k1 ¼
½NBAꢂ
The first-order dependence of the reaction on NBA at
its lower concentration tends to zero order at its higher
concentrations as demonstrated by the plot of ꢀdc/dt
versus [NBA] (Fig. 1). This result is further verified by
2.2. Kinetic measurements
The requisite volumes of standard solutions of NBA,
Pd(II), HClO4, Hg(OAc)2, KCl, NHA and NaClO4 were
taken in a black-coated reaction vessel and equilibrated
at 40 ꢁC. A measured volume of sugar solution, also
equilibrated separately at the same temperature, was
poured into the reaction vessel, and the reaction was
started by vigorous shaking of the reaction mixture.
The progress of the reaction was monitored iodometri-
cally by estimating the amount of the remaining NBA
at regular time intervals.
12
10
8
Fru
Glc
6
4
2
2.3. Stoichiometry and product analysis
0
0
5
10
15
20
25
30
35
[NBA]×104 (mol dm-3)
Various sets of experiments were performed with differ-
ent [NBA]/[sugar] ratios under kinetic conditions for
72 h. Determination of unconsumed NBA revealed that
for the oxidation of each mole of sugar 2 mol of NBA
were required. Accordingly, the following stoichiometric
equations could be formulated:
Figure 1. [PdCl2] = 28.20 · 10ꢀ6 mol dmꢀ3
,
[sugar] = 2.50 · 10ꢀ2
mol dmꢀ3 (Glc) and 2.00 · 10ꢀ2 mol dmꢀ3 (Fru), [HClO4] = 13.33 ·
10ꢀ3 mol dmꢀ3
10ꢀ4 mol dmꢀ3
,
,
[Hg(OAc)2] = 3.00 · 10ꢀ3 mol dmꢀ3
[NHA] = 3.50 · 10ꢀ3 mol dmꢀ3
,
[KCl] = 4.00 ·
,
l = 3.50 · 10ꢀ2
mol dmꢀ3
.
2 MeCONHBr + C H O + 2 H O
C H O + 2 MeCONH + 2 HBr + HCO H
5 10 6 2 2
→
→
6
12
6
2
(NBA)
(Glc)
(arabinonic acid)
2 MeCONHBr + C6H12O6 + 2 H 2O
(NBA) (Fru)
C5H10O6 + 2 MeCONH2 + 2 HBr + HCO2H
(arabinonic acid)

A. K. Singh et al. / Carbohydrate Research 341 (2006) 397–409
399
Table 1. Effect of variation of [NBA] and [sugar] on the first-order rate
16
14
12
10
8
constant (k1) at 40 ꢁCa
[NBA] · 104 (mol dmꢀ3
Fru
Glc
)
[sugar] · 102 (mol dmꢀ3
)
k1 · 105
(sꢀ1
)
Glc Fru
2.50
3.75
5.00
6.25
7.50
8.75
10.00
12.50
15.00
17.50
20.00
22.50
8.33Glc/10.00Fru
8.33Glc/10.00Fru
8.33Glc/10.00Fru
8.33Glc/10.00Fru
8.33Glc/10.00Fru
8.33Glc/10.00Fru
8.33Glc/10.00Fru
8.33Glc/10.00Fru
8.33Glc/10.00Fru
8.33Glc/10.00Fru
8.33Glc/10.00Fru
8.33Glc/10.00Fru
8.33Glc/10.00Fru
8.33Glc/10.00Fru
8.33Glc/10.00Fru
2.50Glc/2.00Fru
2.50Glc/2.00Fru
2.50Glc/2.00Fru
2.50Glc/2.00Fru
2.50Glc/2.00Fru
2.00Fru
2.50Glc/2.00Fru
2.50Glc/2.00Fru
2.50Glc/2.00Fru
2.50Glc/2.00Fru
2.50Glc/2.00Fru
2.50Glc/2.00Fru
1.00
1.25
1.50
1.66
2.00
2.50
3.00
3.33
4.00
4.78
4.92
4.88
4.91
4.95
—
4.92
4.77
4.28
3.93
3.62
3.30
1.33
2.29
—
2.66
3.51
4.55
4.85
5.55
6.25
—
5.54
5.54
5.70
5.32
5.33
5.39
5.45
5.32
5.37
5.09
4.63
4.38
2.25
—
3.47
—
5.00
5.18
6.02
—
7.94
9.12
6
4
2
0
0
3
6
9
12
[Sugar]×102 (mol dm-3)
Figure 2. [NBA] = 8.33 · 10ꢀ4 mol dmꢀ3 (Glc) and 10.00 · 10ꢀ4
mol dmꢀ3 (Fru), [PdCl2] = 28.20 · 10ꢀ6 mol dmꢀ3, [HClO4] = 13.33 ·
10ꢀ3 mol dmꢀ3
10ꢀ4 mol dmꢀ3
,
,
[Hg(OAc)2] = 1.25 · 10ꢀ3 mol dmꢀ3
,
[KCl] = 4.00 ·
[NHA] = 9.11 · 10ꢀ4 mol dmꢀ3 (Glc), 11.11 · 10ꢀ4
mol dmꢀ3 (Fru), l = 3.50 · 10ꢀ2 mol dmꢀ3
.
the constant values of the first-order rate constant, k1, in
the lower range of [NBA] and decreasing values of k1
in the higher range of [NBA] (Table 1). Since through-
out the study the concentration of NBA was fixed in
its lower concentration range, for the purpose of calcu-
lation of the first-order rate constant, k1, the order with
respect to [NBA], has been taken as unity. Figure 2
shows first-order kinetics with respect to each sugar at
lower concentration, but the reaction tends to zero order
at higher concentrations of sugar. The plots of k1
against [Pd(II)] were linear passing through origin,
suggesting first-order dependence of the rate of reaction
on the [Pd(II)]. At the same time, it also shows that the
reaction does not proceed with measurable velocity in
the absence of Pd(II) (Table 2, Fig. 3). The reactions
4.50
5.00
6.00
7.00
8.00
10.00
7.25 10.96
8.90 12.95
—
9.35 14.25
9.75 14.95
13.98
a Conditions: [PdCl2] = 28.20 · 10ꢀ6 mol dmꢀ3, [Hg(OAc)2] (Glc and
Fru) in [NBA] variation and 1.25 · 10ꢀ3 mol dmꢀ3 (Glc and Fru),
[KCl] = 4.00ꢀ·310ꢀ4 mol dmꢀ3 (Glc and Fru), [NHA] = 3.50 ·
10ꢀ3 mol dm
(Glc and Fru) for [NBA] variation and 9.11 ·
10ꢀ4 mol dmꢀ3 (Glc), 11.11 · 10ꢀ4 mol dmꢀ3 (Fru), and l = 3.50 ·
10ꢀ2 mol dmꢀ3 (Glc and Fru).
