Alkaline KMnO4 oxidation of reducing sugars in microemulsions: Inhibition effect of surfactants

Article abstract of DOI:10.1134/S0023158415030210

The kinetics of oxidation of reducing sugars viz. D-glucose and D-fructose by alkaline KMnO₄ in microemulsion media was investigated. The aqueous, cationic microemulsion was prepared from cetyl trimethyl ammonium bromide, n-butanol, n-hexane and water, whereas n-decane, aerosol-OT and water were used to prepare the anionic microemulsion. The order of reaction in oxidant was always found to be unity, while that in substrate and alkali was decreased from unity to zero at higher concentrations substrate and alkalirespectively. On decreasing [H₂O]/[Surfactant] ratio (increasing surfactant content) in microemulsion, the observed rates constants of oxidation (k _obs) were decreased. The inhibition effect on the rate of oxidation was greater in cationic microemulsion. A mechanism consistent with kinetic data is proposed.

Full text of DOI:10.1134/S0023158415030210

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2015, Vol. 56, No. 3, pp. 261–266. © Pleiades Publishing, Ltd., 2015.
Alkaline KMnO Oxidation of Reducing Sugars in Microemulsions:
4
1
Inhibition Effect of Surfactants
R. Tripathi and S. K. Upadhyay*
Department of Chemistry, H. B. Technological Institute, Kanpur, India
*
eꢀmail: upadhyay_s_k@rediffmail.com
Received June 18, 2014
Abstract—The kinetics of oxidation of reducing sugars viz. Dꢀglucose and Dꢀfructose by alkaline KMnO in
4
microemulsion media was investigated. The aqueous, cationic microemulsion was prepared from cetyl trimꢀ
ethyl ammonium bromide, nꢀbutanol, nꢀhexane and water, whereas nꢀdecane, aerosolꢀOT and water were
used to prepare the anionic microemulsion. The order of reaction in oxidant was always found to be unity,
while that in substrate and alkali was decreased from unity to zero at higher concentrations substrate and alkaꢀ
lirespectively. On decreasing [H O]/[Surfactant] ratio (increasing surfactant content) in microemulsion, the
2
observed rates constants of oxidation (kobs) were decreased. The inhibition effect on the rate of oxidation was
greater in cationic microemulsion. A mechanism consistent with kinetic data is proposed.
DOI: 10.1134/S0023158415030210
1
In last two decades, the microemulsion was used as
(2) a pH of the microemulsion was the same as that
a novel reaction media for the organic reactions due to of the aqueous solutions indicating that the presence
their large solubilisation capacities [1, 2]. The organic of microemulsion in reaction mixture does not affect
reactions in microemulsion media [3–12] were widely its pH;
investigated and it was observed that the rate of chemꢀ
(
3) the reagents are the ionic species of same sign as
ical reaction can be drastically modified by introducꢀ
tion of colloidal particles changing the local microenꢀ
vironment. It was also found [13] that the enhanceꢀ
ment of the rate of reaction in microemulsion is
usually larger than that in micellar media.
well as of opposite sign.
In the present communication, the kinetics results
of oxidation of reducing sugars vis. glucose and frucꢀ
tose by alkaline KMnO in cationic microemulsion
4
(
prepared from cetyl trimethyl ammonium bromide,
Recently it was reported [14–16] that an addition CTAB) and anionic microemulsion (prepared from
of a surfactant (anionic, cationic, nonꢀionic) in the aerosolꢀOT, AOT) are reported and discussed.
reaction mixture resulted in the retardation of the rate
of oxidation of reducing sugars/amino acids by alkaꢀ
EXPERIMENTAL
line KMnO . The formation of an inactive complex
4
between surfactant and KMnO was found to be
responsible for the inhibition effect of the surfactant
4
Materials
on the rate of oxidation in the reaction. Spectrophotoꢀ
metric evidence also supported complexation/associꢀ
Permanganate solution was prepared by dissolving
potassium permanganate (GR, Loba, India) in doubly
distilled water. Freshly prepared solutions of Dꢀgluꢀ
cose and Dꢀfructose (AR, Thomas Baker, India) were
used in the experiments. The surfactants viz. cetyl triꢀ
methyl ammonium bromide and aerosolꢀOT (Thomas
Baker) were used as received. However, their critical
micelles concentrations (CMC) were determined by
surface tension measurements and were in agreement
with their reported values. All other reagents and solꢀ
vents used were of analytical grade. Doubly distilled
ation between the surfactant and KMnO . Since the
4
presence of the droplets can multiply enhance or
retard chemical reaction rates, the application of
microemulsions as molecular scale reactors in the
above reactions may be interesting. In this context we
chose to study the kinetics of the oxidation of some
reducing sugars viz. glucose and fructose by alkaline
4
KMnO in microemulsion media. The goal of the
study was motivated by the following conditions:
water, prepared from alkaline KMnO , was used
4
throughout the experiments for the preparation of
solutions.
