Kinetic and mechanistic investigation of Pd(II)-catalysed and Hg(II)-co-catalysed oxidation of d(+)melibiose by N-bromoacetamide in acidic medium

Article abstract of DOI:10.1016/j.jorganchem.2010.06.009

The kinetic and mechanistic study of homogeneously Pd(II)-catalysed and Hg(II)-co-catalysed oxidation of d(+)melibiose (mel) by N-bromoacetamide (NBA) in perchloric acid medium have been made at temperature 40 °C ± 0.1 °C. Kinetic results show first-order kinetics with respect to NBA at its low concentrations, tending to zero-order at its high concentrations. The oxidation rate is directly proportional to [Pd(II)] and [sugar], indicating first-order kinetics with respect to Pd(II) and sugar. Zero effect of Cl^- and H⁺ ions throughout their variation have been noted. First-order kinetics with respect to Hg(II) at its low concentration tends to zero-order at its higher concentration. Addition of acetamide (NHA) decreases the first-order rate constant (k₁) while the rate of reaction is not influenced by the change in ionic strength (μ) of the medium. The first-order rate constant decreases with an increase in dielectric constant of the medium. Arabinonic acid, lyxonic acid and formic acid were identified as oxidation products of the reaction. Various activation parameters including the entropy of activation were also calculated. A plausible mechanism conforming to kinetic data, spectrophotometric observations, reaction stoichiometry and product analysis has been proposed.

Full text of DOI:10.1016/j.jorganchem.2010.06.009

Journal of Organometallic Chemistry 695 (2010) 2213e2219
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Kinetic and mechanistic investigation of Pd(II)-catalysed and Hg(II)-co-catalysed
oxidation of
D
(þ)melibiose by N-bromoacetamide in acidic medium
*
Ashok Kumar Singh , Sarita Yadav, Rashmi Srivastava, Jaya Srivastava, Shahla Rahmani
Department of Chemistry, University of Allahabad, Allahabad-211002, UP, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The kinetic and mechanistic study of homogeneously Pd(II)-catalysed and Hg(II)- co-catalysed oxidation
Received 24 February 2010
Received in revised form
15 May 2010
Accepted 8 June 2010
Available online 17 June 2010
of
D
(þ)melibiose (mel) by N-bromoacetamide (NBA) in perchloric acid medium have been made at
temperature 40 ꢀC ꢁ 0.1 ꢀC. Kinetic results show first-order kinetics with respect to NBA at its low
concentrations, tending to zero-order at its high concentrations. The oxidation rate is directly propor-
tional to [Pd(II)] and [sugar], indicating first-order kinetics with respect to Pd(II) and sugar. Zero effect of
Clꢂ and Hþ ions throughout their variation have been noted. First-order kinetics with respect to Hg(II) at
its low concentration tends to zero-order at its higher concentration. Addition of acetamide (NHA)
decreases the first-order rate constant (k1) while the rate of reaction is not influenced by the change in
Keywords:
Kinetics
N-Bromoacetamide
ionic strength (m) of the medium. The first-order rate constant decreases with an increase in dielectric
constant of the medium. Arabinonic acid, lyxonic acid and formic acid were identified as oxidation
products of the reaction. Various activation parameters including the entropy of activation were also
calculated. A plausible mechanism conforming to kinetic data, spectrophotometric observations, reaction
stoichiometry and product analysis has been proposed.
D
(þ)melibiose
Pd(II)-catalysed oxidation
Perchloric acid
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
Thesestudies haveprompted usto probethecatalyticactivityofPd(II)
chloride in the oxidation of
oxidative mode of NBA along with the role of biologically important
reducing sugar (
D
(þ)melibiose and to ascertain the
N-Haloimides such as N-bromosuccinimide (NBS), N-bromoace-
tamide (NBA), N-bromophthalamide and N-chlorosuccinimide (NCS)
have been used as analytical reagents and also as very effective
oxidants due to their specific oxidizing character. The use of N-bro-
moacetamide (NBA) has been made as an oxidizing and brominating
reagent [1] in preparative organic chemistry and also to estimate
various organic and inorganic compounds. Reports are available
where NBA was used as an oxidant in presence of certain transition
metal ions viz Ir(III) [2,3], Ru(III) [4e8], Rh(III) [9,10] and Os(VIII)
[11,12]. Palladium-basedcatalysts [13]are found to havehighcatalytic
activity in clean deionized water. Tobias Schalow et al. [14] have
described the formation and catalytic activity of partially oxidized
palladium-nanoparticles. Their report clearly shows that oxide sup-
ported metal nanoparticles are commonly used as catalysts for
oxidationreactions inchemical industry, emissioncontroland energy
technology. Few reports [15e18] are also available where chloro-
complex of Pd(II) has been used as homogeneous catalyst in the
oxidation of monosaccharides using either NBA or NBS as an oxidant.
D
(þ)melibiose) in the redox system under investi-
gation. In this study our main aim was to ascertain whether-
(1) NBA as an oxidant in acidic medium behaves in the same way
as it behaved in the reported Ru(III) [5]- and Ir(III) [2]- cata-
lyzed oxidation of reducing sugars.
