Oxidation of D-glucose in the presence of 2,2′-bipyridine by Cr VI in aqueous micellar media: A kinetic study

Article abstract of DOI:10.1016/j.carres.2005.07.002

The kinetics of Cr^VI oxidation of d-glucose to the corresponding lactone in the presence and absence of 2,2′-bipyridine (bipy) has been carried out under the conditions, [d-glucose]_T Gt; [Cr ^VI]_T at different temperatures in aqueous micellar media. The monomeric Cr^VI species has been found to be kinetically active in the absence of bipy whereas in the bipy-catalysed path, the Cr ^VI-bipy complex has been found to be the active oxidant. In the bipy-catalysed path, the Cr^VI-bipy complex undergoes nucleophilic attack by the substrate to form a ternary complex. The ternary complex spontaneously experiences a redox decomposition (through two-electron transfer) in the rate-determining step leading to the product lactone and Cr ^IV-bipy complex. The Cr^IV-bipy complex then takes part in faster steps in the further oxidation of d-glucose and is ultimately converted into a Cr^III-bipy complex. In the uncatalysed path, the Cr ^VI-substrate ester experiences acid catalysed redox decomposition (two-electron transfer) in the rate-determining step. The uncatalysed path shows a second order dependence on [H⁺] and a first order dependence on each of the reactants [d-glucose]_T and [Cr^VI]_T. In contrast, the bipy-catalysed path shows a first order dependence on each of the reactants [H⁺], [d-glucose]_T and [Cr ^VI]_T. The bipy-catalysed path is first order in [bipy]_T. These observations remain unaltered in the presence of externally added surfactants. The effect of the cationic surfactant, N-cetylpyridinium chloride (CPC) and anionic surfactant, sodium dodecyl sulfate (SDS) on both the uncatalysed and bipy-catalysed path has been studied. CPC inhibits both the uncatalysed and bipy-catalysed path, while SDS catalyses these reactions. The observed micellar effects have been explained by considering hydrophobic and electrostatic interactions between the surfactants and reactants.

Full text of DOI:10.1016/j.carres.2005.07.002

Carbohydrate
RESEARCH
Carbohydrate Research 340 (2005) 2163–2170
0
Oxidation of D-glucose in the presence of 2,2 -bipyridine by
VI
Cr in aqueous micellar media: a kinetic study
Ruhidas Bayen, Monirul Islam, Bidyut Saha and Asim K. Das*
Department of Chemistry, Visva-Bharati University, Santiniketan 731235, India
Received 19 April 2005; received in revised form 7 July 2005; accepted 10 July 2005
Available online 28 July 2005
VI
Abstract—The kinetics of Cr oxidation of D-glucose to the corresponding lactone in the presence and absence of 2,2 -bipyridine
0
VI
(
bipy) has been carried out under the conditions, [D-glucose]
T
ꢀ [Cr
T
] at different temperatures in aqueous micellar media. The
VI
VI
monomeric Cr species has been found to be kinetically active in the absence of bipy whereas in the bipy-catalysed path, the Cr
bipy complex has been found to be the active oxidant. In the bipy-catalysed path, the Cr –bipy complex undergoes nucleophilic
attack by the substrate to form a ternary complex. The ternary complex spontaneously experiences a redox decomposition (through
two-electron transfer) in the rate-determining step leading to the product lactone and Cr –bipy complex. The Cr –bipy complex
then takes part in faster steps in the further oxidation of D-glucose and is ultimately converted into a Cr –bipy complex. In the
uncatalysed path, the Cr -substrate ester experiences acid catalysed redox decomposition (two-electron transfer) in the rate-deter-
mining step. The uncatalysed path shows a second order dependence on [H ] and a first order dependence on each of the reactants
T T
and [Cr ] . In contrast, the bipy-catalysed path shows a first order dependence on each of the reactants [H ], [D-glu-
cose] and [Cr ] . The bipy-catalysed path is first order in [bipy] . These observations remain unaltered in the presence of externally
–
VI
IV
IV
III
VI
+
VI
+
[
D-glucose]
VI
T
T
T
added surfactants. The effect of the cationic surfactant, N-cetylpyridinium chloride (CPC) and anionic surfactant, sodium dodecyl
sulfate (SDS) on both the uncatalysed and bipy-catalysed path has been studied. CPC inhibits both the uncatalysed and bipy-catal-
ysed path, while SDS catalyses these reactions. The observed micellar effects have been explained by considering hydrophobic and
electrostatic interactions between the surfactants and reactants.
