Micellar effect on the reaction of chromium(VI) oxidation of D-fructose in the presence and absence of picolinic acid in aqueous media: A kinetic study

Article abstract of DOI:10.1002/poc.374

The kinetics and mechanism of the Cr(VI) oxidation of D-fructose in the presence and absence of picolinic acid (PA) in aqueous acid media were studied under the conditions [D-fructose]_T ?[Cr(VI)]_T at different temperatures. Under the kinetic conditions, the monomeric species of Cr(VI) was found to be kinetically active in the absence of PA whereas in the PA-catalysed path, the Cr(VI)-PA complex was considered to be the active oxidant. In this path, the Cr(VI)-PA complex undergoes a nucleophilic attack by the substrate to form a ternary complex which subsequently experiences a redox decomposition through glycol splitting leading to the lactone of C₅-aldonic acid along with formaldehyde and the Cr(IV)-PA complex. The primary product formaldehyde undergoes further oxidation (in part) to form formic acid. Then the Cr(IV)-PA complex participates further in the oxidation of D-fructose and ultimately is converted into the inert Cr(III)-PA complex. In the uncatalysed path, the Cr(VI)-substrate ester experiences an acid-catalysed redox decomposition (2e transfer) in the rate-determining step giving rise to the products. The uncatalysed path shows a second-order dependence on [H⁺] whereas the PA catalysed path shows a fractional order in [H⁺]. Both paths show a first-order dependence on [D-fructose]_T and [Cr(VI)]_T. The PA-catalysed path is first order in [PA]_T. All these patterns remain unaltered in the presence of externally added surfactants. The effects of a cationic surfactant, N-cetylpyridinium chloride (CPC), and an anionic surfactant, sodium dodecyl sulfate (SDS), on both the uncatalysed and PA-catalysed paths were studied. CPC inhibits both the uncatalysed and PA-catalysed paths whereas SDS catalyses the reactions. The observed micellar effects are explained by considering a distribution pattern of the reactants between the micellar and aqueous phases. The applicability of different kinetic models, e.g. the pseudo-phase ion-exchange model, the Menger-Portnoy model and the Piszkiewicz cooperative model, was tested to explain the observed micerllar effects. The effect of [surfactant]_T on the activation parameters was explored to rationalize the micellar effect. Copyright

Full text of DOI:10.1002/poc.374

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2001; 14: 333–342
DOI:10.1002/poc.374
Micellar effect on the reaction of chromiumꢀVI) oxidation of
D-fructose in the presence and absence of picolinic acid in
aqueous media: a kinetic study
Asim K.Das,* Aparna Roy, Bidyut Saha, Rajani K.Mohanty* and Mahua Das
Department of Chemistry, Visva Bharati University, Santiniketan 731235, India
Received 09 October 2000; revised 25 January 2001; accepted 15 February 2001
ABSTRACT: The kinetics and mechanism of the Cr(VI) oxidation of D-fructose in the presence and absence of
picolinic acid (PA) in aqueous acid media were studied under the conditions [D-fructose]T ꢀ[Cr(VI)]T at different
temperatures. Under the kinetic conditions, the monomeric species of Cr(VI) was found to be kinetically active in the
absence of PA whereas in the PA-catalysed path, the Cr(VI)–PA complex was considered to be the active oxidant. In
this path, the Cr(VI)–PA complex undergoes a nucleophilic attack by the substrate to form a ternary complex which
subsequently experiences a redox decomposition through glycol splitting leading to the lactone of C5-aldonic acid
along with formaldehyde and the Cr(IV)–PA complex. The primary product formaldehyde undergoes further
oxidation (in part) to form formic acid. Then the Cr(IV)–PA complex participates further in the oxidation of D-
fructose and ultimately is converted into the inert Cr(III)–PA complex. In the uncatalysed path, the Cr(VI)–substrate
ester experiences an acid-catalysed redox decomposition (2e transfer) in the rate-determining step giving rise to the
‡
products. The uncatalysed path shows a second-order dependence on [H ] whereas the PA catalysed path shows a
‡
fractional order in [H ]. Both paths show a first-order dependence on [D-fructose]T and [Cr(VI)]T. The PA-catalysed
path is first order in [PA]T. All these patterns remain unaltered in the presence of externally added surfactants. The
effects of a cationic surfactant, N-cetylpyridinium chloride (CPC), and an anionic surfactant, sodium dodecyl sulfate
(SDS), on both the uncatalysed and PA-catalysed paths were studied. CPC inhibits both the uncatalysed and PA-
catalysed paths whereas SDS catalyses the reactions. The observed micellar effects are explained by considering a
distribution pattern of the reactants between the micellar and aqueous phases. The applicability of different kinetic
models, e.g. the pseudo-phase ion-exchange model, the Menger–Portnoy model and the Piszkiewicz cooperative
model, was tested to explain the observed micellar effects. The effect of [surfactant]T on the activation parameters
was explored to rationalize the micellar effect. Copyright  2001 John Wiley & Sons, Ltd.
KEYWORDS: kinetics; oxidation; catalysis; D-fructose; chromium(VI); picolinic acid; surfactants
INTRODUCTION
aspects5 of interaction of Cr(VI) with reducing sugars and
their different metabolic intermediates. This paper deals
with the kinetics and mechanism of the Cr(VI) oxidation
of D-fructose in the presence and absence of picolinic
acid (PA), which is unique5e,6,7 in catalysing the Cr(VI)
oxidation of organic substrates containing OH groups. D-
Fructose is basically a polyol but the reactivities of the
OH groups are markedly influenced by the adjacent
carbonyl group. Hence the effect of PA in the present
kinetic study is of interest. Micellar effects on both the
uncatalysed and PA-catalysed reactions were studied to
substantiate the proposed mechanism.
