Sodium dodecylsulphate-catalyzed hetero-aromatic nitrogen base-promoted chromium(VI) oxidation of 2-propenol to 2-propenal in aqueous media

Article abstract of DOI:10.1007/s11164-015-2019-0

A kinetic study of homogeneously micelle-catalyzed, hetero-aromatic Nbase (2-picolinic acid, 2,2'-bipyridine and 1,10-phenanthroline)-promoted oxidation of 2-propenol by chromium(VI) in sulfuric acid medium was carried out at 303 K. The pseudo first-order rate constant (k_obs) of the oxidation reaction is directly proportional to [H⁺] and [promoters], which indicates that the reaction is first order with respect to H⁺ and promoters. The reaction was conducted at four different temperatures from which the activation parameters were calculated. Proton nuclear magnetic resonance (¹HNMR) studies were performed to detect the location of reactant molecules in nanometersized micelles. From the observed kinetic results, a suitable mechanism consistent with the rate law has been proposed. The micelles produced a catalytic effect in the entire range of sodium dodecylsulphate (SDS) concentrations used, and the effect is explained by pseudo-phase ion exchange. The combination of 2,2'-bipyridine (bipy) and SDS exhibited a 15-fold rate enhancement of Cr(VI) oxidation of 2-propenol to 2-propenal.

Full text of DOI:10.1007/s11164-015-2019-0

Res Chem Intermed
DOI 10.1007/s11164-015-2019-0
Sodium dodecylsulphate-catalyzed hetero-aromatic
nitrogen base-promoted chromium(VI) oxidation of
2-propenol to 2-propenal in aqueous media
Pintu Sar1 Aniruddha Ghosh1 Susanta Malik1
Bidyut Saha1
•
•
•
Received: 23 December 2014 / Accepted: 13 March 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract A kinetic study of homogeneously micelle-catalyzed, hetero-aromatic N-
base (2-picolinic acid, 2,20-bipyridine and 1,10-phenanthroline)-promoted oxidation of
2-propenol by chromium(VI) in sulfuric acid medium was carried out at 303 K. The
pseudo first-order rate constant (kobs) of the oxidation reaction is directly proportional to
[H?] and [promoters], which indicates that the reaction is first order with respect to H?
and promoters. The reaction was conducted at four different temperatures from which
the activation parameters were calculated. Proton nuclear magnetic resonance (1H-
NMR) studies were performed to detect the location of reactant molecules in nanometer-
sized micelles. From the observed kinetic results, a suitable mechanism consistent with
the rate law has been proposed. The micelles produced a catalytic effect in the entire
range of sodium dodecylsulphate (SDS) concentrations used, and the effect is explained
by pseudo-phase ion exchange. The combination of 2,20-bipyridine (bipy) and SDS
exhibited a 15-fold rate enhancement of Cr(VI) oxidation of 2-propenol to 2-propenal.
Electronic supplementary material The online version of this article (doi:10.1007/s11164-015-2019-
0) contains supplementary material, which is available to authorized users.
&
Bidyut Saha
b_saha31@rediffmail.com
1
Homogeneous Catalysis Laboratory, Department of Chemistry, The University of Burdwan,
Golapbag, Burdwan 713104, WB, India
123

