Mechanistic aspects of oxidation of dextrose by N-bromophthalimide in acidic medium: A micellar kinetic study

Article abstract of DOI:10.1007/s11164-012-0571-4

Kinetic investigations of oxidation of dextrose by N-bromophthalimide (NBP) in acidic medium in the presence of mercuric(II) acetate as a scavenger have been studied. In both the absence and presence of surfactants, the oxidation kinetics of dextrose by NBP shows a first-order dependence on NBP, fractional order on dextrose, and negative fractional order dependence on sulfuric acid. The determined stoichiometric ratio was 1:1 (dextrose:NBP). The variation of Hg(OAC)₂ and phthalimide (reaction product) have an insignificant effect on reaction rate. Effects of surfactants, added acrylonitrile, added salts, and solvent composition variation have been studied. Activation parameters for the reaction have been evaluated from Arrhenius plot by studying the reaction at different temperature. The rate law has been derived on the basis of obtained data. A plausible mechanism has been proposed from the results of kinetic studies, reaction stoichiometry and product analysis. The role of anionic and non-ionic micelle was best explained by the Berezin's model.

Full text of DOI:10.1007/s11164-012-0571-4

Res Chem Intermed (2013) 39:469–484
DOI 10.1007/s11164-012-0571-4
Mechanistic aspects of oxidation of dextrose by
N-bromophthalimide in acidic medium: a micellar
kinetic study
Minu Singh
Received: 31 January 2012 / Accepted: 21 April 2012 / Published online: 30 June 2012
Ó Springer Science+Business Media B.V. 2012
Abstract Kinetic investigations of oxidation of dextrose by N-bromophthalimide
(NBP) in acidic medium in the presence of mercuric(II) acetate as a scavenger have
been studied. In both the absence and presence of surfactants, the oxidation kinetics
of dextrose by NBP shows a first-order dependence on NBP, fractional order on
dextrose, and negative fractional order dependence on sulfuric acid. The determined
stoichiometric ratio was 1:1 (dextrose:NBP). The variation of Hg(OAC)2 and
phthalimide (reaction product) have an insignificant effect on reaction rate. Effects
of surfactants, added acrylonitrile, added salts, and solvent composition variation
have been studied. Activation parameters for the reaction have been evaluated from
Arrhenius plot by studying the reaction at different temperature. The rate law has
been derived on the basis of obtained data. A plausible mechanism has been pro-
posed from the results of kinetic studies, reaction stoichiometry and product anal-
ysis. The role of anionic and non-ionic micelle was best explained by the Berezin’s
model.
Keywords Kinetics ꢀ Oxidation ꢀ Dextrose ꢀ N-Bromophthalimide ꢀ
Micellar kinetics
Introduction
The application of N-halo compounds in the field of organic synthesis is very
extensive, such as oxidation reactions, deprotection and protection of different
functional groups, halogenations of saturated and unsaturated compounds, acylation
of alcohols, phenols, amines or thiols, epoxidation of alkenes, aziridination etc.
M. Singh (&)
Department of Chemistry, Kalyan Post Graduate College, Bhilai Nagar, Durg 490006, Chhattisgarh,
India
e-mail: minu_0511@yahoo.co.in
123