(
1) the kinetics behavior of the above reactions in
aqueous and micellar media is well known [14–16];
Two types of microemulsion were used and were
prepared as follows.
1
The article is published in the original.
261

262
TRIPATHI, UPADHYAY
–
The cationic microemulsion was prepared from MnO . The results may be represented by the followꢀ
4
combination (in weight percent) of cationic surfactant ing stoichiometric equations:
CTAB (30%), coꢀsurfactant
nꢀbutanol (4.5%), oil
–
–
RCHOHCHO + 2OH + 2MnO4
phase ꢀhexane (1.5%) and water (64%). By mixing
n
the components in the appropriate proportion the
microemulsion was prepared and the solution was
stirred vigorously until a clear transparent solution was
obtained [17].
The anionic microemulsion was prepared [18] by
mixing decane : aerosolꢀOT : water in weight percent
composition 80 : 10 : 10. The solution was stirred vigꢀ
2–
2
MnO4 + H O + RCHOHCOOH,
2
–
–
RCOCH OH + 2OH + 2MnO4
2
2–
2
MnO4 + H O + HCHO + RCOOH.
2
The presence of corresponding aldonic acid and
orously to obtain a perfectly microemulsion. The valꢀ formaldehyde (in the case of fructose) as the oxidation
ues of pH of microemulsions were determined by products was identified by the spot tests. The results
directly immersing a glass electrode into microemulꢀ are also in agreement with the earlier reported data
sion solutions. The pH of microemulsion was same as [14–16] on the oxidation of reducing sugars by
that of the aqueous solutions.
KMnO in aqueous and micellar media. The reaction
4
mixture, when stored for a long time, did not contain
formaldehyde in case of fructose and stoichiometric
results showed that a mole of fructose consumed more
–
Method
To a reaction mixture, containing appropriate
quantities of KMnO , NaOH, NaNO , microemulꢀ
than 3 moles of MnO , indicating that formaldehyde
4
4
3
sion was added so that the total volume of mixture was is further oxidized in the presence of excess of oxidant.
0 ml after adding substrate. The reaction mixture was
stirred well and then placed in a water bath maintained
at a desired temperature (±0.1 C). The reaction mixꢀ
5
RESULTS AND DISCUSSION
°
ture was allowed to attain the bath temperature. The
reaction then was initiated by adding requisite amount
of substrate placed separately in the same bath.
In all kinetics measurements, the substrate (gluꢀ
cose or fructose) was used in excess relative to KMnO4
to ensure pseudoꢀfirstꢀorder conditions. Although the
oxidation of glucose and fructose by alkaline KMnO4
in aqueous and micellar media are reported [14], in
order to compare the results with those in microemulꢀ
sions media, the kinetic results obtained in aqueous
media are included in the present communication.
The reaction has been studied at various initial conꢀ
centrations of the reactants and pseudo firstꢀorder rate
constants (kobs) were evaluated from the slopes of
straight lines plotted as log of absorbance vs. time.
Determination of Rate Constant
The rates were measured spectrophotometrically, by
monitoring the absorbance due to KMnO as a function
4
of time at 520 nm (λ of KMnO ) on a spectrophoꢀ
max 4
tometer Toshniwal TVSPꢀ25 (India). The absorbance
due to other reagents was negligible at 520 nm. The
concentration of KMnO was kept within the limits of
4
Beer’s law.
The observed rate constants (kobs) at various initial
The reactions were studied at different initial conꢀ
centrations of the reactants. When the natural log of
absorbance vs. time plots was plotted, each reaction
showed a linear relationship with a negative slope up to
concentrations of KMnO are shown in Table 1. The
4
identical values of kobs at various [KMnO ] confirmed
4
the firstꢀorder dependence of the reaction rate with
–
8
5–90%, suggesting a firstꢀorder dependence of reacꢀ
respect to [MnO ].
4
–
tion rate with respect to MnO . Therefore, pseudo
4
–
The results of effect the [Substrate] and [OH ] on
–
firstꢀorder rate constants in MnO4 (kobs) were evaluꢀ
the observed rate constant were identical. The plots of
–
ated from the slopes (slope = kobs/2.303) of the straight kobs vs. [Substrate] or [OH ] showed a deviation from
lines (r > 0.99) plotted as log of absorbance vs. time. The
reported rate constant data, represent as an average of
duplicate runs, were reproducible to within ±5%.
