Journal of Molecular Catalysis A: Chemical 236 (2005) 260–266

Kinetics and mechanism of 2,2′-bipyridyl and1,10-phenanthroline-catalysed chromium(VI)

oxidation ofd-fructose in aqueous micellar media

Monirul Islam, Bidyut Saha, Asim K. Das∗

Department of Chemistry, Visva Bharati University, Santiniketan, West Bengal 731235, India

Received 18 April 2005; received in revised form 23 April 2005; accepted 24 April 2005

Abstract

The kinetics and mechanism of CrVI oxidation of fructose in the presence and absence of 2,2′-bipyridyl (bipy) and 1,10-phenanthroline(phen) in aqueous acid media have been studied under the condition [sugar]T � [CrVI ]T at different temperatures. Under the kinetic conditions,both the slower uncatalysed and faster catalysed (by heteroaromatic N-bases i.e., bipy, phen) paths go on. The monomeric species of CrVI hasb rCu icN factants.E e-catalysedp d effects haveb©

K

1

eeueiagohfe

n ofdif-ut

con--s.kerswellt-f dif-einecat-

est.and

1d

een found to be kinetically active in the absence of bipy and phen whereas in the heteroaromatic N-base catalysed path, the CVI -bipy andrVI -phen complexes have been found to be the active oxidants. Both the paths show the first-order dependence on [sugar]T and [CrVI ]T. Thencatalysed path shows a second-order dependence on [H+]. But the catalysed path shows a first-order dependence on [H+]. The heteroaromat-base-catalysed path is first-order in [bipy]T or [phen]T. These observations remain unaltered in the presence of externally added surffect of the cationic surfactant (i.e. CPC) and anionic surfactant (i.e. SDS) on both the uncatalysed and heteroaromatic N-basaths has been studied. CPC inhibits both the uncatalysed and catalysed paths while SDS accelerates the reactions. The observeeen explained by considering the hydrophobic and electrostatic interaction between the surfactants and reactants.2005 Elsevier B.V. All rights reserved.

eywords:Kinetics; Oxidations; Catalysis;d-Fructose; Chromium(VI); 1,10-Phenanthroline; 2,2′-Bipyridine; Surfactants

. Introduction

The kinetics of oxidative degradation of sugars by differ-nt metal ions have been the subject of numerous studies toxplore the chelating and reducing properties of the sugarsnder different conditions[1,2]. Reduction of CrVI by differ-nt reducing sugars is relevant in understanding the chem-

stry of chromium in the environment where CrVI appearss a hazardous one because of its mutagenic and carcino-enic activity [1,3]. The kinetics and mechanistic aspectsf the interactions of different types of sugars with CrVI

ave been investigated by different workers[4,5] under dif-erent conditions. The proposed mechanisms involve two-lectron transfer at the rate-determining step and CrV has

∗ Corresponding author. Tel.: +91 3463 262293; fax: +91 3463 262756.E-mail address:ak das3@rediffmail.com (A.K. Das).

also been proposed to exist in the oxidative degradatiothe sugars. Comparison of the kinetics of oxidation offerent sugars by CeIV , CrVI and VV has been carried oby Virtanen and Lindroos-Heinanen[2a]. Here it is worthmentioning that regarding the reaction mechanism, theclusions drawn by Sengupta and Basu[4a]are different compared to those given by Sala et al.[4d] in many aspectIt may be pointed out that these two groups of worused different types of sugars. Picolonic acid (PA) isknown[5–8] to catalyse the CrVI oxidation reactions. Kineics and mechanistic aspects of PA-catalysed oxidation oferent sugars have been reported by us recently[5]. Becausof the structural similarity among bipyridyl, phenanthroland picolinic acid (all are heteroaromatic N-bases), thealytic effects of bipyridyl and phenanthroline are of interThe present paper deals with the mechanism of bipyphen catalysis in CrVI oxidation of a ketohexose (d-fructose)

381-1169/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.molcata.2005.04.019

M. Islam et al. / Journal of Molecular Catalysis A: Chemical 236 (2005) 260–266 261

along with the micellar effect on the bipy and phen-catalysedpaths.