From the standpoint of the biochemical importance of D-
fructose in metabolic systems,1 its physico-chemical
properties in solution have been extensively studied.2 The
kinetics and mechanism of the oxidation of ketohexoses
by different transition metal ions have been reported.3
Different ketohexoses including D-fructose have been
subjected3c,e to kinetic studies of Cr(VI) oxidation in
aqueous acidic media. Because of the carcinogenic and
mutagenic activity of Cr(VI), the chemistry of chromium
in biological systems4 has become important to both
biochemists and inorganic chemists. It is believed that
reducing sugars including D-fructose play an important
role in monitoring the toxicity4 of chromium. Hence fresh
interest has arisen among kineticists to explore the kinetic
EXPERIMENTAL
Materials and reagents. PA (Fluka) was used after
repeated crystallization from methanol (m.p. 136°C). D-
Fructose (SRL), K2Cr2O7 (BDH), sodium dodecyl sulfate
*Correspondence to: A. K. Das and R. K. Mohanty, Department of
Chemistry, Visva Bharati University, Santiniketan 731235, India.
Contract/grant sponsor: CSIR.
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 333–342

334
A. K. DAS ET AL.
(SDS) (SRL), N-cetylpyridinium chloride (CPC) (SRL)
and all other chemicals used were of highest purity
available commercially. Solutions were prepared in
doubly distilled water.
Procedure and kinetic measurements. Solutions of the
oxidant and reaction mixtures containing known quantities
of the substrate (S) (D-fructose), catalyst (PA) {under the
conditions [S]T ꢀ[Cr(VI)]T and [PA]T ꢀ[Cr(VI)]T}, acid
and other necessary chemicals were separately thermo-
stated (Æ0.1°C). The reaction was initiated by mixing the
requisite amounts of the oxidant with the reaction mixture.
Progress of the reaction was monitored by following the
rate of disappearance of Cr(VI) by a titrimetric quenching
technique as discussed earlier.7 The pseudo-first-order rate
constants (kobs) were calculated as usual. Under the
experimental conditions, the possibility of decomposition
of the surfactants by Cr(VI) was investigated and the rate
of decomposition in this path was found to be kinetically
negligible. To circumvent the solubility problem, different
acids (HClO4 and H2SO4) were used to follow the effects
of the anionic surfactant and cationic surfactant (CPC).
The catalytic efficiencies of PA in aqueous H2SO4 and
HClO4 were compared. Errors associated with the
different rate constants and activation parameters were
estimated as usual.8
Figure 1. Effect of [PA]T on kobs+T) for the Cr+VI) oxidation of
D-fructose in the presence of PA in aqueous HClO4 media.
[Cr+VI)]T = 1.13 Â 10À3 mol dmÀ3, [HClO4] = 0.45 mol dmÀ3
[D-fructose]T =
,
I = [HClO4] ‡ [NaClO4] = 1.0 mol dmÀ3
,
0.017 mol dmÀ3. +A) 25°C; +B) 35°C; +C) 40°C
experimental conditions, [S]T ꢀ[PA]T ꢀ103[Cr(VI)]T,
and [PA]T ꢀ103[Cr(VI)]T, the rate of disappearance of
Cr(VI) shows a first-order dependence on [Cr(VI)]. In the
presence of surfactants, the first-order dependence on
[Cr(VI)] remains unaltered. The pseudo-first-order rate
constants (kobs) were evaluated from the linear plot of
log[Cr(VI)]t vs time (t) as usual.
RESULTS AND DISCUSSION
Product analysis and stoichiometry
Dependence on [PA]T
Under the kinetic conditions (i.e. [S]T ꢀ[Cr(VI)]T),
qualitative identification of the reaction products was
carried out by paper chromatography.5a,b,9 To characterize
At [S]T = 0.017 mol dmÀ3 and [Cr(VI)]T = 1.13 Â
10À3 mol dmÀ3, the effect of [PA]T on kobs was followed
in both aqueous HClO4 and H2SO4 media. The plots of
kobs vs [PA]T are linear (r >0.99) with positive intercepts
measuring the contribution of the relatively slower
uncatalysed paths (Fig. 1). The pseudo-first-order rate
constants [kobs(u)] directly measured in the absence of PA
agree well with those obtained from the intercepts of the
plots of kobs(T) vs [PA]T. The relevant equation is
the oxidation products of D-fructose, a series of aldopen-
toses and aldohexoses were oxidized with nitric acid and
bromine water10 separately and the purified products were
taken as the standards in the chromatographic procedure.
The aldonic acid (C5-acid) corresponding to arabinose was
identified as the main product of
D-fructose oxidation.
Paper chromatography was effected by using butan-1-ol–
acetic acid–water (4:1:5) as eluent. Formaldehyde was
detected in the reaction mixture as such by the
chromotropic acid test.11 After reduction of the reaction
mixture with Zn–HCl, the solution was subjected to the
chromotropic acid test under identical conditions in a
control experiment, and the intensity of the colour (at
ꢀ = 570 nm) was found to be higher than that obtained
from the direct reaction mixture not subjected to reduction
with Zn–HCl. This indicates that formic acid is also
produced in part in the reaction mixture.
kobsꢁT† ˆ kobsꢁu† ‡ kobsꢁc† ˆ kobsꢁu† ‡ kcat‰PAŠ
ꢁ1†
T
The values of kcat with the activation parameters are
given in Table 1. During the progress of the reaction, PA
is lost owing to the formation of an inert Cr(III)–PA
complex. The nature of the dependence on [PA]T remains
unaltered also in the presence of the surfactants. Under
the condition [PA]T ꢀ[Cr(VI)]T during the progress of
the reaction, [PA]T remains more or less constant.