–
linearity at higher concentrations of substrate or OH .
However, the plots of 1/kobs vs. 1/[Substrate] (Fig. 1)
–
and plots of 1/kobs vs. 1/[OH ] (Fig. 2) were linear with
positive intercepts. It can be therefore inferred that the
order of reaction in substrate and alkali decreases from
Stoichometry and Product Analysis
–
unity at higher [Substrate] and [OH ], respectively.
The reaction mixture containing an excess amount
Varying the concentration of NaNO and keeping
3
of KMnO over the substrate in the presence of alkali other reactants at a fixed concentration the effect of
4
in aqueous as well as in microemulsion media were ionic strength on the rate of oxidation was stadied. The
kept for 72 h at 40
°
C.
observed rate constants are reported in Table 2. An
The stoichiometric results showed that a mole of increase in of kobs was observed with an increasó in
substrate (glucose or fructose) consumes 2 moles of [NaNO3].
KINETICS AND CATALYSIS Vol. 56
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ALKALINE KMnO OXIDATION OF REDUCING SUGARS IN MICROEMULSIONS
263
4
Table 1. Effect of [KMnO ] on the observed rate constants at 35°C
4
4
–1
kobs × 10 , s
[
KMnO4]
×
104,
glucose
anionic
fructose
anionic
3
mol/dm
aqueous
medium
cationic
aqueous
medium
cationic
microemulsion microemulsion
microemulsion microemulsion
3.0
4.0
5.0
6.0
6.15
6.15
5.90
6.15
4.45
4.40
4.60
4.40
4.05
3.85
3.85
4.05
6.50
6.35
5.50
5.40
5.50
5.40
4.40
4.60
4.60
4.40
6.35
6.50
10–3 mol/dm , [NaNO ] = 0.2 mol/dm , [NaOH] = 10.0
3
3
×
10–4 mol/dm3.
[
Substrate] = 20.0
×
3
Table 2. Effect of added [NaNO ] on the observed rate constants at 35
°
C
3
4
–1
kobs
×
10 , s
[
NaMnO4]
×
104,
glucose
anionic
fructose
anionic
3
mol/dm
aqueous
medium
cationic
aqueous
medium
cationic
microemulsion microemulsion
microemulsion microemulsion
0.05
0.10
0.20
0.30
2.95
4.40
6.15
4.90
2.60
3.65
4.40
4.90
2.45
2.80
3.85
3.50
5.00
6.35
6.50
3.00
4.00
5.50
5.65
2.75
3.20
4.60
5.20
4.20
10–3 mol/dm , [KMnO ] = 4.0
3
×
10–4 mol/dm , [NaOH] = 10.0
3
×
10–4 mol/dm3.
[
Substrate] = 20.0
×
4
The effect of microemulsion on the rate of oxidaꢀ increasing the surfactant content or on decreasing the
tion was studied at three different temperatures (35, 40 water content in the microemulsion, the kobs
and 45
C) and at different [H O]/[Surfactant] ratio in decreases.
°
2
microemulsions. The kobs at different [H O]/[Surfacꢀ
tant] ratio in case of cationic and anionic microemulꢀ
sions are given in Tables 3 and 4, respectively. It can be
2
The reactions have also been studied at different
temperatures (35, 40 and 45
°
C) to investigate the
seen that kobs decrease on decreasing [H O]/[Surfacꢀ
2
tant] ratio. Thus, it is evident from the result that on
1
1
1
4
2
0
8
6
4
2
Glucose
3
2
Fructose
3
2
1
1
1
1
4
2
0
8
6
4
2
Glucose
Fructose
1
3
2
3
2
1
1
0
40 80 120 160 200 0 40 80 120 160 200
10 , dm / mol
0
10 20 30 40
(1/[OH ])
0
10 , dm / mol
10 20 30 40 50
–3
3
–
–3
3
(
1/[Substrate])
×
×
–
Fig. 1. Plots of 1/kobs versus 1/[Substrate] in aqueous
medium ( ), in the presence of anionic microemulsion (
and cationic microemulsion ( ). [KMnO ] = 4.0
Fig. 2. Plotsof1/kobs versus 1/[OH ] in aqueous medium (
1
),
1
2
)
in the presence of anionic microemulsion (
ionic microemulsion ( ). [Substrate] = 20.0
2) and catꢀ
–
3
3
×
3
×
10
mol/dm , [KMnO ] = 4.0 × 10 mol/dm , [NaNO ] =
4
4
⎯
4
3
–3
3
3
–4
3
1
0
mol/dm , [NaOH] = 10.0
×
10
mol/dm ,
3
3
3
[
NaNO ] = 0.2 mol/dm , 35
°
C.
