Hetero-aromatic Nitrogen Base Promoted Cr(VI) Oxidation of Butanal in Aqueous Micellar Medium at Room Temperature and Atmospheric Pressure

Article abstract of DOI:10.1007/s10953-016-0434-5

The kinetics of the oxidation of butanal by chromic acid in aqueous and aqueous surfactant (sodium dodecyl sulfate, SDS, Triton X-100 (TX-100) and N-cetylpyridinium chloride, CPC) media have been investigated in the presence of a promoter at 303 K. The pseudo-first-order rate constants (k _obs) were determined from a logarithmic plot of absorbance as a function time. The rate constants were found to increase with introduction of heteroaromatic nitrogen base promoters such as picolinic acid (PA), 2, 2′-bipyridine (bipy) and 1,10-phenanthroline (phen). The product, butanoic acid, was characterized by ¹H-NMR. Three promoters PA, bpy and phen are used in combination with SDS, TX-100 and CPC surfactants showing an increase in the rate of oxidation compared to the unpromoted pathway. The mechanism of the reaction path has been proposed with the help of kinetic results and spectroscopic studies. The observed net enhancement of rate effects has been explained by considering the hydrophobic and electrostatic interaction between the surfactants and reactants. The TX-100 and PA combination is suitable for butanal oxidation.

Full text of DOI:10.1007/s10953-016-0434-5

J Solution Chem (2016) 45:109–125
DOI 10.1007/s10953-016-0434-5
Hetero-aromatic Nitrogen Base Promoted Cr(VI)
Oxidation of Butanal in Aqueous Micellar Medium
at Room Temperature and Atmospheric Pressure
Susanta Malik1 Aniruddha Ghosh1 Bidyut Saha1
•
•
Received: 2 April 2015 / Accepted: 31 October 2015 / Published online: 18 January 2016
Ó Springer Science+Business Media New York 2016
Abstract The kinetics of the oxidation of butanal by chromic acid in aqueous and
aqueous surfactant (sodium dodecyl sulfate, SDS, Triton X-100 (TX-100) and N-
cetylpyridinium chloride, CPC) media have been investigated in the presence of a pro-
moter at 303 K. The pseudo-first-order rate constants (kobs) were determined from a log-
arithmic plot of absorbance as a function time. The rate constants were found to increase
with introduction of heteroaromatic nitrogen base promoters such as picolinic acid (PA), 2,
20-bipyridine (bipy) and 1,10-phenanthroline (phen). The product, butanoic acid, was
1
characterized by H-NMR. Three promoters PA, bpy and phen are used in combination
with SDS, TX-100 and CPC surfactants showing an increase in the rate of oxidation
compared to the unpromoted pathway. The mechanism of the reaction path has been
proposed with the help of kinetic results and spectroscopic studies. The observed net
enhancement of rate effects has been explained by considering the hydrophobic and
electrostatic interaction between the surfactants and reactants. The TX-100 and PA com-
bination is suitable for butanal oxidation.
Keywords Kinetics Á Oxidation Á Butanal, Cr(VI) Á Micellar effects
1 Introduction
Oxidation is a fundamental transformation in organic synthesis and will likely play a
significant role in the growth of value-added chemicals from biomass. However, direct
conversion of butanal to butanoic acid is still a challenge. Butanoic acid has many potential
applications in different industries, and currently there is a great interest in using it as a
&
Bidyut Saha
b_saha31@rediffmail.com
1
Homogeneous Catalysis Laboratory, Department of Chemistry, The University of Burdwan,
Burdwan, WB 713104, India
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J Solution Chem (2016) 45:109–125
precursor to biofuels. Biofuels in general offer many advantages including sustainability, a
reduction of greenhouse gas emissions, and security of supply. In addition, butanoic acid
can be used to produce ethyl butyrate and butyl butyrate, both of which can be used as
fuels. In addition to its use as a biofuel, butanoic acid has also many applications in the
pharmaceutical and chemical industries [1].