2. Results and discussion

2.1. Dependence on [CrVI]T

Both in the presence and absence of heteroaromaticN-bases bipy and phen, under the experimental conditions:[d-fructose]T � [bipy]T and [phen]T � [CrVI ]T, the rate ofdisappearance of CrVI shows a first-order dependence onCrVI . In the presence of surfactants, the first-order depen-dence on CrVI remains also unaltered. The pseudo-first-orderrate constants (kobs) were evaluated from the linear plot oflog[CrVI ]T versus time (t) as usual.

2.2. Dependence on [bipy]T and [phen]T

The plots ofkobsversus [bipy]T andkobsversus [phen]T arelinear (r > 0.99) with positive intercepts measuring the con-tribution of the relatively slower uncatalysed path (cf.Fig. 1).The pseudo-first-order rate constants (kobs(u)) directly mea-sured in absence of bipy or phen nicely agree with thoseobtained from the intercepts of the plots ofkobs(T) versus[ ref

k

T eni y,pa y]a ion[

Ftm n:[(H n:[(H Ta

ble

1K

inet

icpa

ram

eter

san

dso

me

repr

esen

tativ

era

teco

nsta

nts

for

the

Cr

VI

oxid

atio

nof

d-fr

ucto

sein

the

pres

ence

of1,

10-p

hena

nthr

olin

ein

aque

ous

H2S

O4

med

ia

Tem

pera

ture

(◦ C)

k obs

(u)(

w)

(×10

4s−

1)a

k cat

(w)

(×10

2dm

3m

ol−1

s−1)a

k cat

(cpc

)

(×10

2dm

3m

ol−1

s−1)a

k eff(

w)

(×10

2)a

k H(c

)(w

)

(dm

3m

ol−1

s−1)b

k s(c

)(w

)

(×10

2dm

3m

ol−1

s−1)c

k s(c

)(sd

s)

(×10

2dm

3m

ol−1

s−1)c

k s(c

)(cp

c)

(×10

2dm

3m

ol−1

s−1)c

304.

1±

0.05

5.8±

0.15

1.2±

0.04

350.

1±0.

015.

0±0.

1849

17.7±

0.30

450.

3±0.

037.

2±0.

265.

3±0.

2024

550.

4±0.

049.

1±0.

3323

�H

=(k

Jm

ol−1

)19

.8±

0.91

�S=

(JK

−1m

ol−1

)−2

08±

9.5

Sub

scrip

t(u)

for

unca

taly

sed

path

;(c)

for

1,10

-phe

n-ca

taly

sed

path

;(w

)fo

rth

eva

lue

inth

eab

senc

eof

surf

acta

nt;(

CP

C)

or(S

DS

)fo

rth

eva

lue

inpr

esen

ceof

the

resp

ectiv

esu

rfac

tant

.a

[CrV

I ]T

=4.

0×

10−4

mol

dm−3

,[H

2S

O4]=

0.25

mol

dm−3

,[S

] T=

5.0×

10−3

mol

dm−3

,[1,

10-p

hen] T

=(0

–1.3

)×10

−2m

oldm

−3,[

CP

C] T

=2×

10−3

mol

dm−3

.kef

f(w

)=

(kob

s(T

)−k o

bs(u

))/k o

bs(u

)and

k eff(

w)

calc

ulat

edat

[1,1

0-ph

en]

T=

1.0×

10−2

mol

dm−3

.b

[CrV

I ]T

=4.

0×

10−4

mol

dm−3

,[S

] T=

6.0×

10−3

mol

dm−3

,[1,

10-p

hen] T

=0.