ꢀ
2005 Elsevier Ltd. All rights reserved.
0
Keywords: Kinetics; Oxidations; Catalysis; D-Glucose; Chromium(VI); 2,2 -Bipyridine (bipy); Surfactants
1
. Introduction
kinetics of oxidative degradation of different sugars
VI
by metal ions including Cr has been studied under
VI 1–3
5–11
Due to the carcinogenic and toxic effects of Cr ,
studies on the kinetics and mechanism of Cr oxida-
tion of biologically relevant reducing agents is of inter-
est to both biochemists and inorganic chemists.
During the reduction of Cr to Cr , the intermediate
oxidation states of chromium may interact with biolog-
ically active molecules to induce toxicity. Thus, in
chromate toxicity, it is reasonable to assume that the
reducing agents may have an important role. The
different conditions.
The present studies aims to
VI
contribute in this direction and we note that in regard
to the reaction mechanism, the conclusions drawn by
3
,4
7
10
Sengupta and Basu differ from those of Sala et al.
VI
III
in many aspects. The present investigations have been
carried out in micro-heterogeneous systems relevant
to biological systems in the presence of the chelating
agent bipyridine (bipy). Picolinic acid (PA) is a chelat-
3
1
2–23
VI
ing agent that is well known
to catalyse Cr oxi-
dation reactions. The structure of bipyridine is
comparable to that of picolinic acid in many respects
and, in particular, both chelating agents are heteroaro-
matic nitrogen bases.
*
Corresponding author. Tel.: +91 03463 262293; fax: +91 03463
62756; e-mail: ak_das3@rediffmail.com
2
0
008-6215/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2005.07.002

2164
R. Bayen et al. / Carbohydrate Research 340 (2005) 2163–2170
2
. Results and discussion
VI
.1. Dependence on [Cr ]T
2
VI
The rate of disappearance of Cr shows a first order
VI
dependence on Cr in the presence and absence of bipy
VI
under the conditions, [D-glucose] ꢀ [bipy] ꢀ [Cr ]
T
T
T
VI
and [bipy] ꢀ [Cr ] . In the presence of surfactants,
T
T
VI
the first order dependence on Cr remains unaltered.
The pseudo first order rate constants (kobs) were evalu-
VI
ated from the linear plot of log [Cr ] versus time (t)
t
as usual.
2
.2. Dependence on [bipy]T
ꢁ
3
3
VI
At [D-glucose] = 6 · 10 mol/dm and [Cr ] = 4 ·
T
T
ꢁ
4
3
3
1
0
mol/dm , [H SO ] = 0.5 mol/dm , the effect of bipy
2 4
on kobs has been studied. The plots of kobs versus [bipy]T
are linear (r > 0.99) with positive intercepts measuring
the contribution of the relatively slower uncatalysed
path (Fig. 1). The pseudo first order rate constants
(
kobs(u)) directly measured in the absence of bipy agree
well with those obtained from the intercepts of the plots
of kobs(T) versus [bipy] . The observation is formulated
T
as follows:
k
obsðTÞ ¼ kobsðuÞ þ kobsðcÞ ¼ kobsðuÞ þ kcat½bipyꢂT
ð1Þ
The values of kcat with the activation parameters are
given in Table 1. During the progress of the reaction,
III
bipy is lost due to the formation of the inert Cr –bipy
VI
complex. Under the conditions, [bipy] ꢀ [Cr ] , the
T
T
[
bipy] remains more or less constant over the course
T
of the reaction.
1
1
6
2
8
4
0
C
B
A
0
5
10
15
20
3
-3
1
0 [bipy] mol dm
T
VI
Figure 1. Effect of [bipy]
T
on kobs(T) for the Cr oxidation of D-glucose
VI
in the presence of bipy in aqueous H
2
SO
4
media. [Cr
= 6 · 10 mol/dm , H SO = 0.5 mol/dm .
= 3 · 10 mol/dm , T = 35 ꢁC. (B) CPC = 0 mol/dm ,
] = 4 ·
T
ꢁ
4
3
ꢁ3
3
3
1
(
0
mol/dm , [D-glucose]
T
2
4
ꢁ
3
3
3
A) [CPC]
T
3
T = 35 ꢁC. (C) CPC = 0 mol/dm , T = 55 ꢁC.