Table 2. Comparison of observed rates in the variation of [PdCl2] with the calculated ratesa
[PdCl2] · 106 (mol dmꢀ3
)
ꢀdc/dt · 108 (mol dmꢀ3 sꢀ1
)
D-Glucose
D-Fructose
Calculatedb
Experimental
Calculatedb
Calculatedc
Experimental
Calculatedc
5.64
0.85
2.08
2.78
3.72
4.13
4.97
5.53
6.23
7.40
8.33
—
0.85
1.70
2.54
3.38
4.23
4.65
5.08
5.92
6.77
7.62
—
0.82
1.61
2.38
3.15
4.29
4.68
5.43
6.18
6.94
7.92
—
1.05
2.02
2.78
4.04
5.18
5.80
6.66
7.40
8.07
9.25
10.24
11.11
0.96
1.92
2.82
3.84
4.79
5.27
5.75
6.71
7.62
8.63
9.59
10.35
0.99
1.98
2.98
3.98
4.98
5.48
5.49
6.99
7.99
9.00
10.01
11.02
11.28
16.92
22.56
28.20
31.02
33.84
39.48
45.12
50.76
56.40
62.04
—
—
—
a Conditions: [NBA] = 8.33 · 10ꢀ4 mol dmꢀ3 (Glc) and 10.00 · 10ꢀ4 mol dmꢀ3 (Fru), [sugar] = 2.50 · 10ꢀ2 mol dmꢀ3 (Glc) and 2.00 · 10ꢀ2
mol dmꢀ3 (Fru), [HClO4] = 13.33 · 10ꢀ3 mol dmꢀ3 (Glc and Fru), [Hg(OAc)2] = 1.25 · 10ꢀ3 mol dmꢀ3 (Glc and Fru), [KCl] = 4.00 · 10ꢀ4
mol dmꢀ3 (Glc and Fru), [NHA] = 9.11 · 10ꢀ4 mol dmꢀ3 (Glc) and 11.11 · 10ꢀ4 mol dmꢀ3 (Fru), l = 3.50 · 10ꢀ2 mol dmꢀ3 (Glc and Fru), and
t = 40 ꢁC.
b Calculated on the basis of rate law (14).
c Calculated with the help of multiple regression analysis.

400
A. K. Singh et al. / Carbohydrate Research 341 (2006) 397–409
30
25
20
15
10
5
Fru
12
10
8
Glc
6
4
Fru
Glc
2
0
0
0
10
20
30
40
50
60
70
1
2
3
4
5
6
7
8
9 10 12
[PdCl2]×106 (mol dm-3)
[HClO4]×103 (mol dm-3)
Figure 3. [NBA] = 8.33 · 10ꢀ4 mol dmꢀ3 (Glc) and 10.00 · 10ꢀ4
mol dmꢀ3 (Fru), [sugar] = 2.50 · 10ꢀ2 mol dmꢀ3 (Glc) and 2.00 ·
10ꢀ2 mol dmꢀ3 (Fru), [HClO4] = 13.33 · 10ꢀ3 mol dmꢀ3, [Hg(OAc)2] =
Figure 4. [NBA] = 8.33 · 10ꢀ4 mol dmꢀ3 (Glc) and 10.00 · 10ꢀ4
mol dmꢀ3 (Fru), [sugar] = 2.50 · 10ꢀ2 mol dmꢀ3 (Glc) and 2.00 ·
10ꢀ2 mol dmꢀ3 (Fru), [PdCl2] = 28.20 · 10ꢀ6 mol dmꢀ3, [Hg(OAc)2] =
1.25 · 10ꢀ3 mol dmꢀ3
,
[KCl] = 4.00 · 10ꢀ4 mol dmꢀ3
,
[NHA] =
9.11 · 10ꢀ4 mol dmꢀ3 (Glc) and 11.11 · 10ꢀ4 mol dmꢀ3 (Fru),
l = 3.50 · 10ꢀ2 mol dmꢀ3
1.25 · 10ꢀ3 mol dmꢀ3
,
[NHA] = 9.11 · 10ꢀ4 mol dmꢀ3 (Glc) and
11.11 · 10ꢀ4 mol dmꢀ3 (Fru), [KCl] = 4.00 · 10ꢀ4 mol dmꢀ3
3.50 · 10ꢀ2 mol dmꢀ3
,
l =
.
.
follow negative fractional order in [H+] up to nearly
8.00 · 10ꢀ3 mol dmꢀ3, but thereafter with the increase
in concentration of H+ ions, no significant change in
the first-order rate constant (k1) was observed (Table
3, Fig. 4). An increasing effect of [Hg(OAc)2] on the
rate constant of the reaction was observed (Table 4) as
indicating the involvement of Hg(II) as co-catalyst in
addition to its role as a Brꢀ ion scavenger.2,15,16 Succes-
sive addition of acetamide (NHA) (Table 5) and Clꢀ
(Fig. 5) shows a negative effect on the rates of reaction
in oxidation of both Glc and Fru. Change in ionic
strength of the medium did not bring about any change
in the rate of reaction. Change in dielectric constant of
the medium showed that on increasing the dielectric
constant of the medium, the rates were decreased.
The reactions were studied in the temperature range
of 303–318 K, and the values of activation parameters
such as energy of activation (Ea), entropy of activation
(DS#), enthalpy of activation (DH#) and Gibbs free
energy of activation (DG#) at 313 K are given in
Table 6. The identical kinetics results obtained for the
oxidation of Glc and Fru suggest that the reactions fol-
low a common mechanism.
Table 3. Effect of variation of [HClO4] and [KCl] on the first-order
rate constant (k1) at 40 ꢁCa
[HClO4] · 103 (mol dmꢀ3
)
[KCl] · 104 (mol dmꢀ3
)
k1 · 105 (sꢀ1
)
3.2. Test for free radicals
Glc Fru
To test for the presence of free radicals in the reaction, a
reaction mixture containing acrylamide was kept for
24 h in an inert atmosphere. When the reaction mixture
was diluted with methanol, the formation of a precipi-
tate was not seen. This suggests that there is no possibil-
ity of formation of free radicals in the reaction.
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
4.00
4.00
4.00
4.00
4.00
4.00
4.00
4.00
4.00
4.00
4.00
2.40
3.40
4.40
5.40
6.40
7.40
21.38 22.15
13.80 14.29
12.47 10.84
8.81
7.50
6.76
6.10
5.62
5.43
5.34
5.16
6.12
5.23
4.68
4.44
4.08
3.66
9.13
—
7.01
—
5.82
—
5.54
5.35
6.05
5.35
4.85
4.25
3.75
3.65
9.00
3.3. Suggested mechanism
10.00
12.00
13.33
13.33
13.33
13.33
13.33
13.33
It has been reported17 earlier by several workers
that NBA in acidic medium exists in the following
equilibria:
CH3CONHBr + H2O
(NBA)
CH3CONH2 + HOBr
(NHA)
a Condtions: [NBA] = 8.33 · 10ꢀ4 mol dmꢀ3 (Glc) and 10.00 ·
10ꢀ4 mol dmꢀ3 (Fru), [sugar] = 2.50 · 10ꢀ2 mol dmꢀ3 (Glc) and
2.00 · 10ꢀ2 mol dmꢀ3 (Fru), [PdCl2] = 28.20 · 10ꢀ6 mol dmꢀ3 (Glc
and Fru), [Hg(OAc)2] = 1.25 · 10ꢀ3 mol dmꢀ3 (Glc and Fru),
[NHA] = 9.11 · 10ꢀ4 mol dmꢀ3 (Glc) and 11.11 · 10ꢀ4 mol dmꢀ3
(Fru), and l = 3.50 · 10ꢀ2 mol dmꢀ3 (Glc and Fru).