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M. Singh
[1–3]. The diverse nature of the chemistry of these compounds is due to their ability
to act as sources of halonium cations, hypohalite species and nitrogen anions, which
act both as bases and nucleophiles [4].
Carbohydrates make up most of the organic matter on Earth because of their
extensive roles in all forms of life. First, carbohydrates serve as energy stores, fuels,
and metabolic intermediates. Second, ribose and deoxyribose sugars form part of the
structural framework of RNA and DNA. Third, polysaccharides are structural
elements in the cell walls of bacteria and plants. In fact, cellulose, the main
constituent of plant cell walls, is one of the most abundant organic compounds in the
biosphere. Fourth, carbohydrates are linked to many proteins and lipids, where they
play key roles in mediating interactions among cells and interactions between cells
and other elements in the cellular environment. Monosaccharides are the major
source of fuel for metabolism, being used both as an energy source (glucose being
the most important in nature) and in biosynthesis.
This study will enable to understand the complicated biological reaction in living
systems and will also help to understand the catalytic activity of surfactants along
with oxidative capacity of NBP in acidic medium. Kinetic studies of oxidation of
different types of organic substrate by NBP have also been investigated by different
workers [5–17] to explore the effect of the substituent on the redox activity of NBP.
In the search for more environmental friendly processes, the use of self-assembled
surfactant structures opens up a whole range of new possibilities. This organized
media, such as vesicles or micelles can enhance/decrease the reaction rate and
selectivity [18]. The unique ability of micelles to solubilize organic substrates in
water and to catalyze chemical reactions forms the basis for their use as a medium
for investigating organic and bioorganic processes [19–28]. In this regard, micellar
effect as a powerful probe has been utilized by different workers to explore the
redox activity of NBP.
The present paper deals with the micellar effects on NBP oxidation of dextrose in
the presence of acidic medium. The micellar effects have been studied to substantiate
the proposed reaction mechanisms.
Experimental
Materials and methods
Analytical reagent grade chemicals and triple distilled water were used throughout
the investigation. The solution of NBP (Aldrich, 95 %) was prepared in 80 % acetic
acid (AR, S.D. Fine-Chem Limited, Mumbai, India) and stored in a black-coated
flask to prevent any photochemical deterioration. The prepared solution of NBP was
standardized by reported method [29]. Solution of dextrose (freshly prepared),
sodium dodecylsulfate, tritonX-100, and phthalimide (all are, S.D. Fine-Chem
Limited, Mumbai, India) were prepared with triple distilled water. Freshly prepared
starch solution used as an indicator. Mercuric acetate (S.D. Fine-Chem Limited,
Mumbai, India) solution was acidified with 20 % acetic acid and sulfuric acid (AR,
s. d. fine chem. limited, Mumbai, India) was diluted with triple distilled water for
123

Mechanistic aspects of oxidation of dextrose
471
the present investigation. All the kinetic measurements were carried out at constant
temperature 40 °C.
Procedure and kinetic measurements
Solutions of the oxidant and reaction mixtures containing known quantities of the
substrates (i.e., dextrose), surfactants, acid, and other necessary chemicals were
separately thermostated ( 0.1 °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 NBP by an iodometric
determination method. The pseudo first order rate constants (kobs) were calculated as
usual. Under the experimental conditions, the possibility of decomposition of the
surfactants by NBP was investigated and the rate of decomposition in this path was
found to be kinetically negligible. To circumvent the solubility problem, sulfuric
acid was used to follow the effects of the anionic surfactant (SDS) and non-ionic
surfactant (TX-100). The pseudo-first order rate constants (kobs) were reproducible
within the experimental error limit.
Product identification and stoichiometry
The reaction mixture is [NBP] ꢁ [dextrose] were kept in presence of Hg(OAc)2,
H2SO4, surfactants and acetic acid at room temperature for 72 h. Determination of
unconsumed NBP indicated that one mole of dextrose is oxidized by 1 mol of NBP.
This result showed 1:1 stoichiometry according to equation for dextrose could be
formulated as in Scheme 1.
Under the kinetic conditions, [dextrose] ꢁ [NBP], the oxidized reaction mixture
was completely neutralized by sodium bicarbonate and then extracted with ether.
The aqueous layer was used to detect and estimate the main product, lactone of
gluconic acid. The oxidation product of dextrose was detected by FeCl3-HCl blue
test, paper chromatography, and dinapthol-sulfuric acid test.
(i) FeCl3-HCl blue test [30, 31]: After the kinetic experiment was completed, a part
of the oxidized reaction mixture was treated with alkaline hydroxylamine
solution. To the other part of the reaction mixture, barium carbonate was added
to make the solution neutral [32]. FeCl3 solution that had been colored violet
with phenol when added to this reaction mixture gave a bright-yellow coloration
[33], indicating the presence of gluconic acid. It is concluded that lactone,
formed in the rate-determining step, is hydrolyzed to gluconic acid in neutral
medium in a fast step.
(ii) Paper chromatography [34]: Generally, paper chromatographic technique is
used to identify the oxidation products of carbohydrates. Therefore, paper
C6H12O6 + NBP + H2O
dextrose oxidant
D-glucono-1,5-lactone + NHP + HBr
phthalimide
Scheme 1 Stoichiometry of the reaction
123