R. Bayen et al. / Carbohydrate Research 340 (2005) 2163–2170
2165
2
.3. Dependence on [D-glucose]T
4
12
9
From the plot of kobs versus [D-glucose] (Fig. 2) it was
established that the catalysed path shows a first order
dependence on [S] where S denotes D-glucose, that is
T
3
2
1
0
B
T
6
k
k
obsðcÞ ¼ kobsðTÞ ꢁ kobsðuÞ ¼ ksðcÞ½SꢂT
obsðuÞ ¼ ksðuÞ½SꢂT
ð2Þ
ð3Þ
3
A
0
+
.4. Dependence on [H ]
0
0.4
0.8
1.2
2
+
-3
[
H ] mol dm
Acid dependence patterns for the uncatalysed and catal-
ysed paths are totally different (Fig. 3). From the exper-
imental fit, the observations are
VI
Figure 3. Effect of [HClO
oxidation of D-glucose in the presence and absence of bipy. [Cr ]T
4 T
] on kobs(u) and kobs(c) for the Cr
VI
=
] +
ꢁ4
3
ꢁ4
3
4 · 10 mol/dm ,
[D-glucose]
T
= 80 · 10 mol/dm ,
[HClO
4
3
[
NaClO ] = 1.5 mol/dm , T = 35 ꢁC. (A) for kobs(u): [bipy] = 0 mol/
4
T
obsðuÞ ¼ kHðuÞ½Hþꢂ2
3
ꢁ4
3
dm ; (B) for kobs(c): [bipy] = 50 · 10 mol/dm .
T
k
k
ð4aÞ
ð4bÞ
þ
obsðcÞ ¼ kHðcÞ½H ꢂ
2.6. Mechanism of the reaction
The mechanism of the reaction can be divided in two
parts as (i) the uncatalysed path (Scheme 1) and (ii)
the catalysed path (Scheme 2).
2
.5. Test for free radical formation
The existence of free radicals has been indicated by poly-
merisation of acrylonitrile under a nitrogen atmosphere
under the experimental conditions.
K1
ꢁ
þ
Glu-OH þ HCrO þ H ¢ Glu-OCrO H þH O ð5Þ
4
3
2
ðAÞ
K2
þ
þ
þ
A þ H ¢ Glu-OCrO
3
H ðAH Þ
ð6Þ
ð7Þ
ð8Þ
2
1
1
5
0
k1
þ
IV
AH ! ProductðPÞ þ Cr
C
VI
Cr ;Glu OH
-
B
IV
III
Cr ƒƒƒƒ ƒƒ ! ProductðPÞ þ Cr
where
6
OH
A
O
5
+
HO
HO
AH =
O
OH2
Cr
OH
O
O
H
0
0
3
6
9
2
-3
1
0 [D-Glucose] mol dm
T
2
.6.1. Uncatalysed path. In aqueous acidic media,
VI
on kobs(c) for the Cr oxidation of D-glucose in
Figure 2. Effect of [S]
T
D-glucose exists predominantly in hemiacetal cyclic
pyranose forms as a mixture of a (OH-1, axial) and
b (OH-1, equatorial) anomers.
of the open chain aldehyde form,
VI
ꢁ4
the presence of bipy in aqueous H
2
SO
] = 0.25 mol/dm , [bipy]
4
media. [Cr
= 4 · 10 mol/dm , T = 30 ꢁC.
]
T
= 4 · 10 mol/
3
3
ꢁ3
3
dm , [H
2
SO
4
T
7
,24,25
ꢁ
3
3
3
(
A) [CPC]
T
= 3 · 10 mol/dm , (B) CPC = 0 mol/dm , SDS = 0 mol/
The concentration
3
ꢁ3
3
26,27
dm , (C) SDS = 6 · 10 mol/dm .
the intermediate
6
HO
6
6
OH
O
OH
OH
HO
4
H O
4
- H O
5
2
2
O
OH
HO
HO
HO
HO
2
2
4
O
5
5
P =
1
1
1
CO H
2
3
3
OH
O
OH
2
3
(
δ lactone)
(free D-gluconic acid)
OH
γ lactone)
(
VI
Scheme 1. Cr oxidation of D-glucose (denoted by Glu–OH) in the absence of 2,2 -bipyridine.