ðiÞ
HOBr + Hþ ꢀ (H2OBr)þ
ðiiÞ

A. K. Singh et al. / Carbohydrate Research 341 (2006) 397–409
401
Table 4. Comparison of observed rates in the variation of [Hg(OAc)2] with the calculated ratesa
[Hg(OAc)2] · 103 (mol dmꢀ3
)
(ꢀdc/dt) · 108 (mol dmꢀ3 sꢀ1
)
D-Glucose
D-Fructose
Calculatedb
Experimental
Calculatedb
Calculatedc
Experimental
Calculatedc
0.90
1.00
1.11
1.50
1.75
2.00
2.25
2.50
3.00
3.50
4.00
5.00
6.00
7.00
3.32
3.64
3.75
4.92
—
5.35
—
—
5.95
—
7.05
7.45
8.04
8.52
3.42
3.66
3.91
4.66
—
5.39
—
—
6.41
—
7.07
7.54
7.92
8.16
3.35
3.52
3.69
4.27
—
4.89
—
—
5.92
—
6.78
7.53
8.21
8.83
—
—
4.89
5.45
6.90
7.17
7.63
8.05
8.45
9.46
—
—
—
—
4.62
5.54
6.07
6.56
7.03
7.48
8.33
9.11
4.38
5.40
5.97
6.48
6.94
7.36
8.09
8.71
9.24
10.10
10.77
11.31
10.25
10.96
12.05
12.35
9.86
11.23
12.50
13.69
a Conditions: [NBA] = 8.33 · 10ꢀ4 mol dmꢀ3 (Glc) and 10.00 · 10ꢀ4 mol dmꢀ3 (Fru), [sugar] = 2.50 · 10ꢀ2 mol dmꢀ3 (Glc) and
2.00 · 10ꢀ2 mol dmꢀ3 (Fru), [PdCl2] = 28.20 · 10ꢀ6 mol dmꢀ3 (Glc and Fru), [HClO4] = 13.33 · 10ꢀ3 mol dmꢀ3 (Glc and Fru),
[KCl] = 4.00 · 10ꢀ4 mol dmꢀ3 (Glc and Fru), [NHA] = 9.11 · 10ꢀ4 mol dmꢀ3 (Glc) and 11.11 · 10ꢀ4 mol dmꢀ3 (Fru), l = 3.50 · 10ꢀ2 mol dmꢀ3
(Glc and Fru), and t = 40 ꢁC.
b Calculated on the basis of rate law (14).
c Calculated with the help of multiple regression analysis.
Table 5. Comparison of observed rates in the variation of [NHA] with the calculated ratesa
[NHA] · 103 (mol dmꢀ3
)
(ꢀdc/dt) · 108 (mol dmꢀ3 sꢀ1
)
D-Glucose
D-Fructose
Calculatedb
Experimental
Calculatedb
Calculatedc
Experimental
Calculatedc
1.00
2.00
4.00
6.00
8.00
10.00
3.95
3.70
2.52
2.19
1.99
1.66
4.14
3.56
2.78
2.28
1.93
1.68
3.86
3.05
2.45
2.16
1.97
1.83
5.24
4.50
3.02
2.67
2.05
1.85
4.88
4.05
3.02
2.41
2.03
1.72
5.21
3.89
2.91
2.46
2.18
1.98
a Conditions: [NBA] = 8.33 · 10ꢀ4 mol dmꢀ3 (Glc) and 10.00 · 10ꢀ4 mol dmꢀ3 (Fru), [sugar] = 2.50 · 10ꢀ2 mol dmꢀ3 (Glc) and
2.00 · 10ꢀ2 mol dmꢀ3 (Fru), [PdCl2] = 28.20 · 10ꢀ6 mol dmꢀ3 (Glc and Fru), [HClO4] = 13.33 · 10ꢀ3 mol dmꢀ3 (Glc and Fru),
[Hg(OAc)2] = 1.25 · 10ꢀ3 mol dmꢀ3 (Glc and Fru), [KCl] = 4.00 · 10ꢀ4 mol dmꢀ3 (Glc and Fru), l = 3.50 · 10ꢀ2 mol dmꢀ3 (Glc and Fru), and
t = 40 ꢁC.
b Calculated on the basis of rate law (14).
c Calculated with the help of multiple regression analysis.
or
reported23 NBS oxidation of arabinose, xylose and
galactose in acidic medium. But the mechanistic path-
ways are different for NBA and NBS oxidations of
reducing sugars. HOBr, the reactive species in both
NBA and NBS oxidations, participates in the reaction
in an entirely different way. In the reported NBS oxida-
tion, PdCl24ꢀ, the reactive species of palladium(II) chlo-
ride, combines with HOBr, the reactive species of
CH3CONHBr + Hþ ꢀ (CH3CONH2Br)þ
ðiiiÞ
(CH3CONH2Br)þ + H2O ꢀ CH3CONH2 + (H2OBr)þ
ðivÞ
Perusal of the above equilibria (i–iv) shows that there
can be four possible reactive species, that is, NBA itself,
HOBr, (CH3CONH2Br)+ and (H2OBr)+. In the light of
the kinetic observations, especially the inverse fractional
order in [NHA], HOBr can be assumed to be the main
reactive species of NBA for the present investiga-
tion. Such an observation has also been noted in the
NBS, to form a complex of the type [PdCl3ÆHOBr]1ꢀ
,
which interacts with the sugar molecule in the form of
[PdCl3ÆOBr]2ꢀ in the rate-determining step. Contrary
to this, in the present investigation PdCl24ꢀ first forms
a complex with a sugar molecule with a composition
of [PdCl3ÆS]1ꢀ. This complex in the rate-determining

402
A. K. Singh et al. / Carbohydrate Research 341 (2006) 397–409
b4
8
6
4
2
0
Pd2þ þ 4Clꢀ ꢀ PdCl42ꢀ
ðixÞ
In most of the studies, using Pd(II) as the homoge-
neous catalyst, it has been employed in the form of
Pd(II) chloride. Pd(II) chloride, when dissolved in
hydrochloric acid, exists in the form of ( PdCl4)2ꢀ as
represented by equilibrium19 (viii). The existence of
Pd(II) chloride exclusively in the form of (PdCl4)2ꢀ is
also supported by Ayres,20 who has observed that when
a reaction ratio of 2:1 for sodium chloride (Clꢀ ion) to
palladium(II) chloride is maintained, the result is the
formation of well-known tetrachloropalladate(II),
(PdCl4)2ꢀ. From the UV spectrum of the solution con-
taining Pd(II) chloride, H+ and Clꢀ ions, it is clear that
there are two peaks, one at 240 and the other at 215 nm
(Fig. 6, Panel A, line 1). The peak observed at 240 nm
can be considered as due to the presence of (PdCl4)2ꢀ
in the solution, whereas the peak observed at 215 nm
can be considered as due to the presence of
PdCl3(H2O)ꢀ in solution. On the basis of values of kh
determined at various ionic strengths, it is reported21
that the hydrolysis of (PdCl4)2ꢀ takes place according
to the following equilibrium:
Fru
Glc
0
2
4
6
8
10
[Cl–]×104 (mol dm-3)
Figure 5. [NBA] = 8.33 · 10ꢀ4 mol dmꢀ3 (Glc) and 10.00 · 10ꢀ4
mol dmꢀ3 (Fru), [sugar] = 2.50 · 10ꢀ2 mol dmꢀ3 (Glc) and 2.00 ·
10ꢀ2 mol dmꢀ3 (Fru), [PdCl2] = 28.20 · 10ꢀ6 mol dmꢀ3
, [HClO4] =
13.33 · 10ꢀ3 mol dmꢀ3, [Hg(OAc)2] = 1.25 · 10ꢀ3 mol dmꢀ3, [NHA] =
9.11 · 10ꢀ4 mol dmꢀ3 (Glc) and 11.11 · 10ꢀ4 mol dmꢀ3 (Fru),
l = 3.50 · 10ꢀ2 mol dmꢀ3
.