The oxidation of butanal by several oxidants has been reported earlier [2]. A similar
version for the oxidation of butanal to butanoic acid in aqueous micellar medium is not
known. The dichromate anion (Cr2O7À, Cr6?) is commonly used to oxidize butanal. Upon
reaction with butanal, the chromium (VI) in dichromate (yellow) is reduced to Cr3? and
this species is observed in solution with the appearance of a blue color. The kinetic aspects
of oxidation of different aldehydes by several higher valent metal ions including Cr(VI)
have been studied by different workers under different conditions [3–7].
The mechanistic aspects of oxidation of different aliphatic and aromatic aldehydes by
different transition metal ions in aqueous acid media have been reported [8–10]. Various
chelating agents are known to catalyze the Cr(VI) oxidation of different organic substrates.
In this regard, picolinic acid (PA) is unique and quite efficient. A diversity of selective
chromium (VI) oxidizing agents, including pyridinium chlorochromate (PCC), pyridinium
dichromate (PDC), imidazolium dichromate (IDC), etc. have been developed, which allow
for the oxidation of a large range of compounds [11]. All these reagents required organic
solvents such as dichloromethane (CH2Cl2, DCM), acetone (CH3COCH3) or dimethyl
formamide (Me2NCHO, DMF) which are hazardous in the case of skin contact (irritant,
permeator), eye contact (irritant), ingestion or inhalation. Solvents in particular make a
large negative contribution to the environment. So we have selected water as a solvent to
avoid such hazards. Simple chromic acid oxidation of aldehydes is a very slow process [12,
13, 22]. Three representative surfactants (anionic, cationic and neutral) and three promoters
containing pyridine moieties: picolinic acid, 2,20-bipyridine, and 1,10-phenanthroline are
used for the oxidation of butanal.
Although water is an eco-friendly solvent for reactions, due to solubility problems with
organic substrates, researchers select aggregated surfactant media which have two different
parts: one is hydrophobic and another is hydrophilic. Aggregated surfactant media are
composed of different nano sized aggregates, including micellea, vesiclea, large unil-
amellar vesiclea, etc., above the critical micelle concentration (CMC) in aqueous medium.
Among these, micelles catalyze several reactions, including oxidative transformations,
hydroformylation, Diels–Alder reactions, several organic synthesis, etc. [14]. In this study
we have selectively chosen three different types of surfactants: cationic (cetyl pyridinium
choloride, CPC), anionic (sodium dodecylsulfate, SDS), and nonionic (octylphenol-poly-
oxyethylene ether, TX-100).
The promoters are used as chelating agents and found to be similar in reactivity as they
contain one or two hetero atoms, i.e., nitrogen in the aromatic ring. Naturally, they are
strong bases and act as an effective oxidation catalysts. One of the key conclusions from
our work is that the effects of both surfactant head groups and of the surfactant tails need to
be taken into account to fully reproduce medium effects as reported by a variety of kinetic
and spectroscopic probes of the micellar pseudophase [15]. In the present work, some
preliminary investigations into effects on the rate of oxidation reaction in micellar medium
are reported and, for this study, several kinetics measurements were performed. Product
identification was by different spectroscopic studies. This work essentially involved
determination of the rate of the reaction and establishing the proposed mechanism of the
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111
reaction. Dynamic light scattering (DLS) can be used. This oxidation takes place in
environmentally friendly aqueous medium rather than hazardous organic solvents.
2 Experimental
2.1 Materials and Reagents
All chemicals used were of analytical reagent (AR) grade. Butanal (99.0 %, SRL, India),
K2Cr2O7 (99.9 %, BDH), H2SO4 (98 %, Merck), picolinic acid (99 %, Sigma Aldrich), 2,
20-bipyridine (99 %, Spectrochem, India), 1,10-phenanthroline (99.5 %, Merck), sodium
dodecyl sulfate (SRL, India), N-cetylpyridinium chloride (98.0 %, SRL, India), Triton
X-100 (98.0 %, HIMEDIA) and double distilled water for the preparation of required
solutions. All other used chemicals have been purchased in their highest purity state
available commercially.