472
M. Singh
chromatography was carried out using n-butanol-acetic acid–water (4:1:5) as
eluent to confirm the lactone formation. Silver nitrate, sodium hydroxide, and
sodium thiosulfate were used as detectors.
(iii) Dinapthol-sulfuric acid test [35]: The test was carried out by treating a little
amount of the final reaction mixture with a few drops of b,b’-dinapthol
solution in concentrated sulfuric acid and then heating for an hour in a water
bath at 85 °C. It did develop a characteristic green fluorescence of gluconic
acid.
Determination of CMC values
Surfactants are made up of a hydrophilic headgroup and a hydrophobic tail. In aqueous
solution the hydrophobic surfactant tail is encapsulated by a highly structured water
layer, which is one or two molecules thick. This structured layer has extremely low
entropy compared to bulk water. Above the critical micelle concentration (CMC), the
surfactants aggregate into micelles or differently shaped assemblies. This surfactant
self-assembly destroys the ice-like water layer and results in a positive entropy
change, stabilizing the assemblies. Nonionic surfactants generally have a hydrophilic
poly(ethylene oxide) or carbohydrate headgroup. Micelles formed by nonionic surfac-
tants contain a hydrophobic core and a hydrophilic palisade layer, which contains
water-swollen poly(ethylene oxide) chains, while anionic surfactants can have
phosphate, sulfate, sulfonate, or carboxylate headgroups. Micelles generally appear
approximately spherical at surfactant concentrations close to the CMC, and this shape
is geometrically constrained. Increasing surfactant concentrations and addition of
salts results in micellar growth and change of shape to ellipsoidal [36]. CMCs are also
affected by the temperature, pressure, and added solubilizates, reduced by a factor
proportional to the mole fraction of the solubilizate in the micelle and increased by the
reduction of hydrophobic interaction/decrease the ionic strength. CMC value of
surfactants (SDS, TX-100) in the presence and absence of substrate and oxidants were
determined from plots of the specific conductivity (r) versus surfactant concentration
using conductometric determination method and carried out with a digital conduc-
tivity meter, model 611E at 400 °C. The values of CMC of surfactants are sensitive to
the nature of the reactants and also depend upon reaction conditions. The break point
of nearly two straight-line portions in the plot is taken as an indication of micelle
formation, and this corresponds to the CMC of surfactant (Table 1).
Results and discussion
In order to propose a probable reaction mechanism for micelle catalyzed oxidation
of dextrose by NBP, it is necessary to study the effect of variation of concentra-
tion of different reactants on the rate of reaction. Kinetics due to molecular bro-
mine intervention was removed by the addition of mercury(II) ions, which removed
Br- ions either as HgBr2 or as HgBr24-. Here, mercuric acetate was added as a
scavenger. Oxidation of dextrose by NBP in the presence of SDS and TX-100 were
123

Mechanistic aspects of oxidation of dextrose
473
Table 1 Critical micellar
Solutions
CMC of SDS
CMC of TX-100
concentration (CMC) values of
surfactants at 40 °C
(i) Water
9.15 9 10-3
4.29 9 10-3
8.20 9 10-3
2.26 9 10-3
1.57 9 10-3
2.90 9 10-3
3.20 9 10-4
3.0 9 10-4
2.60 9 10-4
1.66 9 10-5
1.27 9 10-5
2.69 9 10-5
(ii) Dextrose
(iii) NBP
(iv) Acetic acid% (v/v)
(v) H2SO4
Literature value at 25 °C:
(a) SDS = 8.3 9 10-3
mol dm-3; (b) TX-
100 = 0.2 9 10-3 mol dm-3
(vi) Dextrose ? NBP
? acetic acid% (v/v)
? H2SO4
investigated under varying conditions of [dextrose], [NBP], solvent, mercuric
acetate, phthalimide, salts, [SDS], [TX-100] and temperature.
Dependence on [S]T i.e., [dextrose]T
From the plot of kobs vs. [S]T (cf. Fig. 1; Table 2), it has been established that this
shows the first order dependence on [S]T in both, the absence and presence of SDS
and TX-100.
Dependence on [NBP]T
Under the experimental conditions [dextrose]T ꢁ [NBP]T, the presence and absence
of SDS and TX-100, the rate of disappearance of NBP shows a first order dependence
on NBP. The pseudo first order rate constants (kobs) have been evaluated from the
linear plot of log[NBP]T vs time (t) as usual (Fig. 2; Table 2).
2
AQ
1.8
SDS
y = 0.464x + 0.707
R² = 0.995
1.6
1.4
1.2
1
TX-100
y = 0.427x + 0.244
R² = 0.988
0.8
0.6
0.4
0.2
0
y = 0.367x + 0.260
R² = 0.992
0
0.5
1
1.5
2
2.5
3+ log [dextrose]
Fig. 1 Plots between log [dextrose] versus log kobs at 40 °C: [NBP] = 2.0 9 10-4 mol dm-3
,
[Hg(OAc)2] = 5.0 9 10-4 mol dm-3, [H2SO4] = 0.5 9 10-4 mol dm-3, acetic acid (30 %) (v/v).
Filled diamond without surfactant (aqueous), filled square SDS, filled triangle TX-100
123