4×

10−2

mol

dm−3

,[H

+] T

=(0

.2–1

.0)m

oldm

−3.

c[C

rVI ]

T=

4.0×

10−4

mol

dm−3

,[S

] T=

(0.6

–4.0

)×10

−2m

oldm

−3,[

1,10

-phe

n] T=

0.4×

10−2

mol

dm−3

,[H

2S

O4]=

0.25

mol

dm−3

,[S

DS

] T=

0.75

×10

−2m

oldm

−3,[

CP

C] T

=0.

4×

10−2

mol

dm−3

.

bipy]T andkobs(T) versus [phen]T. These observations aormulated as follows:

obs(T) = kobs(u) + kobs(c)

= kobs(u)+ kcat[L] T (L = bipy, phen) (1)

he values ofkcat with the activation parameters are givn Tables 1 and 2. During the progress of reaction biphen are lost due to the formation of inert CrIII -bipynd CrIII -phen complexes. Under the conditions: [bipTnd [phen]T � [CrVI ]T, during the progress of the react

bipy]T and [phen]T remain more or less constant.

ig. 1. Effect of [bipy]T and [phen]T on kobs(T) for the CrVI oxida-ion of d-fructose in the presence of bipy and phen in aqueous H2SO4

edia, [CrVI ]T = 4× 10−4 mol dm−3. (A) For bipy-catalysed reactiod-fructose]T = 60× 10−4 mol dm−3, H2SO4 = 0.5 mol dm−3, T= 30◦C;B) for bipy-catalysed reaction: [d-fructose]T = 60× 10−4 mol dm−3,

2SO4 = 0.5 mol dm−3, T= 40◦C; (C) for phen-catalysed reactiod-fructose]T = 50× 10−4 mol dm−3, H2SO4 = 0.25 mol dm−3, T= 45◦C;D) for phen-catalysed reaction: [d-fructose]T = 50× 10−4 mol dm−3,

2SO4 = 0.25 mol dm−3, T= 55◦C.

262 M. Islam et al. / Journal of Molecular Catalysis A: Chemical 236 (2005) 260–266

Tabl

e2

Kin

etic

para

met

ers

and

som

ere

pres

enta

tive

rate

cons

tant

sfo

rth

eC

rV

Iox

idat

ion

ofd-

fruc

tose

inth

epr

esen

ceof

2,2

′ -bip

yrid

ylin

aque

ous

H 2S

O4

med

ia

Tem

pera

ture

(◦ C)

k obs

(u)(

w)

(×10

4s−

1)a

k cat

(w)

(×10

2dm

3m

ol−1

s−1)a

k cat

(cpc

)

(×10

2dm

3m

ol−1

s−1)a

k cat

(sds

)

(×10

2dm

3m

ol−1

s−1)a

k eff(

w)

(×10

2)a

k H(c

)(w

)

(dm

3m

ol−1

s−1)b

k s(c

)(w

)

(×10

2dm

3m

ol−1

s−1)c

k s(c

)(cp

c)

(×10

2dm

3m

ol−1

s−1)c

k s(c

)(sd

s)

(×10

2dm

3m

ol−1

s−1)c

300.

3±

0.03

4.6±

0.15

15.3

4.0±

0.03

1.1±

0.10

5.7±

0.15

3510

.1±

0.10

400.

4±

0.04

5.9±

0.20

4.4±

0.20

8.4±

0.30

13.8

500.

6±

0.06

7.5±

0.25

12.3

�H

=(k

Jm

ol−1

)18

.3±

0.87

�S=

(JK

−1m

ol−1

)−2

13±

10

Sub

scrip

t(u)

for

unca

taly

sed

path

;(c)

for

2,2

′ -bip

y-ca

taly

sed

path

;(w

)fo

rth

eva

lue

inth

eab

senc

eof

surf

acta

nt;(

CP

C)

or(S

DS

)fo

rth

eva

lue

inpr

esen

ceof

the

resp

ectiv

esu

rfac

tant

.a

[CrV

I ]T

=4.