P. Sar et al.
Graphical Abstract Ultraviolet-visible (UV-Vis) spectrum of reaction mixture for the
Cr(VI) oxidation of 2-propenol with varying solution colour at different reaction times.
Right: Location of reactant molecule in the micellar pseudophase region.
Keywords 2-Propenol Á Chromium(VI) Á SDS Á Kinetics Á Promoter
Introduction
Chromium has both beneficial and detrimental properties. The two oxidation states of
chromium are naturally occurring; one is trivalent chromium and another is hexavalent
chromium. The behavior of chromium species depends strongly on their oxidation
states: Cr(III) is essential for human nutrients, whereas most hexavalent compounds
are toxic, several of which even cause lung cancer [1, 2]. From an economical point of
view, there are several advantages of chromium application in a wide array of
chemical industries [3, 4], such as the production of stainless steel, the refractory
industry, electroplating, chromium pigments of paints and textiles and some wood
preservation. These applications contrast with its chromium’s external effects as a
hazardous pollutant. As compared to other high valency transition metals like
-
Cerium(IV), Vanadium(V), Manganese(VII), and Osmium(VIII), the E0 of HCrO4
/
Cr3? couple (1.33 V) is high enough to oxidize most organic functional groups [5–9].
The oxidation of various organic compounds by Cr(VI) was found to be more
pronounced in the presence of metal ions, micellar catalyst and promoters [8–13].
We used 2-propenol as an organic substrate which is oxidized to 2-propenal by
the powerful oxidant Cr(VI) with subsequent reduction to non-toxic Cr(III).
2-propenol is the simplest unsaturated alcohol containing two different functional
groups: –CH=CH2 and –OH groups at the terminal carbon. The oxidized product
2-propenal is widely used in chemical industry as an intermediate in the
manufacture of chemical compounds, colloidal metals, plastics, and perfumes
[14]. The most important use of 2-propenal is as an aquatic biocide to control the
growth of bacteria, fungi, algae and mollusks in cooling-water systems.
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Sodium dodecylsulphate-catalyzed hetero-aromatic…
Kinetic studies on the oxidation of 2-propenol with different oxidants such as
potassium permanganate, quinaldinium chlorochromate, vanadium(V), bromate(V),
cerium(IV), chloramine-T, diperiodatonickelate(IV), etc. have been reported [15–
21]. The reported literature followed only simple oxidation kinetics and rate law
with probable mechanisms. In this paper, our target is to speed up the oxidation
reaction in a simple kinetic way by using different hetero-aromatic N-base
promoters and homogeneous micellar medium. We are also interested in determin-
ing the role of promoters on the reaction rate by discerning the actual mechanism
and location of the reactant molecules inside the nanometer-sized micelle. The
promoters used, 2-picolinic acid (PA), 2,20-bipyridine (bipy) and 1,10-phenanthro-
line (phen), are well known for chromium(VI) oxidation of organic substances.
Micellar media has several important roles in the field of organic chemistry and in
various fields of research, e.g., solar energy conversion, nanoparticle synthesis, and
drug delivery in biological systems [22, 23]. Another critical role of micellar media
is to supervise different chemical reactions as multifunctional nano-reactors. A
substantial volume of research on the systematic kinetics of micelles and their
effects on various organic reactions has been reported during the past few decades
[24].
To achieve the target, several kinetic measurements were performed. Product
identification was characterized by different spectroscopic studies. Micellar medium
as a nano-reactor in combination with different promoting agents is a very efficient
facilitator of rapid 2-propenol oxidation. From the view point of green chemistry,
water would be the perfect solvent to carry out chemical reactions, provided that the
reactants, as well as products, are soluble in water and no polluting waste waters are
dispersed into the environment. This oxidation procedure also takes place in
environmentally friendly aqueous medium rather than in hazardous organic
solvents.
Experimental section
Materials and solution preparation
2-propenol, sulfuric acid and Cr2(SO4)3 were purchased from Hi-Media (Mumbai,
India). 2-picolinic acid and D2O were purchased from Sigma Aldrich (USA). 1,10-
phenanthroline and 2,20-bipyridine were purchased from Merck (Mumbai, India).
Potassium dichromate was purchased from BDH Chemical (Mumbai, India),
sodium dodecylsulfate from Fischer Scientific (Mumbai, India) and other required
chemical reagents of analytical (AR) grade were used without further purification.
Double distilled water was used to prepare reaction mixtures in each case, and for
other studies, unless other systems are mentioned specifically.
Kinetic measurements
Kinetics measurements were carried out by monitoring the spectral changes over a
range of 250–650 nm on a UV–Vis spectrophotometer (Shimadzu UV-3600) with
123