step combines with HOBr and Hg(II) to form a most
reactive activated complex, of the composition
Kh
PdCl24ꢀ þ H2O ꢀ PdCl3ðH2OÞꢀ þ Clꢀ
ðxÞ
. In this case Hg(II), which is used in the
reaction as a scavenger for Brꢀ ions also acts as co-cat-
alyst. However, in NBS oxidation mercury(II) acetate
was found to have a limited role as a Brꢀ ion scavenger
only.
When spectra for Pd(II), H+, Clꢀ, sugar and NBA
solutions and Pd(II), H+, Clꢀ, sugar, NBA and Hg(II)
solutions were collected, it has been found, as will be
discussed in detail below, that the additional peak at
215 nm completely disappears, and the species
(PdCl3ÆS)ꢀ observed at 240 nm combines with the reac-
tive species of NBA to form another complex,
(PdCl3ÆSÆOBr)2ꢀ, which subsequently combines with
It is reported18 that Pd(II) chloride is rather insoluble
in aqueous solution but does dissolve in the presence of
Clꢀ ion as PdCl3(H2O)ꢀ and PdCl24ꢀ. The equilibrium
constants corresponding to the following equilibria have
been determined by several workers, and all are in fair
agreement with a value of logb4 between 11 and 12 at
25 ꢁC.
Hg(II) to form a complex of the type
. In
both the cases, an increase in absorbance was noted
(Fig. 6). This spectral information led us to conclude
that the species PdCl3(H2O)ꢀ initially observed in the
solution is converted into (PdCl4)2ꢀ with the formation
K1
Pd2þ þ Clꢀ ꢀ PdClþ
ðvÞ
ðviÞ
K2
PdClþ þ Clꢀ ꢀ PdCl2
K3
PdCl2 þ Clꢀ ꢀ PdClꢀ
ðviiÞ
ðviiiÞ
3
of complexes, (PdCl3ÆSÆOBr)2ꢀ
,
, and on
K4
PdClꢀ3 þ Clꢀ ꢀ PdCl42ꢀ
Elding has studied both the stability constants and
rates of reaction and determined values of logK1 to
logK4 and also of logb4. b4, is the equilibrium constant
for the following equilibrium:
the basis of this, it is reasonable to assume that
(PdCl4)2ꢀ is the reactive species of Pd(II) chloride in
the oxidation of Glc and Fru by NBA in the presence
of perchloric acid.
Table 6. Activation parameters of Pd(II)-catalysed oxidation of D-glucose and D-fructose by NBA in acidic mediaa
Reducing sugars
Ea (kJ molꢀ1
)
k (molꢀ1 dm3 sꢀ1
)
DS# (J Kꢀ1 molꢀ1
)
DH# (kJ molꢀ1
)
DG# (kJ molꢀ1
)
A (molꢀ1 dm3 sꢀ1
)
D-Glucose
D-Fructose
91.83
86.96
1.21
1.38
29.89
12.37
89.16
83.47
79.79
70.02
21.44 · 1014
2.63 · 1014
a Conditions: t = 40 ꢁC.

A. K. Singh et al. / Carbohydrate Research 341 (2006) 397–409
403
centrations was added to the solution of Pd(II), Clꢀ and
H+ ions, it was observed that with the increase in concen-
tration of sugar there is an increase in absorbance from
2.64 to 2.68 and 2.77 at 240 nm (Fig. 6, Panel B, lines
9, 10 and 3, respectively). This increase in absorbance
with the increase in sugar concentration is due to more
and more formation of a complex between reactive spe-
cies of Pd(II), that is, (PdCl4)2ꢀ and a sugar molecule
according to equilibrium (xi)
2ꢀ
½PdCl4ꢂ þ S ꢀ ½PdCl3 ꢃ Sꢂꢀ þ Clꢀ
ðxiÞ
The formation of [Pd(II)–sugar] complexes also finds
support from the observed negative effect of Clꢀ ion
concentrations on the first-order rate constant of the
reaction.
