Kinetics and mechanism of Pd(II) catalysed oxidation of D-arabinose, D-xylose and D-galactose by N-bromosuccinimide in acidic solution

Article abstract of DOI:10.1016/S0008-6215(98)00322-X

The kinetics of Pd(II) catalysed oxidation of Ara, Xyl and Gal by N-bromosuccinimide (NBS) in acidic medium has been studied using Hg(OAc)₂ as a scavenger for the Br^- ion. The reaction data show that first-order kinetics in each pentose and hexose at low concentrations tend to zero-order at high concentrations. First-order kinetics with respect to NBS and Pd(II) and inverse fractional order, i.e., decreasing effect of [H⁺] and [Cl^-], were observed, whereas ionic strength, Hg(OAc)₂ and succinimide did not influence the oxidation rate. Various activation parameters have been calculated and recorded. The corresponding acids, arabonic, xylonic and galactonic, were identified as the main oxidation products of the reactions. On the basis of the experimental findings, a suitable mechanism has been proposed. Copyright (C) 1998 Elsevier Science Ltd.

Full text of DOI:10.1016/S0008-6215(98)00322-X

Carbohydrate Research 314 (1998) 157–160
Kinetics and mechanism of Pd(II) catalysed oxidation of
D-arabinose,
D-xylose and
D-galactose by
N-bromosuccinimide in acidic solution
Ashok Kumar Singh *, Deepti Chopra, Shahla Rahmani, Bharat Singh
Department of Chemistry, Uni6ersity of Allahabad, Allahabad 211 002, India
Received 7 July 1998; accepted 7 December 1998
Abstract
The kinetics of Pd(II) catalysed oxidation of Ara, Xyl and Gal by N-bromosuccinimide (NBS) in acidic medium
has been studied using Hg(OAc)2 as a scavenger for the Br− ion. The reaction data show that first-order kinetics in
each pentose and hexose at low concentrations tend to zero-order at high concentrations. First-order kinetics with
respect to NBS and Pd(II) and inverse fractional order, i.e., decreasing effect of [H+] and [Cl−], were observed,
whereas ionic strength, Hg(OAc)2 and succinimide did not influence the oxidation rate. Various activation parameters
have been calculated and recorded. The corresponding acids, arabonic, xylonic and galactonic, were identified as the
main oxidation products of the reactions. On the basis of the experimental findings, a suitable mechanism has been
proposed. © 1998 Published by Elsevier Science Ltd. All rights reserved.
Keywords: Mechanism; Pd(II) catalysis; Acidic medium; Oxidation; Reducing sugars; N-bromosuccinimide
ever, bromine oxidation is obviated by using
1. Introduction
Hg(II) [1]. The oxidation of sugars [9–12] by
alkaline NBS solution has also been reported,
but no study has examined the role of Pd(II)
chloride as a catalyst in NBS oxidation of
sugars. This prompted the authors to perform
the present work, which constitutes an investi-
gation of the kinetics and mechanism of Pd(II)
catalysed oxidation of reducing sugars like
Ara, Xyl and Gal by NBS in acidic medium.
N-Bromosuccinimide (NBS) has been used
as an oxidising and halogenating agent in the
quantitative estimation of a large number of
compounds. Various investigations on oxida-
tion kinetics involving NBS and esters [1],
alcohols [2,3], ketones [4–6], polyhydric alco-
hols [7] and glycol [8] have been reported.
Although the oxidative capacity of NBS has
been examined in several uncatalysed and
catalysed reactions in acidic media, it appears
from the literature that no attempt has so far
been made to explore the catalytic potential of
Pd(II) in NBS oxidation. It is known that
NBS oxidation of organic compound is com-
plicated by parallel bromine oxidation. How-
2. Experimental
Materials.—NBS solution was always made
up fresh and stored in a black-coated flask to
prevent photochemical deterioration. It was
prepared from E. Merck grade recrystallised
material dissolved in double-distilled water.
The prepared solution was then standardised
* Corresponding author. Tel.: +91-532-609-936.
0008-6215/98/$ - see front matter © 1998 Published by Elsevier Science Ltd. All rights reserved.
PII: S0008-6215(98)00322-X