Dependence on [CrꢀVI)]T
Dependence on [S]T
Both in the presence and absence of PA, under the
From the plot of kobs vs [S]T (Fig. 2), it is established that
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 333–342

MICELLAR EFFECT ON Cr(VI) OXIDATION OF D-FRUCTOSE
335
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 333–342

336
A. K. DAS ET AL.
From the experimental fit, the observations (in both the
presence and absence of SDS) are
2
‡
kobsꢁu† ˆ kHꢁu†‰H Š
ꢁ4†
‡
‡
kobsꢁc† ˆ kobsꢁT† À kobsꢁu† ˆ a‰SŠ ‰PAŠ ‰H Š=ꢁb ‡ m‰H Š†
T
T
ꢁ5†
where a, b and m are constants, or
‡
1=kobsꢁc† ˆ b=ꢁa‰SŠ ‰PAŠ ‰H Š†
T
T
‡ ꢁm=a†ꢁ1=‰SŠ ‰PAŠ †
ꢁ6†
T
T
Figure 2. Effect of [D-fructose]T on kobs for the Cr+VI)
‡
‡
oxidation of D-fructose in the presence of PA Àin3 aqueous
The linear plots (Fig. 3) of kobs(u)/[H ] vs [H ] and 1/
HClO4 medÀia3. [Cr+VI)]T = 1.13 Â 10À3 mol dm
, [PA]T =
I = [HClO4] ‡
‡
kobs(c) vs [H ] support the above equations.
0.02 mol dm
,
[HClO4] = 0.45 mol dmÀ3
,
[NaClO4] = 1.0 mol dmÀ3, 35°C
Test for acrylonitrile polymerization
both the uncatalysed and catalysed paths show a first-
order dependence on [S]T, i.e.
Under the experimental conditions, polymerization of
acrylonitrile was observed under a nitrogen atmosphere.
kobsꢁc† ˆ kobsꢁT† À kobsꢁu† ˆ ksꢁc†‰SŠ
ꢁ2†
ꢁ3†
T
kobsꢁu† ˆ ksꢁu†‰SŠ
Mechanism of the reaction
T
In the presence of surfactants, the same dependence
pattern is also found. The values of ks(c) and ks(u) under
different conditions are given in Table 1.
In aqueous solution, D-fructose exists mainly as cyclic
hemiacetals which are in dynamic equilibrium with the
acyclic form (where C2 is present as a keto group). Of the
pyranoid and furanoid cyclic forms, the former is more
stable.12 Hence the preponderant form of the D-fructose is
b-pyranoid. Kinetically, the monosaccharides can be
considered as polyols in which the reactivities of alcohol
groups are expected to be increased by the presence of the
carbonyl group. In fact, in the case of oxidations by V(V),
the rate constant for hydroxyacetone is about 104
times3b,13 greater than that for glycol, but the rate
constants for D-glucose and D-mannose are only about 15
times3b,14 higher than that for glycol. A similar observa-
tion has been noted by us5e,15 for Cr(VI) oxidation of
glycols and D-glucose.
‡
Dependence on [H ]
The acid concentration dependence patterns for the
uncatalysed and catalysed paths are different (Fig. 3).
By considering the open-chain structures of the sugars,
it is reasonable to expect that the sugars should react
much faster than glycol. This decreased rate constant for
the sugars has been explained by some workers14 by
considering the fact that the oxidation goes predomi-
nantly through the open-chain form whose concentration
is very small. Thus the oxidation process needs the
conversion of cyclic forms into open-chain forms through
a dynamic pre-equilibrium step. The carbonyl group
present in the open-chain form will undergo hydra-
tion15,16 and the hydrated compound is kinetically
active.17 In fact, this pre-oxidative hydration step is well
documented17 for carbonyl compounds.
Figure 3. Effect of [HClO4]T on kobs for the Cr+VI) oxidation of
D-fructose in the presence of PA in aqueous HClO4 media.
[Cr+VI)]T = 1.0 Â 10À3 mol dmÀ3
,
I = [HClO4] ‡ [NaClO4] =
The apparent low reactivities3b,e (compared with the
pure acyclic compound, e.g. hydroxyacetophenone) of
the monosaccharides and the increase in the reactivity of
1.5 mol dmÀ3, [D-fructose]T = 0.011 mol dmÀ3, [PA]T = 0.02
mol dmÀ3, 35°C +A) For kobs+u); +B) and +C) for kobs+c), i.e. PA-
catalysed path
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 333–342

MICELLAR EFFECT ON Cr(VI) OXIDATION OF D-FRUCTOSE
337
the sugars with increase in the relative amount of the
open-chain form [e.g. for Cr(VI) oxidation, the rate
sequences D-galactose >D-mannose >D-glucose; D-ribose
that for the cyclic b-pyranoid form of D-glucose, the
reactive OH groups are not suitably oriented for chelation
with Cr(VI). In the case of the b-form of D-glucose, the
equatorial HO-1 group is readily attacked by Cr(VI) to
generate the corresponding ester.9 The glycol splitting in
the case of D-fructose generates formaldehyde, which
may be further oxidized to formic acid. The Cr(IV)
generated may participate in the subsequent faster
reactions in different ways [Eqns. ((10)–(14))].9 It is
important to mention that Cr(IV) and Cr(V) intermediates
have also been argued18b as efficient oxidants to cause
glycol splitting.
>L-arabinose >D-xylose and D-fructose >L-sorbose
follow the same sequence of relative amounts of the
open-chain forms of the sugars] strongly indicate that the
kinetic path involving the open-chain form of the sugar
definitely makes a significant contribution to the overall
reaction. However, some workers have also pro-
posed3c,5a,b,9 the participation of the cyclic forms in the
redox reaction mechanism. It has been argued that in the
cyclic hemiacetal forms, the hydroxyl groups are better
exposed to interact with Cr(VI).5a Scheme 1 presents the
reaction mechanism by considering the b-pyranoid form
of D-fructose. In reality, the pseudo-first-order rate
constants are the sum contribution of each pyranoid and
furanoid form in addition to the possible contribution of
the open-chain form remaining in dynamic equilibrium
with the cyclic forms.