0.2 mol/dm , 35°C.
3
KINETICS AND CATALYSIS Vol. 56
No. 3 2015

264
TRIPATHI, UPADHYAY
Table 3. Effect of [H O]/[Surfactant] ratio in cationic microemulsion on the observed rate constants at different temperatures
2
4
–1
kobs × 10 , s
[
H O]/[Surfactant]
glucose
40°C
fructose
40°C
2
35°C
45°C
35°C
45°C
Aqueous medium
6.15
5.71
5.50
4.70
4.10
3.83
6.70
6.10
5.65
5.18
4.85
4.41
7.29
6.38
5.85
5.45
5.20
4.90
6.50
6.00
5.79
5.01
4.83
4.60
6.92
6.60
6.36
5.83
5.75
4.82
7.86
7.21
6.60
6.41
6.01
33.25
15.95
10.11
7
.28
.55
5
5.41
10–3 mol/dm , [KMnO ] = 4.0
3
×
10–4 mol/dm , [NaOH] = 10.0
3
×
10–4 mol/dm , [NaNO ] = 0.2 mol/dm .
3
3
[Substrate] = 20.0
×
4
3
Table 4. Effect of [H O]/[Surfactant] ratio in anionic microemulsion on the observed rate constants at different temperaꢀ
2
tures
4
–1
kobs × 10 , s
[
H O]/[Surfactant]
glucose
40°C
fructose
40°C
2
35°C
45°C
35°C
45°C
Aqueous medium
6.15
5.80
5.20
5.00
4.60
4.41
6.70
6.33
5.80
5.37
5.20
4.80
7.29
6.90
6.33
5.80
5.60
5.37
6.50
6.14
6.01
5.90
5.80
5.70
6.90
6.71
6.52
6.20
6.00
5.60
7.86
7.48
6.71
6.33
6.14
10.65
5
.10
.25
.33
.78
3
2
1
6.00
10–3 mol/dm , [KMnO ] = 4.0
3
×
10–4 mol/dm , [NaOH] = 10.0
3
×
10–4 mol/dm [NaNO ] = 0.2 mol/dm .
3,
3
[Substrate] = 20.0
×
4
3
Table 5. Secondꢀorder rate constants at different temperatures
Secondꢀorder rate constants, dm mol s–1
3
–1
glucose
anionic
fructose
anionic
Temperature, K
aqueous
medium
cationic
aqueous
medium
cationic
microemulsion microemulsion
microemulsion microemulsion
308
313
318
1.53
1.67
1.82
1.10
1.11
1.34
0.95
1.10
1.22
1.58
1.72
1.96
1.34
1.39
1.15
1.20
1.35
1.48
10–3 mol/dm , [KMnO ] = 4.0
3
×
10–4mol/dm , [NaOH] = 10.0
3
×
10–4mol/dm , [NaNO ] = 0.2 mol/dm .
3
3
[Subdtrate] = 20.0
×
4
3
effect of temperature and to evaluate the activation comparison to those found in aqueous medium are
parameters. The second order rate constants consistent with the accepted view that slower reactions
–
4
E# or H#
.
(
kobs/
[MnO ] ) at different temperatures are reported (in microemulsion) would require higher
Δ
Δ
E#
H#
S#
A lower negative value derived from experiments in
microemulsions indicates a relatively higher extent of
in Table 5. The activation parameters
Δ
,
Δ
,
Δ
,
#
and G , evaluated with the help of Arrehenius and
Δ
Eyring plots can be also found in Table 6. The large rigidness for the reactants in microemulsions. Nearly
#
#
obtained in microemulsion in the same value of
G# is in favour of suggestion that a
values of
ΔE
and
Δ
H
Δ
KINETICS AND CATALYSIS Vol. 56
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ALKALINE KMnO OXIDATION OF REDUCING SUGARS IN MICROEMULSIONS
265
4
Table 6. Activation parameters evaluated by the Arrenius and Eyring plots
Glucose
anionic
Fructose
anionic
Parameter
aqueous
cationic
aqueous
cationic
medium microemulsion microemulsion medium microemulsion microemulsion
Δ
E
* ± 0.25, kJ/mol
17.5
14.8
21.4
18.8
24.2
21.4
19.6
17.0
22.0
19.4
23.0
20.5
Δ
H # ± 0.25, kJ/mol
* ± 1.00, J K–1 mol–1
G # ± 0.50, kJ/mol
–
Δ
S
243.4
91.0
235.5
92.6
229.7
93.0
236.2
90.9
236.5
93.2
228.7
92.0
Δ
common mechanism is operative in the oxidation KMnO in microemulsions, the following observaꢀ
4
reactions.
