CarbohydrateRESEARCH

Carbohydrate Research 340 (2005) 2163–2170

Oxidation of DD-glucose in the presence of 2,2 0-bipyridine byCrVI 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 2005Available online 28 July 2005

Abstract—The kinetics of CrVI oxidation of DD-glucose to the corresponding lactone in the presence and absence of 2,2 0-bipyridine(bipy) has been carried out under the conditions, [DD-glucose]T � [CrVI]T at different temperatures in aqueous micellar media. Themonomeric CrVI species has been found to be kinetically active in the absence of bipy whereas in the bipy-catalysed path, the CrVI–bipy complex has been found to be the active oxidant. In the bipy-catalysed path, the CrVI–bipy complex undergoes nucleophilicattack by the substrate to form a ternary complex. The ternary complex spontaneously experiences a redox decomposition (throughtwo-electron transfer) in the rate-determining step leading to the product lactone and CrIV–bipy complex. The CrIV–bipy complexthen takes part in faster steps in the further oxidation of DD-glucose and is ultimately converted into a CrIII–bipy complex. In theuncatalysed path, the CrVI-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[DD-glucose]T and [CrVI]T. In contrast, the bipy-catalysed path shows a first order dependence on each of the reactants [H+], [DD-glu-cose]T and [CrVI]T. The bipy-catalysed path is first order in [bipy]T. These observations remain unaltered in the presence of externallyadded surfactants. The effect of the cationic surfactant, N-cetylpyridinium chloride (CPC) and anionic surfactant, sodium dodecylsulfate (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 andelectrostatic interactions between the surfactants and reactants.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Kinetics; Oxidations; Catalysis; DD-Glucose; Chromium(VI); 2,20-Bipyridine (bipy); Surfactants

1. Introduction

Due to the carcinogenic and toxic effects of CrVI,1–3

studies on the kinetics and mechanism of CrVI oxida-tion of biologically relevant reducing agents is of inter-est to both biochemists and inorganic chemists.3,4

During the reduction of CrVI to CrIII, the intermediateoxidation states of chromium may interact with biolog-ically active molecules to induce toxicity.3 Thus, inchromate toxicity, it is reasonable to assume that thereducing agents may have an important role. The

0008-6215/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.carres.2005.07.002

* Corresponding author. Tel.: +91 03463 262293; fax: +91 03463262756; e-mail: ak_das3@rediffmail.com

kinetics of oxidative degradation of different sugarsby metal ions including CrVI has been studied underdifferent conditions.5–11 The present studies aims tocontribute in this direction and we note that in regardto the reaction mechanism, the conclusions drawn bySengupta and Basu7 differ from those of Sala et al.10

in many aspects. The present investigations have beencarried out in micro-heterogeneous systems relevantto biological systems in the presence of the chelatingagent bipyridine (bipy). Picolinic acid (PA) is a chelat-ing agent that is well known12–23 to catalyse CrVI oxi-dation reactions. The structure of bipyridine iscomparable to that of picolinic acid in many respectsand, in particular, both chelating agents are heteroaro-matic nitrogen bases.

epresence

of2,20 -bipyridyl

inaq

ueousH

2SO

4media

(precessionin

theconcentrationof

eff(w

)akH(c)(w)(·

104,

dm

3/m

ols)c

ks(c)(w

)(·

102,

dm

3/m

ols)d

ks(c)(sds)(·

102,

dm

3/m

ols)d

ks(c)(cpc)(·

102,

dm

3/m

ols)d

1.9±

0.1

2.9±

0.1

0.5±

0.01

6.3

11.7

±0.2

1.3

7.3

tant;(C

PC)or(SDS)fortheva

luein

presence

oftherespective

surfactant.

2mol/dm

3,

[CPC] T

=3·10

�3mol/dm

3,

[SDS] T

=5·10

�3mol/dm

3an

dkeff(w

)=

ol/dm

3,[H

ClO

4] T

+[N

aClO

4] T

=1.5mol/dm

3.

ol/dm

3,[CPC] T

=3·10

�3mol/dm

3,[SDS] T

=6·10

�3mol/dm

3.