P. Sar et al.
fully thermostatted cell compartments. Reaction mixtures were previously ther-
mostatted and transferred into a cell of 1 cm path-length immediately after mixing,
and disappearance of Cr(VI) was monitored to 85 % conversion. The pseudo first-
order conditions were attained by maintaining a large excess (*15 times) of
2-propenol over the dichromate. Experiments were performed at 303 K unless
otherwise stated. Pseudo first-order rate constants (kobs) were calculated from the
slopes of plots of -ln(A450) versus time (t), which were linear (Fig. S1). The half life
(t1/2) values were directly calculated by using relation t1/2 = (ln2/kobs). No other
reactant or product had any significant absorbance at this wavelength.
Product analysis and stoichiometry
Under experimental conditions, [2-propenol]T ꢀ [Cr(VI)]T (T = total concentration),
2-propenol oxidized to 2-propenal (separated from mixture by fractional distillation),
as confirmed by a 2,4-dinitrophenylhydrazine (DNP) test [19]. After completion of the
reaction, the mixture solution was treated with an excess of a saturated solution of 2,4-
DNP in dilute H2SO4 medium. After storage in an ice bath for approximately 1 h, the
precipitate was filtered off, dried and recrystallized from ethanol. The micelle-
catalyzed reactions were performed in the presence of promoters in the same manner as
mentioned before. The infrared (IR) spectrum of the 2,4-DNP derivative of the product
(Fig. S2) was recorded by the a Perkin-Elmer FTIR spectrometer (model RX1). The
characteristic IR bands of the oxidized product at 1615 cm-1 support the formation of
corresponding aldehydes (Table S1). The observed IR spectrum of the 2-propenal-
DNP derivative was similar to previous literature [25].
To determine the stoichiometry of the oxidation reaction, several reaction
mixtures taking [chromium(VI)] ꢀ [2-propenol] (7.5 9 10-3 mol dm-3 oxidant:
1.5–6.0 9 10-3 mol dm-3 reductant) at fixed [H?] (0.5 mol dm-3) were prepared
and kept for 24 h at 303 K. After completion of the reaction, the unconsumed
oxidant was determined spectrophotometrically. The overall stoichiometry of the
reaction may be represented as:
3CH2¼CHÀCH2ÀOH þ 2HCrOÀ4 þ 8Hþ ¼ 3CH2¼CHÀCHO þ 2CrIII þ 8H2O
Test for free radicals
During oxidation of the aldehyde in an atmosphere of nitrogen, attempts at inducing
polymerization with the addition of acrylonitrile were unsuccessful. Further, the
addition of acrylonitrile did not affect the rate. This indicates that one-electron
oxidation, giving rise to free radicals, is unlikely in this reaction.
1H-NMR measurements
1H NMR experiments were performed on a Bruker spectrometer (Ascend 400). All
measurements were performed at 303 K. Samples were prepared in D2O. SDS
solutions were pre-equilibrated for 10 min before measurements.
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Sodium dodecylsulphate-catalyzed hetero-aromatic…
Results and Discussion
UV–Vis spectrophotometric study
During the reaction, the yellow-orange colour of the dichromate solution of Cr(VI)
quickly disappeared and a green coloured solution was obtained (Fig. 1). The
orange color of the dichromate was reduced to green Cr(III). The gradual production
of the Cr(III)-aqua complex was confirmed spectrophotometrically. In the unpro-
moted path, the final color that appeared as green (kmax = 579 nm Fig. 1) was quite
different from the color found in the promoted path, which was pale violet. In the
unpromoted path, the two bands were observed due to the transitions kmax = 581 nm
4
4
4
for A2g(F) ? T2g(F) and kmax = 408 nm (appeared as shoulder) for A2g(F) ?
4T1g(F) of Cr(III), respectively [26, 27]. The spectra of the final solution produced in
the unpromoted path after completion of the reaction and the spectra of the pure
chromic sulfate solution in aqueous sulfuric acid media were almost similar (Fig. 1),
but were different in the promoted reaction path due to the presence of different
types of Cr(III) species. This simply suggests that the final Cr(III) species are a
Cr(III)-promoter complex in which the promoter behaves as a ligand. PA, bipy and
phen as a ligand contain one or more heteroatom, i.e., nitrogen donor sites are in the
aromatic ring with vacant p* (antibonding) M.O., highly assisting the metal-to-
ligand charge transfer (MLCT).
Cr(VI) oxidation of 2-propenol in aqueous acid media may generate three
possible oxidized products: 2-propenal, 2-propenoic acid and glycerol (Scheme 1).
Fractional distillation products from the reaction mixture were divided into two
parts. The first part was treated with freshly prepared permanganate solution. The
Fig. 1 UV–Visible spectrum with corresponding colours of the reaction mixture [Cr(VI)]T = 5 9
10-4 mol dm-3, [H2SO4] = 0.5 9 10-3 mol dm-3, [2-propenol]T = 1.5 9 10-3 mol dm-3 at 303 K.
[The green spectrum, for pure Cr(III) solution, is almost identical to the spectrum after completion of the
reaction.]
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P. Sar et al.
OH
2-propenol
O
O
H
OH
2-propenoic acid
2-propenal
OH
OH
HO
Glycerol
Scheme 1 Possible oxidized product from Cr(VI) oxidation of 2-propenol
Fig. 2 Scanned absorption spectra of the reaction mixture in the presence of SDS (8.0 mol dm-3) at
regular time intervals. [Cr(VI)]T = 5 9 10-4 mol dm-3
, , [2-
[H2SO4] = 0.5 9 10-3 mol dm-3
propenol]T = 1.5 9 10-3 mol dm-3 at 303 K. Inset Similar scanned absorbance spectra in absence
of SDS and promoter
pink KMnO4 colour disappeared, indicating that the product included an unsaturated
component. The second part was treated with freshly prepared 2,4-DNP solution and
dil H2SO4, yielding a hydrazone product. The presence of orange hydrazone
confirms the presence of the [C=O group. Previously we stated that the reaction
occurred under a pseudo first-order condition, i.e., formation of 2-propenal from
single-step oxidation of 2-propenol is possible, but oxidation of 2-propenol to
2-propenoic acid is not possible under the reaction condition. Therefore, oxidation
of 2-propenol by Cr(VI) in acidic media only produces 2-propenal. Product analysis
studies ruled out the possibility of an attack on the double bond. Again, the IR
spectrum of this hydrazone (Fig S2, Table S1) superimposed on the spectrum of a
corresponding hydrazone of a standard sample of 2-propenal.
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Sodium dodecylsulphate-catalyzed hetero-aromatic…
Table 1 kobs and half life of the reaction in the presence and absence of a promoter and non-functional
micellar catalyst
Promoter [104 mol dm-3
]
Micellar catalyst [103 mol dm-3
]
104 9 kobs [s-1
]
t1/2 [h]
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
3.308 0.04
4.698 0.01
5.428 0.04
5.616 0.02
5.998 0.06
6.468 0.03
7.163 0.02
6.068 0.05
9.331 0.05
11.900 0.02
13.600 0.01
18.600 0.04
26.700 0.05
4.920 0.06
5.661 0.06
6.151 0.05
7.418 0.04
8.716 0.01
10.100 0.02
3.251 0.03
3.863 0.05
4.285 0.04
4.148 0.04
4.132 0.02
3.990 0.06
6.090 0.02
8.015 0.04
9.387 0.05
31.372 0.05
42.367 0.04
49.552 0.01
5.438 0.03
6.985 0.04
9.125 0.06
0.582
0.410
0.354
0.342
0.321
0.297
0.269
0.317
0.206
0.162
0.141
0.103
0.072
0.391
0.340
0.313
0.259
0.221
0.191
0.592
0.498
0.449
0.464
0.466
0.482
0.316
0.240
0.205
0.061
0.045
0.038
0.354
0.275
0.211
PA
5
10
15
20
30
40
5
PA
PA
PA
PA
PA
Bipy
Bipy
Bipy
Bipy
Bipy
Bipy
Phen
Phen
Phen
Phen
Phen
Phen
–
10
15
20
30
40
5
10
15
20
30
40
SDS
SDS
SDS
SDS
SDS
SDS
SDS
SDS
SDS
SDS
SDS
SDS
SDS
SDS
SDS
4
6
–
–
8
–
12
16
20
14
14
14
14
14
14
14
14
14
–
–
PA
20
30
40
20
30
40
20
30
40
PA
PA
Bipy
Bipy
Bipy
Phen
Phen
Phen
[Cr(VI)]T = 5 9 10-4 mol dm-3
,
[H2SO4] = 0.5 9 10-3 mol dm-3
,
[2-propenol]T = 1.5 9
10-3 mol dm-3, PA = 2-picolinic acid, bipy = 2,20-bipyridine, phen = 1,10-phenanthroline, Temp =
303 K
The reaction mixture was scanned in the range of 300–600 nm in both the
presence and absence of a promoter and surfactant at regular time intervals to follow
the gradual development of the reaction intermediates (if any) and the product. The
123