474
M. Singh
Table 2 Dependence of rate constants (kobs) on [NBP], [dextrose], [H2SO4], and acetic acid% (v/v) in
the absence and presence of surfactants
102 [dextrose]
mol dm-3
104 [NBP]
mol dm-3
104
Acetic acid%
(v/v)
104,a
s-1
104,b
s-1
104,c
s-1
[H2SO4]mol dm-3
1.0
2.0
0.5
0.5
30
30
30
0.44
0.59
0.72
1.03
1.29
0.73
0.72
0.72
0.72
0.71
0.72
0.55
0.40
0.31
0.23
1.59
1.04
0.72
0.50
0.46
0.48
0.65
0.98
1.33
1.61
0.66
0.65
0.65
0.64
0.64
0.65
0.54
0.34
0.22
0.17
0.86
0.78
0.65
0.58
0.48
1.50
2.25
3.02
4.60
5.92
2.27
2.25
2.25
2.24
2.23
2.25
1.63
0.98
0.60
0.42
2.58
2.39
2.25
2.05
1.52
2.5
5.0
10.0
20.0
2.5 (For a 5.0)
1.0
2.0
3.0
4.0
5.0
2.0
0.5
1.0
2.5
5.0
10.0
20
25
30
40
50
a
Without surfactant (aqueous); [Hg(OAc)2] = 5.0 9 10-4 mol dm-3, acetic acid% (v/v) 30 %,
Temp. = 40 °C
b
[SDS] = 1.0 9 10-2 mol dm-3; [Hg(OAc)2] = 5.0 9 10-4 mol dm-3, acetic acid% (v/v) 30 %,
Temp. = 40 °C
c
[TX-100] = 1.0 9 10-3 mol dm-3; [Hg(OAc)2] = 5.0 9 10-4 mol dm-3, acetic acid% (v/v) 30 %,
Temp. = 40 °C
Effect of dielectric constant and calculation of the size of the activated complex
In order to find out the effect of the dielectric constant of the medium on the rate of
the reaction, the reaction was studied at different dielectric constants (D) of the
medium with constant concentrations of all other reactants and at a constant
temperature. The dependence of the rate constant on the dielectric constant of the
medium is given by the following equation:
ZAZBe2N
1
log kobs ¼ log k00 ꢂ
ꢃ
ð1Þ
2:303ð4pe0ÞdABRT
e
where ko is the rate constant in a medium of infinite dielectric constant, ZA and ZB
are the charges of the reacting ion, dAB represents the size of the activated com-
plex, T is the absolute temperature and e is the dielectric constant of the medium.
123

Mechanistic aspects of oxidation of dextrose
475
1.2
1
0.8
0.6
0.4
AQ
0.2
SDS
TX-100
0
0
10
20
30
40
t (time) sec.
Fig. 2 Plots between t (s) versus log [NBP] at 40 °C: [dextrose] = 2.5 9 10-2 mol dm-3 (For filled
diamond 5.0 9 10-2 mol dm-3), [Hg(OAc)2] = 5.0 9 10-4 mol dm-3
,
[H2SO4] = 0.5 9 10-4
mol dm-3, acetic acid (30 %) (v/v). Filled diamond without surfactant (aqueous), filled square SDS,
filled triangle TX-100
This equation suggests that if a plot is made between log kobs vs. 1/e, a straight line
can be attained (Table 2; Fig. 3). A negative dielectric effect supports the proposed
mechanism. The effect of changing the solvent composition on the rate of reaction
has been discussed in detail [37]. For the limiting case of the zero angle approach
between two dipoles or an ion–dipole system, it has been shown that a negative
slope of a linear line in the plot of log kobs versus 1/e is a general result for a reaction
between a negative ion and a dipole or between two dipoles. A positive slope relates
to a positive ion–dipole interaction [38]. The former concept agrees with the
observations in the present study.
1.6
AQ
y = -0.169x + 1.635
1.4
1.2
1
R² = 0.833
SDS
TX-100
y = -0.310x + 1.604
R² = 0.988
0.8
0.6
0.4
0.2
0
y = -0.205x + 1.187
R² = 0.973
0
0.5
1
1.5
2
2.5
3
102/ε
Fig. 3 Plots between 1/e versus log kobs at 40 °C: [NBP] = 2.0 9 10-4 mol dm-3, [dextrose] =
2.5 9 10-2 mol dm-3 (For filled diamond 5.0 9 10-2 mol dm-3), [Hg(OAc)2] = 5.0 9 10-4mol dm-3
,
[H2SO4] = 0.5 9 10-4 mol dm-3. Filled diamond without surfactant (aqueous), filled square SDS, filled
triangle TX-100
123