2164 R. Bayen et al. / Carbohydrate Research 340 (2005) 2163–2170

2. Results and discussion

2.1. Dependence on [CrVI]T

The rate of disappearance of CrVI shows a first orderdependence on CrVI in the presence and absence of bipyunder the conditions, [DD-glucose]T � [bipy]T � [CrVI]Tand [bipy]T � [CrVI]T. In the presence of surfactants,the first order dependence on CrVI remains unaltered.The pseudo first order rate constants (kobs) were evalu-ated from the linear plot of log [CrVI]t versus time (t)as usual.

2.2. Dependence on [bipy]T

At [DD-glucose]T = 6 · 10�3 mol/dm3 and [CrVI]T = 4 ·10�4 mol/dm3, [H2SO4] = 0.5 mol/dm3, the effect of bipyon kobs has been studied. The plots of kobs versus [bipy]Tare linear (r > 0.99) with positive intercepts measuringthe contribution of the relatively slower uncatalysedpath (Fig. 1). The pseudo first order rate constants(kobs(u)) directly measured in the absence of bipy agreewell with those obtained from the intercepts of the plotsof kobs(T) versus [bipy]T. The observation is formulatedas follows:

sein

th

,k 1 1

surfac

.6·10

�

.2–1.0

m0.25

m

Figur

in th10�4 m(A) [T = 3

kobsðTÞ ¼ kobsðuÞ þ kobsðcÞ ¼ kobsðuÞ þ kcat½bipy�T ð1Þ

CrV

IoxidationofDD-gluco

(·10

2,

ls)b

kcat(sds)(·

102

dm

3/m

ols)b

16.8±

0.3

eva

luein

theab

sence

of

mol/dm

3,

[bipy]

T=

0–1

dm

3.

10�2mol/dm

3,[H

+] T

=0

10�2mol/dm

3,[H

2SO

4]=

The values of kcat with the activation parameters aregiven in Table 1. During the progress of the reaction,bipy is lost due to the formation of the inert CrIII–bipycomplex. Under the conditions, [bipy]T � [CrVI]T, the[bipy]T remains more or less constant over the courseof the reaction.

Table1.Kineticparam

etersan

dsomerepresentative

rate

constan

tsforthe

reag

ents

areusedwithin

±2.0%

)a

Tem

p(�C)

kobs(u)(w)(·

104,

s�1)b

kcat(w)(·

102,

dm

3/m

ols)b

kcat(cpc)

dm

3/m

o

30 350.3

4.9±

0.2

2.2±

0.45

0.6

6.8±

0.3

551.2

8.8±

0.4

DH

�(kJmol�

1)

20±

0.2

DS�(J

K�1mol�

1)

�20

6±

11

aSubscript(u)foruncatalysedpath;(c)forbipycatalysedpath;(w

)forth

b[CrV

I ]T=

4·10

�4mol/dm

3,

[S] T

=6.0·10

�3mol/dm

3,

[H2SO

4]=

0.5

[kobs(T)�

kobs(u)]/k

obs(u)an

dkeff(w

)calculatedat

[bipy]

T=

1.0·10

�2mol/

c[CrV

I ]T=

4.0·10

�4mol/dm

3,[S] T

=8.0·10

�3mol/dm

3,[bipy]

T=

0.5·

d[CrV

I ]T=

4·10

�4mol/dm

3,[S] T

=1–

10·10

�2mol/dm

3,[bipy]

T=

0.4·

0

4

8

12

16

0 10 15 20

103 [bipy]T mol dm-3

104 k o

bs (T

) s-1

A

B

C

5

e 1. Effect of [bipy]T on kobs(T) for the CrVI oxidation of DD-glucose

e presence of bipy in aqueous H2SO4 media. [CrVI]T = 4 ·ol/dm3, [DD-glucose]T = 6 · 10�3 mol/dm3, H2SO4 = 0.5 mol/dm3.

CPC]T = 3 · 10�3 mol/dm3, T = 35 �C. (B) CPC = 0 mol/dm3,5 �C. (C) CPC = 0 mol/dm3, T = 55 �C.

