Choice of suitable micellar catalyst for 2,2′-bipyridine-promoted chromic acid oxidation of glycerol to glyceraldehyde in aqueous media at room temperature

Article abstract of DOI:10.1007/s11164-013-1415-6

Abstract The micellar catalyzed 2,2′-bipyridine (bipy)-promoted oxidation of glycerol to glyceraldehyde by chromic acid is investigated under the criteria [glycerol]_T [Cr(VI)]_T at 30 °C. The critical micellar concentrations values of the three representative surfactants, N-cetylpyridinium chloride (CPC), sodium dodecyl sulphate (SDS), and TX-100, are determined by conductometric and spectrophotometric methods. The oxidized product glyceraldehyde is identified by 2,4-DNP test and FTIR spectral measurement. The pseudo-first-order rate constants (k_obs, s^-1) are calculated from the slope of plots of ln(A₄₅₀) versus time (t) which are linear. From these plots, the kinetic parameter k_eff values are calculated and the k_eff value of SDS-catalyzed bipy-promoted reaction path was found to be highest among all the combinations. In the bipy-promoted oxidation path, Cr(VI)-bipy complex is the main active oxidant which undergoes attack by the substrate to form the product. The active oxidant Cr(VI)-bipy complex reacts with glycerol to form a ternary complex which undergoes redox decomposition in a rate-limiting step. Here, the anionic surfactant SDS and the neutral surfactant TX-100 both catalyze the reaction in the presence of bipy, whereas the cationic surfactant CPC and neutral surfactant TX-100 inhibit the reaction in the absence of bipy. SDS is found to be the most suitable micellar catalyst for the bipy-promoted chromic acid oxidation of glycerol.

Full text of DOI:10.1007/s11164-013-1415-6

Res Chem Intermed
DOI 10.1007/s11164-013-1415-6
0
Choice of suitable micellar catalyst for 2,2 -bipyridine-
promoted chromic acid oxidation of glycerol
to glyceraldehyde in aqueous media at room
temperature
Aniruddha Ghosh
Kakali Mukherjee
•
Rumpa Saha
Sumanta K. Ghosh
Bidyut Saha
•
•
•
Pintu Sar
•
Susanta Malik
•
Received: 16 August 2012 / Accepted: 19 September 2013
Ó Springer Science+Business Media Dordrecht 2013
0
Abstract The micellar catalyzed 2,2 -bipyridine (bipy)-promoted oxidation of
glycerol to glyceraldehyde by chromic acid is investigated under the criteria
[
glycerol] ꢀ [Cr(VI)] at 30 °C. The critical micellar concentrations values of the
T
T
three representative surfactants, N-cetylpyridinium chloride (CPC), sodium dodecyl
sulphate (SDS), and TX-100, are determined by conductometric and spectropho-
tometric methods. The oxidized product glyceraldehyde is identified by 2,4-DNP
-
1
test and FTIR spectral measurement. The pseudo-first-order rate constants (kobs, s
)
are calculated from the slope of plots of ln(A450) versus time (t) which are linear.
From these plots, the kinetic parameter keff values are calculated and the keff value
of SDS-catalyzed bipy-promoted reaction path was found to be highest among all
the combinations. In the bipy-promoted oxidation path, Cr(VI)–bipy complex is the
main active oxidant which undergoes attack by the substrate to form the product.
The active oxidant Cr(VI)–bipy complex reacts with glycerol to form a ternary
complex which undergoes redox decomposition in a rate-limiting step. Here, the
anionic surfactant SDS and the neutral surfactant TX-100 both catalyze the reaction
in the presence of bipy, whereas the cationic surfactant CPC and neutral surfactant
TX-100 inhibit the reaction in the absence of bipy. SDS is found to be the most
suitable micellar catalyst for the bipy-promoted chromic acid oxidation of glycerol.
0
Keywords 2,2 -Bipyridine (bipy) ꢁ Glycerol ꢁ [Cr(VI)] ꢁ CMC ꢁ CPC ꢁ SDS ꢁ
T
TX-100
A. Ghosh ꢁ R. Saha ꢁ K. Mukherjee ꢁ S. K. Ghosh ꢁ P. Sar ꢁ S. Malik ꢁ B. Saha (&)
Homogeneous Catalysis Laboratory, Department of Chemistry, The University of Burdwan,
Golapbag, Burdwan 713104, WB, India
e-mail: b_saha31@rediffmail.com
R. Saha
Bishnupur K.G. Engineering Institute, Bankura, WB, India
123

A. Ghosh et al.
Introduction
Chromium(VI)-based oxidizing agents have been used to develop a good number of
reagents, some of which have become quite popular and perform well as oxidizing
agents [1]. Some of the important entries in the list of the reagents are pyridinium
chlorochromate, pyridinium dichromate, pyridinium fluorochromate, pyridinium
bromochromate, quinolinium fluorochromate, and prolinium chlorochromate. The
syntheses involving these oxidants have been carried out mostly in less polar
solvents, such as dichloromethane and trichloromethane, though the majority of
kinetic measurements have been made only in polar solvents like aqueous acetic
acid [2]. Problems in the use of chromium(VI) complexes as oxidation reagents
include the lack of selectivity in oxidations, safety hazards associated with the use
of large quantities of toxic compounds, cumbersome preparation, and potential
danger (ignition) in handling, difficulties in terms of product, waste disposal, and
the need for aqueous acidic or basic conditions for reactions of chromate salts [3].
Hexavalent chromium compounds are widely used as oxidizing agents mainly for
carbonyls and alcohols in aqueous and non-aqueous media [4, 5]. A non-aqueous
medium is introduced for one-step oxidation. Chromium chemistry is mainly
dominated by two oxidation states, ?3 and ?6. In addition to non-toxic trivalent
chromium, an amount of hexavalent chromium remains in the industrial effluents.
There are two problems. Hexavalent chromium is carcinogenic [6–8] and a non-
aqueous medium is not environmentally friendly [9, 10]. Use of large excesses of
substrate over hexavalent chromium and aqueous medium can solve the problem.
Academia and industry have been focused on the use of biorenewables for the
production of chemicals and clean fuels. One biorenewable feedstock is glycerol,
which can be found naturally in the form of fatty acid esters and also as important
intermediates in the metabolism of living organisms. Glycerol is a highly
functionalized compound and, in particular, its oxidation leads to a complex
reaction network, in which a large number of products can be formed. Therefore,
control of selectivity to the desired product would be highly advantageous [11]. The
choice of K Cr O in aqueous micellar media is due to the control and selective
2
2 7
oxidation of glycerol to glyceraldehyde by maintaining pseudo-first-order criteria.
Glyceraldehyde is an important ingredient in the organic synthesis of glyceralde-
hyde-3-phosphate dehydrogenase which is a well-studied glycolytic protein with
energy production as its implied occupation [12]. Glyceraldehyde is an example of a
small chiral building block of great interest because of the wide applicability of its
structural motif [13]. Water is one of the universal green solvents compared to other
hazardous organic solvents, therefore the oxidation of glycerol has been carried out
in aqueous media. This will produce one-step oxidation and total conversion of
hexavalent chromium to trivalent chromium. Micellar solutions have versatile uses
in the fields of chemistry, biochemistry, pharmacy, medicine, and industry.
Different kinds of reactions, such as hydrolytic, oxidation, reduction, and
photochemical, are significantly influenced by micelles [14–16]. The property of
micelle formation in solution gives surfactants excellent solubilization properties
[17]. A fundamental property of surfactants is their ability to form micelles in
solution. This property is due to the presence of both hydrophobic and hydrophilic
1
23