158
A.K. Singh et al. / Carbohydrate Research 314 (1998) 157–160
Table 1
against sodium thiosulphate solution using
starch as an indicator. Aqueous solutions of
Ara, Xyl and Gal (AR grade) were also pre-
pared fresh each day. A solution of Pd(II)
chloride (Qualigens Chemical India Ltd.) was
prepared in HCl of known strength. The over-
all strength of HCl was maintained at 1.00×
10−3 M and the strength of Pd(II) chloride
was 5.64×10−3 M. An aq standard solution
of mercuric acetate (E. Merck), acidified with
acetic acid and 20% perchloric acid diluted
with water, was standardised through acid–
base titration.
Effect of variation of [NBS] on reaction rate at 40 °C
[NBS]×104 Ma
k1×105 (s−1
)
Ara
Xyl
Gal
4.00
6.00
8.00
10.00
16.00
20.00
5.66
5.45
5.00
5.16
5.33
5.03
6.75
6.00
6.66
7.01
6.50
6.02
5.25
5.83
5.92
5.65
4.83
4.83
a Solution conditions: 2.00×10−3 M perchloric acid, 5.00×
10−3 M mercuric acetate, 19.74×10−6 M palladous chloride,
5.00×10−2 M (Ara and Gal), 10.00×10−2 M (Xyl).
Method.—The
reactants,
i.e.,
NBS,
Hg(OAc)2, HClO4 and Pd(II), were mixed in a
black-coated conical flask and equilibrated for
ca. 30 min at 40 °C. The sugar solution at
40 °C was then rapidly poured into the reac-
tion mixture and the reaction was started by
vigorous shaking of the reaction mixture and
its progress followed by measuring uncon-
sumed NBS iodometrically.
der dependence of the reactions on Pd(II)
chloride (Fig. 2). The order of reaction with
respect to hydrogen ions (obtained from
perchloric acid) was determined as −0.38 for
Ara, −0.34 for Xyl and −0.13 for Gal from
the slope of the plot between log k1 and
log [H]+ (Fig. 3).
The results of Table 2 indicate the negative
effect of the addition of KCl and the zero
effect of the addition of succinimide (NHS) on
the rate constant. The reactions were studied
at different temperatures and the rate con-
stants at 35, 40, 45 and 50 °C used to calculate
Ea, DSc and DGc in the oxidation of Ara, Xyl
and Gal as 6.86, 10.49 and 12.20 kcal/mol,
respectively, −30.18, −17.62 and −10.77
3. Results and discussion
The kinetics of the oxidation of sugars were
investigated at several initial concentrations of
the reactants. The rate of reaction in each
kinetic run was determined by the slope of the
tangent drawn at a fixed concentration of
NBS. The effect of NBS variation on the
reaction rate (−dc/dt) has been determined
by the formula
k1=(−dc/dt)/[NBS]
A close perusal of the results indicates that
the rate of reaction (−dc/dt) is almost di-
rectly proportional to the concentration of
NBS. The first-order kinetics with respect to
NBS are also verified by the constant values
of k1 obtained at various initial concentrations
of NBS (Table 1).
A plot of (−dc/dt) versus [sugar] shows
that first-order kinetics with respect to sugars
in their lower range of concentration shifts to
zero-order in the higher range of their concen-
trations (Fig. 1). A log k1 versus log [Pd(II)]
plot gives a straight line with a slope near
unity (1.02, 0.92 and 0.94 in Ara, Xyl and
Gal, respectively), which confirms the first-or-
Fig. 1. Plots of (−dc/dt) vs. [substrate] at 40 °C. [NBS]=
10.00×10−4 M, [HClO4]=2.00×10−3 M, [PdCl2]=
19.74×10−6 M, [Hg(OAc)2]=1.25×10−3 M; A, Ara; X,
Xyl; G, Gal.