476
M. Singh
Effect of [H2SO4]
To understand the effect of H?, H2SO4 was used. The rate constant decreased with
increasing [H2SO4]. A plot of log kobs vs. log [H2SO4] was linear with a slope is
negative (Fig. 4; Table 2) and less than unity indicating a negative fractional order
dependence of the rate on [H2SO4]. The findings can be explained by considering
the proposed mechanism.
Effect of [Hg(OAc)2]
Mercuric acetate was added to the reaction mixture as scavenger to eliminate Br-
formed in the reaction, which could have produced Br2 in the reaction. The Br2 thus
formed might cause another parallel oxidation. Hg(OAc)2 thus ensures the oxidation
purely through NBP.
Effect of [phthalimide]
The reaction rate is retarded by the addition of phthalimide (NHP) which is one of
the products of the reaction. This point to the existence of a pre-equilibrium step as
shown below:
K1
NBP + H2O ꢀ HOBr + NHP
ð2Þ
The positive bromine from NBP can be transferred to the dextrose through the
intermediate formation of hypobromous acid (HOBr).
1.6
AQ
1.4
SDS
y = -0.572x + 1.768
TX-100
1.2
1
R² = 0.996
y = -0.378x + 1.124
R² = 0.998
0.8
0.6
0.4
0.2
0
y = -0.470x + 1.167
R² = 0.987
0
0.5
1
1.5
2
2.5
5+ log [H2SO4]
Fig. 4 Plots between log [H2SO4] versus log kobs at 40 °C: [NBP] = 2.0 9 10-4 mol dm-3
,
[Hg(OAc)2] = 5.0 9 10-4mol dm-3
,
[dextrose] = 2.5 9 10-2 mol dm-3 (For filled diamond
5.0 9 10-2 mol dm-3), acetic acid 30 % (v/v). Filled diamond without surfactant (aqueous), filled
square SDS, filled triangle TX-100
123

Mechanistic aspects of oxidation of dextrose
477
Effect of salt
Added salts inhibit the micellar catalysis, which is a general phenomenon. The
inhibition has been explained by assuming that the counterions compete with an
ionic reactant for a site on the micelle. Electrolytes have also been found to increase
the aggregation number, decrease the CMC, and cause shape change from spherical
to rod-like micelles. The added salts (KCl, KBr, and Na2SO4) inhibit the rate of
reaction. As the concentration of these electrolytes increases, the concentration of
reactants at the reaction site decreases due to salting-out effect of the salts. Addition
of electrolytes, in general, is responsible for rate inhibition of micellar-mediated
reactions due to the exclusion of the reagent(s) from the micellar pseudo-phase.
Test for free radicals
Addition of acrylonitrile to the reaction mixture does not induce polymerization.
Therefore, the reaction does not involve the formation of free radicals. The
observations demonstrate that no free radicals are formed in the reaction mechanism.
Temperature effect and activation parameters
The oxidation of dextrose has been studied at 30–50 °C and the data are presented
in Table 3. Arrhenius parameters evaluated from linear plots of log kobs versus
Table 3 Effect of temperature and activation parameters
Temperature °C
104 kobs s-1
a
b
c
30
0.39
0.54
0.72
1.08
1.52
0.35
0.48
0.65
0.96
1.42
1.20
1.63
2.25
2.89
3.81
35
40
45
50
Activation parameters
Ea (kJ mol-1
log pZ (dm3mol-1 s-1
DH#(kJ mol-1
DS#(JK-1mol-1
DG#(kJ mol-1
)
53.34
4.76
53.86
4.80
45.67
3.97
)
)
50.74
324.54
152.32
51.26
43.06
)
325.39
153.10
315.09
141.69
)
a
Without surfactant (aqueous); [NBP] = 2.0 9 10-4 mol dm-3, [dextrose] = 5.0 9 10-2 mol dm-3
,
,
[Hg(OAc)2] = 5.0 9 10-4 mol dm-3, [H2SO4] = 0.5 9 10-4 mol dm-3, acetic acid = 30 % (v/v)
b[SDS] = 1.0 9 10-2 mol dm-3; [NBP] = 2.0 9 10-4 mol dm-3, [dextrose] = 2.5 9 10-2 mol dm-3
[Hg(OAc)2] = 5.0 9 10-4 mol dm-3, [H2SO4] = 0.5 9 10-4 mol dm-3, acetic acid = 30 % (v/v)
c
[TX-100] = 1.0 9 10-3 mol dm-3
;
[NBP] = 2.0 9 10-4 mol dm-3
[Hg(OAc)2] = 5.0 9 10-4 mol dm-3 [H2SO4] = 0.5 9 10-4 mol dm-3
acid = 30 % (v/v)
,
[dextrose] = 2.5 9 10-2
mol dm-3
,
,
,
acetic
123