P. Sar et al.
scanned spectra in Fig. 2 indicates the gradual disappearance of Cr(VI) species at a
wavelength of 450 nm.
The reaction kinetics were found to obey the Michaelis–Menten equation with
respect to [2-propenol]; that is, a complex formed between the oxidant and substrate
prior to the rate determining step. The spectrum with an isosbestic point at
kmax = 533 nm (Fig S3) shows the intermediate complex subsequently decomposed
into the products.
The kinetic data (kobs and t1/2) in Table 1 shows kobs for the SDS-catalyzed
reaction to be greater than that of the corresponding non-catalyzed reaction. In the
case of the promoted path, this observation was also true as compared to the
unpromoted path. From Fig. 3, it was found that the kobs value increased with SDS
concentration, up to 8 mol dm-3, after which the kobs value decreased to some
extent. For each promoter, we followed the reaction in the concentration range
(5–40) 9 10-4 mol dm-3. In this oxidation, bipy is the best promoter, as compared
to the other two promoters (PA and phen) used (Table 1; Fig. 4).
Effect of [2-propenol]
The pseudo first-order rate constants kobs (s-1) were measured (Table S2) at
constant [Cr(VI)] (5 9 10-4 mol dm-3), [H?] (0.5 9 10-3 mol dm-3) and
temperature (303 K). The kobs values thus obtained were found to increase with
an increasing concentration of 2-propenol (Fig. 5).
Fig. 3 Effect of [SDS] on the oxidation reaction. [2-propenol]T = 1.5 9 10-3 mol dm-3
[Cr(VI)]T = 5 9 10-4 mol dm-3, [H2SO4] = 0.5 9 10-3 mol dm-3, Temp = 303 K
,
123