0

2166
R. Bayen et al. / Carbohydrate Research 340 (2005) 2163–2170
in the dynamic equilibrium between the anomers, is very
small. The rate of mutarotation (i.e., a-D-glucose «
+
N
+
2
8
b-D-glucose) is known to be acid catalysed and,
O
N
under these oxidation conditions, the mutarotation
k
2
Product (P) +
CrIV - bipy
(13)
(14)
2
8
equilibrium is immediately attained. Thus, the rate of
oxidation of D-glucose is the summation of rates con-
tributed by each of the a- and b-anomers in addition
to the possible contribution from the open chain alde-
Cr
O
-
O
OH2
H
Glu
(C
CrIV - bipy
1
1
hyde form. The cyclic hemiacetal forms are expected
to be more reactive species because the hydroxy groups
)
2
CrVI ,Glu OH
( fast )
Product (P) + CrIII
VI
10
are better exposed to interact with the Cr species.
VI
Scheme 2. Cr oxidation of D-glucose denoted by (Glu–OH) in the
presence of 2,2 -bipyridine.
In fact, esterification of the –OH group of the substrate
VI
0
with chromic acid is the first step of Cr oxidation of
D-glucose. Among the a- and b-anomers, the b-form
bearing the –OH group at C1 at the equatorial position
þ
k
obsðcÞ ¼ ð2=3Þk
2
K
3
K
4
½Sꢂ ½Lꢂ ½H ꢂ
ð15Þ
2
9
T
T
is more suitably exposed
to attack the chromic
III
The final Cr –bipy complex has been characterised
acid. Thus, D-glucose is mainly oxidised as the b-pyra-
nose form and the initial product is the corresponding
lactone. We have already established the kinetics
III
3
2g
spectroscopically and are outlined below. As Cr ðt Þ
is an inert species, it is reasonable to consider that the
heteroaromatic nitrogen base (i.e., bipy) does not bind
1
8
for the uncatalysed path (Scheme 1) for D-glucose as
follows:
III
to the Cr -centre after its formation from the reduction
of Cr . Therefore, we suggest that the heteroaromatic
VI
þ 2
nitrogen base (denoted by L) undergoes complexation
with the higher oxidation states of chromium, which
are labile. The Cr -species (i.e., chromic acid) is typi-
k
obsðuÞ ¼ ð2=3Þk
1
K
1
K
2
½Sꢂ ½H ꢂ
ð9Þ
T
VI
cally labile and undergoes complexation at the first step
with the chelating agent (L) to produce the chelate com-
Kb
+
+
plex (C ), which is believed to be the kinetically active
H
1
31
+
N
oxidant. Under these reaction conditions, the first
N
N
N
ð10Þ
order dependence on [bipy]T is strictly maintained
4
(
L)
(Kb = 2.8 x 10 Ref. 30)
H
throughout the range of [bipy] used. Thus, it is reason-
T
+
LH
)
(
able to conclude that the equilibrium constant for the
VI
reaction leading to Cr –bipy complex (C ) is low. In
1
VI
the next step, the Cr –bipy complex reacts with the sub-
K
3
-
strate to form a ternary complex (Eq. 12), which experi-
ences redox decomposition through a cyclic transition
state in the rate-limiting step giving rise to the organic
+
+
HCrO + 2H + LH
4
+
N
+
N
O
IV
ð11Þ
product and Cr –bipy complex. The negative value of
Cr
à
DS (entropy of activation, Table 1) of the composite
OH2
H O
rate constant kcat supports the suggested cyclic transi-
2
O
à
tion state. The fairly high DH (enthalpy of activation)
(
C )
1
value (Table 1) indicates that the bipy-catalysed path
is favoured mainly due to the very high negative value
K
4
(
C ) + Glu
à
IV
1
OH
+
of DS . The Cr -species produced in the rate-limiting
step participates in the next faster steps to give the final
product. The different possible routes are given below:
+
+
+
H O
3
O
N
N
Cr
O
Path I
-
O
OH2
IV
VI
V
Cr þ Cr ! 2CIrII
H
Glu
C )
V
2
Cr þ 2S ! 2Cr þ Products
(
2
ð12Þ
Path II
IV
III
ꢃ
Cr þ S ! Cr þ S
2
.6.2. Catalysed path. For the bipy-catalysed path, the
VI
ꢃ
V
Cr þ S ! Cr þ Products
reactions in Scheme 2 can explain the experimental find-
ings and lead to the following rate law:
V
III
Cr þ S ! Cr þ Products

R. Bayen et al. / Carbohydrate Research 340 (2005) 2163–2170
2167
Path III
(C1)W
(C1)m
IV
II
Dn
+
+
+
w
Cr þ S ! Products þ Cr
(
D-glucose)
(
D-glucose)
m
II
VI
III
V
Cr þ Cr ! Cr þ Cr
V
III
Cr þ S ! Products þ Cr
Products
In the aforementioned possible paths, S denotes the
substrate acting as a two-electron reductant and Sꢃ
stands for the partially oxidised substrate. In both the
Scheme 3. Partitioning of the reactive species between the aqueous and
micellar phases.