Oxidation of glycerol to glyceraldehyde
groups in each surfactant molecule. As the title reaction is very slow, suitable
combinations of micellar catalyst [18–22] and promoter [23–28] are used to speed
up the reaction. In this work, we have studied the efficiency of the suitable
0
combination of 2,2 -bipyridine (bipy) and micellar catalyst for the chromic acid
oxidation of glycerol to glyceraldehyde. The micellar catalyzed picolinic acid and
phenanthroline-promoted glycerol oxidation by chromic acid has been previously
studied [27]. During the oxidation, three different types of surfactants: N-
cetylpyridinium chloride (CPC) as cationic, sodium dodecyl sulphate (SDS) as
anionic, and Triton X-100- as neutral non-functional micellar catalyst, have been
used.
The chromic acid oxidation of glycerol is represented as follows:
ꢂ
4
þ
3
CH2ðOHÞCH(OH)CH ðOH) + 2HCrO þ 8H ! 3CH2ðOH)CH(OH)CHO
2
þ 2Cr(III) + 8H O
2
Experimental
Instrumentation
Solutions of the oxidant and reaction mixtures containing known quantities of the
substrate (i.e. glycerol), and promoter (bipy) were prepared under the kinetic
conditions [glycerol] ꢀ [Cr(VI)] . Acid and other necessary chemicals were
T
T
separately thermostated (± 0.10 °C). The reaction was initiated by mixing requisite
amounts of the oxidant with the reaction mixture. Progress of the reaction was
monitored by following the decay of Cr(VI) at 450 nm wave length at different time
intervals with a UV–Vis (UV–Vis–NIR-3600; Shimadzu) spectrophotometer.
Quartz cuvettes of path length 1 cm were used. The pseudo-first-order rate
-
1
constants (kobs, s ) were calculated from the slope of plots of ln(A450) versus time
t) (Fig. 1), which were linear [5]. A large excess (C15-fold) of reductant was used
(
in all kinetic runs. No interference was observed due to other species at 450 nm. The
scanned spectra, the spectrum after completion of the reaction and other spectra,
were recorded with a UV–Vis spectrophotometer (UV-1800; Shimadzu). Rate
constants of fast reactions are determined with the help of astopped flow
spectrophotometer (SX20 Spectrometer).
Determination of critical micellar concentration (CMC) of CPC, SDS and
TX-100
The CMC values of CPC and SDS were measured by the conductometric method.
The conductivity of the different concentrated solutions of CPC and SDS was
-
1
measured with a conductivity cell (CON 6000, cell constant 1 cm ) provided with
a built-in temperature sensor immersed in the beaker containing the surfactant
solutions. The temperature was set to 30 °C (Table 1). All the conductivity
measurements were recorded with the help of a water analyzer kit (Eutech
instruments, CyberScan PCD 6500 Series Meters). Typical experimental
123

A. Ghosh et al.
Fig. 1 Representative first-order plot to evaluate the pseudo-first-order rate constant (kobs) for Cr(VI)
-4
-3
T
oxidation of glycerol in an unpromoted path in aqueous medium. [glycerol] = 75 9 10 mol dm ,
-
4
-3
= 5 9 10 mol dm , [H
-3
[
Cr(VI)]
T
2
SO
4
]
T
= 0.5 mol dm , Temp = 30 °C
Table 1 CMC values at 30 °C temperature
Surfactant
Name of surfactant
Method
CMC (mM)
Cationic
Anionic
Neutral
CPC
Conductometric
Conductometric
Spectrophotometric
8.2
SDS
1.1
TX-100
0.24
Fig. 2 Conductance against
CPC] plot; the intersection
point is CMC of CPC at
.1 mM. Temp = 30 °C
[
T
1
observations are presented in Figs. 2 and 3. The breaks obtained in the region of the
CMC for solutions containing these two surfactants separately. From the plot of
conductance versus concentration of CPC and SDS (Figs. 2, 3), the CMC of SDS
and CPC were found at 1.1 and 8.2 mM, respectively, at 30 °C. These CMC values
agree nicely with the established values established earlier [29, 30].
The CMC of TX-100 was analyzed by interaction with a saturated aqueous
solution of iodine with varying concentration (below and above CMC) of TX-100
solution. The CMC value of TX-100 was determined by recording the absorbance
values of the solutions containing the mixture of TX-100 and I solution after
2
1
23

Oxidation of glycerol to glyceraldehyde
Fig. 3 Conductance against
[
SDS]
point is CMC of SDS at
.2 mM. Temp = 30 °C
T
plot; the intersection
8
Fig. 4 Plot of absorbance
versus concentration of TX-100
with iodine solution at 460 nm.
The break point is the CMC of
TX-100. Temp = 30 °C
temperature equilibration at 30 °C at 460 nm with a UV–Vis spectrophotometer
(
UV-1800; Shimadzu). The CMC values obtained for TX-100 (non-ionic) is
.24 mM from the plot (Fig. 4) of absorbance (A) versus [TX-100]T at
0
kmax = 460 nm. The plot has a break in the region of CMC for solutions containing
the mixture of TX-100 and I solution. It has similarity with the plots reported in
earlier literature [31, 32]. The peak at 460 nm for iodine is continuously blue-shifted
with increasing the concentration of TX-100 (Fig. 5).
2
Product analysis
Substrate glycerol was taken 15 times higher than [Cr(VI)] and dissolved in
T
-
3
3
0
.5 mol dm H SO making a total volume of the solution of 200 cm . The
2 4
reaction mixture was heated and evaporated to a small volume to get a concentrated
solution. It is important to mention that the substrate was taken at least 10 times
higher than the oxidant maintaining pseudo-first-order criteria. The selectivity of the
oxidation was maintained such that no further oxidation of the aldehyde would
occur. Oxidation was made selective from glycerol to glyceraldehdye by stopping
the over-oxidation of glyceraldehdye. The change in the color from yellow to pale
greenish blue ensures the completion of the reaction. The appearance of the first
123