Sodium dodecylsulphate-catalyzed hetero-aromatic…
Fig. 4 Effect of [promoter] on the oxidation reaction. [2-propenol]T = 1.5 9 10-3 mol dm-3
[Cr(VI)]T = 5 9 10-4 mol dm-3, [H2SO4] = 0.5 9 10-3 mol dm-3, Temp = 303 K
,
Fig. 5 Effect of [2-propenol] on the oxidation reaction. [Cr(VI)]T = 5 9 10-4 mol dm-3
[H2SO4] = 0.5 9 10-3 mol dm-3, Temp = 303 K
,
Effect of [H1]
The effect of [H?] on the rate of 2-propenol oxidation was studied at constant
Cr(VI) and substrate (2-propenol) concentrations. The results obtained show that the
pseudo first-order rate constant of oxidation increased significantly with an increase
in the [H?] of the reaction medium.
123

P. Sar et al.
The plot of kobs against [H?] was found to be linear (Fig. 6). This shows that the
reactions were acid-catalyzed. As the sulfuric acid concentration increased in the
reaction mixture, the [H?] increased, but there was also a corresponding increase in
[HSO4-]. If the rate is proportionally dependent on the [HSO4-] to a great extent,
the overall effect of adding sulfuric acid would increase the rate.
Effect of temperature
Oxidation of the 2-propenol was carried out at different temperatures, ranging from
293 K to 313 K, maintaining constant concentrations of 2-propenol, Cr(VI) and H?.
The pseudo first-order rate constant increased with an increase in temperature for
uncatalyzed, SDS-catalyzed and bipy-promoted paths, as shown in Table 2. The
Arrehenius parameters (that is, Ea) were calculated using the Eyring equation
ꢀ
ꢁ
¼
¼
kobs
h
DH
RT
DS
ln
¼ À
þ
KBT
R
¼
DH ¼ Ea À RT
The energy of activation Ea for uncatalysed, catalysed and promoted paths
(Table 3) as kobs(bipy) ꢀ kobs(SDS) ꢀ kobs(u). The Ea values clearly suggest that SDS
and bipy act as catalysts and provide new reaction paths with lower activation
energy. The variation of the activation parameters in SDS micelles compared to
water is as expected because one may expect stabilization of the transition state
owing to the presence of micelles that assist the reaction path. The negative value of
DS# indicates formation of a complex in the reaction, and the complex is more
ordered than the reactants [28]. The observed large decrease in DS# further
Fig. 6 Effect of [H?] on the oxidation reaction. [2-propenol]T = 1.5 9 10-3 mol dm-3
[Cr(VI)]T = 5 9 10-4 mol dm-3, Temp = 303 K
,
123