0×

10−4

mol

dm−3

,[S

] T=

6.0×

10−3

mol

dm−3

,[2

,2′ -b

ipy]

T=

(0–2

.5)×

10−2

mol

dm−3

,[H

2S

O4]=

0.5

mol

dm−3

,[C

PC

] T=

2×

10−3

mol

dm−3

,[S

DS

] T=

1.5×

10−2

mol

dm−3

.k e

ff(w

)=

(kob

s(T

)–k o

bs(u

))/k o

bs(u

)and

k eff(

w)

calc

ulat

edat

[2,2′ -

bipy

] T=

1.0×

10−2

mol

dm−3

.b

[CrV

I ]T

=4.

0×

10−4

mol

dm−3

,[S

] T=

6.0×

10−3

mol

dm−3

,[2,

2′-b

ipy]

T=

0.5×

10−2

mol

dm−3

,[H

] T=

(0.2

–1.0

)mol

dm−3

.c

[CrV

I ]T

=4.

0×

10−4

mol

dm−3

,[S

] T=

(0.6

–3.0

)×10

−2m

oldm

−3,[

2,2′

-bip

y]T

=0.

5×

10−2

mol

dm−3

,[H

2S

O4]=

0.5

mol

dm−3

,[C

PC

] T=

6×

10−3

mol

dm−3

,[S

DS

] T=

4×

10−3

mol

dm−3

.

ig. 2. Effects of [S]T on kobs(c) for the CrVI oxidation of d-fructosein the presence of bipy and phen in aqueous H2SO4 media, [CrVI ]T

= 4× 10−4 mol dm−3: (A) [bipy]T = 5× 10−3 mol dm−3, [H2SO4] =0.5 mol dm−3, [CPC]T = 6× 10−3 mol dm−3, T= 30◦C; (B) [bipy]T =5× 10−3 mol dm−3, [H2SO4] = 0.5 mol dm−3, T= 30◦C; (C) [phen]T = 4× 10−3 mol dm−3, [H2SO4] = 0.25 mol dm−3, T= 30◦C.

2.3. Dependence on [d-fructose]T, i.e. [S]T

From the plot ofkobs versus [S]T (cf. Fig. 2), it is estab-lished that the catalysed path shows the first-order depen-dence on [S]T, i.e.:

kobs(c) = kobs(T)− kobs(u) = ks(c)[S]T (2)

kobs(u) = ks(u)[S]T (3)

2.4. Dependence on [H+ ]

Acid dependence patterns for the uncatalysed and catal-ysed paths are different (cf.Fig. 3). From the experimentalfit, the observations are:

kobs(u) = kH(u)[H+]

2(4)

kobs(c) = kH(c)[H+] (5)

Fom[==

ig. 3. Effect of [HClO4]T on kobs(x) (x= u or c) for the CrVI oxidationf d-fructose in the presence and absence of bipy, phen in aqueous H2SO4

edia, [CrVI ]T = 4× 10−4 mol dm−3, [d-fructose]T = 60× 10−4 mol dm−3,HClO4] + [NaClO4] = 1.5 mol dm−3,T= 35◦C: (A) x= u, [bipy]T = [phen]T0 mol dm−3; (B) x= c, [bipy]T = 50× 10−4 mol dm−3; (C) x= c, [phen]T40× 10−4 mol dm−3.

M. Islam et al. / Journal of Molecular Catalysis A: Chemical 236 (2005) 260–266 263

Scheme 1. CrVI oxidation ofd-fructose in the absence of catalyst.

2.5. Test for free radical formation

Under the experimental conditions, existence of free rad-ical was indicated by polymerisation of acrylonitrile under anitrogen atmosphere.