2.2 Instrumentation
1H-NMR spectra have been recorded on a Bruker Ascend 500 MHz spectrometer at
ambient temperature. UV–vis spectra have been recorded on a UV-1601PC (SHIMADZU)
spectrometer equipped with a temperature controller (TCC SHIMADZU). The balance
used was a Sartorius BSA224S-CW. The solutions have been prepared using a Digital
Ultrasonic Cleaner CD 4820 sonicator and a Z206A, Hermle Labortechnik GmbH cen-
trifuge. DLS studies have been investigated in a Malvern Zetasizer Nano ZS-90
instrument.
2.3 Procedure and Kinetics Measurements
Solutions of the oxidant and reaction mixtures containing known quantities of the substrate
(butanal = 75 9 10-4 molÁdm-3), promoter (picolinic acid = 150 9 10-4 molÁdm-3
,
2,20-bipyridine = 125 9 10-4 molÁdm-3, 1,10-phenanthroline = 75 9 10-4 molÁdm-3
)
under the kinetics conditions [butanal]T ꢀ [Cr(VI)]T and [promoter]T ꢀ [Cr(VI)]T. The
reactions were followed under pseudo-first-order conditions, using an excess of butanal
over Cr(VI). Reactant solutions were previously thermostated and transferred into 1 cm
path length cell immediately after mixing. Experiments were performed using 0.5
(molÁdm-3) H2SO4 at 30 °C [16]. The pseudo-first-order rate constant were calculated
from the slopes of the plot of log10[Cr(VI)]T versus time (t), which were linear at least for
three half lives. The scanned spectra and spectrum after completion of the reaction were
recorded with a UV–vis spectrophotometer [UV-1601PC (SHIMADZU)]. Under experi-
mental conditions, the possibility of decomposition of the surfactants by Cr(VI) was
investigated and the rate of decomposition in this path was negligible.
2.4 Product Analysis and Stoichiometry
Product analysis was carried out under mineral acid ([H2SO4] = 0.5 molÁdm-3) catalyzed
conditions in butanal. Keeping the concentration of butanal in excess over the oxidizing
reagent (dichromate), the two solutions were mixed and sulfuric acid was added to
maintain the solution pH at 2. Under our experimental conditions, [butanal]T ꢀ [Cr(VI)]T
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Fig. 1 The 1H NMR spectrum of butanoic acid in CDCl3 solvent
(T = total concentration). In the case of the oxidation of butanal by Cr(VI), after keeping
the reaction mixtures, K2Cr2O7 and sulfuric acid in proper composition for 72 h in water at
room temperature, the volume of the reaction mixture was reduced to 20 % under vacuum.
The butanoic acid produced was first separated from the reaction mixture by fractional
distillation.
Estimation of the un-reacted Cr(VI) showed that 1 mol of each butanal consumed 1 mol
of Cr(VI) for oxidation to take place, on the assumption that all the butanal was consumed
for the oxidations to take place. Our present work was intended to find the best combi-
nation of promoter and micellar catalyst in the presence of which the reaction occurs most
rapidly. The overall stoichiometry of the oxidation can be represented as:
3CH3CH2CH2CHO þ 2HCrO4À þ 8Hþ À! 3CH3CH2CH2COOH þ 2CrðIIIÞ þ 5H2O
2.4.1 1H NMR Spectrum
1
The H NMR spectrum (Fig. 1) of the separated product was recorded at 500 MHz in
CDCl3. Chemical shifts (ppm) are internally referenced to the TMS signal (0 ppm) in all
cases and are listed in Table 1.