3

4

(u) s

-1 9

12

(c) s

-1

B

R. Bayen et al. / Carbohydrate Research 340 (2005) 2163–2170 2165

2.3. Dependence on [DD-glucose]T

From the plot of kobs versus [DD-glucose]T (Fig. 2) it wasestablished that the catalysed path shows a first orderdependence on [S]T where S denotes DD-glucose, that is

104 k

s-1

Figure 2. Effthe presencedm3, [H2SO(A) [CPC]Tdm3, (C) SD

1

2

104 k o

bs

3

6

104 k o

bs

A

Scheme 1. C

kobsðcÞ ¼ kobsðTÞ � kobsðuÞ ¼ ksðcÞ½S�T ð2Þ

kobsðuÞ ¼ ksðuÞ½S�T ð3Þ

00 0.4 0.8 1.2

[H+] mol dm-3

0

Figure 3. Effect of [HClO4]T on kobs(u) and kobs(c) for the CrVI

oxidation of DD-glucose in the presence and absence of bipy. [CrVI]T =4 · 10�4 mol/dm3, [DD-glucose]T = 80 · 10�4 mol/dm3, [HClO4] +

3

2.4. Dependence on [H+]

Acid dependence patterns for the uncatalysed and catal-ysed paths are totally different (Fig. 3). From the exper-imental fit, the observations are

0

5

10

15

0

obs

(c)

ect of [Sof bipy

4] = 0.25= 3 · 10S = 6 ·

[NaClO4] = 1.5 mol/dm , T = 35 �C. (A) for kobs(u): [bipy]T = 0 mol/dm3; (B) for kobs(c): [bipy]T = 50 · 10�4 mol/dm3.

rVI oxid

kobsðuÞ ¼ kHðuÞ½Hþ�2 ð4aÞ

kobsðcÞ ¼ kHðcÞ½Hþ� ð4bÞ

2.5. Test for free radical formation

The existence of free radicals has been indicated by poly-merisation of acrylonitrile under a nitrogen atmosphereunder the experimental conditions.

6102 [D-Glucose]T mol dm-3

A

BC

3 9

]T on kobs(c) for the CrVI oxidation of DD-glucose in

in aqueous H2SO4 media. [CrVI]T = 4 · 10�4 mol/mol/dm3, [bipy]T = 4 · 10�3 mol/dm3, T = 30 �C.

�3 mol/dm3, (B) CPC = 0 mol/dm3, SDS = 0 mol/10�3 mol/dm3.

OOH

O

HOP = HO

OH

12

3

4

5

6

(δ lactone)

H2OHO

HO3

46

(free D-g

ation of DD-glucose (denoted by Glu–OH) in the absence

2.6. Mechanism of the reaction

The mechanism of the reaction can be divided in twoparts as (i) the uncatalysed path (Scheme 1) and (ii)the catalysed path (Scheme 2).

Glu-OHþHCrO�4 þHþ

¢K1

Glu-OCrO3HðAÞ

þH2O ð5Þ

AþHþ¢K2

Glu-OCrO3Hþ2 ðAHþÞ ð6Þ

AHþ !k1 ProductðPÞ þ CrIV ð7Þ

CrIV ������!CrVI;Glu-OHProductðPÞ þ CrIII ð8Þ

where

OOH

OHO

HO

OHH

CrOH2

O O

AH+ =

6

2.6.1. Uncatalysed path. In aqueous acidic media,DD-glucose exists predominantly in hemiacetal cyclicpyranose forms as a mixture of a (OH-1, axial) andb (OH-1, equatorial) anomers.7,24,25 The concentrationof the open chain aldehyde form,26,27 the intermediate

CO2H

OHOH

OH

125

luconic acid)

- H2O OO

OH

OHHO

HO

1

23

4

5

6

(γ lactone)

of 2,2 0-bipyridine.

NN

Cr

OO

GluH

O

-

+ +

(C2)

k 2

CrIV - bipy Glu

CrIV - bipyProduct (P) + (13)

(14)

OH2

OHCrVI ,Product (P)( fast ) + CrIII

Scheme 2. CrVI oxidation of DD-glucose denoted by (Glu–OH) in thepresence of 2,2 0-bipyridine.

2166 R. Bayen et al. / Carbohydrate Research 340 (2005) 2163–2170

in the dynamic equilibrium between the anomers, is verysmall. The rate of mutarotation (i.e., a-DD-glucose«b-DD-glucose) is known to be acid catalysed28 and,under these oxidation conditions, the mutarotationequilibrium is immediately attained.28 Thus, the rate ofoxidation of DD-glucose is the summation of rates con-tributed by each of the a- and b-anomers in additionto the possible contribution from the open chain alde-hyde form.11 The cyclic hemiacetal forms are expectedto be more reactive species because the hydroxy groupsare better exposed to interact with the CrVI species.10