A. Ghosh et al.
Fig. 5 Absorption spectra of saturated aqueous solution of I
2
in the absence and presence of TX-100 at
-5
3
(
2 T
0 °C. Spectra-1 saturated I solution in the absence of TX-100; spectra 2–13, [TX-100] = 10 9 10
-
5
-5
-5
-5
-5
-5
M), 15 9 10 (M), 20 9 10 (M), 25 9 10 (M), 30 9 10 (M), 35 9 10 (M), 40 9 10 (M),
-5
-5
-5
-5
5 9 10 (M), 50 9 10 (M), 75 9 10 (M), 100 9 10 (M), and 125 9 10 (M), respectively
-5
4
prominent greenish blue color confirms the total conversion of Cr(VI) to Cr(III).
After completion of the reaction, the product was isolated by fractional distillation.
A small portion of the distilled product was treated with 2,4-dinitrophenyl hydrazine
(
DNP) in ethanolic H SO media. The mixture was allowed to settle down for half
2 4
an hour. The yellowish orange-colored precipitate of the 2,4-dinitrophenyl
hydrazone derivative was filtered off, dried in an oven, and recrystallized from
ethanol. The same was also performed for the micelle-catalyzed reactions in the
presence of promoters. The melting point of the recrystallized hydrazone substance
was obtained at 169 °C which matches with the reported [27, 33–35] value of the
glyceraldehyde-DNP derivative. The IR spectrum (Fig. 6) of the 2,4-DNP
derivative of the product was recorded by the Perkin-Elmer FTIR model RX1
spectrometer. The IR spectrum of glyceraldehyde-DNP derivative observed is
similar to the earlier literature work [33]. Melting point data and IR spectrum of
hydrazone strongly support the identity of the product as glyceraldehyde from
oxidation of glycerol.
Materials
Glycerol (AR, SRL) was purified by refluxing with an excess of freshly burnt
quicklime followed by distillation, and purity was checked by density measurement.
0
2
,2 -bipyridine (AR, Qualigens), SDS (AR, SRL), CPC (AR, SRL), TX-100 (AR,
SRL), 2,4-dinitrophenyl hydrazine (DNP) (AR, Qualigens), and Iodine (AR,
Merck), and all other necessary chemicals used were of highest purity available
commercially. Solutions were prepared by using double-distilled water.
1
23

Oxidation of glycerol to glyceraldehyde
1
1
1
4
2
0
8
6
4
2
0
2
9
25
835
2945
1399
1508
618
1
618
1
060
1
093
1
340
1
214
3427
-
4000
3500
3000
2500
2000
1500
1000
500
-
1
cm
Fig. 6 IR spectrum of 2,4-dinitrophenylhydrazone of glyceraldehyde
Results and discussion
Dependence on Cr(VI)
Under the kinetic and experimental conditions [glycerol] ꢀ [Cr(VI)] both in the
T
T
presence and absence of bipy, the rate of disappearance of Cr(VI) showed a first-
order dependence on [Cr(VI)] . The first-order dependence on Cr(VI) was also
T
maintained in the presence of the three non-functional surfactants, CPC, SDS and
TX-100. The pseudo-first-order rate constants kobs were directly evaluated from the
slope of the linear plot of log [Cr(VI)] versus time (t) [27, 36].
T
Dependence on substrate
From previous work, it has been observed that the rate of the reaction was first-order
with respect to substrate [27, 36], and this was also true in the presence of
surfactants.
?
Dependence on H
?
The reaction is second-order with respect to H ion concentration which has been
previously observed [27].
Dependence on [bipy]T
The plots of kobs(T) versus [bipy] are linear with positive intercepts measuring the
T
contribution of rate enhancement over the unpromoted path (Fig. 7). The pseudo-first-
123

A. Ghosh et al.
Fig. 7 Dependence of kobs(T) on [bipy]
aqueous H SO
media at 30 °C. [Cr(VI)]
SO = 0.5 mol dm
T
for Cr(VI) oxidation of glycerol (in the absence of surfactant) in
-3
-
4
-4
-3
2
4
T T
= 5 9 10 mol dm , [glycerol] = 75 9 10 mol dm ,
-3
[H
2
4 T
]
order rate constant kobs(u) calculated in the absence of bipy under the same conditions
nicely matches with those obtained from the intercepts of plots of kobs(T) versus [bipy]T
[
27]. The observation can be formulated as: kobs(T) = kobs(u) ? k [bipy]
cat T.
Spectrophotometric analysis of the reaction
UV–Vis spectra
The gradual production of the Cr(III) species was confirmed spectrophotometrically.
In the unpromoted path, the final color appeared as pale blue (kmax = 583 and
2
60 nm; Fig. 8) whcih is quite different from the color observed in the bipy-
T
Fig. 8 Absorption Spectrum of reaction mixture (after completion of reaction): [glycerol] =
-
4
-3
5 9 10 mol dm , [Cr(VI)]
-4
-3
-3
7
(
T 2 4 T
= 5 9 10 mol dm , [H SO ] = 0.5 mol dm , Temp = 30 °C
i) unpromoted: (The spectrum of the chromic sulfate is identical with this under the experimental
condition), (ii) bipy promoted: [bipy] = 0.01 mol dm
T
-3
1
23