2.5 Determination of the Size of Aggregates by Dynamic Light Scattering
(DLS)
DLS is the most versatile and useful techniques for measuring the sizes, size distribution
and shapes of nanoparticles. The hydrodynamic diameters (Dh) of TX-100 micelles with
reactant were determined by DLS. DLS characterization was performed to obtain micelle/
vesicle size and morphology on a DLS instrument with a capillary cell. Size measurements
were performed at a 90° angle, in triplicate. DLS analyzes the velocity distribution of
particle movement by measuring dynamic fluctuations of light scattering intensity caused
by the Brownian-motion of the particle. The light scattering study was done with TX-100
surfactant by taking its concentration above its CMC value in acidic medium.
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113
Table 1 Chemical shift values
for different protons of butanoic
acid
Butanoic acid
Moiety
Chemical shift (d ppm)
Multiplicity
CH3
CH2
CH2
OH
0.96
1.62
2.25
10.8
3H, triplet
2H, multiplet
2H, triplet
1H, singlet
3 Result and Discussion
All the kinetic measurements were carried out at 30 °C. The reaction rate increases linearly
with increasing butanoic acid concentration showing that the reaction is first order with
respect to butanoic acid. The rate of oxidation increases linearly with the acidity of the
solution [8]. The pseudo-first-order rate constants (kobs, s-1) were determined from the
slopes of plots of ln(A450) against time (t), where A450 is the absorbance at 450 nm. The t1/2
values are directly calculated in Table 1 by using the relation t1/2 = ln 2/kobs (Fig. 2).
The data in Table 2 show the kobs for SDS, CPC and TX-100 micelle catalyzed reac-
tions to be greater than for the corresponding non-catalyzed reaction. This observation is
also true for promoted path compared to the unpromoted path. It was observed that the
reactions of organic compounds were significantly enhanced in the aqueous micellar
solutions of ionic surfactants.
The reaction mixtures were scanned in the range of 200–800 nm in the presence and
absence of promoter and surfactant at regular time intervals (3 min) to follow the gradual
development of the reaction intermediates (if any) and the product. The scanned spectra
(Fig. 3a–g) indicate the gradual disappearance of Cr(VI) species and appearance of Cr(III).
The same also appeared for micellar catalyzed reactions in the presence and absence of
promoter.
The colors of the final solutions in the absence and presence of promoters are different
due to the presence of different types of Cr(III) species. The color of the final solution in
the absence of the promoter under the experimental condition is pale blue (kmax = 407 nm
4
4
and 584 nm) and the corresponding transitions [17–19] are 584 nm for A2g (F) ? T2g
4
(F) and 407 nm for 4A2g (F) ? T1g (F) of Cr(III) species (Fig. 4a). The spectra of the final
Fig. 2 Representative first order
plot to evaluate the pseudo-first-
order rate constant (kobs) for SDS
catalyzed phen-promoted Cr(VI)
oxidation of butanal in aqueous
medium at 30 °C:
[butanal]T = 75 9 10-4
molÁdm-3
,
[Cr(VI)]T = molÁdm-3
,
[H2SO4]T = 0.5 molÁdm-3
.