In fact, esterification of the –OH group of the substratewith chromic acid is the first step of CrVI oxidation ofDD-glucose. Among the a- and b-anomers, the b-formbearing the –OH group at C1 at the equatorial positionis more suitably exposed29 to attack the chromicacid. Thus, DD-glucose is mainly oxidised as the b-pyra-nose form and the initial product is the correspondinglactone. We have already established the kineticsfor the uncatalysed path (Scheme 1) for DD-glucose18 asfollows:

kobsðuÞ ¼ ð2=3Þk1K1K2½S�T½Hþ�2 ð9Þ

NN

+ H+ Kb

NN+

H(L)(LH )+

(Kb = 2.8 x 104 Ref. 30)

ð10Þ

HCrO4-

+ 2H++ LH +K3

NN+ +

Cr

O

O(C1)

OH2H2O

ð11Þ

(C1) +K 4

NN

Cr

O

OO

GluH

-

+ + + +

(C2)

OH2

H3OGlu OH

ð12Þ

2.6.2. Catalysed path. For the bipy-catalysed path, thereactions in Scheme 2 can explain the experimental find-ings and lead to the following rate law:

kobsðcÞ ¼ ð2=3Þk2K3K4½S�T½L�T½Hþ� ð15Þ

The final CrIII–bipy complex has been characterisedspectroscopically and are outlined below. As CrIII ðt32gÞis an inert species, it is reasonable to consider that theheteroaromatic nitrogen base (i.e., bipy) does not bindto the CrIII-centre after its formation from the reductionof CrVI. Therefore, we suggest that the heteroaromaticnitrogen base (denoted by L) undergoes complexationwith the higher oxidation states of chromium, whichare labile. The CrVI-species (i.e., chromic acid) is typi-cally labile and undergoes complexation at the first stepwith the chelating agent (L) to produce the chelate com-plex (C1), which is believed to be the kinetically activeoxidant.31 Under these reaction conditions, the firstorder dependence on [bipy]T is strictly maintainedthroughout the range of [bipy]T used. Thus, it is reason-able to conclude that the equilibrium constant for thereaction leading to CrVI–bipy complex (C1) is low. Inthe next step, the CrVI–bipy complex reacts with the sub-strate to form a ternary complex (Eq. 12), which experi-ences redox decomposition through a cyclic transitionstate in the rate-limiting step giving rise to the organicproduct and CrIV–bipy complex. The negative value ofDS� (entropy of activation, Table 1) of the compositerate constant kcat supports the suggested cyclic transi-tion state. The fairly high DH� (enthalpy of activation)value (Table 1) indicates that the bipy-catalysed pathis favoured mainly due to the very high negative valueof DS�. The CrIV-species produced in the rate-limitingstep participates in the next faster steps to give the finalproduct. The different possible routes are given below:

Path I

CrIV þ CrVI ! 2CrV

2CrV þ 2S ! 2CrIII þ Products

Path II

CrIV þ S ! CrIII þ S�CrVI þ S� ! CrV þ Products

CrV þ S ! CrIII þ Products

(C1)W (C1)m

R. Bayen et al. / Carbohydrate Research 340 (2005) 2163–2170 2167

Path III

0

3

6

9

12

104 k o

bs (T

) (s-1

)

Figure 4. Effectglucose in the p4 · 10�4 mol/dmmol/dm3, [bipy]

+(D-glucose )w

+ Dn +(D-glucose)m

Products

CrIV þ S ! Productsþ CrII

CrII þ CrVI ! CrIII þ CrV

CrV þ S ! Productsþ CrIII

Scheme 3. Partitioning of the reactive species between the aqueous andmicellar phases.

In the aforementioned possible paths, S denotes thesubstrate acting as a two-electron reductant and S�stands for the partially oxidised substrate. In both theWatanabe–Westheimer mechanism32 (i.e., Path I) andthe Perez-Bennito mechanism33,34 (i.e., Path III), the ti-tle organic substrate acts in all steps as a two-electronreductant, while it acts both as a two-electron reductantand a one-electron reductant in the Rocek mechanism35