Oxidation of glycerol to glyceraldehyde
promoted path which was pale violet (kmax = 544 nm, Fig. 8). In the unpromoted
4
= 583 nm for A
path, two peaks were found due to the transitions [37, 38] k
max
2g
4
4
4
(
F) ? T (F) and kmax = 260 nm for A (F) ? T (F) of Cr(III), respectively.
2
g
2g
1g
The spectra of the final solution in the absence of promoter and pure chromic
sulphate solution in aqueous sulphuric acid media are almost similar, but different
due to the presence of different types of Cr(III)-species (kmax = 544 nm for the
bipy-promoted path; Fig. 8). This simply suggests that the final Cr(III)-aqua species
are the Cr(III)-bipy complex. Similar results were reported by earlier workers [39].
In the bipy-promoted reaction path, there is a hypsochromic (blue) shift for the
4
4
transition A (F) ? T (F) compared to the final solution of the unpromoted
2
g
2g
4
4
reaction path (Fig. 7a). The blue shift for the transition A (F) ? T (F) in the
2
g
2g
Cr(III)-bipy complex is due to the presence of the strong field bipy ligand compared
to the aqueous ligand. For the Cr(III)-bipy complex [23, 39], the band due to the
4
4
transition A (F) ? T (F) merges with a charge-transfer band (Fig. 8). Here,
2
g
1g
the appearance of the charge-transfer at much lower energy (visible range) for the
proposed Cr(III)-bipy complex is significant due to the favored metal to ligand
charge-transfer. The vacant p* (antibonding) M.O. of the ligand (bipy) switches the
metal to ligand electron-transfer (MLCT). These facts are supported by previous
work [38–40]. The presence of the charge-transfer band (MLCT) at this lower
energy (higher wave length) for the bipy-promoted reaction supports the formation
of the Cr(III)-bipy complex in the final solution.
The reaction solutions were scanned in the range 400–700 nm in both the
presence and absence of promoter and surfactant at regular time intervals (3–4 min)
to follow the gradual development of the reaction intermediates (if any) and the
product. The scanned spectra (Fig. 9a) indicate the gradual disappearance of Cr(VI)
species and the appearance of Cr(III) species with an isobestic point at
kmax = 533 nm for the unpromoted reaction. In the presence of only the promoter
(
bipy), the isobestic points appeared at kmax = 519 nm (Fig. 9b). In the presence of
SDS and bipy, the scanned spectrum could not be recorded as the reaction was too
fast to monitor. The isobestic point of the TX-100 catalyzed bipy-promoted reaction
path was found at kmax = 512 nm (Fig. 9c). It is interesting to note that, under the
present experimental and kinetic conditions, the appearance of the single isobestic
point in each scanned spectra firmly suggests the formation of very low
concentrations or short-lived intermediates like Cr(V) and Cr(IV) [41] during the
oxidation. In such cases, because of the very low concentration of the species, it
may not always be possible to detect the species spectrophotometrically [42]. In the
bipy-promoted reaction path, the active oxidant involved in the oxidation process is
Cr(VI)–bipy (Fig. 10) [5, 23, 26–28].
Mechanism of the reaction and rate law
Product analysis and spectroscopic results indicate that glyceraldehyde was the
major oxidized product, i.e. oxidation of the single primary hydroxyl group
occurred. The mechanism of the reaction can be divided into two parts: (1) the
unpromoted path and (2) the promoted path. The unpromoted path for chromic acid
oxidation of glycerol (Scheme 1) has been established earlier [27].
123

A. Ghosh et al.
Fig. 9 a Scanned absorption spectra of the reaction mixture in absence of promoter at regular time
c
-4
-3
-4
-3
intervals
SO
reaction mixture at regular time intervals (3 min). [glycerol]
(5 min).
[glycerol]
T
= 75 9 10 mol dm
,
T
[Cr(VI)] = 5 9 10 mol dm ,
-3
[H
2
4
]
T
= 0.5 mol dm . kisobestic = 533 nm. Temp = 30 °C. b Scanned absorption spectra of the
-
4
-3
-3
T
= 75 9 10 mol dm
-4
,
.
-
4
-3
-3
[
Cr(VI)]
T
= 5 9 10 mol dm
,
[H
2
SO
4
]
T
= 0.5 mol dm
,
T
[bipy] = 125 9 10 mol dm
k
isobestic = 519 nm. Temp = 30 °C. c Scanned absorption spectra of the reaction mixture at regular
-
4
-3
-4
= 5 9 10 mol dm
-3
-3
time intervals (3 min). [glycerol]
]
T
= 75 9 10 mol dm
-3
,
[Cr(VI)]
[TX-100]
T
,
.
-3
-4
= 75 9 10 mol dm
-2
= 2 9 10 mol dm
[H
2
SO
4
T
= 0.5 mol dm
,
[bipy]
T
,
T
kisobestic = 512 nm. Temp = 30 °C
?
In this path, attachment of H with the neutral ester (1) formed during the
process facilitates the decomposition of the protonated ester (2) and gives rise to the
product glyceraldehyde [27]. In the unpromoted path, the rate law is: k = (2/3)
obs
?
k K K [glycerol] [H ] . In the bipy-promoted reaction path, the formation of the
2
1
1
2
T
Cr(III)-bipy complex characterized spectrophotometrically indicates that the
promoter (bipy) used here undergoes complexation with the highest oxidation
0
2
0
g
states of chromium (labile, t e ). Based on this concept, it is believed that the
g
Cr(VI)–bipy complex (complex C ) produced in the pre-equilibrium step is the
1
?
active oxidant (mentioned as AO in Scheme 2). This complex species reacts with
the substrate glycerol to produce a ternary complex C , and it would undergo redox
2
decomposition via several steps to produce the Cr(III)-bipy complex. The
mechanism of the bipy-promoted reaction path can be established as follows
Derivation of rate law (considering Scheme 2):
½
C1ꢃ
K ¼
Â
Ã
3
þ
þ 2
ꢂ
4
½
½
bipyH ꢃ½H ꢃ HCrO
þ
½
C2ꢃ½H3O ꢃ
K ¼
4
glycerolꢃ½C1ꢃ
Â
Ã
ꢂ
d½glycerolꢃ
dt
dln HCrO
dt
4
rate =
¼ ꢂ
¼ k ½C ꢃ
3
2
k3K4½glycerolꢃ½C1ꢃ
¼
þ
½
H3O ꢃ
Â
Ã
þ
þ 2
ꢂ
4
k3K3K4½glycerolꢃ½bipyH ꢃ½H ꢃ HCrO
¼
þ
½
The total concentration of bipy is given by:
H3O ꢃ
þ
þ
½
bipyH ꢃ ¼½bipyꢃ þ ½bipyH ꢃ
T
þ
þ
bipyH
½bipyH ꢃ
Kb ¼
; or ½bipyꢃ ¼
þ
þ
½
bipyꢃ½H ꢃ
bipyH
Kb½Hþꢃ
þ
Kb½H ꢃ
Â
Ã
ꢀ
ꢁ
2
1
þ
þ
or ½bipyꢃ ¼
þ ½bipyH ꢃ ¼ ½bipyH ꢃ ꢄ 1 þ
T
Kb½Hþꢃ
ꢀ
ꢁ
þ
Kb½H ꢃ þ 1
¼
½bipyH ꢃ ꢄ
ꢅ ½bipyHþꢃ
þ
Kb½H ꢃ
þ
4
Since, K ½H ꢃ ꢀ 1; K ¼ 2:8 ꢄ 10 for bipy
b
b
1
23