Cr(VI)
S
Route I: Cr(IV) À! 2Cr(V) À! 2Cr(III) ꢁ10†
S
Route II: Cr(IV) À! Cr(III) ‡ SÁ
ꢁ11†
ꢁ12†
SÁ
S
Cr(VI) À! Cr(V) À! Cr(III)
K1
Cr(VI)
S
À
‡
A ‡ HCrO4 ‡ H „ B ‡ H2O
ꢁ7†
Route III: Cr(IV) À! Cr(II) À! Cr(III)
‡ Cr(V)
ꢁ13†
ꢁ14†
K2
‡
‡
B ‡ H „ BH
ꢁ8†
ꢁ9†
S
Cr(V) À! Cr(III)
k1
‡
BH À! product (P) ‡ HCHO ‡ Cr(VI)
Á
In the above routes S is a 2e reductant and S is the
partially oxidized product. In both the Watanabe–
Westheimer mechanism19a (i.e. Route I) and Perez-
Benito Mechanism20 (i.e. route III), the title organic
substrate behaves as a 2e reductant throughout the
reaction sequence, but in the Rocek mechanism19b (i.e.
route I), the organic substrate acts as a 1e reductant
towards Cr(IV) and as a 2e reductant towards both Cr(VI)
and Cr(V). The Rocek mechanism has been widely
accepted for the Cr(VI) oxidation of many organic
compounds. Previously, the route involving Cr(II) (i.e.
Perez-Benito mechanism) was discarded because of the
instability of Cr(II), but in the recent past it has been
established20 that this mechanism operates for the
different organic compounds known to act as 2e
reductants toward Cr(VI). It has been suggested that in
such cases, Cr(IV) is reduced to Cr(II) through a hydride
transfer mechanism. By considering the oxidation of D-
fructose to formaldehyde and the lactone of C5-aldonic
acid, Scheme 1 leads to the following rate equation:
where A = cyclic form of b-D-fructose and
Scheme 1. Cr+VI) oxidation of D-fructose in the absence of
PA
In Scheme 1, B denotes the Cr(VI) ester formed
through chelation. It is important to mention that the
intermediate ester could not be characterized and no
kinetic evidence for the intermediate ester formation was
available. However, such an ester formation mechanism
for the oxidation of alcohols is well documented.17,18
B
experiences acid-catalysed redox decomposition through
glycol splitting18 (i.e. splitting of the C—C bond) giving
rise to the lactone of the corresponding C5-aldonic acid
and formaldehyde. The acid-catalysed redox decomposi-
tion of C may be considered to involve protonation on the
2
‡
kobsꢁu† ˆ ꢁ2=3†k1K1K2‰SŠ ‰H Š
ꢁ15†
T
For the PA-catalysed path, Scheme 2 is reasonable to
explain the findings. Here, PA readily forms a reactive
cyclic Cr(VI)–PA complex (D) which is the active
oxidant. Under the experimental conditions, the first-
order dependence on [PA]T is strictly maintained
throughout the range of [PA]T used. Hence it is reason-
able to conclude that the equilibrium constant for the
‡
oxygen of the Cr—O bond to generate BH before the
electron movement. In fact, such protonation will favour
the electron movement towards the chromium centre. A
similar observation has been noted by Sala and co-
workers.5a,b It is worth noting that in the case of glucose,
no glycol splitting occurs. This is probably due to the fact
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 333–342

338
A. K. DAS ET AL.
reaction leading to the cyclic Cr(VI)–PA complex is low.
In the next step, the Cr(VI)–PA complex reacts with the
substrate to form a ternary complex (E) which experi-
ences a redox decomposition through glycol splitting
with a rate-limiting step giving rise to the organic
products and the Cr(IV)–PA complex. Then the Cr(IV)–
PA complex may participate in the next faster steps as
outlined for the uncatalysed reactions.
(21) conforms to the experimentally observed equation
[Eqn. (5)]. Comparison of Eqns (5) and (21) leads to
a = (2/3)(KaK3k2k3), b = k3 and m = kÀ2. Polymerization
of acrylonitrile under the experimental conditions was
observed in both the presence and absence of PA.
Probably the reaction between formaldehyde and Cr(IV)
produced in the first step initiates acrylonitrile polymer-
ization. In terms of the Rocek mechanism,19b it is
believed that Cr(IV) acts as a 1e oxidant towards the 2e
reductant giving rise to the organic free radical which
initiates the polymerization. On the other hand, according
to the Perez-Benito mechanism,20 Cr(IV) reacts with the
2e reductant through a hydride ion transfer mechanism
yielding Cr(II) and a carbocation centre, which is
responsible22 for acrylonitrile polymerization.
The different rate parameters for both the uncatalysed
and catalysed paths are given in Table 1. To represent
the rate constants in the presence of surfactants, the
subscripts CPC and SDS are used and for the values in
the absence of surfactants the subscript W is used. The
catalytic efficiency of PA is believed to be due to the
enhanced ECr(VI)/Cr(IV) and ECr(VI)/Cr(III) potentials in
the presence of PA. The catalytic efficiency measured
by keff [= kobs(c)/kobs(u)
]
at 0.04 mol dmÀ3 and
‡
H ꢂ 0.5 mol dmÀ3 is about 10 (cf. Table 1), which is
considerably lower than for acyclic alcohol oxidation.6a
The highly negative value of DS is inconsistent with the
ˆ
proposed cyclic transition state for glycol splitting (cf.
Scheme 2).
Effect of CPC
CPC, a representative cationic surfactant, is found to
retard both the uncatalysed and catalysed paths. The plot
of kobs vs [CPC]T (Fig. 4) shows a continuous decrease
and finally it tends to level off at higher concentrations of
CPC. This observation is similar to that observed by
Scheme 2. Mechanistic aspects of Cr+VI) oxidation of D-
fructose in the presence of PA
By considering the glycol splitting as the main
reaction, Scheme 2 leads to the following rate equation
under the steady-state conditions of the ternary complex
E:
2
‡
‡
kobsꢁc† ˆ ꢁ2=3†ꢁKaK3k2k3‰PAŠ ‰SŠ ‰H Š †=fꢁk ‰H Š
À2
T
T
‡
‡ k3†ꢁ‰H Š ‡ Ka†g
ꢁ20†
Neglecting Ka (= 0.025 mol dmÀ3 at 25°C)21 compared
‡
with [H ] (= 0.45 –1.20 mol dmÀ3), Eqn. (20) reduces to
‡
‡
kobsꢁc† ˆ ꢁ2=3†ꢁKaK3k2k3‰PAŠ ‰SŠ ‰H Š†=ꢁk ‰H Š ‡ k3†
À2
T
T
Figure 4. Effect of [CPC]T on kobs+u) for the Cr+VI) oxidation of
D-fructose in the presence of CPC in aqueous H2SO4 media.