Sodium dodecylsulphate-catalyzed hetero-aromatic…
Table 2 Temperature effect and thermodynamic activation parameters for the oxidation of 2-propenol
by Cr(VI)
Temperature
Without surfactant
and promoter
With SDS
104 9 kobs [s-1
With promoter
104 9 kobs [s-1
]
]
104 9 kobs [s-1
]
293 K
1.150
2.335
3.308
4.331
5.485
2.357
3.270
4.364
5.200
6.892
18.400
22.000
26.700
29.386
32.841
298 K
303 K
308 K
313 K
Thermodynamic parameters
DH# (kJ mol-1
DS# (J K-1 mol-1
DG# (kJ mol-1
Ea (kJ mol-1
)
54.795
-131.772
94.722
37.322
-186.573
93.854
19.617
-229.962
89.296
)
)
)
2573.937
2556.464
2538.759
[Cr(VI)]T = 5 9 10-4 mol dm-3
,
[H2SO4] = 0.5 9 10-3 mol dm-3
,
[2-propenol]T = 1.5 9
10-3 mol dm-3, SDS = 14 9 10-3 mol dm-3, Promoter (bipy) = 4 9 10-3 mol dm-3
Uncertainties in the thermodynamic parameters DH#, DS#, DG# and Ea are B 0.1 kJ mol-1
,
0.1 J K-1 mol-1
,
0.1 kJ mol-1 and 0.1 kJ mol-1 , respectively
Table 3 Proton NMR shifts d
(ppm) of SDS in the presence of
2-propenol
2-Propenol
a
b
5.168 5.912
c
4.195
a-CH2 b- CH2 Bulk -CH2 Terminal-CH3
SDS
3.982 1.634
1.250
1.274
0.024
0.850
0.845
0.005
SDS ? 2-propenol 4.000 1.656
Dd (shift in ppm) 0.018 0.022
strengthens the point. A meaningful, mechanistic explanation of the apparent values
of DS# and DH# is not possible because the kobs does not represent a single
elementary kinetic step; it is a function of true rate constants.
Location of reactant molecule inside the micelle
Aggregated surfactant media of any type consist of several solubilization sites
where the reactant molecules prefer to occur through different type of interactions
[29–31]. Actually, an aqueous surfactant solution looks like a micro-heterogeneous
media and is considered a two-phase system, namely, an aqueous phase and a
micellar pseudophase. Due to the different properties of the micellar pseudo-phase,
it is not possible to precisely locate the exact site of the reaction but, at least, the
location of the reactants can be considered. It is well established that most of the
micellar reactions involving an ionic and a neutral reactant, as well as ionic
micelles, are believed to take place either inside the Stern layer or at the interface
between the micellar surface and the bulk solvent water.
123

P. Sar et al.
To explain micellar catalysis, some authors consider exchange of the reactants
between the micelles. This exchange is a function of the thickness of the layer where
the substrate and the surfactant are located. Taking into account that, for both H?
and Na? (for the SDS case) ions, there is no difference between the specific
interactions with the surfactant head group, all the H? can be considered located in
the micellar interface. Changes in the micro-environment of the surfactant solution
can be easily monitored by the analysis of the 1H NMR spectrum of the micelle. The
1H-NMR spectra of SDS alone and in the presence of 2-propenol are represented in
Fig. 7a, b, respectively. The signals for four different protons of SDS, i.e., a-CH2,
b-CH2, 9 bulk methylene (bulk-CH2) and the terminal methyl proton (–CH3), in
Fig. 7 a 1H-NMR spectrum of SDS in D2O. b 1H-NMR spectrum of SDS in the presence of 2-propenol
in D2O. [Inset]: 1H-NMR spectrum of 2-propenol in D2O. [SDS] = 12 9 10-3 mol dm-3, [2-propenol] =
1 9 10-2 mol dm-3
123