(i.e., Path II). Previously, the Rocek mechanism35 wasaccepted widely in explaining the CrVI oxidation of dif-ferent organic substrates and the Perez-Bennito mecha-nism33,34 was discarded because of the instability ofCrII. Recently, however, it has been proven33,34 that forthe oxidation of different two-electron organic reductants,CrII is produced from CrIV through hydride transfer.Thus, the carbocationic centre generated is responsiblefor acrylonitrile polymerisation.36 It may be noted thatin the Rocek mechanism,35 the free radical S� is supposedto 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.The plot of kobs(T) versus [CPC]T (Fig. 4) indicates thatthe rate decreases in a continuous manner, and it tendsto level off at higher CPC concentration. Bunton andCerichelli37 noted a similar observation in the oxidationof ferrocene by FeIII salts in the presence of cationic sur-factant cetyl trimethyl ammonium bromide (CTAB).The present observation is also similar to those observedby Panigrahi and Sahu38 in the oxidation of acetophe-

0 4 8

103 [CPC]T (mol dm-3)102 6

of [CPC]T on kobs(T) for the CrVI oxidation of DD-resence of bipy in aqueous H2SO4 media, [CrVI]T =3, H2SO4 = 0.5 mol/dm3. [DD-glucose]T = 180 · 10�4

T = 60 · 10�4 mol/dm3, T = 35 �C.

none by CeIV, by Sarada and Reddi39 in the oxalic acidcatalysed oxidation of aromatic azo-compounds byCrVIin the presence of surfactant sodium dodecyl sulfate(SDS). In the uncatalysed path, the neutral CrVI-sub-strate ester (A) formed (Eq. 5) can be partitioned inthe micellar pseudo-phase of the surfactant but the cat-ionic surfactant repelling H+ needed for the redoxdecomposition of the ester (Eqs. 6 and 7) inhibits thereaction. In the bipy-catalysed path, CPC restricts thepositively charged CrVI–bipy complex (C1), the activeoxidant, in the aqueous phase and thus the accumulatedneutral substrate in the micellar phase (Stern layer) can-not participate in the reaction. Therefore in both theuncatalysed and bipy-catalysed paths, the reaction ismainly 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. Partitioningof the reactants between the aqueous and micellar phaseis shown in Scheme 3 in which Dn represents micellisedsurfactants where n is the aggregation number.

2.8. Effect of SDS

Sodium dodecyl sulfate (SDS), a representative anionicsurfactant, catalyses the oxidation reaction both in thepresence and absence of bipy. In the bipy-catalysedpath, the rate acceleration arises due to the preferentialpartitioning of the positively charged CrVI–bipy com-plex (C1) by an electrostatic attraction and the neutralsubstrate in the micellar surface Stern layer. In the unca-talysed path, the neutral CrVI-substrate ester (A) is par-titioned in the micellar phase (Stern layer) and the H+

ions needed for the redox decomposition of the ester(A) (Eqs. 6 and 7) are also preferentially accumulatedin the micellar interphase due to an electrostatic attrac-tion. Thus, SDS allows the reaction to proceed in boththe aqueous and micellar interphases. However, thereaction is more favoured in the micellar pseudo-phasebecause of the enhanced concentration of the reactants(i.e., A and H+) in the micellar interphase. In thebipy-catalysed reaction, the plot of kobs(T) versus [SDS]T(Fig. 5) indicates that the rate increases in a continuousfashion up to the SDS concentration used. An increasein [SDS]T increases the micellar solubilisation of thereactants but at the same time an increase in [SDS]Tincreases the concentration of the micellar counter ions

0

4

8

12

16

0 0.4 0.8 1.2

102 [SDS]T mol dm-3

104 k o

bs(T

) s-1

Figure 5. Effect of [SDS]T on kobs(T) for the CrVI oxidation of DD-glucose in the presence of bipy in aqueous H2SO4 media, [CrVI]T =4 · 10�4 mol/dm3, [bipy]T = 4 · 10�3 mol/dm3, [H2SO4] = 0.25 mol/dm3, [DD-glucose]T = 6 · 10�3 mol/dm3, T = 30 �C.

2168 R. Bayen et al. / Carbohydrate Research 340 (2005) 2163–2170

(i.e., Na+), which may displace H+ and OX2+ ions (C1)out of the micellar surface.

2NaþW þOX2þM ¢ 2NaþM þOX2þ

W ð16Þ

NaþW þHþM ¢NaþM þHþ

W ð17Þ

The plot of kobs(T) versus [SDS]T indicates that thesolubilisation effect is greater than the counter-ion effectfor the bipy-catalysed path up to the SDS concentrationused. Here, it may be pointed out that the organic prod-uct DD-gluconic acid is also partitioned between themicellar (both cationic and anionic) and aqueousphases. However, this partitioning does not have anyeffect on the rate process or reaction mechanism.