(2) HOBr is the reactive species of NBA in acidic medium in the
present investigation as is reported in Ru(III) [5]- and Ir(III) [2]-
catalyzed oxidation of reducing sugars by NBA in acidic
medium or not?
(3) The role of reducing sugar molecule in the presence of Pd(II)
chloride as catalyst and NBA as an oxidant in acidic medium is
similar to the reported role of reducing sugar in the presence of
Ir(III)/Ru(III) chloride and NBA in acidic medium.
(4) Pd(II) chloride in acidic medium participates in the reaction
under investigation in the same way as it is reported in Ru(III)
[5]- and Ir(III) [2]- catalyzed oxidation of reducing sugars by
NBA in acidic medium.
(5) The role of Hg(II) is similar to its reported role as Brꢂ ion
scavenger and co-catalyst in Ru(III) [5]- and Ir(III) [2]- catalyzed
oxidation of reducing sugars by NBA in acidic medium or not?
* Corresponding author. Tel.: þ91 532 2462266(O), þ91 532 2640434(R).
E-mail
addresses:
ashokeks@rediffmail.com,
ashok_au2@yahoo.co.in
(A.K. Singh).
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.06.009

2214
A.K. Singh et al. / Journal of Organometallic Chemistry 695 (2010) 2213e2219
(6) Effect of [Hþ] on the rate of oxidation in the present investi-
gation is same as is reported in Ru(III) [5]- and Ir(III) [2]-
catalyzed oxidation of reducing sugars or not?
3. Kinetic results
Kinetic study of Pd(II)-catalysed oxidation of
D
(þ)melibiose by
NBA in perchloric acid medium was made at 40 ꢀC ꢁ 0.1 ꢀC. Initial
rate (ꢂdc/dt) of reaction in each kinetic run was determined by the
slope of the tangent of the plot drawn between remaining [NBA]
and time. The method of initial rates was adopted to avoid possible
complications due to interference by products during the course of
reaction. The first-order rate constant (k1) was calculated by the
following equation:
(7) The formation of most reactive activated complex results by the
interaction of two oppositely charged species in the same way
as it is reported for Ru(III) [5]- and Ir(III) [2]- catalyzed oxida-
tion of reducing sugars by NBA in acidic medium.
(8) The role of Clꢂ ions and the effect of [NHA] on the rate of
oxidation in the present investigation are similar to the role
reported in Ru(III) [5]- and Ir(III) [2]- catalyzed oxidation of
reducing sugars by NBA in acidic medium or not?
ꢂdc=dt
(9) There isanenhancement inthe catalyticactivityof Pd(II) chloride
in comparison to the catalytic activities of Ru(III) and Ir(III) in the
oxidation of reducing sugars by NBA in acidic medium or not?
k1
¼
½NBAꢄ
Kinetic study of oxidation of
D
(þ)melibiose was made at several
initial concentrations of all reactants. The reaction under investi-
gation follows first-order kinetics with respect to N-bromoaceta-
mide at its low concentration, which tends to zero-order at its
higher concentration as shown by the plot of ꢂdc/dt vs [NBA]
(Fig. 1). This result is further verified by the constant values of k1 in
lower [NBA] which decreases at higher NBA concentrations
(Table 1). The order with respect to the [NBA] was taken as unity, as
throughout the study, the concentration of NBA was fixed in its
lower concentration. Straight line passing through the origin in the
plot of k1 vs [Pd(II)], indicates first-order dependence of reaction
rate on [Pd(II)] throughout its variation (Fig. 2). The first-order rate
constant (k1) increases with the increase in [mel], suggesting first-
order dependence of the reaction on [mel]. This observation is
further confirmed by the straight line passing through the origin in
the plot of k1 vs [mel] (Fig. 3, Table 1). Effect of [Hþ] on the rate of
reaction was found to be zero. Stoichiometric study indicates Brꢂ
ion as reaction product in the present investigation. During prog-
ress of reaction Brꢂ ion interacts with NBA to form molecular
2. Experimental
2.1. Materials
Stock solution of Palladium(II) chloride (Qualigens ‘Glaxo’ Chem.)
was prepared by dissolving its sample in known volume of hydro-
chloric acid. The overall strength of hydrochloric acid was maintained
at 11.71 ꢃ 10ꢂ2 M and the strength of Pd(II) chloride was
5.64 ꢃ 10ꢂ3 M. N-bromoacetamide (NBA) solution was prepared by
weighing and dissolving it in doubly distilled water every day. Stan-
dardization of NBA solution was done iodometrically by titrating it
against standard sodium thiosulphate (hypo) solution. Reducing
substrate i.e.,
D
(þ)melibiose solution was prepared daily by weighing
and dissolving it in doubly distilled water. The aqueous solution of
mercuric acetate (Loba chemicals) was acidified with 20% acetic acid
solution. All other reagents namely NaClO4, HClO4, acetamide and KCl
werepreparedfromtheirsamples(E. Merck)indoublydistilledwater.