tions can be emphasized:
1) The kinetic results are similar to those observed
in the presence of surfactants. The reaction order in
oxidant was always unity while that in alkali and subꢀ
strate was found to decrease from unity to zero at
higher [Alkali] and [Substrate]. The effect of ionic
strength on the rate of oxidation was positive.
(
In alkaline medium, the formation of enediol
anion of monosaccharides proceeds as follows:
−
H OH
C
H O
C
H
H C OH
C O
C OH
R
C OH
(
2) When a reaction occurs in the microemulsion,
R
R
its rate is lower than when it takes place in a convenꢀ
tional aqueous medium or in the presence of surfacꢀ
tant. The inhibition effect was also found to be greater
in microemulsion medium.
–
Fructose
Enediol (E) Enediol anion (E )
In the absence of other reactants these anions
undergo epimerization and isomerization to form a
mixture of aldoses and ketoses (Lobry de Bruyn–
Alberda–van Ekenstion transformation). Aldoses and
(
3) With decreasing [H O]/[Surfactant] ratio in the
2
microemulsion, the kobs (and hence the rate of oxidaꢀ
tion) also decreases indicating that an increase in the
surfactant contents in microemulsion is responsible
ketoses generally yield a mixture of Zꢀ and Eꢀenediols,
the proportion of which differs from one sugar to
another and are dependent on experimental condiꢀ
tions such as the strength and the nature of alkali and
temperature. In the presence of an oxidant or a cataꢀ
lyst, the enediol anion was considered as a reactive
species of the reducing sugar. A higher rate of fructose
for decreasing in kobs
.
(
4) The inhibition effect is more pronounced in
case of cationic microemulsion in comparison to that
in anionic microemulsion.
On the basis of the above facts and making use of
enolization was explained [19] on the basis of an easy known mechanism for the oxidation process in aqueꢀ
attack on the two active primary H atoms at C(2) of ous surfactant medium, a common mechanism for
aldoses. The observed rate constants follow the order: oxidation in microemulsion may be proposed as
kobs(fructose) > kobs(glucose).
K1
–
Sugar + OH–
E ,
(Enediol anion)
(I)
The oxidation of reducing sugars by alkaline
KMnO in aqueous medium and in the presence of
4
–
K
2
–
–
E + MnO4
I ,
Intermediate)
II)
anionic, cationic and nonꢀionic surfactants was
reported [14–16]. The presence of the surfactant in
the reaction mixture retards the rate of oxidation of
(
k3
I
Products (slow),
(III)
reducing sugars by KMnO . The formation of an inacꢀ
4
–
4
Surfactant + MnO
K2
X.
tive complex between the surfactant and oxidant,
(
IV)
(
in microemulsion
)
(Complex)
KMnO was responsible for the inhibition effect of
4
On the basis of above mechanism, the rate of disapꢀ
pearance of KMnO may be obtained as
surfactant on the rate of oxidation. A spectrophotoꢀ
metric evidence for the complexation/association
4
–
4
–
d[MnO ]
between surfactant and MnO4 also confirmed this
–ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀ
dt
suggestion. The inhibition effect of cationic surfactant
(
CTAB) on the rate of oxidation was more proꢀ
–
–
k K K [Sugar][OH ][MnO ]
3
1
2
4 T
nounced due to a more favourable association/comꢀ
=
ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀ ꢀꢀ ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀ ꢀꢀ ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀ ꢀꢀ ,
–
–
1 + K K [Sugar]([OH ] + K [Surfactant])
1
2
4
plexation between negatively charged MnO4 and posꢀ
–
–
itively charged surfactant.
where [MnO4 ]T = [MnO4 ] + [I] + [X].
From the results obtained in the present investigaꢀ
This rate law explains all the experimental results
tion, i.e. in the oxidation of reducing sugars by such as the firstꢀorder dependence of reaction rate
KINETICS AND CATALYSIS Vol. 56
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266
TRIPATHI, UPADHYAY
–
REFERENCES
with respect to MnO4 , decrease in the order of reacꢀ
1
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2
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,
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