400 450 500 550 600 650 700

0.0

0.1

0.2

0.3

0.4

(b)

(a)

Wavelength (nm)

Abso

rban

ce

Figure 6. (a) Absorption spectra (absorbance per cm) of the reactionmixture (after completion of reaction): [CrVI]T = 3.85 · 10�4 mol/dm3,[DD-glucose]T = 60 · 10�3 mol/dm3, [bipy]T = 0 mol/dm3 (i.e., uncatal-ysed path), [H2SO4] = 0.5 mol/dm3. (The spectrum of the chromicsulfate is identical under the experimental conditions.) (b) Absorptionspectra (absorbance per cm) of the reaction mixture (after completionof reaction): [CrVI]T = 3.85 · 10�4 mol/dm3, [DD-glucose]T = 60 · 10�3

mol/dm3, [bipy]T = 8 · 10�3 mol/dm3, [H2SO4] = 0.5 mol/dm3.

3. Experimental

3.1. Materials and reagents

2,2 0-Bipyridine (Qualigens, India) DD-glucose (SRL),K2Cr2O7 (BDH), sodium dodecyl sulfate (SRL), N-cetylpyridinium chloride (SRL), H2SO4 (E. Merck),HClO4 (E. Merck) and all other chemicals used wereof highest purity available commercially. The solutionswere prepared in doubly distilled water.

3.2. Procedure and kinetic measurements

Solutions of the oxidant and reaction mixtures contain-ing known quantities of the substrate (S) (i.e., DD-glu-cose), catalyst (bipy) (under the conditions [S]T �[CrVI]T and [bipy]T � [CrVI]T) acid and other necessarychemicals were separately thermostated (±0.1 �C). Thereaction was started by mixing the requisite amountsof the oxidant with the substrate. The progress of thereaction was monitored by following the rate of disap-pearance of CrVI by a titrimetric quenching technique.40

The pseudo first order rate constants (kobs) were calcu-lated as usual. Under the experimental conditions, the

possibility of decomposition of the surfactants by CrVI

was investigated and the rate of decomposition in thispath was found to be kinetically negligible.

3.3. Product analysis and stoichiometry

Under the kinetic conditions (i.e., [DD-glucose]T �[CrVI]T), qualitative identification of the organic reac-tion product was carried out by paper-chromatogra-phy,8,10,11 using n-butyl alcohol–acetic acid–water(4:1:5) as the eluant. To characterise the oxidation prod-ucts, a series aldopentoses and aldohexoses were oxi-dised with nitric acid and bromine water41 separatelyand the purified products were used as authentic stan-dards in the chromatographic procedure. DD-Gluconicacid and the corresponding lactone were identified42 asthe only reaction products. The observed Rf value forDD-gluconic acid is also in good agreement with thereported value.43 The paper chromatogram developedfrom the product solution also indicates the presenceof unreacted substrate DD-glucose (Rf = 0.18). Theobserved Rf value of DD-glucose was also compared withthat of authentic sample under the same conditions. Theobserved Rf value for glucose agrees well with thereported value (Rf = 0.19).43 The paper chromatogramobtained from the product solution indicates theabsence of any other organic compounds. Thus, it isevident that during the reaction the substrate did notdegrade under these conditions.The final fate of CrIII-species was confirmed spectro-

scopically. The UV–vis spectra (Fig. 6) were recordedon a UV–vis–NIR scanning spectrophotometer, (UV-3101PC, Shimadzu). The characteristic part of electronic

R. Bayen et al. / Carbohydrate Research 340 (2005) 2163–2170 2169

absorption spectra of CrIII species lies in the range 360–600 nm. The colour of the final solution of bipy-cataly-sed reaction in aqueous H2SO4 media is pale violet[kmax = 548 nm for 4A2g!4T2g] while the colour of thefinal solution for the uncatalysed reaction (i.e., in theabsence of bipy) under identical conditions is pale blue[kmax = 578 nm for 4A2g!4T2g; and 412 nm for4A2g!4T1g(F)]. The spectrum of the final solution ofthe uncatalysed reaction and pure chromic sulfate solu-tion in aqueous sulfuric acid media is identical. Thisindicates that the final CrIII-species is simply chromicsulfate for the uncatalysed reaction while for the bipy-catalysed reaction, the final CrIII-species is a CrIII–bipycomplex. Similar results have been noted by the earlierworkers.31,44 It is interesting to note that for the finalsolution of the bipy-catalysed reaction, there is a blueshift (Fig. 6) for the peak due to the transition4A2g!4T2g. This blue shift is due to the presence ofthe strong field ligand like bipy. For the CrIII–bipy com-plex, the peak due to the transition 4A2g!4T1g(F)merges with a charge transfer band (Fig. 6).