bromine which sets parallel oxidation of
D
(þ)melibiose and this
complicates the kinetic study of
D(þ)melibiose by NBA in the
2.2. Kinetic measurements
presence of Pd(II). In order to ensure pure NBA oxidation, mercuric
acetate has been used which scavenges Brꢂ ions and thus obviates
parallel Br2 oxidation. Hg(II) has also been reported earlier to act as
an oxidant as well as co-catalyst. Therefore, here an attempt has
also been made to study the role of Hg(II) in addition to its role as
Brꢂ ions scavenger. Kinetic results of few experiments without NBA
but with Hg(II) under similar kinetic conditions indicated that
A black coated reaction vessel containing requisite volumes of
solution of NBA, HClO4, Pd(II) chloride, KCl, NaClO4 and Hg(OAc)2 was
placed in an electrically operated thermostatic water bath main-
tained at 40 ꢀC ꢁ 0.1 ꢀC for thermal equilibrium. Another conical
flask containing
D(þ)melibiose solution was also placed in the same
thermostat at the same temperature for thermal equilibrium.
Requisite amount of
D
(þ)melibiose solution was added to the reac-
tion vessel containing all other thermally equilibrated reactants to
initiate the reaction. The progress of the reaction was determined by
estimating the unconsumed N-bromoacetamide iodometrically.
30
25
20
15
10
5
2.3. Stoichiometry and product analysis
Several sets of experiments with different [NBA]: [sugar] ratio
under the condition [NBA]>>[sugar], were performed at room
temperature for 72 h. Estimation of unconsumed [NBA] was made
iodometrically. On the basis of stoichiometric experiments it was
found that 4 mol of NBA were required to oxidize 1 mol of
melibiose. Accordingly, the following stoichiometric equation can
be formulated:
D
(þ)
PdðIIÞ=Hþ
/
C12H22O11 þ 4CH3NCBOANHBr þ5H2O
dðþÞ melibiose
0
ꢃ 2HCOOH þ C5H10O6 þ C5H10O6 þ4HBr þ 4CH3CONH2
0
5
10
15
20
25
Formic acid
NHA
arabinoic acid lyxonic acid
4
[NBA]×10 M
Kinetic and equivalence studies together with spot test [19a] and thin
layer chromatography [19b] experiments helped us to identify formic
acid and other reaction products of the reaction under investigation.
Fig.
[mel]
1. Plot
between
ꢃ
ꢂdc/dt
and
[PdCl2]
[NBA]
¼
at
02.82
40
ꢀCꢁ0.1
ꢀC.
¼
02.00
10ꢂ2 mol dmꢂ3
,
ꢃ
10ꢂ5 mol dmꢂ3
,
[HClO4] ¼ 01.33 ꢃ 10ꢂ2 mol dmꢂ3, [Hg(OAc)2] ¼ 22.22 ꢃ 10ꢂ4 mol dmꢂ3
.

A.K. Singh et al. / Journal of Organometallic Chemistry 695 (2010) 2213e2219
2215
Table 1
12
Effect of variation of [NBA] and [mel] on the first-order rate constant at
40 ꢀC ꢁ 0.1 ꢀC.a
[NBA] ꢃ 104 (mol dmꢂ3
)
[D
(þ)melibiose] ꢃ 102 (mol dmꢂ3
)
k1 ꢃ 104
(secꢂ1
)
9
6
3
0
2.00
4.00
6.00
8.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
1.67
1.52
1.52
1.62
1.43
1.47
1.32
1.19
0.71
1.54
2.50
3.33
4.17
5.20
6.30
6.94
8.57
10.00
12.00
16.00
20.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
0
3
6
9
12
15
2
D(+) melibiose×10 M
10.00
a
Conditions:
[PdCl2]
¼
2.82
ꢃ
10ꢂ5
mol
dmꢂ3
,
[Hg
Fig. 3. Plot between k1 and [mel] at 40 ꢀCꢁ0.1 ꢀC. [NBA] ¼ 10.00 ꢃ 10ꢂ4 mol dmꢂ3
,
(OAc)2] ¼ 22.22 ꢃ 10ꢂ4 mol dm3 for [NBA] variation and 12.50 ꢃ 10ꢂ4 mol dmꢂ3 for
[PdCl2]
¼
02.82
ꢃ
10ꢂ5 mol dmꢂ3
,
[HClO4]
¼
01.33
ꢃ
10ꢂ2 mol dmꢂ3
, [Hg
[mel] variation, [HClO4] ¼ 01.33 ꢃ 10ꢂ2 mol dmꢂ3
.