478
M. Singh
1.8
1.6
1.4
1.2
1
AQ
SDS
TX-100
y = -2.385x + 8.955
R² = 0.999
0.8
0.6
0.4
0.2
0
y = -2.786x + 9.778
R² = 0.994
y = -2.813x + 9.820
R² = 0.996
3
3.1
3.2
3.3
3.4
3.5
103 /T
Fig. 5 Plots between 1/T versus log kobs, [NBP] = 2.0 9 10-4 mol dm-3, [dextrose] = 2.5 9 10-2
mol dm-3 (For filled diamond 5.0 9 10-2 mol dm-3), [Hg(OAc)2] = 5.0 9 10-4 mol dm-3
[H2SO4] = 0.5 9 10-4 mol dm-3
acetic acid 30 % (v/v). Filled diamond without surfactant
(aqueous), filled square SDS, filled triangle TX-100
,
,
1/T are presented (Fig. 5) in Table 3. The large negative value of DS# in the
presence of TX-100 indicates that more ordered activated complex is formed. The
fairly high positive values of DH# and DG# indicate that the transition state is highly
solvated. Here, Ea is the energy of activation, DH# is the enthalpy of activation, DS#
is the entropy of activation, DG# is the free energy of activation.
Mechanism
The monosaccharides are considered as a polyol and the reactivities of –OH groups
can be influenced by the presence of the carbonyl group. Monosaccharides exist
mainly as pyranoid and furanoid forms, the former being more stable [39] (Scheme 2).
On the other hand, NBP exists in the following equilibria [40] as in Scheme 3.
Since on assumption of NBP or (NBPH)? as the reactive species, the rate law
fails to explain the negative effect of phthalimide. Hence, neither of these species,
NBP and (NBPH)? can be taken as the reactive species. When HOBr is taken as the
reactive species, the rate law explains the negative effect of [H?] and [NHP]. Thus,
HOBr is taken as the most reactive species, which gave the rate law capable of
explaining all the kinetic observations and other effects.
Scheme 2 Species of glucose (dextrose)
123

Mechanistic aspects of oxidation of dextrose
479
Scheme 3 Reactive species of NBP in aqueous medium
According to the experimental conditions above, HOBr is the most reactive
species of NBP and considering the fact that one mole of dextrose is oxidized by
one mole of NBP in Scheme 4.
Scheme 4 Mechanism of the reaction
123