4. Conclusions

The cationic CrVI–bipy complex has been found to act asthe active oxidant for the bipy-catalysed path of oxida-tion of DD-glucose to DD-gluconic acid in aqueous acidicmedia. This complex is produced in a rapid pre-equilib-rium step in the reaction of CrVI and bipy, which then re-acts with DD-glucose to produce a ternary complex thatexperiences the redox decomposition (two-electrontransfer) in the rate-determining step. The reactions havebeen carried out in aqueous micellar media. The effects ofboth the cationic and anionic surfactants have been fol-lowed and the observed micellar effects are in agreementwith the proposed reaction mechanism. The mechanisticpath of the uncatalysed and bipy-catalysed chromic acidoxidation of DD-glucose have been compared.The organic product, DD-gluconic acid, was identified

by paper chromatography and the observed Rf valuehas been found in good agreement with the reportedvalue. This was further supported by comparisonwith an authentic sample of DD-gluconic acid underthe same conditions. In the reaction, CrVI is finallyreduced to CrIII. The fate of CrIII-species in the finalsolution was determined by following the UV–visspectra. In the uncatalysed reaction, the species issimply chromic sulfate (pale blue, kmax = 412 and578 nm) while for the bipy-catalysed reaction, the corre-sponding species is a CrIII–bipy complex (pale violetcolour, kmax = 548 nm). In the CrIII–bipy complex,the peak at 548 nm (due to the transition 4A2g!4T2g)experiences a blue shift (with respect to the simply chro-mic sulfate species obtained in the case of uncatalysedreaction) due to the presence of the strong field ligand

bipy. The existence of CrIII–bipy complex in thefinal product solution for the bipy-catalysed reactionsupports the formation of CrVI–bipy complex in thepre-equilibrium step. This is a reasonable proposal asthe CrVI centre is kinetically labile and the CrVI–bipycomplex is finally reduced to a CrIII–bipy complex. Incontrast, formation of CrIII–bipy complex after thereduction of CrVI to CrIII is ruled out as CrIII ðt32gÞ is akinetically inert centre.

Acknowledgements

Thanks are due to Visva-Bharati (Santiniketan) forfinancial assistance. The authors are thankful to Dr.D. Mondal, Dr. B. C. Bag, Saugata Pal and BiplabGoswami of this department for their help.

Appendix A. Derivation of rate law for DD-glucose

(considering Scheme 2)

Rate ¼ k2½C2� ðiÞ

K4 ¼½C2�½Hþ�½C1�½S�

or; ½C2� ¼K4½C1�½S�½Hþ� ðiiÞ

K3 ¼½C1�

½HCrO�4 �½Hþ�2½LHþ�

ðiiiÞ

or; ½C1� ¼ K3½HCrO�4 �½Hþ�2½LHþ�

The total concentration of L (= bipy) is given by

½L�T ¼ ½L� þ ½LHþ� ðivÞ

Kb ¼½LHþ�½L�½Hþ� ; or; ½L� ¼ ½LHþ�

Kb½Hþ� ðvÞ

or; ½L�T ¼ ½LHþ�Kb½Hþ� þ ½LHþ� ¼ ½LHþ� 1þ 1

Kb½Hþ�

� �ðviÞ

¼ ½LHþ� Kb½Hþ� þ 1

Kb½Hþ�

� �� ½LHþ�

Since Kb[H+] � 1 (Kb = 2.8 · 104 for bipy) (Ref. 30)

It leads to: ½C2� � K3K4½S�T½HCrO�4 �½Hþ�½L�T

Rate ¼ K3K4k2½S�T½HCrO�4 �½L�T½H

þ�2

½Hþ�¼ K3K4k2½S�T½HCrO�

4 �½L�T½Hþ�

and,

� d ln½HCrO�4 �

dt¼ kobsðcÞ

¼ ð2=3ÞK3K4k2½S�T½L�T½Hþ� ð15Þ

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

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