(OAc)2] ¼ 12.50 ꢃ 10ꢂ4 mol dmꢂ3
.
oxidation of
D
(þ)melibiose in the presence of homogeneous cata-
4. Discussion and mechanism
lyst Pd(II) chloride in acidic medium does not occur. The non-
feasibility of the reaction negates the role of Hg(II) as an oxidant or
co-oxidant. Further, when [Hg(II)] was varied in the presence of
N-bromoacetamide has been used as an oxidant in Ru(III) [5]-, Ir
(III) [2]- and Pd(II) [15]-catalysed oxidation of reducing sugars in
acidic medium. In each case, it has been observed that HOBr is the
main reactive species of NBA. It is also reported [4] that NBA in
acidic medium exists in the following equilibria:
NBA as an oxidant in Pd(II)-catalysed oxidation of
D(þ)melibiose
under similar conditions, the first-order rate constant (k1) increases
linearly in lower concentration range of Hg(II) but it tends to zero-
order in its higher range. This experimental findings indicate that
Hg(II) acts as co-catalyst (Fig. 4, Table 2) in addition to its role as Brꢂ
ions scavenger. Negative effect of acetamide concentration on first-
order rate constant (k1) was observed (Fig. 5). Decreasing effect of
CH3ðCNOBANÞHBr þH2O# CH3CONH2 þHOBr
(a)
ðNHAÞ
HOBr þ Hþ#ðH2OBrÞþ
(b)
dielectric constant of the medium was found in the oxidation of D(+)
melibiose by NBA (Fig. 6). Variation in ionic strength of the medium
has no effect on the rate of reaction. The reaction was studied at
four different temperatures i.e., 30, 35, 40 and 45 ꢀC. The rate
constants calculated at different temperatures were utilized to
calculate various activation parameters such as energy of activation
or
CH3CONHBr þ Hþ#ðCH3CONH2BrÞþ
(c)
ðCH3CONH2BrÞþH2O#CH3CONH2 þ ðH2OBrÞþ
(d)
(Ea), entropy of activation (
D
S#), enthalpy of activation (
D
H#) and
Gibb’s free energy of activation (
D
G#) (Table 3).
60
50
40
30
20
10
0
0
5
10
15
20
25
5
[PdCl2]×10 M
Fig. 2. Plot between k1 and [Pd(II)]at 40 ꢀCꢁ0.1 ꢀC, [NBA] ¼ 10.00 ꢃ 10ꢂ4 mol dmꢂ3
,
Fig. 4. Plot between k1 and [Hg(II)] at 40 ꢀCꢁ0.1 ꢀC. [NBA] ¼ 10.00 ꢃ 10ꢂ4 mol dmꢂ3
,
,
[mel]
¼
02.00
ꢃ
10ꢂ2 mol dmꢂ3
,
[HClO4]
¼
02.00
ꢃ
10ꢂ2 mol dmꢂ3
,
[Hg
[mel]
¼
02.00
ꢃ
10ꢂ2 mol dmꢂ3
,
[PdCl2]
¼
02.82
ꢃ
10ꢂ5 mol dmꢂ3
(OAc)2] ¼ 12.50 ꢃ 10ꢂ4 mol dmꢂ3, [KCl] ¼ 06.00 ꢃ 10ꢂ3 mol dmꢂ3
.
[HClO4] ¼ 01.33 ꢃ 10ꢂ2 mol dmꢂ3, [
m
] ¼ 50.00 ꢃ 10ꢂ3 mol dmꢂ3
.

2216
A.K. Singh et al. / Journal of Organometallic Chemistry 695 (2010) 2213e2219
Table 2
Effect of variation of [PdCl2] and [Hg(OAc)2] on the first-order rate constant at
40 ꢀCꢁ0.1 ꢀC.a
[PdCl2] ꢃ 105 (mol dmꢂ3
3.5
3
)
[Hg(OAc)2] ꢃ 103 (mol dmꢂ3
)
k1ꢃ104 (secꢂ1
2.90
5.82
11.70
16.67
23.33
31.00
37.00
50.00
2.50
3.00
4.20
5.83
6.67
7.50
7.80
8.30
)
10
9
1.41
2.82
5.62
8.46
11.28
14.10
16.92
22.56
2.82
2.82
2.82
2.82
2.82
2.82
2.82
2.82
1.25
1.25
1.25
1.25
1.25
1.25
1.25
1.25
1.25
1.50
2.00
2.50
3.00
3.50
4.00
4.50
8
7
6
5
4
3
2.5
2
a
Conditions:
[NBA]
¼
10.00
ꢃ
10ꢂ4
mol
dmꢂ3
,
D
(þ)
melibiose ¼ 2.00 ꢃ 10ꢂ2 mol dmꢂ3, [HClO4] ¼ 02.00 ꢃ 10ꢂ2 mol dmꢂ3 for [PdCl2]
variation and 01.33
ꢃ
10ꢂ2 mol dmꢂ3 for [Hg(OAc)2] variation,
[KCl] ¼ 06.00 ꢃ 10ꢂ3 mol dmꢂ3
,
m
¼ 50.00 ꢃ 10ꢂ3 mol dmꢂ3
.
From the above two sets of equilibria (a,b and c,d), it is clear that
NBA in acidic medium exists in four different forms i.e., NBA itself,
HOBr, (CH3CONH2Br)þ and (H2OBr)þ. In view of observed fractional
negative order in [NHA] and zero-order with respect to [Hþ], HOBr
can be assumed as the reactive species of NBA in Pd(II)-catalysed
2
1.5
1
oxidation of
D
(þ) melibiose out of aforesaid four possible oxidizing
species of NBA. This result is further supported by the observed
spectrum of NBA in perchloric acid where a single peak is observed
at 205 nm (Fig. 6 (2)).