Further, when NBA solutions of concentrations
1.00 · 10ꢀ3 and 1.33 · 10ꢀ3 mol dmꢀ3 were added to
the solution of Pd(II), Clꢀ, H+ and reducing sugar, it
was again observed that with the increase in [NBA] there
is an increase in absorbance from 2.77 to 2.91 and
2.94 nm with a shift in kmax value towards longer wave-
length (248 nm) (Fig. 6, Panel B, line 10, Panel A, lines 4
and 5, respectively). This increase in absorbance can be
considered as due to the formation of a complex
between (PdCl3.S)ꢀ and the reactive species of NBA
according to equilibrium (xii)
Figure 6. (1) [NBA] = 1.00 · 10ꢀ3 mol dmꢀ3, (2) [Hg(OAc)2] = 1.00 ·
10ꢀ3 mol dmꢀ3 (3) [Pd(II)] = 5.64 · 10ꢀ4 mol dmꢀ3 [H+] = 2.25 ·
10ꢀ3 mol dmꢀ3, [Clꢀ] = 4.00 · 10ꢀ3 mol dmꢀ3 [sugar] = 0.2 mol dmꢀ3
(4) [Pd(II)] = 5.64 · 10ꢀ4 mol dmꢀ3 [H+] = 2.25 · 10ꢀ3 mol dmꢀ3
,
,
,
,
,
[Clꢀ] = 4.00 · 10ꢀ3 mol dmꢀ3
,
[sugar] = 0.2 mol dmꢀ3
(5) [Pd(II)] = 5.64 · 10ꢀ4 mol dmꢀ3
[Clꢀ] = 4.00 · 10ꢀ3 mol dmꢀ3
,
[NBA] =
,
[sugar] =
1.00 · 10ꢀ3 mol dmꢀ3
2.25 · 10ꢀ3 mol dmꢀ3
,
,
[H+] =
,
0.2 mol dmꢀ3 [NBA] = 1.33 · 10ꢀ3 mol dmꢀ3
,
(6) [Pd(II)] = 5.64 ·
10ꢀ4 mol dmꢀ3
,
[H+] = 2.25 · 10ꢀ3 mol dmꢀ3 [Clꢀ] = 4.00 ·
,
[PdCl3 ꢃ S]ꢀ + HOBr ꢀ [PdCl3 ꢃ S ꢃ OBr]2ꢀ + Hþ ðxiiÞ
10ꢀ3 mol dmꢀ3, [sugar] = 0.2 mol dmꢀ3 [NBA] = 1.00 · 10ꢀ3 mol dmꢀ3
[Hg(II)] = 1.25 · 10ꢀ3 mol dmꢀ3, (7) [Pd(II)] = 5.64 · 10ꢀ4 mol dmꢀ3
,
,
When Hg(II) solutions of concentrations 1.25 · 10ꢀ3
and 1.66 · 10ꢀ3 mol dmꢀ3 were added to the solution
of Pd(II), Clꢀ, H+, reducing sugar and NBA, it was very
clearly observed that with the addition of Hg(II) solu-
tion there was an increase in absorbance from 2.91 to
3.06 and 3.27 at 258 nm (Fig. 6, Panel A, lines 4, 6
and 7, respectively). The increase in absorbance with a
shift in kmax towards longer wavelength can be consid-
[H+] = 2.25 · 10ꢀ3 mol dmꢀ3, [Clꢀ] = 4.00 · 10ꢀ3 mol dmꢀ3, [sugar] =
0.2 mol dmꢀ3
10ꢀ3 mol dmꢀ3
10ꢀ3 mol dmꢀ3
[NBA] = 1.00 · 10ꢀ3 mol dmꢀ3
,
[Hg(II)] = 1.67 ·
,
,
(8) [Pd(II)] = 5.64 · 10ꢀ4 mol dmꢀ3
,
,
[H+] = 2.25 ·
[H+] = 2.25 ·
(9) [Pd(II)] = 5.64 · 10ꢀ4 mol dmꢀ3
10ꢀ3 mol dmꢀ3, [Clꢀ] = 4.00 · 10ꢀ3 mol dmꢀ3, (10) [Pd(II)] = 5.64 ·
10ꢀ4 mol dmꢀ3
,
[H+] = 2.25 · 10ꢀ3 mol dmꢀ3
,
[Clꢀ] = 4.00 · 10ꢀ3
mol dmꢀ3, [sugar] = 1.00 · 10ꢀ2 mol dmꢀ3
.
It is reported22a,12 that Pd(II) forms a complex with
allyl alcohol/reducing sugar. In order to verify the
existence of a Pd(II)–sugar complex and the formation
of other probable complexes, several experiments were
performed to collect the UV spectra at room temperature
of Pd(II) chloride, H+ and Clꢀ solutions, Pd(II), H+, Clꢀ
and sugar solutions, Pd(II), H+, Clꢀ, sugar and NBA
solutions and Pd(II), H+, Clꢀ, sugar, NBA and Hg(II)
solutions (Fig. 6). When spectra for Pd(II) chloride with
two different solutions of Clꢀ ions were collected, it was
found that with the increase in concentration of Clꢀ ions
there was a significant increase in absorbance from 2.55
to 2.64 at 240 nm (Fig. 6, Panel B, lines 8 and 9), whereas
there was an insignificant increase in absorbance at
215 nm. The peak observed at 215 nm, in addition to
the peak observed at 240 nm is due to the presence of
PdCl3(H2O)ꢀ in the solution of Pd(II) chloride, Clꢀ ions
and H+ ions. When a sugar solution of two different con-
ered as due to formation of
a new complex,
, which is the result of the combination
of the complex (PdCl3ÆSÆOBr)2ꢀ and the reactant Hg2+
according to equilibrium (xiii)
ðxiiiÞ
On the basis of the above discussion made for the
reactive species of NBA and Pd(II) chloride, spectral
information collected for the possible formation of com-
plex or complexes in the reaction and kinetic informa-
tion about the effect of [NBA], [substrate], [Pd(II)],
[H+], [Hg(II)], [Clꢀ], [acetamide], l and dielectric con-
stant (D) of the medium on the rate of reaction, the reac-
tions shown in Scheme 1 may be proposed.

404
A. K. Singh et al. / Carbohydrate Research 341 (2006) 397–409
K1
CH3CONHBr
(NBA)
+
S
H2O
CH3CONH2
(NHA)
+
+
HOBr
Cl −
(I)
k2
[PdCl3·S]−
[PdCl4]2−
(C1)
+
(II)
k−2
(C2)
k3
PdCl 3·S·OBr
[PdCl3 ·S]−
+
Hg2+
+
HOBr
(III)
(IV)
+
H+
k−3
Hg
(C3)
H
C
H
k4
slow and rate-
+ Hg2+
O−
[PdCl3(H2O)]−
+
PdCl3·S·OBr
+ H2O
R
C
determining step
OBr
OH
Hg
(C3)
where S stands for the sugar molecule and R stands for C4H9O4 for both Glc and Fru
H
C
H
C
fast
O−
+
H+
R
C
O
H
H
C
O
OH
(V)
+
+ HBr
R
O−Br
H−O
OH
C
NBA/Pd(II)/Hg(II)
H+
(VI)
R
C
O
H
O
R
H
O−Br
OH
C
(VII)
R
Br
R
C
O
OH
+
HBr
H
(Arabinonic acid)
[PdCl3(H2O)]−
+
Cl−
[PdCl4]2−
+
H2O
(VIII)
Scheme 1.
On the basis of the above mechanism, the rate in
terms of disappearance of NBA can be expressed as
change in the first-order rate constant, k1, with the in-
crease in the concentration of H+ ions (Fig. 4). Since
for the observed order with respect to [NBA], [sugar],
[Pd(II)], [Hg(II)], [Clꢀ], [NHA] and for the effect of l
and dielectric constant (D) of the medium on the rate
of reaction, the concentration of H+ has been fixed at
13.33 · 10ꢀ3 M where no effect of [H+] on the rate of
reaction was observed; hence, the proposed mechanism
needs modification. With zero order in [H+], step (III)
d½NBAꢂ
rate ¼ ꢀ
¼ 2k4½C3ꢂ
ð1Þ
dt
On applying the law of chemical equilibrium to step (I)
and steady-state approximation to steps (II) and (III),
the rate law in terms of concentration of reaction com-
ponents can be derived as
d½NBAꢂ
rate ¼ ꢀ
dt
2K1k2k3k4½Sꢂ½HgðIIÞꢂ½NBAꢂ½PdðIIÞꢂT
¼
ðk ½Clꢀꢂ½NHAꢂ þ K1k3½HgðIIÞꢂ½NBAꢂÞðk ½Hþꢂ þ k4Þ þ ðk ½Hþꢂ þ k4Þk2½Sꢂ½NHAꢂ þ K1k2k3½Sꢂ½NBAꢂ½HgðIIÞꢂ
ꢀ2
ꢀ3
ꢀ3
ð2Þ
The above rate law is well in accordance with the ob-
served order with respect to each reactant (except in the
higher range of [H+]) taking part in the reaction. In the
present investigation, although the negative effect of
[H+] in Glc and Fru is observed but in the higher range
(10.00 · 10ꢀ3 mol dmꢀ3 and above), there is almost no
of Scheme 1 can be considered as an irreversible step,
and under this condition, steps (III) and (IV) of Scheme
1 will be shown as follows.