Oxidation of glycerol to glyceraldehyde
123

A. Ghosh et al.
Fig. 10 Absorption spectrum of reaction mixture with and without promoter (in the absence of
-4
-
4
-3
= 5 9 10 mol dm , [H
-3
substrate): (i) [Cr(VI)]
T
2
SO
4
]
T
T
= 0.5 mol dm . (ii) [Cr(VI)] = 5 9 10
-3
-3
-3
mol dm , [H
2
SO
4
]
T
= 0.5 mol dm , [bipy] = 0.01 mol dm , Temp = 30 °C
T
CH OH
H C
O
CrO OH
2
2
2
_
K1
H+
CHOH
+
+
HCrO4
CHOH
CH OH
CH OH
2
2
(
Neutral ester)
(
1)
+
H C
O
CrO OH
H C
O
CrO (OH )
2
2
2
2 2
K2
+
CHOH
+
H O
CHOH
+ H O
3
2
CH OH
CH OH
2
2
(Neutral ester)
(Protonated ester)
(1)
HC
CHOH
O
O
O
k1
rds
+ H+
H CrO
2 3
HOH C
CH
Cr
+
2
+
H
O
OH2
CH OH
2
(
Protonated ester)
(
2)
Scheme 1 Cr(VI) oxidation of glycerol in the absence of promoter
1
23

Oxidation of glycerol to glyceraldehyde
Kb
H+
4
+
;K = 2.8 × 10 , [19, 40]
b
+
N
N
N
N
H
(
bipyH+)
_
4
+
K3
+
2 H + HCrO
O
+
N
+
+
N
N
N
H
Cr
(
bipyH+)
H O
OH2
[C1]
2
O
AO )
+
(
+
N
+
N
+
N
+
N
O
CH OH
2
_
K4
Cr
+ H O+
3
O
O
Cr
+
CHOH
O
OH2
H O
OH2
2
O
H
CH OH
2
+
C
(
AO )
C ]
[
C ]
2
H
CHOH
[
1
CH OH
2
rds k3
HC
O
CHOH + Cr(IV)-bipy
CH OH
2
HC
CHOH
CH OH
O
CH OH
2
Cr(VI)
fast
Cr(IV)-bipy +
+
Cr(III)-bipy
CHOH
CH OH
2
2
Scheme 2 Cr(VI) oxidation of glycerol in presence of bipy
123

A. Ghosh et al.
Â
Ã
ꢂ
þ
þ 2
ꢂ
d½HCrO4ꢃ
dt
k3K3K4½glycerolꢃ½bipyH ꢃ½H ꢃ HCrO
4
rate ¼ ꢂ
¼
þ
½H O ꢃ
3
Â
d HCrO4
Ã
ꢂ
þ
¼
k3K3K4½glycerolꢃ½bipyꢃ HCrO4 ½H ꢃ
and
Â
ꢂÃ
1
1 d½glycerolꢃ
ꢂ 2
dt
¼
3
dt
Â
ꢂÃ
d ln HCrO4
2
ꢂ
¼ kobsðcÞ ¼ 3 k3K3K4½glycerolꢃ½bipyꢃ½Hþꢃ
dt
The rate law of bipy-promoted reaction path can be proposed as:
k
¼ ð2=3Þk3K3K4½glycerolꢃ½bipyꢃ½Hþꢃ
obsðcÞ
Effect of the non-functional micellar catalysts on the reaction rate
The surfactant comprises a hydrophilic and a hydrophobic group. The different
interaction of these two moieties with water is an important cause for surfactants to
aggregate into micelles and other nanometer-scale structures in aqueous solution [43].
Partitioning of reactant in unpromoted path
In the unpromoted reaction path, partitioning of the neutral ester [(1) Scheme 1] in all
types of the surfactants is almost equally probable (Fig. 11). The phenomenon of
partitioning of proton in the unpromoted reaction path is highest for the anionic
surfactant SDS due to favorable and very effective electrostatic attraction. Thus, for
SDS, the reaction goes on in both aqueous and micellar phases where the active
reactants are preferably accumulated [42]. Thus, SDS allows the reaction to proceed in
both aqueous and micellar interphases [44]. TX-100, a neutral surfactant, retards the
rate of oxidation process slightly lower than pure water, as there are no charge head
groups present [28]. Again, with gradual increasing of the concentration of TX-100,
the rate of oxidation is decreased. These data are not shown in the text. The decrease in
?
electrostatic attraction between the approaching H ion and the large hydrophobic
group of neutral TX-100 is significant for the observed rate retardation. The rate of
oxidation of glycerol is first-order with respect to substrate, i.e. glycerol as observed
from previous literature [27]. A severe electrostatic repulsion between the approach-
?
ing H ion and the positive hydrophilic head group of cationic CPC makes unavailable
?
the effective concentration of neutral ester and H within the micellar core.
Consequently, the reaction is restricted only in aqueous phase where the concentration
of the ester species is depleted due to its preferential partitioning in the micellar
pseudo-phase. Thus, CPC shows the rate-retarding effect even from pure water [42].
Partitioning of reactant in the promoted path
The most pronounced acceleration of the oxidation process is observed in the SDS-
catalyzed bipy-promoted path (Table 3). In this path, the Cr(VI)–bipy complex,
1
23