1
It is reported [20] that Pd(II) is the most common oxidation state
of palladium and certainly the most important in the chemistry of
its homogeneous catalysis. It is also reported that much of its
catalytic chemistry is related to the fact that Pd(II) is an oxidant. Pd
(II) is a d8 ion and it prefers to form four co-ordinate square planar
complexes. Reports [21] are available where the equilibrium
constants for the following equilibria have been determined and
found in agreement with a value of log b4 between 11 and 12 at
25 ꢀC.
0.5
0
200
240
280
Wavelength (nm)
320
360
400
Fig. 6. spectra of solutions [1,2,3,4,5,6,7,8,9 and 10] recorded at room temperature.
1. [Sugar] ¼ 0.20 mol dmꢂ3, 2. [NBA] ¼ 1.00 ꢃ 10ꢂ3 mol dmꢂ3, [Hþ] ¼ 1.50 mol dmꢂ3
,
3. [NBA]
¼
1.00
ꢃ
10ꢂ3 mol dmꢂ3
,
[Hg(II)]
[Hþ
5. [Pd(II)]
10ꢂ2 mol dmꢂ3 [Clꢂ
10ꢂ2 mol dmꢂ3
6. [Pd(II)]
¼
1.25
1.20
5.64
1.20
5.64
ꢃ
10ꢂ3 mol dmꢂ3
,
4. [ꢂP3d
(II)]
¼
5.64
1.20
1.20
ꢃ
10ꢂ4 mol dmꢂ3
10ꢂ2 mol dmꢂ3
,
]
¼
ꢃ
10ꢂ2 mol dm
10ꢂ4 mol dmꢂ3
10ꢂ2 mol dmꢂ3
10ꢂ4 mol dmꢂ3
,
,
,
,
,
,
,
,
,
,
,
[Clꢂ
]
¼
ꢃ
,
¼
ꢃ
[Hþ
]
¼
ꢃ
,
]
¼
ꢃ
[Sugar]
¼
1.00
ꢃ
,
¼
ꢃ
[Hþ] ¼ 1.20 ꢃ 10ꢂ2 mol dmꢂ3, [Clꢂ] ¼ 1.20 ꢃ 10ꢂ2 mol dmꢂ3, [Sugar] ¼ 0.20 mol dmꢂ3
7. [Pd(II)]
¼ 5.64
1.20
1.00
ꢃ
10ꢂ4 mol dmꢂ3
,
[Hþ
[Sugar]
8. [Pd(II)]
]
¼
1.20
¼
ꢃ
10ꢂ2 mol dmꢂ3
[Clꢂ
[NBA]
]
¼
ꢃ
10ꢂ2
10ꢂ3 mol dmꢂ3
mol
dmꢂ3
,
0.20
mol
dmꢂ3
¼
ꢃ
,
¼
5.64
ꢃ
10ꢂ4 mol dmꢂ3
[Hþ] ¼ 1.20 ꢃ 10ꢂ2 mol dmꢂ3, [Clꢂ] ¼ 1.20 ꢃ 10ꢂ2 mol dmꢂ3, [Sugar] ¼ 0.20 mol dmꢂ3
[NBA]
¼
3.00
ꢃ
10ꢂ3 mol dmꢂ3
,
9. [Pd(II)]
¼
5.64
ꢃ
10ꢂ4 mol dmꢂ3
[Hþ] ¼ 1.20 ꢃ 10ꢂ2 mol dmꢂ3, [Clꢂ] ¼ 1.20 ꢃ 10ꢂ2 mol dmꢂ3, [Sugar] ¼ 0.20 mol dmꢂ3
[NBA]
(II)]
¼
1.00
5.64
1.20
ꢃ
10ꢂ3 mol dmꢂ3
,
[Hg(II)]
10ꢂ4 mol dmꢂ3 [Hþ
10ꢂ2 dmꢂ3
mol
¼
1.25
ꢃ
10ꢂ3 mol dmꢂ3
, 10. Pd
ꢃ
¼
ꢃ
,
]
¼
1.20
10ꢂ2 mol dmꢂ3
0.20 mol
dmꢂ3
,
,
[Clꢂ
]
¼
ꢃ
,
[Sugar]
¼
[NBA] ¼ 1.00 ꢃ 10ꢂ3 mol dmꢂ3, [Hg(II)] ¼ 3.00 ꢃ 10ꢂ3 mol dmꢂ3
.
Fig. 5. Plot between [Pd (II)]T/rate and [NHA]. [NBA] ¼ 10.00 ꢃ 10ꢂ4 mol dmꢂ3
,
,
[mel]
¼
02.00
ꢃ
10ꢂ2 mol dmꢂ3
,
[PdCl2]
¼
02.82
ꢃ
10ꢂ5 mol dmꢂ3
[HClO4] ¼ 01.33 ꢃ 10ꢂ2 mol dmꢂ3, [Hg(OAc)2] ¼ 12.50 ꢃ 10ꢂ4 mol dmꢂ3
.