On the basis of (1) the reactions in Scheme 1, where
reaction step (III) is taken as irreversible and is the
rate-determining step, and (2) the stoichiometric equa-

A. K. Singh et al. / Carbohydrate Research 341 (2006) 397–409
405
. .
k3
PdCl3 S OBr
−
H+
.
(III)
(IV)
[PdCl3 S]
+ + HOBr
Hg2+
+
slow and rate-
determining step
Hg
(C3)
H
H
C
fast
+ Hg2+
O−
[PdCl3(H2O)]−
+
. .
PdCl3 S OBr
+ H2O
R
C
OBr
OH
Hg
(C3)
tions for both Glc and Fru, the rate in terms of decrease
in concentration of NBA, can be expressed as follows:
Total concentration of NBA, that is, [NBA]T can be
expressed as Eq. 11
d½NBAꢂ
½NBAꢂT ¼ ½NBAꢂ þ ½HOBrꢂ
ð11Þ
rate ¼ ꢀ
¼ 2k3½C2ꢂ½HgðIIÞꢂ½HOBrꢂ
ð3Þ
dt
On applying the law of chemical equilibrium to step (I),
we have
On substituting the value of [HOBr] from Eq. 4 to Eq.
11, we get Eq. 12
K1½NBAꢂ
½NBAꢂT½NHAꢂ
½HOBrꢂ ¼
ð4Þ
½NBAꢂ ¼
ð12Þ
½NHAꢂ
½NHAꢂ þ K1
On applying steady-state approximation to [C2], we get
On putting the value of [NBA] from Eq. 12 to Eq. 10,
the final rate law can be obtained as Eq. 13
d½C2ꢂ
O ¼
dt
d½NBAꢂ
¼ k2½C1ꢂ½Sꢂ ꢀ k ½C2ꢂ½Clꢀꢂ ꢀ k3½C2ꢂ½HgðIIÞꢂ½HOBrꢂ
rate ¼ ꢀ
dt
ꢀ2
2K1k2k3½Sꢂ½HgðIIÞꢂT½NBAꢂT½PdðIIÞꢂT
¼
ðK1 þ½NHAꢂÞðk ½Clꢀꢂþk2½SꢂÞþk3K1½NBAꢂ ½HgðIIÞꢂ
k2½C1ꢂ½Sꢂ
ꢀ2
T
or ½C2ꢂ ¼
ð5Þ
k
½Clꢀꢂ þ k3½HgðIIÞꢂ½HOBrꢂ
ꢀ2
ð13Þ
The rate law (13) is in complete agreement with our
experimental findings hence, the proposed mechanism
can be assumed as a valid one.
From Eqs. 4 and 5, we obtain Eq. 6
k2½C1ꢂ½Sꢂ½NHAꢂ
½C2ꢂ ¼
ð6Þ
½Clꢀꢂ½NHAꢂ þ K1k3½HgðIIÞꢂ½NBAꢂ
If k2[S] ꢁ kꢀ2[Clꢀ], then under this condition Eq. 13,
k
ꢀ2
will be reduced to Eq. 14
From Eqs. 3, 4 and 6, we obtain Eq. 7
d½NBAꢂ
d½NBAꢂ
rate ¼ ꢀ
rate ¼ ꢀ
dt
dt
2K1k2k3½Sꢂ½HgðIIÞꢂT½PdðIIÞꢂT½NBAꢂT
2K1k2k3½HgðIIÞꢂ½Sꢂ½NBAꢂ½C1ꢂ
¼
¼
ð7Þ
k
½Clꢀꢂ½NHAꢂ þ K1k3½HgðIIÞꢂ½NBAꢂ
k2K1½Sꢂ þ k2½Sꢂ½NHAꢂ þ k3K1½NBAꢂT½HgðIIÞꢂ
ꢀ2
ð14Þ
According to the proposed mechanism, the total concen-
tration of Pd(II), that is, [Pd(II)]T can be expressed as
On the other hand, if the inequality kꢀ2[Clꢀ] ꢁ k2[S] is
assumed as a valid one, then under this condition Eq.
13 will be reduced to Eq. 15
½PdðIIÞꢂT ¼ ½C1ꢂ þ ½C2ꢂ
ð8Þ
On substituting the value of [C2] from Eq. 6 to Eq. 8, we
obtain Eq. 9
d½NBAꢂ
rate ¼ ꢀ
dt
ðk ½Clꢀꢂ½NHAꢂþK1k3½HgðIIÞꢂÞ½PdðIIÞꢂ
ꢀ2
T
½C1ꢂ ¼
½Clꢀꢂ½NHAꢂþK1k3½HgðIIÞꢂ½NBAꢂþk2½Sꢂ½NHAꢂ
2K1k2k3½Sꢂ½HgðIIÞꢂ½NBAꢂT½PdðIIÞꢂT
k
ꢀ2
¼
K1k ½Clꢀꢂþk ½Clꢀꢂ½NHAꢂþk3K1½NBAꢂ ½HgðIIÞꢂ
ꢀ2
ꢀ2
T
ð9Þ
ð15Þ
On putting the value of [C1] from Eq. 9 to Eq. 7, we have
Eq. 10
Eq. 14 can also be written as Eq. 16
d½NBAꢂ
rate ¼ ꢀ
dt
½PdðIIÞꢂT
1
¼
2K1k2k3½HgðIIÞꢂ½Sꢂ½NBAꢂ½PdðIIÞꢂT
rate
2k3½HgðIIÞꢂ½NBAꢂT
¼
k
½Clꢀꢂ½NHAꢂþK1k3½HgðIIÞꢂ½NBAꢂþk2½Sꢂ½NHAꢂ
ð10Þ
ꢀ2
½NHAꢂ
1
þ
þ
ð16Þ
2k3K1½HgðIIÞꢂ½NBAꢂT 2k2½Sꢂ

406
A. K. Singh et al. / Carbohydrate Research 341 (2006) 397–409
10
8
According to Eq. 16, if a plot is made between
[Pd(II)]T/rate and 1/[Hg(II)] or [NHA], then a straight
line having a positive intercept on [Pd(II)]T/rate axis
should be obtained. When [Pd(II)]T/rate values are plot-
ted against 1/[Hg(II)] and [NHA], straight lines with po-
sitive intercepts on [Pd(II)]T/rate axis were obtained
(Figs. 7 and 8). This clearly proves the validity of rate
law (14) and hence the proposed reaction Scheme 1.
From the slopes and intercepts of straight lines the val-
ues of constants k2, k3 and K1 were calculated for both
the reducing sugars and are presented in Table 7. Using
the values of constants k2, k3 and K1 and the rate law
(14), the initial rates of the reaction, that is, ꢀd[NBA]/dt,
have been calculated for the variation of each [Pd(II)],
[Hg(II)] and [NHA] and found in close conformity with
the observed rates of reaction (Tables 2, 4 and 5). This
further confirms the validity of the rate law (14) and
hence the proposed reaction Scheme 1 where step (III)
has been treated as an irreversible step.