3
2
Watanabe–Westheimer mechanism (i.e., Path I) and
3
3,34
IV
39
the Perez-Bennito mechanism
(i.e., Path III), the ti-
none by Ce , by Sarada and Reddi in the oxalic acid
catalysed oxidation of aromatic azo-compounds by
tle organic substrate acts in all steps as a two-electron
reductant, while it acts both as a two-electron reductant
VI
Cr in the presence of surfactant sodium dodecyl sulfate
SDS). In the uncatalysed path, the neutral Cr -sub-
3
5
VI
and a one-electron reductant in the Rocek mechanism
(
3
5
(
i.e., Path II). Previously, the Rocek mechanism was
accepted widely in explaining the Cr oxidation of dif-
ferent organic substrates and the Perez-Bennito mecha-
strate ester (A) formed (Eq. 5) can be partitioned in
the micellar pseudo-phase of the surfactant but the cat-
VI
+
ionic surfactant repelling H needed for the redox
33,34
nism
was discarded because of the instability of
decomposition of the ester (Eqs. 6 and 7) inhibits the
II
33,34
Cr . Recently, however, it has been proven
that for
reaction. In the bipy-catalysed path, CPC restricts the
VI
the oxidation of different two-electron organic reductants,
positively charged Cr –bipy complex (C ), the active
1
II
IV
Cr is produced from Cr through hydride transfer.
Thus, the carbocationic centre generated is responsible
oxidant, in the aqueous phase and thus the accumulated
neutral substrate in the micellar phase (Stern layer) can-
not participate in the reaction. Therefore in both the
uncatalysed and bipy-catalysed paths, the reaction is
mainly restricted to the aqueous phase in which the con-
centration of the substrate is depleted due to its parti-
tioning in the Stern layer of the micelle. Partitioning
of the reactants between the aqueous and micellar phase
3
6
for acrylonitrile polymerisation. It may be noted that
3
5
ꢃ
in the Rocek mechanism, the free radical S is supposed
to be responsible for acrylonitrile polymerisation.
2
.7. Effect of CPC
Cetyl pyridinium chloride (CPC), a representative cat-
ionic surfactant, has been found to show the rate retard-
ing effect both in the uncatalysed and catalysed paths.
is shown in Scheme 3 in which D represents micellised
surfactants where n is the aggregation number.
n
The plot of kobs(T) versus [CPC] (Fig. 4) indicates that
T
2.8. Effect of SDS
the rate decreases in a continuous manner, and it tends
to level off at higher CPC concentration. Bunton and
Sodium dodecyl sulfate (SDS), a representative anionic
surfactant, catalyses the oxidation reaction both in the
presence and absence of bipy. In the bipy-catalysed
path, the rate acceleration arises due to the preferential
3
7
Cerichelli noted a similar observation in the oxidation
III
of ferrocene by Fe salts in the presence of cationic sur-
factant cetyl trimethyl ammonium bromide (CTAB).