A.K. Singh et al. / Journal of Organometallic Chemistry 695 (2010) 2213e2219
2217
Table 3
Activation parameters for Pd(II)-catalysed oxidation of
D
(þ) melibiose by NBA in acidic medium Temp. ¼ 40 ꢀC ꢁ 0.1 ꢀC.
Reducing sugar
(þ) melibiose
Ea (k cal molꢂ1
20.15 ꢁ 0.39
)
k (molꢂ2 dmꢂ6 sꢂ1
1.74 ꢃ 103
)
D
S# (e.u.)
D
H# (k cal molꢂ1
19.53 ꢁ 0.01
)
D
G
# (k cal molꢂ1
13.71 ꢁ 0.11
)
A (molꢂ2 dmꢂ6 secꢂ1
)
1.65 ꢃ 1017
D
18.58 ꢁ 0.26
K1
addition to this when Hg(II) solution of two different concentra-
tions were added to the solution of Pd(II), Hþ, Clꢂ, sugar and NBA, it
was observed that with the addition of Hg(II) solution there is an
increase in absorbance from 3.16 to 3.23 and 3.31 along with a shift
in lmax value towards longer wavelength i.e., from 250 to 255 nm
(Fig. 6 (7) (9) and (10)). This overall increase in absorbance with
a shift in lmax towards longer wavelength can be considered as due
Pd2þ þ Clꢂ # PdClþ
(e)
(f)
K2
PdClþ þ Clꢂ # PdCl2
K3
PdCl2 þ Clꢂ # PdClꢂ3
(g)
K4
PdClꢂ3 þ Clꢂ # PdCl42ꢂ
(h)
to formation of a new complex
following way-
according to the
Elding [22] studied both the stability constants and rates of
reaction and determined values of log K1 to log K4 as 4.47, 3.29, 2.41
and 1.37, respectively, with log b4 equal to 11.54. Grinberg et al. [23]
were also determined values of aforesaid stability constants and
found as 4.3, 3.54, 2.68 and 1.68, respectively. On the basis of above
equilibria, we can write
b4
On the basis of observed kinetic orders with respect to [NBA],
[reducing sugar], [Pd(II)], [Hþ], [Hg(II)], [NHA] and [Clꢂ], spectro-
photometric evidence collected for the formation of complex species
Pd2þ þ 4Clꢂ # PdCl42ꢂ
(i)
where b4 is equal to K1K2K3K4
Ayres reported [24] that when a concentration ratio 2:1 for
sodium chloride to palladium (II) chloride is maintained, it will
result in the formation of tetrachloropalladate(II), [PdCl4]2ꢂ. Since
[PdCl24ꢂ], [PdCl4$S]2ꢂ and
, a most probable reaction
mechanism in the form of reaction Scheme 1, can be suggested.
According to the above reaction Scheme 1 the rate in terms of
decrease in the concentration of NBA for the oxidation of
melibiose can be expressed as
throughout the study of oxidation of
D
(þ) melibiose, the aforesaid
concentration ratio was maintained, hence it is reasonable to
assume that the catalyst palladium(II) chloride in the reaction under
investigation remains in the form of [PdCl4]2ꢂ. Taking into consid-
eration the observed catalytic role of Pd(II) chloride, it can be
concluded that [PdCl4]2ꢂ is the reactive species of Pd(II) chloride in
D(þ)
d½NBAꢄ
rate ¼ ꢂ
¼ 4k3½C2ꢄ½HgðIIÞꢄ½HOBrꢄ
(1)
dt
the oxidation of
D
(þ)melibiose by NBA inpresence of perchloric acid.
where 4 indicates that 1 mol of
of NBA.
On applying steady-state approximation to the concentration of
C2, we have eq. (2)
D
(þ) melibiose is oxidized by 4 mol
This result is further verified by the single peak observed at 245 nm
for the solution containing Pd(II) chloride, Hþ and Clꢂ ions (Fig. 6(4)).
It is reported that Pd(II) forms a complex with allyl alcohol [25]
and also with sugar molecule [17]. In order to verify the probable
formation of a complex between Pd(II) and
D
(þ) melibiose, spectra
k2½C1ꢄ½Sꢄ
for solutions containing Pd(II), Hþ and Clꢂ, and Pd(II), Hþ and Clꢂ
with a sugar solution of two different concentrations have been
collected. In the spectrum for Pd(II), Hþ and Clꢂ, it is found that
there is a single peak at 245 nm (Fig. 6(4)). When sugar solution of
two different concentrations were added to the solution of Pd(II),
Hþ and Clꢂ, it has been observed that there is an increase in
absorbance from 3.02 to 3.07 and 3.12 (Fig. 6(4,5 and 6)). On the
basis of this observation, it can be concluded that an Organome-
tallic complex between reactive species of Pd(II) i.e., [PdCl4]2ꢂ and
a sugar molecule is formed according to the following equilibrium:
½C2ꢄ ¼
(2)
k
ꢂ2 þ k3½HgðIIÞꢄ½HOBrꢄ
On applying the law of chemical equilibrium to step (i) we have
K1½NBAꢄ
½HOBrꢄ ¼
(3)
½NHAꢄ
According to reaction Scheme 1, the total concentration of Pd(II)
i.e., [Pd(II)]T can be expressed as
½PdðIIÞꢄT ¼ ½C1ꢄ þ ½C2ꢄ
On substituting, the value of C2 from eq. (2) in eq. (4), we get
(4)
½PdCl4ꢄ2ꢂþS#½PdCl4$Sꢄ2ꢂ
(j)
k2½C1ꢄ½Sꢄ
Above equilibrium indicates that with the addition of sugar molecule
to the solution of Pd(II) chloride, the equilibrium will shift towards
½PdðIIÞꢄT ¼ ½C1ꢄ þ
k
ꢂ2 þ k3½HgðIIÞꢄ½HOBrꢄ
right hand side with more and more formation of [PdCl4$S]2ꢂ
.