A.K. Singh et al. / Carbohydrate Research 314 (1998) 157–160
159
Table 2
Effect of addition of KCl and succinimide [NHS] on rate
constant at 40 °C
a
[KCl]×102 M [NHS]×104 M k1 ×105 (s−1
)
Ara
Xyl
Gal
1.42
2.42
3.42
5.42
7.42
11.92 10.00 11.74
11.40
10.95
10.52
9.64 10.00
9.47
9.29
8.76
9.36
6.73
6.31
6.55
6.33
6.58
9.29
8.78
7.89
5.51
5.72
5.89
5.78
5.76
9.42
10.02
5.36
5.24
5.23
5.26
5.30
15.42
15.42
15.42
15.42
15.42
3.34
5.00
10.00
20.00
30.00
Fig. 2. Plots of log [Pd(II)] vs. log k1 at 40 °C. [NBS]=
10.00×10−4 M, [HClO4]=2.00×10−3 M, [Hg(OAc)2]=
1.25×10−3 M, [substrate]=5.00×10−2 M (for Ara and
Gal), 10.00×10−2 M (for Xyl); A, Ara; X, Xyl; G, Gal.
a Solution conditions: 10.00×10−4
M N-bromosuccin-
eu, respectively, and 15.55, 15.30 and 15.21
kcal/mol, respectively. Mercuric acetate was
found to have a limited role as a bromide ion
scavenger.
imide, 19.74×10−6 M palladous chloride, 2.00×10−3
M
perchloric acid, 1.25×10−3 M mercuric acetate, 5.00×10−2
M (Ara and Gal), 10.00×10−2 M (Xyl).
On the basis of the above experimental
findings, a probable scheme (Scheme 1) is
proposed for the oxidation of Ara, Xyl and
Gal. The kinetic data led the authors to as-
sume that HOBr and [PbCl4]2− are reactive
species of NBS and palladous chloride,
respectively.
applying a steady state approximation to [C2]
and [C3], the rate equation (Eq. (1)) may be
expressed finally as Eq. (2) with suitable
approximations.
d[NBS]
dt
kk2k3[PdCl4]2−[NBS][RCHO]
k3[H+]+kk−2[RCHO][Cl−]+k3
(2)
−
=
The rate of oxidation of the title substrate
may be expressed in terms of loss of [NBS] as
The negligible effect of succinimide concen-
tration on the rate of reaction is obvious, as it
does not appear in the rate law (Eq. (2)). The
−d[NBS]/dt=kd[C3][S]
(1)
Considering steps (i)–(iii) of Scheme 1 and on
Fig. 3. Plots of log [H+] vs. log k1 at 40 °C. [NBS]=10.00×
10−4 M, [PdCl2]=19.74×10−6 M, [Hg(OAc)2]=1.25×
10−3 M, [substrate]=5.00×10−2 M (for Ara and Xyl),
10.00×10−2 M (for Gal); A, Ara; X, Xyl; G, Gal.
Scheme 1.

160
A.K. Singh et al. / Carbohydrate Research 314 (1998) 157–160
[3] N. Venkatasubramanian, V. Thiagarajan, Can. J. Chem.,
47 (1969) 694.
[4] L. Pandey, K. Singh, S.P. Mushran, Current Sci., 47
(1978) 611.
[5] K. Singh, J.N. Tiwari, S.P. Mushran, Int. J. Chem Kinet.,
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[7] J.P. Sharma, R.N.P. Singh, A.K. Singh, B. Singh, Tetra-
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Kumar, Oxid. Commun., 16 (1993) 197.
[9] B. Singh, A.K. Singh, V.K. Singh, A.K. Sisodia, M.B.
Singh, J. Mol. Catal., 40 (1987) 49.
proposed mechanism is well supported by the
value of the energy of activation parameters.
A high positive value of free energy of activa-
tion (DGc) indicates that the transition state
is highly solvated, while a negative value of
entropy of activation (DSc) suggests the for-
mation of an activated complex with a reduc-
tion in the degree of freedom of reacting
molecules.
References
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[11] A.K. Singh, A. Singh, R. Gupta, M. Saxena, B. Singh,
Transit. Metal Chem., 17 (1992) 413.
[12] A.K. Singh, A. Singh, M. Saxena, R. Gupta, Oxid.
Commun., 15 (1992) 1.
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