Glc
Fru
6
4
2
0
0
0.2
0.4
0.6
0.8
1
1.2
(1/[Hg(II)])×10-3 (mol-1 dm3)
Figure 7. [NBA] = 8.33 · 10ꢀ4 mol dmꢀ3 (Glc) and 10.00 · 10ꢀ4
mol dmꢀ3 (Fru), [sugar] = 2.50 · 10ꢀ2 mol dmꢀ3 (Glc) and 2.00 ·
10ꢀ2 mol dmꢀ3 (Fru), [PdCl2] = 28.20 · 10ꢀ6 mol dmꢀ3
,
[HClO4] =
, [NHA] =
13.33 · 10ꢀ3 mol dmꢀ3
,
[KCl] = 4.00 · 10ꢀ4 mol dmꢀ3
9.11 · 10ꢀ4 mol dmꢀ3 (Glc) and 11.11 · 10ꢀ4 mol dmꢀ3 (Fru),
l = 3.50 · 10ꢀ2 mol dmꢀ3
.
According to Eq. 17, when [Pd(II)]T/rate values are
plotted against [NHA] for both reducing sugars, straight
lines with positive intercepts on the [Pd(II)]T/rate axis
were obtained (Fig. 8). From the slopes of the straight
20
16
12
8
Glc
Fru
lines the value of k for each reducing sugar has been
ꢀ2
calculated and is presented in Table 7.
Further when kꢀ2 values obtained for the oxidation of
Glc and Fru were utilised for calculating the rate of reac-
tion according to Eq. 15, then the values of calculated
reaction rates show close resemblance with the observed
rates for the variation of [Clꢀ] and [sugar] at 40 ꢁC. The
values of calculated reaction rates at low concentration
of sugar are found to be very close to the experimental
rates of reaction because the rate law that has been used
for the calculation of the rate of reaction is valid only for
the condition where kꢀ2[Clꢀ] ꢁ k2[S].
4
0
0
2
4
6
8
10
12
[NHA]×103 (mol dm-3)
Figure 8. [NBA] = 8.33 · 10ꢀ4 mol dmꢀ3 (Glc) and 10.00 · 10ꢀ4
mol dmꢀ3 (Fru), [sugar] = 2.50 · 10ꢀ2 mol dmꢀ3 (Glc) and 2.00 ·
Since the order of reaction with respect to [NBA]
is unity in the range of low concentration of NBA,
under these conditions the inequality ([K1 + [NHA])
(kꢀ2[Clꢀ] + k2[S]) ꢁ k3K1[NBA]T[Hg(II)] can also be
assumed as valid one, and under these conditions the
rate law (13) will be reduced to rate law (18)
10ꢀ2 mol dmꢀ3 (Fru), [PdCl2] = 28.20 · 10ꢀ6 mol dmꢀ3
,
[HClO4] =
13.33 · 10ꢀ3 mol dmꢀ3, [Hg(OAc)2] = 1.25 · 10ꢀ3 mol dmꢀ3, [KCl] =
4.00 · 10ꢀ4 mol dmꢀ3, l = 3.50 · 10ꢀ2 mol dmꢀ3
.
and again Eq. 15 can also be written as Eq. 17
½PdðIIÞTꢂ
k
½Clꢀꢂ
ꢀ2
d½NBAꢂ
¼
rate ¼ ꢀ
rate
2k2k3½Sꢂ½HgðIIÞꢂ½NBAꢂT
dt
½Clꢀꢂ½NHAꢂ
1
2K1k2k3½Sꢂ½HgðIIÞꢂ½NBAꢂ ½PdðIIÞꢂ
k
ꢀ2
ðK1 þ ½NHAꢂÞðk ½ClꢀꢂTþ k2½SꢂÞ T
þ
þ
ð17Þ
¼
ð18Þ
2k3k2K1½Sꢂ½HgðIIÞꢂ½NBAꢂT 2k2½Sꢂ
ꢀ2
Table 7. Calculation of constants (k2, K1, k3 and kꢀ2) from the plots of [Pd(II)]T/rate versus 1/[Hg(II)] and [Pd(II)T/rate versus [NHA] for the
Pd(II)-catalysed oxidation of D-glucose and D-fructose by NBAa
Plots
Constants
D-Glucose
D-Fructose
k2 (dm3 molꢀ1 sꢀ1
)
7.27 · 10ꢀ2
14.28 · 10ꢀ2
½PdðIIÞꢂT
rate
½PdðIIÞꢂT
rate
1
versus
½HgðIIÞꢂ
versus ½NHAꢂ
K1 (mol dmꢀꢀ32 ꢀ1
)
2.66 · 10ꢀ3
1.63 · 103
4.55
2.41 · 10ꢀ3
1.40 · 103
7.14
k3 (dm6 mol
s
)
k
ꢀ2
(dm3 molꢀ1 sꢀ1
)
a Conditions: see Table 4 for [Hg(II)] variation and Table 5 for [NHA] variation.

A. K. Singh et al. / Carbohydrate Research 341 (2006) 397–409
407
Eq. 18 is valid only at low concentration of NBA for
1.4
1.3
1.2
1.1
1
Fru
Glc
the oxidation of both of the reducing sugars. Using Eq.
18 and utilising the values of constants determined ear-
lier, the values of the rate of reaction for the low concen-
trations of NBA in the oxidation of Glc and Fru have
been calculated and found to be in close agreement with
the observed rates. This gives an additional support to
the proposed reactions in Scheme 1.
1.2
1.3
1.4
(1/D) × 102
1.5
1.6
3.4. Multiple regression analysis
Figure 9. [NBA] = 8.33 · 10ꢀ4 mol dmꢀ3 (Glc) and 10.00 · 10ꢀ4
mol dmꢀ3 (Fru), [sugar] = 2.50 · 10ꢀ2 mol dmꢀ3 (Glc) and 2.00 ·
Multiple regression analysis has also been used to calcu-
late rates for the variation of each [Pd(II)], [Hg(II)] and
[NHA], and in each case it has been found that the reac-
tion rates calculated on the basis of equations of fitted
model for D-glucose and D-fructose are in close agree-
ment with observed rates and the rates calculated on
the basis of the rate law (14). This proves the validity
of rate law (14) and hence the proposed reactions in
Scheme 1. Rate equations, the use of which was made
to calculate rates on the basis of multiple regression
analysis for D-glucose and D-fructose, are (19) and
(20), respectively:
10ꢀ2 mol dmꢀ3 (Fru), [PdCl2] = 28.20 · 10ꢀ6 mol dmꢀ3
,
[HClO4] =
13.33 · 10ꢀ3 mol dmꢀ3, [Hg(OAc)2] = 1.25 · 10ꢀ3 mol dmꢀ3, [KCl] =
4.00 · 10ꢀ4 mol dmꢀ3 [NHA] = 9.11 · 10ꢀ4 mol dmꢀ3 (Glc) and
11.11 · 10ꢀ4 mol dmꢀ3 (Fru), l = 3.50 · 10ꢀ2 mol dmꢀ3
,
.
mechanism for the oxidation of both Glc and Fru is
obvious from the observed order of frequency factor,
which is the same for both of the reducing sugars.