or
Attempts were also made to find out whether there is any
possibility of the formation of a complex between [PdCl4$S]2ꢂ and
reactive species of NBA i.e., HOBr or not? For this, first of all the
spectra after the addition of NBA solution of two different
concentrations to the solution of Pd(II), Hþ, Clꢂ and sugar at room
temperature were collected. From the spectra, it is apparent that
with the addition of NBA solution there is an increase in absorbance
from 3.12 to 3.16 and 3.23 nm with a shift in lmax value towards
longer wavelength i.e., from 245 to 250 nm (Fig. 6(6), (7) and (8)). In
fkꢂ2 þ k3½HgðIIÞꢄ½HOBrꢄg½PdðIIÞꢄ
½C1ꢄ ¼
(5)
(6)
k
ꢂ2 þ k2½Sꢄ þ k3½HgðIIÞꢄ½HOBrꢄ T
With the help of eqs. (2) and (5), we can write
k2½Sꢄ½PdðIIÞꢄ
½C2ꢄ ¼
k
ꢂ2 þ k2½Sꢄ þ k3½HgðTIIÞꢄ½HOBrꢄ
From eqs. (1), (3) and (6), we obtain eq. (7)

2218
A.K. Singh et al. / Journal of Organometallic Chemistry 695 (2010) 2213e2219
Scheme 1.
d½NBAꢄ
4k2k3K1½NBAꢄ½PdðIIÞꢄT½Sꢄ½HgðIIÞꢄ
½PdðIIÞꢄT
k
½NHAꢄ
1
ꢂ2
rate ¼ ꢂ
¼
¼
þ
(10)
dt
½NHAꢄfkꢂ2 þ k2½Sꢄg þ k3K1½NBAꢄ½HgðIIÞꢄ
rate
4k2k3K1½NBAꢄ½Sꢄ½HgðIIÞꢄ 4k2½Sꢄ
(7)
(8)
where, rate ¼ ꢂd½NBAꢄ=dt
Eq. (10) clearly shows that if a plot is made between [Pd(II)]T/
rate and [NHA], it will give a straight line having an intercept on [Pd
(II)]T/rate axis. When a plot was made between [Pd(II)]T/rate and
[NHA], a straight line with intercept on [Pd(II)]T/rate axis was
obtained (Fig. 5). This proves the validity of the rate law eq. (9) and
hence the proposed reaction Scheme 1. From the intercept and the
slope, the values of constants k2 and kꢂ2/k3K1 were calculated and
found as 1.95 ꢃ 10ꢂ1 molꢂ1dm3 secꢂ1 and 6.17 ꢃ 10ꢂ5 mol dmꢂ3
If k2[S] >> kꢂ2, then according to eq. (7)
d½NBAꢄ
4k2k3K1½NBAꢄ½PdðIIÞꢄT½Sꢄ½HgðIIÞꢄ
rate ¼ ꢂ
¼
dt
k2½Sꢄ½NHAꢄ þ k3K1½NBAꢄ½HgðIIÞꢄ
Eq. (8) in the present case cannot be treated as the final rate law
because it shows fractional positive order with respect to [mel]
which is contrary to our observed experimental findings. On the
other hand, if the inequality k >> k2[S] is assumed as valid one
ꢂ2
respectively in the oxidation of
of constants k2 and kꢂ2/k3K1 and taking the help of rate law eq. (9),
the rates of variation of [NHA] in the oxidation of
were calculated and found to be in close agreement with the
observed rates (Table 4). This further proves the validity of rate law
eq. (9) and hence the proposed reaction Scheme 1.
D
(þ) melibiose. Utilizing the values
then eq. (7) will be reduced to eq. (9).