[PA] = 150 9 10-4 molÁdm-3
[TX-100]T = 2 9 10-2
molÁdm-3
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Table 2 Pseudo-first-order rate constant (kobs) and half life of the reaction in the presence and absence of
promoter and non-functional micellar catalyst
104 9 [Promoter] (molÁdm-3
)
104 9 [Micellar catalyst] (molÁdm-3
)
104 9 kobs (sec-1
)
t1/2 (h)
Water
–
1.216 0.03
2.808 0.04
13.23 0.07
9.128 0.01
1.415 0.04
0.33 0.02
1.558 0.06
4.975 0.04
4.51 0.08
5.39 0.01
4.233 0.04
5.26 0.03
6.306 0.03
16.26 0.01
14.46 0.05
14.5 0.03
1.58
PA
150
125
75
None
None
None
0.685
0.145
0.210
1.36
bipy
phen
–
SDS
400
20
–
CPC
1.44
–
TX-100
CPC
200
20
1.23
PA
150
125
75
0.38
bipy
phen
PA
CPC
20
0.42
CPC
20
0.357
0.45
150
125
75
SDS
400
400
400
200
200
200
bipy
phen
PA
SDS
0.365
0.305
0.118
0.133
0.132
SDS
150
125
75
TX-100
TX-100
TX-100
bipy
phen
[Cr(VI)]T = 5 9 10-4 molÁdm-3
temperature = 30 °C
,
H2SO4 = 0.5 molÁdm-3
,
[butanal]T = 75 9 10-3 molÁdm-3
, and
solution in the absence of promoter and pure chromic sulfate solution in aqueous sulfuric
acid media are identical. On the other hand, the color of the final solutions with promoters,
under the identical conditions, is pale violet (kmax = 541 nm for the phen promoted
reaction, 543 nm for the bipy promoted reaction and 561 nm for the PA promoted reaction)
4
4
for A2g (F) ? T2g (F) of Cr(III) species (Fig. 4). For the promoted reaction, there is a
4
4
blue shift for the peak due to the transition A2g (F) ? T2g (F) compared to the final
solution without promoter. This blue shift is due to the presence of the strong field donor
site, i.e. heteroaromatic N-donor site of the promoters. For Cr(III) aqueous species, the
4
band at 260 nm due to the 4A2g (F) ? T1g (P) transition appears as a shoulder on the high
energy charge transfer band [17–20]. The scanned spectrum (Fig. 3c) indicates the gradual
disappearance of Cr(VI) species and appearance of Cr(III) species with an isobestic point
at k = 518 nm for the promoter reaction (phen promoted scan). Observations of this single
isobestic point indicate the very low concentration of Cr(IV) and Cr(V) intermediates
under the present experimental condition. In the presence of promoter, Cr(VI)–A, Cr(VI)–
phen and Cr(VI)–bipy are the active oxidants [21, 22] (Fig. 5).
3.1 Mechanism of the Reaction and Rate Law
3.1.1 Mechanism of the Oxidation Process by Unpromoted Path
The unpromoted path is drawn on the basis that the reaction follows first order dependency
on [Cr(VI)]T, [butanal]T and second order dependency on [H?] is already established [23,
24] (Scheme 1).
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115
Fig. 3 a Scanned absorption spectra of the reaction mixture at regular time intervals (3 min)
[butanal]T = 75 9 10-4 molÁdm-3, [Cr(VI)]T = 5 9 10-4 molÁdm-3, [H2SO4]T = 0.5 molÁdm-3, tem-
perature = 30 °C. b Scanned absorption spectra of the reaction mixture at regular time intervals (3 min)
[butanal]T = 75 9 10-4 molÁdm-3
,
[Cr(VI)]T = 5 9 10-4 molÁdm-3
,
[H2SO4]T = 0.5 molÁdm-3,
[PA] = 150 9 10-4 molÁdm-3, temperature = 30 °C, isosbestic point = 520 nm. c Scanned absorption
spectra of the reaction mixture at regular time intervals (3 min) [butanal]T = 75 9 10-4 molÁdm-3
,
,
[Cr(VI)]T = 5 9 10-4 molÁdm-3
,
[H2SO4]T = 0.5 molÁdm-3
,
[TX-100] = 2 9 10-2 molÁdm-3