[Cr+VI)]T = 2 Â 10À3 mol dmÀ3, [H2SO4] = 1.0 mol dmÀ3, [D-
fructose]T = 0.07 mol dmÀ3. +A) 30°C; +B) 35°C; +C) 40°C
ˆ kcat‰PAŠ
ꢁ21†
T
which explains the hydrogen ion dependence. Equation
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 333–342

MICELLAR EFFECT ON Cr(VI) OXIDATION OF D-FRUCTOSE
339
Bunton and Cerichelli23 in the oxidation of ferrocene by
Fe(III) salts in the presence of cationic cetyltrimethy-
lammonium bromide (CTAB). Similar observations have
also been noted by Panigrahi and Sahu24 in the oxidation
of acetophenone by Ce(IV) and by Sarada and Reddy25
in the oxalic acid-catalysed oxidation of aromatic azo
compounds by Cr(VI) in the presence of SDS. In the
uncatalysed path, the neutral Cr(VI)–substrate ester
formed [cf. Eqn. (7)] can be partitioned in the micellar
pseudo-phase of the surfactant but the cationic surfactant
‡
repelling H needed for the redox decomposition of the
ester [cf. Eqn. (8)] inhibits the reaction. The Cr(VI)–ester
species is likely to be present in the Stern layer. It may be
noted that for the uncatalysed reaction, the other species
H2CrO4 and/or substrate (i.e. D-fructose) may also be
partitioned in the micellar interphase. Simultaneous
partitioning of both H2CrO4 and substrate is equivalent
to the partitioning of the Cr(VI)–ester. In the PA-
catalysed path, CPC restricts the positively charged
Cr(VI)–PA complex [cf. Eqn. (17)] in the aqueous phase
and thus the accumulated neutral substrate in the micellar
phase cannot participate in the reaction. Hence in both the
uncatalysed and PA-catalysed paths the reaction is
mainly restricted to the aqueous phase.
Figure 5. Applicability of the Menger±Portnoy model [i.e
plot of +kw À kobs+x)
)
À1 vs +[CPC]T À cmc)À1, where x = u or c]
to explain the micellar effect on kobs+x) for the Cr+VI) oxidation
of D-fructose in the presence of CPC in aqueous H2SO4
media +takingcmc = 8 Â 10À4 mol dmÀ3). +A) kobs+c), i.e. PA-
catalysed path, [Cr+VI)]T = 1.13 Â 10À3 mol dmÀ3, [H2SO4] =
1.0 mol dmÀ3
,
[PA]T = 0.02 mol dmÀ3
,
[D-fructose]T =
0.04 mol dmÀ3, 35°C. +B) kobs+u), i.e. in the absence of PA,
[Cr+VI)]T = 2 Â 10À3 mol dmÀ3, [H2SO4] = 1.0 mol dmÀ3, [D-
fructose]T = 0.07 mol dmÀ3, 35°C
To interpret the observed kinetic data, the applicability
of the pseudo-phase kinetic model proposed by Menger
and Portnoy26 may be considered. This model considers
the partitioning of one reactant between the aqueous and
micellar phases and leads to the following rate equation:
and kobs(c). Taking km ꢂ 0, Eqn. (22) takes the form
1=kobs ˆ 1=kw ‡ ꢁKB=N†fꢁ‰DŠT À cmc†=kwg ꢁ25†
The linearity of the plot of 1/kobs vs ([D]T À cmc) was
verified (not shown) and indicates that the reaction occurs
predominantly in the aqueous phase. The kw value
obtained from the intercept agrees well with the
experimentally observed value. The magnitude of the
binding constant (KB/N) indicates the depletion of the
concentration of the reactant (which may be either the
neutral Cr(VI)–substrate ester complex or substrate) in
the aqueous phase. This brings about a lowering of the
rate with increasing surfactant concentration. In fact, the
kobs ˆ ꢁkw ‡ kmKB‰DnŠ†=ꢁ1 ‡ KB‰DnŠ†
ꢁ22†
where kw and km are the first-order rate constants in the
aqueous and micellar phase, respectively, and include the
concentration of the other reactant in these pseudo-
phases. KB gives the measure of the binding constant of
the reactant (which is partitioned) with the micelles. [Dn]
is the concentration of the micelles and is related to the
stoichiometric concentration of the surfactant ([D]T),
critical micellar concentration (cmc) and the aggregation
number (N), i.e. [Dn] = ([D]T À cmc)/N. Equation (22)
leads to
‡
reaction is strongly acid catalysed, but H ions are not
available in the cationic micellar phase produced by CPC
owing to electrostatic repulsion. Consequently, the
reaction cannot proceed in the micellar phase even when
‡
Cr(VI)–substrate ester [which needs H for its redox
1=ꢁkw À kobs† ˆ 1=ꢁkw À km† ‡ ꢁN=KB†
f1=ꢁ‰DŠT À cmc†gf1=ꢁkw À km†g ꢁ23†
decomposition, cf. Eqn. (9)] is absorbed in the micellar
pseudo-phase.
The rate data in the presence of surfactants were
subjected to the Piszkiewicz model28 analogous to the
Hill model applied to the enzyme-catalysed reactions.