D
(þ) melibiose
d½NBAꢄ
dt
4k2k3K1½NBAꢄ½PdðIIÞꢄT½Sꢄ½HgðIIÞꢄ
rate ¼ ꢂ
¼
(9)
k
ꢂ2
½NHAꢄ þ k3K1½NBAꢄ½HgðIIÞꢄ
Eq. (9) is the final rate law and is in complete agreement with our
experimental findings
Positive entropy of activation observed in Pd(II)-catalysed
oxidation of
D
(þ) melibiose by NBA in acidic medium clearly
Eq. (9) can also be written as

A.K. Singh et al. / Journal of Organometallic Chemistry 695 (2010) 2213e2219
2219
Table 4
study. On the basis of observed kinetic data and observed positive
entropy of activation, the species, is proposed as the
most reactive species in the present study whereas the species,
Comparison between observed and calculated rates in variation of [NHA] on the
basis of rate law eq. (10) at 40 ꢀCꢁ0.1 ꢀC.a
[NHA] ꢃ 103 (mol dmꢂ3
)
ꢂdc/dt ꢃ 107 (mol dmꢂ3 secꢂ1
)
D
(þ) melibiose
Experimental
Calculated
and the species,
have been
1.00
2.00
4.00
6.00
8.00
10.00
5.83
4.17
3.61
3.30
3.13
3.05
4.19
4.01
3.61
3.39
3.16
2.95
reported as the most reactive species for Ru(III) [5]- and Ir(III) [2]-
catalysed oxidation of reducing sugars, respectively. Negative effect
of [NHA] was observed in all the three catalyzed reactions.
a
Conditions:
[NBA]
¼
10.00
ꢃ
10ꢂ4
mol
ꢃ
dmꢂ3
10ꢂ5 mol dmꢂ3
,
,
D
(þ)
melibiose
¼
02.00
ꢃ
10ꢂ2 mol dmꢂ3
,
[PdCl2]
¼
02.82
References
[HClO4] ¼ 01.33 ꢃ 10ꢂ2 mol dmꢂ3, [Hg(OAc)2] ¼ 12.50 ꢃ 10ꢂ4 mol dmꢂ3
.
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232 (2005) 83e88.
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345e351.
[5] A.K. Singh, Jaya Srivastava, S. Rahmani, J. Mol. Catal. A.: Chem. 271 (2007)
151e160.
supports step (iii) of Scheme 1, where reaction is taking place
between two oppositely charged species i.e., between [PdCl4$S]2ꢂ
and Hg2þ, resulting in the formation of most reactive activated
complex
.
[6] S. Srivastava, A. Awasthi, K. Singh, Int. J. Chem. Kinet. 37 (2005) 275e281.
[7] B. Singh, A.K. Singh, D. Singh, J. Mol. Catal. 48 (1988) 207e215.
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[9] A.K. Singh, Reena Singh, Jaya Srivastava, Shahla Rahmani, Shalini Srinastava,
J. Organomet. Chem. 692 (2007) 4270e4280.
5. Comparsion between the present study and the studies
reported for Ru(III)e and Ir(III)-catalysed oxidation of
reducing sugars
[10] P.S. Radhakrishnamurti, S.A. Misra, Int. J. Chem. Kinet. 14 (1982) 631e639.
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Sci. Acad. 41 part A (No. 4) (1975) 331e338.
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[19] (a) F. Feigl, Spot Tests in Organic Analysis. Elsevier, New York, 1960, p. 368;
(b) F. Gutbauer, J. Chromatogr. 45 (1969) 104.
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Reidel Publishing Company, Holland, 1928, 6.
[21] M. Patrick, Henry, Palladium Catalysed Oxidation of Hydrocarbons, vol. 2, D.
Reidel Publishing Company, Holland, 1928, 11e12.
When an effort was made to compare the findings of Pd(II)-
catalysed oxidation of
D
(þ)melibiose with the results reported for
Ru(III) [5] and Ir(III) [2]- catalyzed oxidation of reducing sugars, it
was found that HOBr is the reactive species of NBA in each case.
First-order kinetics in [sugar] was observed for both Pd(II) and Ru
(III)[5] systems, but the reported reaction path for Ir(III) [2]- cata-
lysed oxidation of reducing sugars clearly shows that there is no
effect of [sugar] on the rate of reaction. The present study shows
similarity with Ru(III) [5]- catalysed oxidation of reducing sugars as
for as formation of a complex between transition metal-catalyst (Pd
(II) or Ru(III)) with a sugar molecule is concerned, but it distin-
guishes itself from Ir(III) [2] system, where zero-order kinetics with
respect to [sugar] was observed. Pd(II) and Ru(III) [5] systems show
same behaviour in respect of the order in [Hg(II)], but observed
second-order tending towards first-order kinetics in [Hg(II)]
provides a reaction path, where two molecules of mercury before
the rate determining step were involved in Ir(III) [2]- catalysed
oxidation of reducing sugars. The present study entirely differs
from other two studies as for as effect of [Hþ] on the rate of reaction
is concerned. Negative effect of [Hþ] was observed in Ru(III) [5]-
and Ir(III) [2]- catalysed oxidation of reducing sugars, whereas zero
effect of [Hþ] on the rate of reaction was observed in the present
[22] L.I. Elding, Inorg. Chim. Acta 6 (1972) 647.
[23] A.A. Grinberg, N.V. Kiseleva, M.I. Gel’fman, Dokl. Akad. Nauk. SSSR 153 (1963)
1327.
[24] G.H. Ayres, Anal. Chem. 24 (1953) 1622e1627;
Anal. Chem. 46 (1981) 2323e2326.
[25] G.A. Hiremath, P.L. Timmanagoudar, S.T. Nandibewoor, J. Phys. Org. Chem. 11
(1998) 31e35.