2.6. Reaction mechanism

The mechanism of the reaction can be divided in two sec-tions: (i) uncatalysed path and (ii) catalysed path. We havealready established the uncatalysed path (Scheme 1) for d-fructose[5a] as:

kobs(u) =(

2

3

)k1K1 K2[S]T[H+]

2(9)

For the bipy- and phen-catalysed paths,Scheme 2can explainall the experimental findings.Scheme 2leads to the followingrate law:

kobs(c) =(

2

3

)K3K4 k2[S]T[L] T[H+] (15)

Formation of CrIII -L complex characterised spectroscopi-cally indicates that the ligand heteroaromatic N-base (L) un-dergoes complexation with the higher oxidation states (whichare labile) of chromium. Because of the inertness of CrIII

Scheme 2. CrVI oxidation ofd-fructos

e in presence of bipy and phen.

264 M. Islam et al. / Journal of Molecular Catalysis A: Chemical 236 (2005) 260–266

(t32g), ligand does not enter to CrIII produced after the reduc-

tion of CrVI . Here bipy and phen readily form the reactivecyclic CrVI -L (where L = bipy, phen) complex (C1), whichis the active oxidant[9]. Under the experimental conditions,the first-order dependence on [bipy]T and [phen]T is strictlymaintained throughout the range of [L]T used. Hence it isreasonable to conclude that the equilibrium constant for thereaction, leading to cyclic CrVI -L complex (C1) is low. Kinet-ically, the monosaccharides can be considered as polyols inwhich the reactivities of alcoholicOH groups were expectedto be increase by the presence of the adjacent carbonyl group.In fact, in the case of oxidations by VV, the rate constant forhydroxy acetone is about 104 times[10,11]greater than thatfor glycol, but the rate constants ford-glucose andd-mannoseare only about 15 times[10,12]higher than that for glycol. Asimilar observation has been noted by us for CrVI oxidation ofglycols[7a]andd-glucose[5c]. In aqueous media,d-fructoseremains mainly as cyclic hemiacetals, which are in a dynamicequilibrium with the acyclic form. Between the furanoid andpyranoid cyclic forms, the latter is more stable[13]. Hencethe preponderant form of thed-fructose is�-pyranoid. Byconsidering the open chain structures of the sugars, it is rea-sonable to expect that the sugars should react much faster thanglycol. The observed decreased rate constant for the sugarshave been explained by some workers[12] by consideringt theo ver,so hasb roxygI sumc tiont ningi xts at si-t alueot stepgT nt

P

C

C

In the above-mentioned possible paths, S denotes the sub-strate acting as a 2e reductant and S• stands for the partiallyoxidised substrate. In both the Watanabe–Westheimer mech-anism[14] (i.e. Path I) and the Perez–Bennito mechanism[15] (i.e. Path III), the title organic substrate acts in all stepsas a 2e reductant, while it may act both as a 2e reductant and1e reductant in the Rocek mechanism[16] (i.e. Path II). Pre-viously, the Rocek mechanism[16] was accepted widely inexplaining the CrVI oxidation of different organic substratesand the Perez–Bennito mechanism[15] was discarded be-cause of the instability of CrII . But recently, it has been es-tablished[15] that for the oxidation of different 2e organicreductants, CrII is produced from CrIV through hydride trans-fer. Thus, the carbocationic centre generated is responsiblefor acrylonitrile polymerisation[17]. It may be noted that inRocek mechanism[16], the free radical S• is supposed to beresponsible for acrylonitrile polymerisation.

2.7. Effect of CPC

CPC, a representative cationic surfactant is found to retardboth the uncatalysed (already established)[5a] and catalysedpaths. The plots ofkobs(T)versus [CPC]T (Fig. 4) shows a con-tinuous decrease and finally it tends to level off at higher con-centrations of CPC. This observation is similar to that notedby Bunton and Cerichelli[19] in the oxidation of ferroceneb etylt gshod ddi[ zo-c theo[ ath,C x( ust Sternl atal-y eousp d due