The Piszkiewicz model relates cooperativity between the
neutral species and surfactant to aggregate to form the
reactive micelles and its contribution to the rate is given
by
ꢁkw À kobs†=ꢁkobs À km† ˆ P
ˆ ꢁKB=N†‰DŠT À ꢁKB=N†ꢁcmc† ꢁ24†
To deal with Eqns (23) and (24) one requires the
knowledge of the cmc, which is not available under the
present kinetic conditions. However, by taking the
literature value27 of cmc (= 8 Â 10À4 mol dmÀ3) under
comparable conditions at 35°C, the plot of 1/(kw À kobs
)
log‰ꢁkobs À kw†=ꢁkm À kobs†Š ˆ log P
vs 1/([D] À cmc) is linear (Fig. 5) at fixed concentrations
ˆ n log‰DŠT À log KD
ꢁ26†
of the other reactants. This leads to km ꢂ 0 for both kobs(u)
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 333–342

340
A. K. DAS ET AL.
Table 2. Kinetic parameters +35°C) of micellar effect for the
Cr+VI) oxidation of D-fructose in the presence and absence of
PA
Effect of CPC
PA
PA
Effect of
SDSg
Parameters
Log(KB/N)
absenta
presentb
2.32c
2.30d
2.30e
2.68f
1.2
ÀLogKD
2.85
2.61
1.8
1.45
1.4
n
1.35
2.13
2.10
ÀLog[D]50 (calculated)
2.23
2.20
ÀLog[D]50 (found)
In the absence of PA [i.e. kobs(u)]: [Cr(VI)]T = 2 Â 10À3 mol dmÀ3
,
a
[S]T = 0.07 mol dmÀ3, [H2SO4] = 1.0 mol dmÀ3
.
Figure 6. Applicability of the Piszkiewicz model +i.e. plot of
logP vs log[CPC]T) to explain the micellar effect on kobs+x)
+where x = u or c) for the Cr+VI) oxidation of D-fructose in the
presence of CPC in aqueous H2SO4 media. +A) PA-catalysed
path, i.e. kobs+c), [Cr+VI)]T = 1.13 Â 10À3 mol dmÀ3, [H2SO4] =
b
For PA-catalysed path [i.e. kobs(c)]: [Cr(VI)]T = 1.13 Â
10À3 mol dmÀ3,[S]T = 0.04 mol dmÀ3,[H2SO4] = 1.0 mol dmÀ3,[PA]T
= 0.02 mol dmÀ3
.
c
cf. Eqn. (23).
d
cf. Eqn. (24).
1.0 mol dmÀ3
,
[PA]T = 0.02 mol dmÀ3
,
[D-fructose]T =
e
cf. Eqn. (25).
0.04 mol dmÀ3,35°C.+B)Uncatalysedpath,i.e.kobs+u),[Cr+VI)]T
= 2 Â 10À3 mol dmÀ3, [H2SO4] = 1.0 mol dmÀ3, [D-fructose]T
= 0.07 mol dmÀ3, 35°C P is de®ned as P = [kw À kobs+x)]/
[kobs+x) À km]
f cf. Eqn. (26).
In the absence of PA [i.e. kobs(u)]: [Cr(VI)]T = 2 Â 10À3 mol dmÀ3
,
g
[S]T = 0.03 mol dmÀ3, [H2SO4] = 1.0 mol dmÀ3
.
where KD is the dissociation constant of micellized
surfactant back to its components and n is the index of
cooperativity. The advantage of Eqn. (26) is that it does
not require the knowledge of the cmc of the surfactant
used. This helps a larger number of data to come under
the purview of analysis. Although Eqn. (26) was
originally developed for micelle-catalysed reactions
showing a maximum rate followed by inhibition, the
model has been applied by different workers24,25,29 to
explain the micellar effect in which the reaction is
inhibited or catalysed by the micelle over the whole range
as observed in the present system. By using Eqn. (26), i.e.
the plot of log P vs log[D]T (Fig. 6), the different
parameters n, log[D]50 (where log[D]50 represents the
concentration of surfactant required for half-maximum
catalysis or inhibition) and logKD have been determined
(cf. Table 2). The calculated log[D]50 values conform
well with the experimentally observed values. The values
of n = 1–2, far less than the aggregation number (20–100)
of the surfactant molecules28 leading to micelles, indicate
the existence of catalytically productive submicellar
aggregates. The non-integral values of n indicate the
existence of multiple equilibria in the formation of
catalytically active submicellar aggregates. When the
interaction (measured by ÀlogKD) is fairly high, it is
appropriate to consider n as representing the average
stoichiometry of the detergent–reactant aggregate. The
simultaneous applicability of the different kinetic models
(i.e. Menger–Portnoy model26 and Piszkiewicz model28)
based on different mathematical assumptions is not
surprising. In fact, both of these models lead to similar
final equations [Eqns (24) and (26)]. From the effect of
[CPC]T on the activation parameters (Fig. 7), it is evident
that the rate retardation in the presence of CPC mainly
arises due to the higher DH values.
ˆ
Effect of SDS
SDS, a representative anionic surfactant, catalyses both
the uncatalysed and PA-catalysed paths. In the PA-
catalysed path, the rate acceleration arises owing to the
preferential partitioning of the positively charged
Cr(VI)–PA complex (by electrostatic attraction) and
neutral substrate in the micellar interphase. Thus SDS
allows the reaction to proceed in both aqueous and
Figure 7. Effect of [CPC]T on the activation parameters
ˆ
ˆ
+DH , DS ) of kobs+u) for the Cr+VI) oxidation of D-fructose in
the presence of CPC in aqueous H2SO4 media. [Cr+VI)]T =
2 Â 10À3 moÀl d3 mÀ3, [H2SO4] = 1.0 mol dmÀ3, [D-fructose]T =
0.07 mol dm
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 333–342

MICELLAR EFFECT ON Cr(VI) OXIDATION OF D-FRUCTOSE
341
micellar interphases. In the absence of PA, binding of
On rearrangement Eqn. (30) yields
H2CrO4 (kinetically active species30a,b) and substrate to
the SDS micelles has been argued by different work-
ers.30c,d However, the possibility of partitioning of the
neutral Cr(VI)–substrate ester [cf. Eqn. (7)] into the
micellar phase cannot be ruled out. However, simulta-
neous partitioning of H2CrO4 and substrate is equivalent
to the partitioning of Cr(VI)–substrate ester. Hence it
leads to higher local concentrations of both the reactants
at the micelle-water interphase compared with their
2
H
H
‡
‡
T
ꢁmH† ꢁKex À 1†‰DnŠ À mHfKex ‰H Š ‡ ‰Na Š
T
‡ ꢁ‰DnŠꢁKex À 1†g ‡ KexHꢁ‰H Š
H
‡
T
ˆ 0
ꢁ31†
‡
Thus [HM ] (= mH[Dn]) can be calculated by using Eqn.