The present observation is also similar to those observed
VI
partitioning of the positively charged Cr –bipy com-
3
8
by Panigrahi and Sahu in the oxidation of acetophe-
plex (C ) by an electrostatic attraction and the neutral
1
substrate in the micellar surface Stern layer. In the unca-
VI
talysed path, the neutral Cr -substrate ester (A) is par-
+
titioned in the micellar phase (Stern layer) and the H
1
2
9
6
3
0
ions needed for the redox decomposition of the ester
(A) (Eqs. 6 and 7) are also preferentially accumulated
in the micellar interphase due to an electrostatic attrac-
tion. Thus, SDS allows the reaction to proceed in both
the aqueous and micellar interphases. However, the
reaction is more favoured in the micellar pseudo-phase
because of the enhanced concentration of the reactants
+
(i.e., A and H ) in the micellar interphase. In the
0
2
4
6
8
10
bipy-catalysed reaction, the plot of kobs(T) versus [SDS]T
(Fig. 5) indicates that the rate increases in a continuous
fashion up to the SDS concentration used. An increase
in [SDS]T increases the micellar solubilisation of the
3
-3
1
0 [CPC] (mol dm )
T
VI
Figure 4. Effect of [CPC]
glucose in the presence of bipy in aqueous H
· 10 mol/dm ,
mol/dm , [bipy]
T
on kobs(T) for the Cr oxidation of D-
VI
2
SO
= 0.5 mol/dm . [D-glucose]
T
4
media, [Cr
]
T
=
ꢁ4
ꢁ
4
3
3
reactants but at the same time an increase in [SDS]T
4
H
2
SO
4
= 180 · 10
3
ꢁ4
3
T
= 60 · 10 mol/dm , T = 35 ꢁC.
increases the concentration of the micellar counter ions

2168
R. Bayen et al. / Carbohydrate Research 340 (2005) 2163–2170
VI
1
1
6
2
8
4
0
possibility of decomposition of the surfactants by Cr
was investigated and the rate of decomposition in this
path was found to be kinetically negligible.
3
.3. Product analysis and stoichiometry
Under the kinetic conditions (i.e., [D-glucose] ꢀ
T
VI
[Cr ] ), qualitative identification of the organic reac-
0
0.4
2
0.8
1.2
T
-3
1
0 [SDS] mol dm
tion product was carried out by paper-chromatogra-
T
8
,10,11
phy,
4:1:5) as the eluant. To characterise the oxidation prod-
ucts, a series aldopentoses and aldohexoses were oxi-
using n-butyl alcohol–acetic acid–water
VI
Figure 5. Effect of [SDS]
glucose in the presence of bipy in aqueous H
· 10 mol/dm , [bipy]
dm , [D-glucose]
T
on kobs(T) for the Cr oxidation of D-
(
VI
2
SO
4
media, [Cr
SO ] = 0.25 mol/
T
] =
ꢁ4
3
ꢁ3
3
3
4
T
= 4 · 10 mol/dm , [H
= 6 · 10 mol/dm , T = 30 ꢁC.
2
4
4
1
3
ꢁ3
dised with nitric acid and bromine water separately
and the purified products were used as authentic stan-
dards in the chromatographic procedure. D-Gluconic
acid and the corresponding lactone were identified as
the only reaction products. The observed R value for
D-gluconic acid is also in good agreement with the
reported value. The paper chromatogram developed
from the product solution also indicates the presence
of unreacted substrate D-glucose (R = 0.18). The
T
4
2
+
+
2+
(
i.e., Na ), which may displace H and OX ions (C1)
out of the micellar surface.
f
þ
2þ
þ
2þ
43
2
Na þ OX ¢ 2Na þ OX
ð16Þ
ð17Þ
W
M
M
W
þ
þ
þ
þ
Na þ H ¢ Na þ H
W
M
M
W
f
observed R value of D-glucose was also compared with
f
The plot of kobs(T) versus [SDS]T indicates that the
solubilisation effect is greater than the counter-ion effect
for the bipy-catalysed path up to the SDS concentration
used. Here, it may be pointed out that the organic prod-
uct D-gluconic acid is also partitioned between the
micellar (both cationic and anionic) and aqueous
phases. However, this partitioning does not have any
effect on the rate process or reaction mechanism.
that of authentic sample under the same conditions. The
observed R value for glucose agrees well with the
f
4
3
reported value (R = 0.19). The paper chromatogram
f
obtained from the product solution indicates the
absence of any other organic compounds. Thus, it is
evident that during the reaction the substrate did not
degrade under these conditions.