Ft1d×

he fact that the oxidation goes predominantly throughpen chain form whose concentration is very low. Howeome workers have also suggested[4b,d,e]the participationf the cyclic forms in the redox reaction mechanism. Iteen assumed that in the cyclic hemiacetal forms, the hydroups are better exposed to interact with CrVI species[4d].

n reality, the pseudo-first-order rate constants are theontribution of each furanoid and pyranoid form in addio the possible contribution of the open chain form remain a dynamic equilibrium with the cyclic forms. In the netep, the CrVI -L complex reacts with the substrate to formernary complex (C2) which experiences a redox decompoion through a cyclic transition state. The large negative vf �S= (entropy of activation, cf.Tables 1 and 2) supports

he suggested cyclic transition state. The rate limitingiving rise to the organic product and the CrIV -L complex.hen the Cr1V-species i.e., CrIV -L complex participates i

he next faster steps as discussed below:

Path I : CrIV + CrVI → 2CrV,

2CrV + 2S → 2CrIII + Products

Path II : CrIV + S → CrIII + S•,

CrVI + S• → CrV + Products,

CrV + S → CrIII + Products

ath III : CrIV + S → CrII + Products,

rII + CrVI → CrIII + CrV,

rV + S → Products+ CrIII

y ferric salts in the presence of cationic surfactant crimethyl ammonium bromide (CTAB). The similar findinave also been reported by Panigrahi and Sahu[20] in thexidation of acetophenone by CeIV in the presence ofN-odecyl pyridinium chloride (NDPC), by Sarada and Re

21] in the oxalic acid catalysed oxidation of aromatic aompounds by CrVI in the presence of SDS and by us inxidation of d-glucose[5c], d-fructose[5a] and l-sorbose

5b] by CrVI in the presence of CPC. In the catalysed pPC restricts the positively charged CrVI -catalyst comple

C1) (cf. Eq. (11),Scheme 2) in the aqueous phase and thhe accumulated neutral substrate in the micellar phase (ayer) cannot participate in the reaction. Hence in the csed path, the reaction is mainly restricted in the aquhase in which concentration of the substrate is deplete

ig. 4. Effect of [CPC]T on kobs(T) for the CrVI oxidation ofd-fructose inhe presence of bipy and phen in aqueous H2SO4 media, [CrVI ]T = 4×0−4 mol dm−3, [H2SO4] = 0.5 mol dm−3: (A) [phen]T = 50× 10−4 molm−3, [d-fructose]T = 140× 10−4 mol dm−3, T= 40◦C; (B) [bipy]T = 12010−4 mol dm−3, [d-fructose]T = 60× 10−4 mol dm−3, T= 50◦C.

M. Islam et al. / Journal of Molecular Catalysis A: Chemical 236 (2005) 260–266 265

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

to its partitioning in the stern layer of the micelle. Partitioningof the reactants between the aqueous and micellar phase isshown inScheme 3.

2.8. Effect of SDS

SDS, a representative anionic surfactant, accelerates boththe uncatalysed and catalysed paths. In the catalysed path, therate acceleration arises owing to the preferential partitioningof the positively charged CrVI -catalyst complex (C1) (cf. Eq.(11), Scheme 2) (by electrostatic attraction) and the neutralsubstrate in the micellar interphase (Stern layer). Thus SDSallows the reaction to proceed in both aqueous and micellarinterphases. The plot ofkobs(T)versus [SDS]T (Fig. 5) showsa continuous increase up to the concentration of SDS used forthe bipy-catalysed reaction. For the phen-catalysed reaction,the rate increases with the increase of [SDS]T and attainsa limiting value followed by a slight rate retardation. Thisrate retardation is probably due to the dilution of reactantsin the micellar phase. An increase in [SDS]T increases themicellar solubilisation of the reactants but at the same time anincrease in [SDS]T increases the concentration of the micellarcounterions (i.e. Na+) which may displace H+ and Ox2+ ions(C1) out of micellar surface:

2NaW+ + OxM

2+ � 2NaM+ + OxW

2+ (16)

N

T[ ctsa n. Int ughly

Ft156

nullify each other at higher concentration to attain the ratesaturation. But in the case of bipy-catalysed path, the formereffect (solubilisation effect) is greater than the later effect(i.e., counterion effect) up to the used SDS concentration.