Sodium dodecylsulphate-catalyzed hetero-aromatic…
1H-NMR spectrum are given in Table 3. In the presence of 2-propenol, the d value
for the terminal -CH3 protons is not shifted much, whereas a significant downfield
shift for a-, b- and bulk-CH2 protons of SDS were found. The Dd value clearly
indicates that the a-protons of SDS are shielded more by the –OH group of
2-propenol. This suggests that the 2-propenol molecule containing hydrophilic
groups (–OH) does not enter the micellar core of SDS; however, it resides in the
stern layer region through ion–dipole interactions. Here, it should be noted that
2-propenol is highly polar and tends to solubilizes in the vicinity of the micellar
surface of SDS due to ion interactions or polar interactions [31].
Reaction mechanism and rate law
-
Aqueous solutions of chromic acid contain mainly HCrO4 species [32]. It has
already been reported that chromium(VI) oxidation of organic substrate (like
alcohol, aldehyde, etc.) occurs via formation of a neutral ester [4, 7, 12, 13] through
binding of the substrate by HCrO4-. The neutral ester protonated by H? ion, present
in the reaction medium, subsequently decomposes to give the product. The
oxidation mechanism in the unpromoted path is given in Scheme 2.
According to the stoichiometry equation, we get
Â
Ã
d HCrOÀ
1
2
1 d½2 À propenolŠ
4
À
¼
3
dt
dt
On the basis of the mechanism in Scheme 2, the following rate equation is
derived:
Â
Ã
d ln HCrOÀ4
KobsðuÞ ¼ À
dt
2
2
¼
k1K1K2½2 À propenolŠ ½HþŠ
T
3
The promoted path of Cr(VI) oxidation of 2-propenol in acid media is not like the
unpromoted path. Chromium(VI) was reported to form chromate esters with ligands
containing O-, N- and S-atoms [9, 13, 26]. Promoters containing such atoms used in
this study behaved as a ligand.
Scheme 2 Chromium(VI) oxidation of 2-propenol in absence of a promoter
123

P. Sar et al.
The promoters bipy/PA/phen promote acid-catalyzed Cr(VI) oxidation of organic
substrates explicitly in an aqueous solution. The mechanistic pathway involves rate-
-
limiting formation of the ‘‘active oxidant’’ through complexation between HCrO4
and the promoter [4, 7, 12, 13]. In general, active oxidant species bearing a positive
charge tend to bind with 2-propenol through the –OH group, forming a ternary
complex, and subsequently decomposing to 2-propenal and Cr(IV)-promoter species
at the rate determining step. Afterward, Cr(IV)-promoter species rapidly reduce to
the Cr(III)-promoter via Cr(VI) and 2-propenol. The mechanism for overall
oxidation of 2-propenol to 2-propenal by Cr(VI) in the presence of a promoter are
proposed in Scheme 3 [13, 26].
In this kinetic study, it was found that bipy, containing N atoms as donor, is the
best among the three promoters used. It has been observed that the orange-yellow
colour of Cr(VI) rapidly disappears to blue-violet in the presence of bipy. In the
presence of PA and phen, the orange-yellow colour disappeared quickly, but not as
rapidly as in the bipy pathway. Bipy, like PA and phen, also forms an active oxidant
species by combining with HCrO4-, but in the ternary complex, electron transfer
from 2-propenol to the active oxidant species occurs more easily in the case of bipy
than that of PA and phen. The active oxidant Cr(VI)-bipy, containing two positively
Scheme 3 Chromium(VI) oxidation of 2-propenol in the presence of a promoter
123