III
The final fate of Cr -species was confirmed spectro-
scopically. The UV–vis spectra (Fig. 6) were recorded
on a UV–vis–NIR scanning spectrophotometer, (UV-
3101PC, Shimadzu). The characteristic part of electronic
3. Experimental
3
.1. Materials and reagents
0
2
,2 -Bipyridine (Qualigens, India) D-glucose (SRL),
K Cr O (BDH), sodium dodecyl sulfate (SRL), N-
0.4
0.3
0.2
2
2
7
cetylpyridinium chloride (SRL), H SO (E. Merck),
2
4
HClO (E. Merck) and all other chemicals used were
4
of highest purity available commercially. The solutions
were prepared in doubly distilled water.
(b)
3
.2. Procedure and kinetic measurements
0.1
(a)
Solutions of the oxidant and reaction mixtures contain-
ing known quantities of the substrate (S) (i.e., D-glu-
0
.0
4
00
450
500 550
600 650
700
cose), catalyst (bipy) (under the conditions [S] ꢀ
T
VI
VI
Wavelength (nm)
[
Cr ] and [bipy] ꢀ [Cr ] ) acid and other necessary
T
T
T
chemicals were separately thermostated (± 0.1 ꢁC). The
reaction was started by mixing the requisite amounts
of the oxidant with the substrate. The progress of the
reaction was monitored by following the rate of disap-
Figure 6. (a) Absorption spectra (absorbance per cm) of the reaction
mixture (after completion of reaction): [Cr
VI
ꢁ4
3
] = 3.85 · 10 mol/dm ,
T
ꢁ
3
3
3
[
D-glucose]
T
= 60 · 10 mol/dm , [bipy]
T
= 0 mol/dm (i.e., uncatal-
4
SO ] = 0.5 mol/dm . (The spectrum of the chromic
3
ysed path), [H
2
sulfate is identical under the experimental conditions.) (b) Absorption
spectra (absorbance per cm) of the reaction mixture (after completion
of reaction): [Cr
VI
40
pearance of Cr by a titrimetric quenching technique.
VI
ꢁ4
3
ꢁ3
The pseudo first order rate constants (kobs) were calcu-
lated as usual. Under the experimental conditions, the
]
T
= 3.85 · 10 mol/dm , [D-glucose] = 60 · 10
T
3
ꢁ3
3
3
mol/dm , [bipy]
T
2 4
= 8 · 10 mol/dm , [H SO ] = 0.5 mol/dm .

R. Bayen et al. / Carbohydrate Research 340 (2005) 2163–2170
2169
III
III
bipy. The existence of Cr –bipy complex in the
final product solution for the bipy-catalysed reaction
absorption spectra of Cr species lies in the range 360–
00 nm. The colour of the final solution of bipy-cataly-
sed reaction in aqueous H SO media is pale violet
6
VI
supports the formation of Cr –bipy complex in the
2
4
4
4
[
kmax = 548 nm for A ! T ] while the colour of the
pre-equilibrium step. This is a reasonable proposal as
2g
2g
VI
VI
final solution for the uncatalysed reaction (i.e., in the
absence of bipy) under identical conditions is pale blue
the Cr centre is kinetically labile and the Cr –bipy
III
complex is finally reduced to a Cr –bipy complex. In
contrast, formation of Cr –bipy complex after the
4
4
III
[
kmax = 578 nm for A ! T ; and 412 nm for
2g
2g
4
4
VI
III
III
3
2g
A ! T (F)]. The spectrum of the final solution of
reduction of Cr to Cr is ruled out as Cr ðt Þ is a
2
g
1g
the uncatalysed reaction and pure chromic sulfate solu-
tion in aqueous sulfuric acid media is identical. This
kinetically inert centre.
III
indicates that the final Cr -species is simply chromic
sulfate for the uncatalysed reaction while for the bipy-
Acknowledgements
III
III
catalysed reaction, the final Cr -species is a Cr –bipy
complex. Similar results have been noted by the earlier
Thanks are due to Visva-Bharati (Santiniketan) for
financial assistance. The authors are thankful to Dr.
D. Mondal, Dr. B. C. Bag, Saugata Pal and Biplab
Goswami of this department for their help.
3
1,44
workers.
It is interesting to note that for the final
solution of the bipy-catalysed reaction, there is a blue
shift (Fig. 6) for the peak due to the transition
4
4
A ! T . This blue shift is due to the presence of
2
g
2g
III
the strong field ligand like bipy. For the Cr –bipy com-
4
4
plex, the peak due to the transition A ! T (F)
2
g
1g
Appendix A. Derivation of rate law for D-glucose
merges with a charge transfer band (Fig. 6).