[PA] = 150 9 10-4 molÁdm-3, temperature = 30 °C, isosbestic point = 518 nm. d Scanned absorption
spectra of the reaction mixture at regular time intervals (3 min) [butanal]T = 75 9 10-4 molÁdm-3
,
,
[Cr(VI)]T = 5 9 10-4 molÁdm-3
,
[H2SO4]T = 0.5 molÁdm-3
,
[TX-100] = 2 9 10-2 molÁdm-3
[Phen] = 75 9 10-4 molÁdm-3, temperature = 30 °C, isosbestic point = 513 nm. e Scanned absorption
spectra of the reaction mixture at regular time intervals (3 min) [butanal]T = 75 9 10-4 molÁdm-3
,
[Cr(VI)]T = 5 9 10-4 molÁdm-3, [H2SO4]T = 0.5 molÁdm-3, [Phen] = 75 9 10-4 molÁdm-3, tempera-
ture = 30 °C, isosbestic point = 519 nm
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Fig. 4 a Absorption spectrum of unpromoted reaction mixture (after completion of reaction): [bu-
tanal]T = 75 9 10-4 molÁdm-3, [Cr(VI)]T = 5 9 10-4 molÁdm-3, [H2SO4] = 0.5 molÁdm-3 (the spec-
trum of chromic sulfate is identical with this under the experimental conditions). Absorption spectra of the
promoted reaction mixture (after completion of reaction);
b
[PA]T = 150 9 10-4 molÁdm-3
;
c [bipy]T = 125 9 10-4 molÁdm-3; d [phen]T = 75 9 10-4 molÁdm-3
Fig. 5 Absorption spectra of reaction mixture with and without promoter (in the absence of substrate): a
[Cr(VI)]T = 5 9 10-4 molÁdm-3
,
[H2SO4] = 0.5 molÁdm-3;
b
[Cr(VI)]T = 5 9 10-4 molÁdm-3
, [H2SO4] =
,
[H2SO4] = 0.5 molÁdm-3
[phen]T = 0.0075 molÁdm-3
,
[PA]T = 0.015 molÁdm-3
;
c
[Cr(VI)]T = 5 9 10-4 molÁdm-3
0.5 molÁdm-3, [bpy]T = 0.0125 molÁdm-3
;
d [Cr(VI)]T = 5 9 10-4 molÁdm-3, [H2SO4] = 0.5 molÁdm-3
,
3.1.2 Mechanism of the Oxidation Process by Promoted Path
Promoted paths are drawn on the basis that the reaction follows first order dependency on
[Cr(VI)]T, [butanal]T and [promoter]T. For bipyridine and phenanthroline first order
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117
K1
HCrO–
+
CH3CH2CH2CH (OH)O-CrO2(OH) + H2O
(Neutral ester)
+ H+
CH3CH2CH2CHO
4
K2
+
+ H+
CH3CH2CH2CH (OH)O-CrO2(OH)
(Neutral ester)
CH3CH2CH2CH(OH)O-CrO2(OH2
)
OH
O
O
k
CH3CH2CH2-COOH + H2CrO3 + H+
Cr
CH3-CH2-CH2-C
OH2+
Scheme 1 Chromic acid oxidation of butanal in absence of promoter
O
H
dependency on [H?] and zero order dependency on [H?] for picolinic acid is observed (not
shown in the paper).
Scheme 2 leads to the following rate law:
ꢀ
ꢁ
2
kobsðcÞ ¼ ð2=3Þ K3Kak3k1½butanalŠ ½PAŠ ½HþŠ =ðkÀ3 þ k1ÞðKa þ ½HþŠÞ
T
T
¼ a½butanalŠ ½PAŠ ½HþŠ
where, a = (2/3) (K3Kak3k1)/(k-3 ? k1)
ð1Þ
T
T
kobsðcÞ ¼ ð2=3Þk2K3K4½butanalŠ ½bipyŠ ½HþŠ
ð2Þ
ð3Þ
T
T
kobsðcÞ ¼ ð2=3Þk3K5K6½butanalŠ ½phenŠ ½HþŠ
T
T
Partition of neutral ester (Scheme 1) and neutral butanal (Fig. 6a, b) are equally
probable for all types of surfactants (Schemes 3, 4).
3.2 DLS Explanation of Micellar Aggregate
Figure 7 shows that, in case of TX-100, the size or diameter of micelles change when
butanal is added to the micellar medium. This helps us to understand that an interaction
occurs between the substrate (butanal) and TX-100 within the ‘‘Stern layer’’ of micelle.
The overall charge of the TX-100 micelle is neutral; therefore TX-100 interacts with the
aggregates and substrate molecules reside in the ‘‘Stern layer’’ of the micelle. The size of
TX-100 was 5.6 nm as found in the plot. Upon interaction of substrate (butanal) with the
outer surface of the aggregates the diameter of the TX-100 micelle increases from 5.6 to
7.6 nm.