480
M. Singh
In Scheme 4 (Eq. 7–10), S stands for dextrose, K1, K2 are the equilibrium constant
for the step 1 and 2, k is the rate constant, A- is intermediate species. The Eq. (11) is
in good agreement with the experimental results. The rate of disappearance of [NBP]
can be expressed in Eq. (12). Eq. (13) and (14) satisfactorily explains the kinetic
results with respect to [NBP], [dextrose], [H2SO4] and [NHP].
d½NBPꢄ
kK1K2½dextroseꢄ½NBPꢄT
ꢂ
¼
ð11Þ
dt
½NHP][Hþꢄ + K1K2½dextrose] + K1½Hþꢄ
where
½NBP]T = [NBP] + [HOBr] + [Aꢂꢄ
rate = kobs½NBP]T
ð12Þ
ð13Þ
ð14Þ
rate
½NBP]T ½NHP][Hþꢄ + K1K2½dextrose] + K1½Hþꢄ
kK1K2½dextroseꢄ½NBPꢄT
Kobs
¼
¼
1
½Hþꢄ½NHPꢄ
½Hþꢄ
1
¼
þ
kobs kK1K2½dextroseꢄ kK2½dextroseꢄ
k
Effect of varying [surfactants]
Plot of kobs versus [TX-100] indicate the rate enhancement at lower concentrations
of TX-100. The rate acceleration is due to preferential partitioning of the negatively
charged dextrose-NBP complex (by hydrogen bonding) and neutral substrate in the
micellar surface (Stern layer). Thus, TX-100 allows the reaction to proceed in both
aqueous and micellar interphases. The partitioning mode leads to higher local
concentration of both the reactants at the micelle-water interphase compared to their
stoichiometric concentrations. TX-100 permits the reaction in both the phases with a
preferential rate enhancement in the micellar phase. At higher concentration, the
value of kobs finally tends to attain a limiting value (Fig. 6; Table 4).
Plot of kobs versus [SDS] shows a continuous decrease in the reaction rate. In
fact, this type of rate retarding effect by the surfactants is due to the accumulated
substrate in the micellar phase (Stern layer) cannot participate in the reaction and
consequently the reaction rate is retarded (Fig. 7; Table 4).
The kinetic model to explain the micellar effects
Micellar catalysis critically depends on the interactions of the micelle with the
substrate(s) and the activated complex. This is an extremely complicated problem
because a number of different interactions are involved including those associated
with the headgroup of the surfactant, different segments of the alkyl chain and the
counterions. In Berezin’s model [41], a solution above the CMC may be considered
as a two-phase system, consisting of an aqueous phase and a micellar pseudo-phase.
The reactants (S and O) may be distributed as shown in Scheme 5. A quantitative
rate expression for a bimolecular reaction occurring only in aqueous (kW path) and
micellar (kM path) phase for the pseudo-first order rate constant is given below:
123

Mechanistic aspects of oxidation of dextrose
481
0.74
0.72
0.7
SDS
0.68
0.66
0.64
0.62
0.6
0
0.5
1
1.5
2
102, [SDS]
Fig. 6 Plot of [SDS] versus kobs, [NBP] = 2.0 9 10-4 mol dm-3, [dextrose] = 2.5 9 10-2 mol dm-3
,
[Hg(OAc)2] = 5.0 9 10-4 mol dm-3, [H2SO4] = 0.5 9 10-4 mol dm-3, acetic acid 30 % (v/v)
Table 4 Effect of surfactants
102 [SDS]
mol dm-3
104, kobs
s-1
103 [TX-100]
mol dm-3
104, kobs
s-1
on the pseudo-first-order rate
constants (kobs) for the reaction
of dextrose with NBP at 40 °C
0
0.72
0.72
0.71
0.69
0.68
0.66
0.65
0.64
0.63
0.62
0.62
0
0.72
0.75
0.85
1.02
1.53
2.25
2.50
2.67
2.69
2.70
0.2
0.4
0.6
0.75
0.8
1.0
1.1
1.2
1.4
1.6
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
[NBP] = 2.0 9 10-4
mol dm-3
,
[dextrose] = 2.5 9 10-2
mol dm-3
[Hg(OAc)2] = 5.0 9 10-4
mol dm-3
,
,
[H2SO4] = 0.50 9 10-4
mol dm-3, acetic acid = 30 %
(v/v)
kw þ k0 KSK0ðCSurf ꢂ CMCÞ
½1 þ KSðCSurf ꢂmCMCÞꢄ½1 þ K0ðCSurf ꢂ CMCÞꢄ
kobs
¼
ð15Þ
where, KS and Ko are the association constants of dextrose and NBP, respectively
with surfactants, CSurf is the analytical concentration of surfactants, k0M = (kM/V),
V being molar volume of the micelle and kW and kM are the pseudo-first order rate
constant in absence and presence of micelles, respectively. Since the oxidant will be
uncharged species and the substrate is large molecules, the hydrophobic and elec-
trostatic interactions will be large and hence it may be expected that KS and K0 will
be high. Since CSurf is small it may be possible that kW ꢁ k0MKSKo(Csurf–CMC) so
that the Eq. (15) takes the form
kw
kobs
¼
ð16Þ
2
1 þ ½ðKS þ K0ÞðCSurf ꢂ CMCÞꢄ1 þ KSK0ðCSurf ꢂ CMCÞ
123