3. Experimental

3.1. Materials and reagents

2,2′-Bipyridyl (bipy) (AR, Qualigens, India), 1,10-phenanthroline (phen) (AR, Qualigens, India),d-fructose(AR, SRL, India), K2Cr2O7 (AR, BDH), sodium dodecyl sul-phate (SDS) (AR, SRL, India),N-cetylpyridinium chloride(CPC) (AR, SRL, India) and all other chemicals used wereof highest purity available commercially. Solutions were pre-pared in doubly distilled water.

3.2. Procedure and kinetic measurements

Solutions of the oxidant and mixtures containing theknown quantities of the substrate (S) (i.e.d-fructose), cat-alyst (bipy or phen) (under the conditions [S]T � [CrVI ]Tand [catalyst]T � [CrVI ]T), acid and other necessary chem-icals were separately thermostated (±0.1◦C). The reactionwas initiated by mixing the requisite amounts of the oxi-d wasmt err ex-p thes m-p le.E ctiva-t

3

t t byp ase elu-e f al-d acida ctsw roce-d asi ctedi test[ l,t nderi sityo hato o re-d lsop

aW+ + HM

+ � NaM+ + HW

+ (17)

he above equilibria lead to decrease the value of [HM]+ andOxM]2+ (C1) to inhibit the rate process. These two effere opposite in nature to determine the rate of reactio

he case of phen-catalysed path, these two effects ro

ig. 5. Effect of [SDS]T on kobs(T) for the CrVI oxidation ofd-fructose inhe presence of bipy and phen in aqueous H2SO4 media, [CrVI ]T = 4×0−4 mol dm−3, [H2SO4] = 0.25 mol dm−3, T= 35◦C: (A) [d-fructose]T =0× 10−4 mol dm−3, [phen]T = 40× 10−4 mol dm−3; (B) [d-fructose]T =0× 10−4 mol dm−3, [bipy]T = 40× 10−4 mol dm−3.

ant with the reaction mixture. Progress of the reactiononitored by following the rate of disappearance of CrVI by

itrimetric quenching technique[8b]. The pseudo-first-ordate constants (kobs) were calculated as usual. Under theerimental conditions, the possibility of decomposition ofurfactants by CrVI was investigated and the rate of decoosition in this path was found to be kinetically negligibrrors associated with the different rate constants and a

ion parameters were estimated as usual[22].

.3. Product analysis and stoichiometry

Under the kinetic conditions (i.e. [S]T � [CrVI ]T), qualita-ive identification of the reaction products was carried ouaper-chromatography[4b,d,e]. Paper chromatograph wffected by using butan-1-ol–acetic acid–water (4:1:5) asnt. To characterise the oxidation products, a series oopentoses and aldohexoses were oxidised with nitricnd bromine water[23] separately and the purified produere taken as the standards in the chromatographic pure. Aldonic acid (C5-acid) corresponding to arabinose w

dentified as the main product. Formaldehyde was deten the reaction mixture as such by the chromotropic acid24]. After reduction of the reaction mixture with Zn–HChe solution was subjected to the chromotropic acid test udentical conditions in a control experiment and the intenf the colour (atλ = 570 nm) was found to be higher than tbtained from the direct reaction mixture not subjected tuction with Zn–HCl. This indicates that formic acid is aroduced in part in the reaction mixture.

266 M. Islam et al. / Journal of Molecular Catalysis A: Chemical 236 (2005) 260–266

Acknowledgements

Financial support from Visva Bharati (Santiniketan) isthankfully acknowledged. Cooperation from Dr. BidhanChandra Bag and Sri Biplab Goswami of this Departmentis thankfully acknowledged.