Sodium dodecylsulphate-catalyzed hetero-aromatic…
charged nitrogen atoms, plays a crucial role in the rate enhancement process
compared to the one positively charged nitrogen atom in the Cr(VI)-PA complex.
The observed pseudo first-order rate constant for the Cr(VI) oxidation of
2-propenol in sulfuric acid media (Table 2; Fig. 5) follows kobs(bipy) ꢀ kobs(phen)
ꢀ
kobs(PA) ꢀ kobs(u). This observation matches well with earlier reports [33–35].
Effect of ‘micelle’ and ‘promoter with micelle’ media on the reaction rate
SDS, as an anionic surfactant, preferably attracts cationic species in its outer stern
layer region. The protonated ester produced from the neutral ester is cationic in
nature [33, 34], which, therefore, is attracted by the outer surface of the negatively
charged head group of the SDS micelle. A slight acceleration of the reaction rate
was observed when the reaction was performed in the presence of anionic SDS
compared to that of aqueous medium. 2-propenol is a neutral molecule with its polar
–OH groups preferably partitioned between the aqueous phase and the stern layer
(or close proximity of head groups) of the SDS micelle. The limited presence of a
reactant towards the outer surface of the micelle results in a slight rate acceleration.
In fact, in the presence of SDS, the reaction occurs simultaneously in both the
micellar and aqueous phase (Scheme 4).
In this study, the maximum rate acceleration was found when promoters were
used in addition to micelles. The active oxidants, Cr(VI)-PA, Cr(VI)-bipy and
Cr(VI)-phen complexes, react with 2-propenol to form a ternary complex which
undergoes redox decomposition in a rate-limiting step [26, 27], giving rise to the
final product, 2-propenal. Formation of the active oxidant (reaction intermediate,
Scheme 3) during the progress of the reaction was already confirmed by obtaining
1
the H NMR spectra in our previous report [36]. The positively charged active
oxidants (AO?) preferably accumulated in the anionic stern layer region of SDS
through electrostatic attraction, as shown in Scheme 5. The counter-ions (Na?) of
the SDS micelle were exchanged by the AO? and H? ions and they were arranged
Micellar pseudophase
Aqueous phase
(Reactant)
m
(Reactant)
w
Product
Scheme 4 Schematic representation of the oxidation reaction occurring in the aqueous and micellar
pseudophase
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P. Sar et al.
Scheme 5 Location of reactant molecule inside the micelle in the presence of a promoter
in an ordered manner, resulting the negative DS# value. Incorporation of reactant
species in the SDS micelle can be regarded both as having micro-environmental
effects on the micelle and solubilization effects. Our kinetic experiment shows
maximum rate acceleration was obtained when combining an SDS-catalyzed and
bipy-promoted pathway than when using an SDS-phen or an SDS-PA pathway.
Combination of SDS and bipy in the reaction mixture leads to almost a 15-fold rate
acceleration compared to the uncatalyzed and unpromoted paths. Therefore, we can
say that bipy in the presence of SDS is the best promoter for the oxidation of
2-propenol by Cr(VI). A table comparing our results (Table S3) explains the
selection of suitable catalysts and promoters over the other oxidants used previously
in 2-propenol to 2-popenal conversion [19–21, 37].
Conclusion
The main target of our study was to carry out the oxidation of 2-propenol to
2-propenal in aqueous micellar media rather than in an hazardous solvent. It is a
growing and challenging preference that synthetic chemists carry out organic
transformations using ‘‘green’’ aqueous solutions. In this context, the use of water as a
solvent features many benefits; not only is water innocuous, but it can also potentially
improve reactivities and selectivities of organic transformations, and simplify the
work up procedures. Micelles act as nano-reactors and efficiently increase the
1
effective local concentration of reactants inside its surface. The H-NMR studies
123

Sodium dodecylsulphate-catalyzed hetero-aromatic…
established that the stern region of micellar reactor is the most probable location
of reactants in this oxidation process. It is interesting to note that the rate of oxidation
increased drastically when SDS was used as the micellar catalyst in combination
with the bipy promoter at a particular concentration. The pseudo first-order rate
constants of the oxidation reaction between Cr(VI) and 2-propenol vary in magnitude
in the order of k(Aqs) \ k(SDS) \ k(PA) \ k(phen) \ k(bipy) \ k(phen?SDS) \ k(PA?SDS)
ꢁ k(bipy?SDS). The aldehyde product was confirmed by a conventional 2,4-DNP test
and IR studies of the DNP derivative. In conclusion, the overall investigation of the
present work is simple, cost effective and eco-friendly, as compared to previous
research. Micellar catalysis by SDS in association with a bipy promoter exhibited a
remarkable *16-fold rate enhancement as compared to that of the uncatalyzed
reaction pathway. Considering all the above facts, it is quite reasonable to select a
combination of SDS micelles and bipy as hetero-aromatic nitrogen base promoters as
being the most effective and suitable for chromic acid oxidation of 2-propenol to
2-propenal in aqueous media.
Acknowledgments Thanks to UGC ((F. No. 33-495/2007 (SR) dated 28.02.2008), New Delhi and CSIR
01 (2463)/11/EMR-II, Dated 16-05-2011), New Delhi for providing financial help in the form of a project
and fellowship. The authors are grateful to Sudipta Bhowmick and Visva-Bharati for fruitful discussions
in the course of this work.
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