Oxidation of glycerol to glyceraldehyde
H+
H+
_
_
_
H+
_
_
H+
+
H+
+
+
+
_
_
H+
+
+
+
H+ H+
+
+
_
_
_
H+
+
+
H+
H+
H+
H+
_
H+
+
+
+
_
+
_
H+
H+
neutral
ester
neutral
ester
H+
_
H+
H+
neutral
ester
+
+
+
+
+
_
_
H+
_
H+
+
_
H+
_
H+
H+
+ + + +
H+
_
_
_
+
_
_
_
_
H+
H+
H+
H+
H+
(a)
(b)
(c)
Fig. 11 Schematic representation of partitioning of neutral ester and proton in a cationic surfactant,
b anionic surfactant, and c neutral surfactant
AO+
AO+
_ _ _
AO+ AO+
AO+
AO+
_
_
AO+
AO+
+
+
+
_
+
_
+
+
AO+
+
+
AO+
_
_
+
+
+
+
+
AO+
AO+
_
_
+
+
AO _
+
+
+
_
glycerol
AO+
AO+
_
glycerol
_
_
+_
_
AO+
glycerol
AO+
AO
+
AO+
AO+
+
+
_
AO+
AO+
AO+
+ + + +
AO+ AO+
+
_
_
+
_
_
_
_
_
AO+
AO+
AO+
AO+
AO+
(a)
(b)
(c)
?
Fig. 12 Schematic representation of the partitioning of substrate and active oxidant [AO = Cr(VI)–
promoter complex] in a cationic surfactant, b anionic surfactant, and c neutral surfactant
T
Fig. 13 Dependence of [SDS] on kobs(T) for Cr(VI) oxidation of glycerol in the absence of the promoter
-
4
-3
-4
in aqueous
mol dm , [H
H
2
SO
SO
4
T T
media at 30 °C. [Cr(VI)] = 5 9 10 mol dm , [glycerol] = 75 9 10
-3
-3
2
4
]
T
= 0.5 mol dm
123

A. Ghosh et al.
Fig. 14 Dependence of kobs(T) on [bipy]
surfactant [SDS] in aqueous H SO
T
= 0.5 mol dm , [SDS] = 5 9 10 mol dm
T
for the Cr(VI) oxidation of glycerol in the presence of the
-
4
-3
T
2
4
T T
media at 30 °C. [Cr(VI)] = 5 9 10 mol dm , [glycerol] =
-4
-3
-3
-2
-3
7
5 9 10 mol dm , [H
2
SO
4
]
T
Fig. 15 Dependence of kobs(T) on [promoter]
surfactant [TX-100] in aqueous SO
4 T T
SO ] = 0.5 mol dm , [TX-100] = 2 9 10 mol dm
T
for the Cr(VI) oxidation of glycerol in the presence of the
-4
-3
T
H
2
4
media at 30 °C. [Cr(VI)]
T
= 5 9 10
mol dm ,
-4
-3
-3
-2
-3
[
glycerol]
T
= 75 9 10 mol dm , [H
2
?
represented as AO (Fig. 12, Scheme 2), a cationic complex, has been argued as the
active oxidant. Because of the electrostatic attraction, it can preferably be
distributed in the micellar pseudo-phase of the anionic surfactant SDS. Thus, in
the presence of SDS, the reaction can propagate in both the micellar pseudo-phase
(
where both the active oxidant and substrate are preferably concentrated) and the
aqueous phase to give the observed rate acceleration [27, 42]. The plots of kobs(T)
versus [SDS] (Fig. 13) and kobs(T) versus [bipy] (Fig. 14) exhibit a continuous
T
T
increase up to the concentration of SDS (for the unpromoted path) and bipy used for
the SDS-catalyzed bipy-promoted reaction path. A similar effect is observed for the
combination of TX-100 micelles and bipy, but the increase of the oxidation rate
(
Fig. 15) is smaller because the degree of glycerol binding to the nonionic surfactant
micelles is probably smaller compared with the degree of binding to the anionic
ones [45]. The active oxidant Cr(VI)–bipy complex reacts with glycerol to form a
ternary complex which undergoes redox decomposition in a rate-limiting step [46,
4
7], giving rise to the product. In the case of TX-100, Cr(VI)–bipy also approaches
1
23

Oxidation of glycerol to glyceraldehyde
Table 2 kobs and half life of the reaction in presence and absence of bipy and non-functional micellar
catalyst
-
3
)
-3
4
-1
Promoter (mol dm
Micellar catalyst (mol dm
)
10 9 kobs (s
)
t1/2 (h)
None
0
None
0.167 ± 0.03
11.54
2
,2 -bipyridine (bipy)
0
0
0
0
0
0
.0025
.005
.075
.01
None
1.358 ± 0.03
2.211 ± 0.04
3.121 ± 0.02
4.218 ± 0.02
4.713 ± 0.03
5.793 ± 0.04
1.41
0.87
0.616
0.45
0.40
0.33
.0125
.015
None
CPC
.002
SDS
0
0.0065 ± 0.02
29.61
None
0
0
0
0
0
.01
.02
.03
.04
.05
0.267 ± 0.04
0.5216 ± 0.04
0.6133 ± 0.05
0.6733 ± 0.06
0.7483 ± 0.03
7.2
3.69
3.14
2.86
2.57
None
TX-100
.02
0
0.3833 ± 0.03
15.83
0
2
,2 -bipyridine (bipy)
SDS
0.05
0
0
0
0
0
0
.0025
.005
.075
.01
38.9 ± 0.04
75.3 ± 0.06
108.2 ± 0.05
128 ± 0.03
0.049
0.0255
0.0177
0.015
.0125
150.6 ± 0.02
161.1 ± 0.04
0.0127
0.0116
.015
0
2
,2 -bipyridine (bipy)
TX-100
0.02
0
0
0
0
0
0
.0025
.005
.075
.01
1.46 ± 0.03
3.017 ± 0.04
3.738 ± 0.02
4.585 ± 0.05
5.213 ± 0.04
7.015 ± 0.03
1.318
0.638
0.515
0.42
.0125
.015
0.37
0.274
-
4
= 5 9 10 mol dm , [H
-3
-3
] = 0.5 mol dm , [glycerol]
-4
= 75 9 10 mol dm
-3
[
Cr(VI)]
T
2
SO
4
T
,
Temp = 30 °C
to the neutral head group of the micelle, but the amount is less compared to SDS
because TX-100 is a neutral surfactant, so no electrostatic attraction takes place [28,
48]. The degree of rate enhancement in the TX-100-catalyzed bipy-promoted path is
most probably due to the continuous increase of Cr(VI)–bipy complexes upon
123