(
considering Scheme 2)
4
. Conclusions
Rate ¼ k ½C ꢂ
ðiÞ
2
2
þ
½
C ꢂ½H ꢂ
VI
2
The cationic Cr –bipy complex has been found to act as
the active oxidant for the bipy-catalysed path of oxida-
tion of D-glucose to D-gluconic acid in aqueous acidic
media. This complex is produced in a rapid pre-equilib-
K ¼
4
½
C
1
ꢂ½Sꢂ
K ½C ꢂ½Sꢂ
4
1
þ
or; ½C
2
ꢂ ¼
ðiiÞ
½H ꢂ
VI
rium step in the reaction of Cr and bipy, which then re-
½C ꢂ
1
K ¼
ðiiiÞ
3
acts with D-glucose to produce a ternary complex that
experiences the redox decomposition (two-electron
transfer) in the rate-determining step. The reactions have
been carried out in aqueous micellar media. The effects of
both the cationic and anionic surfactants have been fol-
lowed and the observed micellar effects are in agreement
with the proposed reaction mechanism. The mechanistic
path of the uncatalysed and bipy-catalysed chromic acid
oxidation of D-glucose have been compared.
The organic product, D-gluconic acid, was identified
by paper chromatography and the observed R value
has been found in good agreement with the reported
value. This was further supported by comparison
with an authentic sample of D-gluconic acid under
ꢁ
þ 2
þ
½
HCrO ꢂ½H ꢂ ½LH ꢂ
4
ꢁ
þ 2
þ
or; ½C
1
ꢂ ¼ K
3
½HCrO ꢂ½H ꢂ ½LH ꢂ
4
The total concentration of L (= bipy) is given by
þ
½
Lꢂ ¼ ½Lꢂ þ ½LH ꢂ
ðivÞ
ðvÞ
T
þ
þ
½
LH ꢂ
½LH ꢂ
K
b
¼
;
or; ½Lꢂ ¼ K
þ
þ
½
Lꢂ½H ꢂ
b
½H ꢂ
ꢀ
ꢁ
þ
½LH ꢂ
1
þ
þ
or; ½Lꢂ ¼
þ ½LH ꢂ ¼ ½LH ꢂ 1 þ
ðviÞ
T
þ
þ
f
K ½H ꢂ
b
K ½H ꢂ
b
ꢀK
ꢁ
þ
½H ꢂ þ 1
¼ ½LHþꢂ
b
ꢄ ½LHþꢂ
þ
K
b
½H ꢂ
VI
the same conditions. In the reaction, Cr is finally
+
4
Since K [H ] ꢀ 1 (K = 2.8 · 10 for bipy) (Ref. 30)
b
b
III
III
reduced to Cr . The fate of Cr -species in the final
solution was determined by following the UV–vis
spectra. In the uncatalysed reaction, the species is
simply chromic sulfate (pale blue, kmax = 412 and
78 nm) while for the bipy-catalysed reaction, the corre-
sponding species is a Cr –bipy complex (pale violet
ꢁ
þ
It leads to: ½C
2
ꢂ ꢄ K
3
K
4
½Sꢂ ½HCrO ꢂ½H ꢂ½Lꢂ
T
4
T
ꢁ
þ 2
K
3
K
4
k
2
½Sꢂ ½HCrO ꢂ½Lꢂ ½H ꢂ
T
4
T
Rate ¼
þ
½H ꢂ
5
ꢁ
þ
¼
K
3
K
4
k
2
½Sꢂ ½HCrO ꢂ½Lꢂ ½H ꢂ
III
T
4
T
III
and,
colour, kmax = 548 nm). In the Cr –bipy complex,
4
4
the peak at 548 nm (due to the transition A ! T )
2
g
2g
ꢁ
d ln½HCrO ꢂ
4
experiences a blue shift (with respect to the simply chro-
mic sulfate species obtained in the case of uncatalysed
reaction) due to the presence of the strong field ligand
ꢁ
¼ kobsðcÞ
dt
þ
¼ ð2=3ÞK
3
K
4
k
2
½Sꢂ
½Lꢂ
½H ꢂ ð15Þ
T
T

2170
R. Bayen et al. / Carbohydrate Research 340 (2005) 2163–2170
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