(31). The positively charged Cr(VI)–PA complex is the
active oxidant (Ox ) in the PA-catalysed path and it may
also participate in a similar exchange equilibrium:
‡
‡
stoichiometric concentrations. The H ions needed for
the reactions are also preferably attracted to the micellar
phase. Thus SDS allows the reaction in both the phases
with a preferential rate enhancement in the micellar
phase. The observed catalysis can be explained by
considering the pseudo-phase ion-exchange (PIE) mod-
el,31 which considers the micellar and aqueous phase as
two distinct phases and in the present system the title
redox reaction occurs in both the phases. The reaction is
acid catalysed and the corresponding exchange equi-
‡
‡
‡
‡
Ox
OxW ‡ NaM „ OxM ‡ NaW ; Kex
ꢁ32†
‡
The magnitude of [OxM ] can also be calculated by using
Ox
the analogous equation [cf. Eqn. (31)] involving Kex
.
For H ions, Kex is close to unity.30d,32 This indicates
‡
H
‡
‡
that there is no specific interaction of H ions or Na ions
with the micellar surface and consequently these ions are
statistically distributed between the aqueous and micellar
phases. If KexH → 1, then Eqn. (31) leads to
‡
‡
librium between H ion and Na ion at the micellar
surface is
‡
‡
‡
‡
T
‰HM Š ˆ ꢁ‰H ŠTꢁ‰DnŠ†=ꢁ‰H Š ‡ ‰Na Š †
ꢁ33†
T
‡
‡
‡
‡
The value of b has been found to be in the range 0.6–0.85
HW ‡ NaM „ HM ‡ NaW
ꢁ27†
from conductivity measurements.30d,31 It is evident that
‡
The ion-exchange equilibrium constant (KexH) is defined
[HM ] increases with increase in [Dn]. It is important to
mention that the change in polarity of the medium
(specifically in the intermicellar zone in which the
reactants are positioned) with the addition of surfactant
may also influence the observed micellar effect.30d From
the plot of kobs vs [SDS]T (not shown), it is evident that
the rate initially increases, but it tends to level off at
higher [SDS]T. In fact, an increase in [SDS]T increases
as
H
‡
‡
‡
‡
Kex ˆ ‰HM Š‰NaW Š=‰HW Š‰NaM
Š
ꢁ28†
where the subscripts M and W denote the micellar and
aqueous phase, respectively. The concentrations are
expressed in terms of the total solution volume and it is
further assumed that the activity coefficient ratios,
‡
the micellar counterions (i.e. Na ) which may displace
‡
‡
H and Ox ions, from the micellar surface to drive the
equilibria (27) and (32) to the left. This reduces [HM ] and
[OxM ] to inhibit the rate process in the micellar phase.
‡
‡
‡
‡
‡
gM(Na )/gM(H ) and gW(Na )/gW(H ), are each equal
‡
‡
to unity. Considering the competition between Na and
H only, the overall micellar binding parameter is given
‡
Owing to these opposing factors (i.e. dilution of all
reactants over micelles at higher [SDS]T), kobs initially
increases with increase in [SDS]T, but it attains a limiting
value at higher [SDS]T.
by
‡
‡
ꢁ ˆ mH ‡ mNa ˆ ‰HM Š=‰DnŠ ‡ ‰NaM Š=‰DnŠ
‡
‡
ˆ ꢁ‰HM Š ‡ ‰NaM Š†=‰DnŠ
ꢁ29†
Acknowledgements
Thus, b gives the fraction of micellar head-groups
neutralized. [Dn] gives the micellized surfactant concen-
tration, i.e. [Dn] = [SDS]T À cmc. The various concentra-
The authors are grateful to CSIR, New Delhi, for
providing financial assistance and Dr G. Brahamachari,
Department of Chemistry, Visva Bharati University, for
helpful discussions.
‡
‡
tion terms are expressed as [HM ] = mH[Dn], [HW ] =
‡
‡
‡
‡
‡
‡
‡
‡
[H ]T À [HM ] = [H ]T À mH[Dn], [NaW ] = [Na ]T À
‡
[NaM ] = [Na ]T À (b À mH)[Dn] and [NaM ] = [Na ]T À
‡
[NaW ] = (b À mH)[Dn].
The ion-exchange equilibrium constant can be ex-
pressed as:
REFERENCES
H
‡
1. White A, Handler P, Smith EL, Stetten DW. Principles of
Biochemistry (2nd edn). McGraw-Hill: New York; 417.
2. (a) Doty TE, Wannien E. Food Technol. 1975; 11: 34; (b) Pawan
Kex ˆ mHf‰Na ŠT À ꢁꢁ À mH†‰DnŠg=ꢁꢁ
‡
À mH†ꢁ‰H ŠT À mH‰DnŠ†
ꢁ30†
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 333–342

342
A. K. DAS ET AL.
GLS, Birch GG, Green LF. Molecular Structure and Function of
14. Virtanen POI, Kurkisuo S, Nevala H, Pohjola S. Acta Chem.
Scand., Sect. A 1986; 40: 200.
Food Carbohydrates. Applied Science: London, 1973; 65.