A. Ghosh et al.
Table 3 Kinetic parameters and some representative keff values for the Cr(VI) oxidation of glycerol in
the presence of bipyand catalyst (surfactant) in aqueous H SO media
2 4
Promoter
Micellar catalyst
k
eff(bipy)(w)
k
eff(s)(w)
k
eff(bipy)(s)
0
2
,2 -bipyridine (bipy)
None
SDS
33.68
–
–
None
–
–
–
3.48
–
–
0
2
2
,2 -bipyridine (bipy)
0
SDS
214.37
17.316
,2 -bipyridine (bipy)
TX-100
–
-
= 5 9 10 mol dm
4
-3
-3
W = water, S = surfactant, [Cr(VI)]
T
,
= 150 9 10 mol dm , [SDS]
[H
-3
2
SO
4
] = 0.5 mol dm
-2
,
[glyc-
-3
-
= 75 9 10 mol dm , [Promoter]
4
-3
-4
erol]
[TX-100]
T
T
T
= 5 9 10 mol dm ,
-2
-3
T
= 2 9 10 mol dm , Temp = 30 °C
increasing the concentration of bipy, forcefully allowing the reaction with glycerol
within the large hydrophobic core. In this study, the combination of CPC and bipy
are not used due to the rate retardation effect of CPC in the absence of the promoter.
From Table 2, it is confirmed that the rate is continuously increasing with the
increase in the concentration of bipy in the presence and absence of micelles
(
Fig. 7). Rate enhancement is small for TX-100 compared to SDS, but CPC retards
the oxidation process as is evident from the t1/2 value in Table 2. Rate acceleration
is highest when SDS and bipy are combined (Fig. 14). The total time taken to
complete the oxidation process gradually shortens with increasing the concentration
of bipy for a fixed concentration of SDS. Plots of kobs(T) versus [bipy] and [SDS]T
T
are linear with positive intercepts (Figs. 12, 13, 14, 15). In each micelle–bipy
combined set of experiments, the values of kobs(T) are increased in a regular fashion
upon further increasing the bipy concentration at the fixed surfactant concentration.
-
1
The pseudo-first-order rate constants (kobs, s ) were determined from the slope of
plots of ln(A450) against time (t) at wave length 450 nm. The t1/2 values are directly
calculated in Table 2 by using the relationship t1/2 = (ln2/kobs), where kobs
pseudo-first-order rate constant.
=
Another kinetic parameter has been introduced in Table 3 which can be defined
as the efficiency of only the promoter or only the catalyst or both (combination of
promoter and catalyst) to catalyze, promote, and accelerate the reaction process, and
is termed k . The k values are directly calculated from the relationship
eff
eff
keff = {kobs(T) - kobs(u)}/kobs(u) under the kinetic and experimental conditions,
where kobs(T) = rate constant of the promoted path and kobs(u) = rate constant of the
unpromoted path. The plot of kobs(T) versus [SDS] (Fig. 13) shows a continuous
T
increase up to the concentration of SDS used for the unpromoted path. From
Table 3 (Fig. 14), the value of keff = 214.37 at the highest concentration of SDS
and bipy strongly suggests that this combination is the most effective one among all
the micelle–bipy combinations. The combination of SDS and bipy in the reaction
mixture leads to almost kilo-fold (994 times faster) acceleration compared to the
uncatalyzed and unpromoted path. From the plot of kobs(T) versus bipy (Fig. 7), it is
clear that kobs is almost proportional to the concentration of bipy added in the
reaction mixture. The value of kobs has been increased linearly with increasing the
concentration of SDS (Fig. 13). The surfactant SDS also has an effect on the bipy-
promoted reaction path, and rate constants are found to be increased (Fig. 14) upon
1
23

Oxidation of glycerol to glyceraldehyde
123

A. Ghosh et al.
increasing the concentration of bipy at a fixed SDS concentration. A similar type of
dependence pattern has also been observed in the presence of TX-100 on the
promoted reaction path with enhanced rate constants (Fig. 15).
The slope obtained from the plot of the combination of SDS and bipy path is very
steep compared with the TX-100–bipy combination path.
A comparison table (Table 4) explains the selection of suitable catalysts and
promoters over the other oxidants used previously in glycerol to glyceraldehydes
conversion:
Conclusion
The main target of this study was to carry out the oxidation in aqueous micellar
media at room temperature. In this context, the use of water as solvent features
many benefits, not only because water itself is innocuous but because it can also
potentially improve reactivities and selectivities, simplify the workup procedures,
enable the recycling of the micellar catalyst, and allowing mild reaction
conditions and protecting group-free synthesis. Oxidation is made selective to
glyceraldehyde by stopping the over-oxidation of glyceraldehyde by maintaining
pseudo-first-order conditions. The remaining glycerol can be re-oxidized to the
deserved product with the addition of chromic acid maintaining the reaction
condition the same. The CMC values of the three surfactants used are very helpful
in determining the particular concentration of the surfactant to be used for the title
reaction. The micelles act as nano-reactors and efficiently increase the effective
concentration of the reactants into the small core inside it. It is interesting to note
that the rate of oxidation is increased drastically when SDS is used as the micellar
catalyst in combination with the bipy promoter at a particular concentration,
whereas the cationic surfactant CPC and the neutral surfactant TX-100 inhibit the
rate of oxidation. The Cr(VI)–bipy complex, a cationic species, has been found to
act as the active oxidant in the bipy-promoted chromic acid oxidation of glycerol.
The generation of Cr(III)-species after the completion of the oxidation of glycerol
has been investigated by a series of spectra observed by UV–Vis spectrophotom-
etry. The product aldehyde has also been confirmed by the conventional 2,4-DNP
test and FTIR spectral analysis of the hydrazone derivative. The effects of the
cationic, anionic, and neutral surfactants have been followed and the observed
micellar effects are in agreement with the proposed reaction mechanism. These
micellar effects are quite important to understand and to substantiate the proposed
mechanistic pathways. The mechanistic paths of uncatalyzed unpromoted and
SDS-catalyzed bipy-promoted chromic acid oxidation of glycerol have been
compared. In conclusion, it can be said that anionic micelle SDS is an efficient
miceller catalyst for the bipy-promoted chromic acid oxidation of glycerol to
glyceraldehyde.
Acknowledgments Thanks to UGC, New Delhi and CSIR, New Delhi for providing financial help in
the form of project and fellowship.
1
23