482
M. Singh
3
2.5
2
TX-100
1.5
1
0.5
0
0
0.5
1
1.5
2
103, [TX-100]
Fig. 7 Plot of [TX-100] versus kobs
, ,
[NBP] = 2.0 9 10-4 mol dm-3 [dextrose] = 2.5 9 10-2
mol dm-3, [Hg(OAc)2] = 5.0 9 10-4 mol dm-3, [H2SO4] = 0.5 9 10-4 mol dm-3, acetic acid 30 %
(v/v)
kW
(Product)W
(C6H12O6)W
+
(NBP)W
(NBP)M
K0
KS
kM
(Product)M
(C6H12O6)M
+
Scheme 5 Berezin’s model
Again, since (Csurf-CMC) is very small, the terms containing (Csurf-CMC)2
may be neglected, and the Eq. (16) may be rearranged to:
1
1
KSþK0
¼
þ
ðCSurf ꢂ CMCÞ
ð17Þ
kobs kW
kW
-1
Plot of k versus (Csurf-CMC) for dextrose is linear (Figs. 8, 9; Table 5).
obs
Conclusions
The oxidation of dextrose by NBP experienced a slow reaction rate in both absence
of surfactants and presence of SDS, but increased in rate in the presence of the TX-
100 surfactant. The reactive species of NBP for the oxidation in an acidic medium
was HOBr. In our body, HOBr formed by the eosinophils in the presence of H2O2,
and eosinophil peroxidase provides a potent mechanism by which eosinophils kill
multicellular parasites and certain bacteria such as tuberculosis bacteria. Aldonic
acid is the main product of the reaction, which is a very important compound. Under
the comparable experimental conditions, the catalytic ability of SDS and TX-100
123

Mechanistic aspects of oxidation of dextrose
483
1.65
SDS
1.6
1.55
1.5
1.45
1.4
1.35
0
5
10
15
20
25
103, (Csurf-CMC) /mol dm-3
Fig. 8 Plots between k versus (Csurf-CMC) at 40 °C: [NBP] = 2.0 9 10-4 mol dm-3, [dextrose] =
-1
obs
2.5 9 10-2 mol dm-3, [Hg(OAc)2] = 5.0 9 10-4 mol dm-3, [H2SO4] = 0.5 9 10-4 mol dm-3, acetic
acid (30 %) (v/v), filled square SDS
1.6
TX -100
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0
5
10
15
20
25
30
104, (Csurf-CMC) /mol dm-3
versus (Csurf-CMC) at 40 °C: [NBP] = 2.0 9 10-4 mol dm-3
,
-1
obs
Fig. 9 Plots between
k
[dextrose] = 2.5 9 10-2 mol dm-3
, ,
[Hg(OAc)2] = 5.0 9 10-4 mol dm-3 [H2SO4] = 0.5 9 10-4
mol dm-3, acetic acid (30 %) (v/v), filled triangle TX-100
Table 5 Other parameters obtained from Berezin’s model
Other parameters obtained from Berezin’s model
kW (s-1
)
(Ks ? Ko) (dm3 mol-1
)
SDS
0.80 9 10-4
0.50 9 10-4
1.28 9 10-5
3.31 9 10-6
TX-100
[NBP] = 2.0 9 10-4 mol dm-3
,
[dextrose] = 2.5 9 10-2 mol dm-3
,
[Hg(OAc)2] = 5.0 9 10-4
mol dm-3, [H2SO4] = 0.50 9 10-4 mol dm-3, acetic acid 30 % (v/v)
123

484
M. Singh
towards the oxidation of dextrose by NBP was in the order: (TX-100) [ (SDS). The
observed results were explained by plausible mechanisms and the related rate laws
were deduced. It can be stated that TX-100 accelerates and SDS decreases the rate
for the oxidation of dextrose by NBP in an acidic medium.
Acknowledgments One of the authors thanks the University Grants Commission, New Delhi for the
Research Project Grant. We also thank Professor Laurence S. Romsted, Department of Chemistry,
Wright-Rieman Laboratories, New Jersey (USA) for valuable discussions and fruitful suggestions.
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