Appendix A

Derivation of rate law (consideringScheme 2):

Rate= k2[C2] (i)

K4 = [C2][H+]

[C1][S]

or, [C2] = K4[C1][S]

[H+](ii)

K3 = [C1]

[HCrO4−][H+]2[LH+]

(iii)

or, [C1] = K3[HCrO4−][H+]

2[LH+]

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

[L] T = [L] + [LH+] (iv)

K

o

Sp

I

R

R

nces

[2] (a) P.O.I. Virtanen, R. Lindroos-Heinanen, Acta Chem. Scand. Ser.B 42 (1988) 411;(b) M. Gupta, S.K. Saha, P. Banerjee, J. Chem. Soc., Perkin Trans.2 (1988) 1781.

[3] (a) S.A. Katz, H. Salem, The Biological and Environmental Chem-istry of Chromium, VCH, New York, 1994, p. 65;(b) M. Mitewa, P.R. Bontchev, Coord. Chem. Rev. 61 (1985) 241;(c) M. Cieslak-Golonka, Polyhedron 15 (1995) 3667.

[4] (a) K.K. Sengupta, S.N. Basu, Carbohydr. Res. 80 (1980) 223;(b) K.K. Sengupta, S. Sengupta, S.N. Basu, Carbohydr. Res. 71(1979) 75;(c) K.K. Sengupta, S. Sengupta, S.N. Basu, Carbohydr. Res. 72(1979) 139;(d) L.F. Sala, S.R. Signorella, M. Rizzotto, M.I. Frascaroli, F. Gan-dolfo, Can. J. Chem. 70 (1992) 2046;(e) M. Rizzotto, M.I. Frascaroli, S. Signorella, L.F. Sala, Polyhedron15 (1996) 1517.

[5] (a) A.K. Das, A. Roy, B. Saha, R.K. Mohanty, M. Das, J. Phys. Org.Chem. 14 (2001) 333;(b) B. Saha, M. Das, R.K. Mohanty, A.K. Das, J. Chin. Chem. Soc.51 (2004) 399;(c) A.K. Das, S.K. Mondal, D. Kar, M. Das, Inorg. React. Mech. 3(2001) 63.

[6] (a) T.Y. Peng, J. Rocek, J. Am. Chem. Soc. 99 (1997) 7622;(b) C. Srinivasan, S. Rajagopal, A. Chellamani, J. Chem. Soc., PerkinTrans. 2 (1990) 1839;(c) T.Y. Lin, J. Chin. Chem. Soc. 28 (1981) 149;(d) T.Y. Lin, Y.L. Mao, C.M. Chuo, J. Chin. Chem. Soc. 38 (1991)167.

[7] (a) A.K. Das, A. Roy, B. Saha, Trans. Met. Chem. 26 (2001) 630;(b) A.K. Das, A. Roy, D. Kar, B. Saha, J. Chem. Res. (S) (2001)

001)

;

for

95)

[ ydr.

[[ em.

[[[ hem.

.[[ rk,

[ Pub-598,

[[ 9.[ .[ nce

[ guin,

[ 6, p.

b = [LH+]

[L][H +]; or, [L] = [LH+]

Kb[H+](v)

r, [L] T = [LH+]

Kb[H+]+ [LH+]

= [LH+]

[1 + 1

Kb[H+]

]

= [LH+]

[Kb[H+] + 1

Kb[H+]

]≈ [LH+] (vi)

inceKb[H+] � 1 (Kb = 2.8× 104 for bipy, = 9.5× 104 forhen)[18]

t leads to : [C2] ≈ K3K4[S]T[HCrO−4 ][H+][L] T

ate= K3K4k2[S]T[HCrO4−][L] T[H+]2

[H+]

= K3K4k2[S]T[HCrO4−][L] T[H+] and,

−d ln[HCrO4−]

dt= kobs(c)

=(

2

3

)K3K4k2[S]T[L] T[H+] (15)

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