Oxidation of glycerol to glyceraldehyde
References
1
. B. Upasana, M.K. Chaudhuri, D. Dey, D. Kalita, W. Kharmawphlang, G.C. Mandal, Tetrahedron 57,
445 (2001)
. S. Sundaram, P.S. Raghavan, Chromium-VI Reagents: Synthetic Applications (Springer, Heidelberg,
011). 47
2
2
2
3. N. Feizi, H. Hassani, M. Hakimi, Bull. Korean Chem. Soc. 26, 2084 (2005)
4. A.K. Das, S.K. Mondal, G. Mukherjee, React. Kinet. Catal. Lett. 73, 257 (2001)
5. R. Saha, S.K. Ghosh, A. Ghosh, I. Saha, K. Mukherjee, A. Basu, B. Saha, Res. Chem. Intermed. 39,
631 (2013)
6
7
8
9
. A.L. Holmes, S.S. Wise, J.P. Wise, Indian J. Med. Res. 128, 353 (2008)
. R. Saha, R. Nandi, B. Saha, J. Coord. Chem. 64, 1782 (2011)
. B. Saha, C. Orvig, Coord. Chem. Rev. 254, 2959 (2010)
. A. Sivaprakash, R. Aravindhan, J.R. Rao, B.U. Nair, Appl. Ecol. Env. Res. 7, 45 (2009)
1
1
1
1
0. A.K. Das, Coord. Chem. Rev. 248, 81 (2004)
1. N. Dimitratos, A. Villa, L. Prati, Catal. Lett. 133, 334 (2009)
2. A.V. Bryksin, P.P. Laktionov, Biochemistry 73, 619 (2008)
3. N. Richter, M. Neumann, A. Liese, R. Wohlgemuth, A. Weckbecker, T. Eggert, W. Hummel,
Biotechnol. Bioeng. 106, 541 (2010)
1
1
1
1
1
4. P.K. Khatua, S. Ghosh, S.K. Ghosh, S.C. Bhattacharya, J Disper Sci Technol 25, 741 (2004)
5. Z. Zhang, C.Y. Lin, J. Photochem. Photobiol. A 130, 139 (2000)
6. M. Almgren, A. Valstar, Langmuir 15, 2635 (1999)
7. M.A. Muherei, R. Junin, J. Appl. Sci. Res. 5, 181 (2009)
8. S.K. Ghosh, R. Saha, K. Mukherjee, A. Ghosh, S.S. Bhattacharyya, B. Saha, J. Korean Chem. Soc.
56, 164 (2012)
1
2
2
2
2
2
9. R. Saha, A. Ghosh, B. Saha, Chem. Eng. Sci. 99, 23 (2013)
0. A. Ghosh, R. Saha, B. Saha, J. Ind. Eng. Chem. (2013). doi:10.1016/j.jiec.2013.03.028
1. A. Ghosh, R. Saha, K. Mukherjee, S.K. Ghosh, B. Saha, Spectrochim. Acta A 109, 55 (2013)
2. K. Mukherjee, R. Saha, A. Ghosh, S.K. Ghosh, B. Saha, Spectrochim. Acta A 101, 294 (2013)
3. J. Mandal, K.M. Chowdhury, K. Paul, B. Saha, J. Coord. Chem. 63, 99 (2010)
4. R. Saha, A. Ghosh, P. Sar, I. Saha, S.K. Ghosh, K. Mukherjee, B. Saha, Spectrochim. Acta Part A.
116, 524 (2013)
25. A. Ghosh, R. Saha, K. Mukhejee, S.K. Ghosh, S.S. Bhattacharyya, S. Laskar, B. Saha, Int. J. Chem.
Kinet. 45, 175 (2013)
26. R. Saha, A. Ghosh, B. Saha, J. Coord. Chem. 64, 3729 (2011)
27. S.K. Ghosh, A. Basu, R. Saha, A. Ghosh, K. Mukherjee, B. Saha, J. Coord. Chem. 65, 1158 (2012)
28. A. Ghosh, R. Saha, K. Mukherjee, S.K. Ghosh, S.S. Bhattacharyya, B. Saha, J. Chem. Res. 36, 347
(2012)
29. A.A. Rafati, H. Gharibi, H. Iloukhani, Phys. Chem. Liq. 39, 521 (2001)
3
0. A. Dom ´ı nguez, A. Fern a´ ndez, N. Gonz a´ lez, E. Iglesias, L. Montenegro, J. Chem. Educ. 74, 1227
(1997)
3
3
3
3
1. S.K. Hait, S.P. Moulik, J. Surf. Deterg. 4, 303 (2001)
2. S. Ross, J.P. Oliver, J. Phys. Chem. 63, 1671 (1959)
3. J.C. Underwood, H.G. Lento, J.C. Underwood, H.G. Lento, Anal. Chem. 32, 1656 (1960)
4. A.I. Vogel, Elementary Practical Organic Chemistry. Part III. Quantitative Organic Analysis, 3rd
edn. (ELBS and Longman Group Ltd., London, 1958), p. 739
35. F. Feigl, Spot Tests in Organic Analysis, vol. 342, 5th edn. (Elsevier, Amsterdam, 1956), p. 121
36. A. Basu, S.K. Ghosh, R. Saha, R. Nandi, T. Ghosh, B. Saha, Tenside Surf. Deterg. 48, 453 (2011)
37. S.P. Meenakshisundaram, M. Gopalakrishnan, S. Nagarajan, N. Sarathi, Catal. Commun. 8, 713
(2007)
3
3
4
4
8. B.N. Figgis, Introduction to Ligand Fields (Wiley Eastern Limited, New Delhi, 1966), p. 222
9. Z. Khan, K. Ud-Din, Transition Met. Chem. 27, 832 (2002)
0. M. Islam, B. Saha, A.K. Das, J. Mol. Catal. A 266, 21 (2007)
1. C.K. Jorgensen, Absorption Spectra and Chemical Bonding in Complexes (Pergamon, London,
1964), p. 290
42. B. Saha, M. Das, R.K. Mohanty, A.K. Das, J. Chin. Chem. Soc. 51, 399 (2004)
123

A. Ghosh et al.
43. K.S. Sharma, S.R. Patil, A.K. Rakshit, K. Glenn, M. Doiron, R.M. Palepu, P.A. Hassan, J. Phys.
Chem. B 108, 12804 (2004)
4. K. Mukherjee, R. Saha, A. Ghosh, S.K. Ghosh, B. Saha, J. Mol. Liq. 179, 1 (2013)
5. S.J. Marta, M. Narkiewicz, Colloid Polym. Sci. 281, 1142 (2003)
6. Z. Khan, S. Masan, Raju, K. Ud-Din, Transition Met. Chem. 28, 881–887 (2003)
7. S. Meenakshisundaram, R. Markkandan, Transition Met. Chem. 29, 308 (2004)
8. A. Basu, S.K. Ghosh, R. Saha, A. Ghosh, K. Mukherjee, B. Saha, Tenside Surf. Deterg. 50, 249
4
4
4
4
4
(2013)
49. S. Shrivasatava, H. Tripathi, K. Singh, Transition Met. Chem. 26, 727 (2001)
1
23