3.6. Calculation of the size of the activated complex (dAB
)
0:72
0:98
0:47
k0 ¼ ½Sꢂ ½PdðIIÞꢂ ½HgðIIÞꢂ ½NHAꢂꢀ0:35½Clꢀꢂꢀ0:40 ð19Þ
In order to determine the effect of the dielectric constant
(D) of the medium on the rate of reaction, the reaction
has been studied with different dielectric constants for
the medium at constant concentration of all other reac-
tants and at a constant temperature of 40 ꢁC. The
change in dielectric constant of the medium has been
made by the addition of ethanol to the reaction mixture.
Although primary alcohols are oxidised by N-chlorosuc-
cinimide, which is treated as strong oxidant, it is repor-
ted22b that NBA or NBS fails to oxidise aliphatic
primary alcohols. It is also reported17 that in the catal-
ysed range of [H+] between 0.2 and 0.75 M, primary
alcohols are oxidised by N-bromoacetamide. Before
conducting experiments for the study of the effect of
dielectric constant of the medium on the rate of reaction,
we have performed experiments taking ethyl alcohol as
an organic substrate instead of reducing sugars in the
usual manner and found that ethyl alcohol under the
conditions of our experiments (at or below
13.33 · 10ꢀ3 mol dmꢀ3 H+ concentration) is not at all
oxidised by NBA in the presence of Pd(II) as the homo-
geneous catalyst. The decrease in the first-order rate
constant with the increase in dielectric constant of the
medium is observed. Since in the rate-determining step
of the mechanism (Scheme 1) proposed for the Pd(II)-
catalysed oxidation of Glc and Fru, the reaction is tak-
ing place between two ions, the calculation of dAB (size
of activated complex) has been made with the help of
the following equation:
0:78
1:01
0:58
k0 ¼ ½Sꢂ ½PdðIIÞꢂ ½HgðIIÞꢂ ½NHAꢂꢀ0:42½Clꢀꢂꢀ0:49 ð20Þ
d½NBAꢂ=dt
½NBAꢂ
for D-fructose.
where k0 ¼ ꢀ
d½NBAꢂ=dt
for D-glucose and k0 ¼
0:87
ꢀ
0:89
½NBAꢂ
3.5. 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
reaction takes place between two ions of opposite
charges, 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. But on
the other hand, when 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 would lead to a decrease in entropy. On the basis
of this information, the observed positive entropy of
activation in the oxidation of both Glc and Fru (Table
6) supports the rate-determining step (III) of the pro-
posed reactions in Scheme 1 where reaction is taking
place between two oppositely charged species
[PdCl3ÆS]1ꢀ and Hg2+, which while interacting with
2
~
HOBr form a neutral complex
. A uniform
ZAZBe N
1
log k ¼ log k0 ꢀ
ꢄ
ð21Þ
2:303ð4pe0ÞdABRT
D

408
A. K. Singh et al. / Carbohydrate Research 341 (2006) 397–409
From the slopes of the straight lines obtained in logk
versus 1/D plots (Fig. 9), the value of dAB has been cal-
culated and found as 6.10 and 5.01 A for Glc and Fru,
respectively. Positive slopes obtained for the oxidation
of both Glc and Fru clearly support the interaction
between two oppositely charged species resulting in the
formation of an activated complex of the type
whereas in the reported Ir(III)-catalysed oxidation of
reducing sugars by NBA, it varies from two to one
and in the Pd(II)-catalysed oxidation of reducing sugars
by NBS, its role is limited to that of a Brꢀ ion scavenger
only. First-order kinetics with respect to [Pd(II)]
throughout its variation in the oxidation of D-glucose
and D-fructose by NBA, as well as in the oxidation of
reducing sugars by NBS, is certainly different from the
first-order kinetics not usually observed at low concen-
tration of Ir(III), which changes to zero-order kinetics
at its high concentration in the oxidation of reducing
sugars by NBA in the presence of perchloric acid.
Although the reported negative fractional order in
[H+] for the Pd(II)-catalysed oxidation of reducing sug-
ars by NBS is similar to the observed negative fractional
order at low concentration of H+, a distinguishing fea-
ture in the present study is that at very high [H+], almost
no effect of [H+] on the rate of reaction is observed. Neg-
ative fractional order in Clꢀ ions and negligible effect of
l on the rate of reaction were observed in the present
case, as well as in the reported Pd(II)- and Ir(III)-catal-
ysed oxidation of reducing sugars.
˚
in the rate-determining step of the pro-
posed reactions in Scheme 1.
3.7. Primary salt effect
The negligible effect of l observed in Pd(II)-catalysed
oxidation of Glc and Fru also supports the rate-deter-
mining step of Scheme 1, where reaction is taking place
between two ions and a neutral molecule (step III).
3.8. Comparative studies
Efforts have also been made to compare the findings of
this paper with the results reported10,23 for the Ir(III)-
catalysed oxidation of reducing sugars by NBA in the
presence of perchloric acid and the Pd(II)-catalysed oxi-
dation of reducing sugars by N-bromosuccinimide
(NBS) in acidic solution. The present paper on the one
hand shows similarity with Ir(III)-catalysed10 oxidation
of reducing sugars being first to zero order with respect
to [NBA] in each case, and on the other hand it differs in
order with respect to [NBS], indicating first-order kinet-
ics throughout the variation of [NBS] in the Pd(II)-catal-
ysed23 oxidation of reducing sugars by NBS in the
presence of perchloric acid. In the present study order
with respect to reducing sugar has been found to be first
order at its lower concentration and zero order at its
very high concentration, whereas in the reported
Ir(III)-catalysed oxidation of reducing sugars by NBA,
order with respect to reducing sugar was found to be
zero, and as a result, participation of the reducing sugar
molecule was not shown in the proposed mechanism
before the rate-determining step. Due to order being
fractionally positive, with respect to reducing sugar con-
centration in the Pd(II)-catalysed oxidation of reducing
sugars by NBA or NBS, the participation of a reducing
sugar molecule in or before the rate-determining step
and its contribution to the reaction rate is in contrast
to the Ir(III)-catalysed oxidation of reducing sugars
where the derived rate law does not relate the rate of
reaction with the concentration of reducing sugar.
The present study entirely differs with the other two
studies10,23 as far as the order with respect to Hg(II) is
concerned. In the Pd(II)-catalysed oxidation of D-glu-
cose and D-fructose by NBA, order with respect to
[Hg(II)] has been found to be fractionally positive,
Positive entropy of activation observed in the present
case supports the interaction between two oppositely
charged species in the rate-determining step and shows
similarity with the reported Ir(III)-catalysed oxidation
of reducing sugars by NBA. But this is in contrast to
the negative entropy of activation observed in Pd(II)-
catalysed oxidation of reducing sugars by NBS. The dis-
tinct feature of the present study is that the proposed
mechanism is consistent with kinetic data and is sup-
ported by the spectra collected for the formation of
complexes during the course of reaction and also by
multiple regression analysis.
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