3. (a) Turney TA. Oxidation Mechanisms. Butterworth: London,
1965; 102; (b) Virtanen POI, Lindroos-Heinanen R, Oikarinen E,
Vaskuri J. Carbohydr. Res. 1987; 167: 29; (c) Sengupta KK, Bose
SN, Sengupta S. Carbohydr. Res. 1981; 97: 1; (d) Singh SV,
Saxena OC, Singh MP. J. Am. Chem. Soc. 1970; 92: 537; (e)
Virtanen POI, Lindroos-Heinanen R. Acta Chem. Scand., Ser. B
1988; 42: 411; (f) Gupta M, Saha SK, Banerjee P. J. Chem. Soc.,
Perkin Trans. 2 1985; 1781.
4. (a) Katz SA, Salem H. The Biological and Environmental
Chemistry of Chromium. VCH: New York, 1994; (b) Goodgame
DML, Joy AM. Inorg. Chim. Acta 1987; 135: L5; (c) Micera G,
Dessi A. J. Inorg. Biochem. 1988; 34: 157.
15. Das AK, Roy A, Saha B. Transition Met. Chem. in press.
16. Pine SH, Hendrickson JB, Cram DJ, Hammond GS. Organic
Chemistry (4th edn). McGraw-Hill: New Delhi, 1981; 254; Finar
IL. Organic Chemistry (6th edn). vol. I, Longman: London, 1973;
228.
17. (a) Lee DG, Spitzer UA. Can. J. Chem. 1975; 53: 3709; (b)
Sengupta KK, Dey S, Sengupta S, Adhikari M, Banerjee A.
Tetrahedron 1990; 46: 2431.
18. (a) Rocek J, Westheimer FH. J. Am. Chem. Soc. 1962; 84: 2241;
(b) Chatterjee AC, Mukherjee SK. Z. Phys. Chem. 1965; 228: 166.
19. (a) Watanabe W, Westheimer FH. J. Chem. Phys. 1949; 17: 61; (b)
Hasan F, Rocek J. Tetrahedron 1974; 30: 21.
5. (a) Sala LF, Signorella SR, Rizzotto M, Frascaroli MI, Gandolfo F.
Can. J. Chem. 1992; 70: 2046; (b) Rizzotto M, Frascaroli MI,
Signorella S, Sala LF. Polyhedron 1996; 15: 1517; (c) Signorella
S, Garcia S, Sala LF. Polyhedron 1997; 16: 701; (d) Signorella SR,
Santoro MI, Mulero MN, Sala LF. Can. J. Chem. 1994; 72: 398;
(e) Das AK, Mondal SK, Kar D, Das M. Inorg. React. Mech. in
press.
6. (a) Peng TY, Rocek J. J. Am. Chem. Soc. 1976; 98: 1026; 1977; 99:
7622; (b) Srinivasan C, Rajagopal S, Chellamani A. J. Chem. Soc.,
Perkin Trans. 2 1990; 1839.
7. (a) Das AK, Mondal SK, Kar D, Das M. J. Chem. Res. (S) 1998;
574; (b) Das AK, Mondal SK, Kar D, Das M. Int. J. Chem. Kinet.
2001; 33: 173.
8. Lyons L. A Practical Guide to Data Analysis for Physical Science
Students. Cambridge University Press: Cambridge, 1991.
9. Sengupta KK, Sengupta S, Basu SN. Carbohydr. Res. 1979; 71:
75.
20. (a) Perez-Benito JF, Arias C, Lamrhari D. J. Chem. Soc., Chem.
Commun. 1992; 472; (b) Perez-Benito JF, Arias C. Can. J. Chem.
1993; 71: 649.
21. Moyne L, Thomas G. Anal. Chim. Acta. 1964; 31: 583.
22. Billmeyer FW. Textbook of Polymer Science. Wiley: New York,
1984; 85.
23. Bunton CA, Cerichelli G. Int. J. Chem. Kinet. 1980; 12: 519.
24. Panigrahi GP, Sahu BP. J. Indian Chem. Soc. 1991; 68: 239.
25. Sarada NC, Reddy IAK. J. Indian Chem. Soc. 1993; 70: 35.
26. Menger FM, Portnoy CE. J. Am. Chem. Soc. 1967; 89: 4698.
27. cf (a) Ghosh KK, Kar SK. J. Indian Chem. Soc. 1998; 75: 39; (b)
Panigrahi GP, Sahu BP. Int. J. Chem. Kinet. 1993; 25: 595.
28. Piszkiewicz D. J. Am. Chem. Soc. 1976; 98: 3053. 1977; 99: 7695.
1977; 99: 1550.
29. (a) Khan Z, Ali SI, Rafique ZA, Kabir-ud-Din. Indian J. Chem.
1997; 36A: 579; (b) Gour DS. J. Indian Chem. Soc. 1997; 74: 545;
(c) Ghosh KK, Sar SK. J. Indian Chem. Soc. 1998; 75: 39.
30. cf. (a) Sengupta KK, Samanta T, Basu SN. Tetrahedron 1986; 42:
681; (b) Rodenas E, Perez-Benito E. J. Phys. Chem. 1991; 95:
9496; (c) Panigrahi GP, Misra SK. Indian J. Chem. 1993; 32A:
956; (b) Sankararaj B, Rajagopal S, Pitchumani K. Indian J. Chem.
1995; 34A: 440.
10. Ferrir RJ, Collins PM. Monosaccharide Chemistry (1st edn).
Penguin: London, 1972; 82.
11. (a) Feigl F. Spot Tests in Organic Analysis (5th edn). Elsevier:
Amsterdam, 1956; (b) Vishnoi M, Sharma K, Mehrotra RN. Int. J.
Chem. Kinet. 1991; 23: 377; (c) O’dea JF, Gibbons RA. Biochem.
J. 1953; 55: 580.
31. cf. Bunton CA, Nome F, Quina FH, Romsted LS. Acc. Chem. Res.
1991; 24: 357.
32. Perez-Benito E, Rodenas E. Langmuir 1991; 7: 232.
12. Mathlouthi M, Luu DV. Carbohydr. Res. 1980; 78: 225.
13. Virtanen POI, Pohjola S. Finn. Chem. Lett. 1984; 155.
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 333–342