3.3 High Resolution Transmission Electron Microscope (HR-TEM) Images
of TX-100 Micelles
The HR-TEM was done at 20 kV acceleration voltage using lacey carbon coated Cu grid of
300 mess size in the HR-TEM microscope (JEOL JEM 2100). Samples were prepared by
placing sample mixture drops directly on the copper grids using a micropipette. The
reactants, including the surfactant present in the aqueous mixture, were allowed to settle.
TEM pictures (Fig. 7) are presented which illustrate the multiple morphologies of the
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Ka
COOH
H+
+
+
N
COOH
N
H
[A]
O
+
N
K3
C
H2O
O
A + HCrO4– + 2 H+
Cr
O
O
OH2
B(Active oxidant)
O
+
N
O
C
+
N
O
_
C
k3
k-3
H2O
+ H3O+
O
Cr
O
+
CH3CH2CH2CHO
O
Cr
O
OH
O
C
H
O
OH2
CH2CH2CH3
[B]
HO
(slow step)
k1
rds
CH3CH2CH2COOH + Cr(IV)-PA
CH3CH2CH2COOH + Cr(III)-PA
Cr(VI)
fast
CH3CH2CH2CHO
Cr(IV)-PA +
Scheme 2 Chromic acid oxidation of butanal in the presence of picolinic acid
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
Neutral ester
Neutral ester
(a)
(b)
Fig. 6 Schematic representation of neutral ester and H? in a neutral surfactant, b anionic surfactant
aggregates made from the surfactant TX-100. Also, the micrograph suggests a relatively
narrow size distribution of the micelle diameters but a widely variable length. TEM images
of blank TX-100 micelles and Cr(VI) ? TX-100 micelles were taken (Fig. 8). From
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119
Kb
+ H+
+
H
N
N
N
N
K3
HCrO– + 2 H+
+
4
+
+
+
H
N
N
O
Cr
O
N
N
H2O
OH2
C (Active oxidant)
+
+
N
N
_
+ 2H3O+
K4
O
Cr
O
C
+
CH3CH2CH2CHO
O
OH2
H
C
CH2CH2CH3
HO
(Slow step)
k2
rds
+ Cr(IV)-bpy
CH3CH2CH2COOH
Cr(VI)
fast
CH3CH2CH2COOH + Cr(III)-bpy
Cr(IV)-bpy + CH3CH2CH2CHO
Scheme 3 Chromic acid oxidation of butanal in presence of 2, 20-bypiridine
Fig. 8a, b it can be observed that the TX-100 micelle is roughly spherical in aqueous acid
media. Small holes are seen in the micellar surface when Cr(VI) is added to the TX-100
aqueous acidic medium. So, we can say that incorporation of Cr(VI) takes place inside the
micelle.
4 Analysis of Catalytic Role of TX-100
In a micellar system, with TX-100 as the substrate, both the oxidant and the butanal are
found to be partitioned between the aqueous and the micellar pseudophase. Thus, the
reaction may proceed according to Scheme 5.
The attractive electrostatic interaction between the negatively charged polar head
groups and positively charged active oxidant is increased, leading to a decrease in the free
energy, which are advantages in regards to the energy requirement for the redox reaction.
The types of interaction occurring between SDS micelles and reactants determine the locus
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Kb
+ H+
+
N
N
N
N
H
+
+
K5
N
N
HCrO4– + 2 H+
O
Cr
O
+
+
N
N
H
H2O
OH2
D (Active oxidant)
+
+
N
N
_
K6
+ H3O+
O
Cr
O
+
D
CH3CH2CH2CHO
O
C
OH2
H
HO
CH2CH2CH3
k3
rds
(slow step)
+ Cr(IV)-phen
CH3CH2CH2COOH
Cr(VI)
CH3CH2CH2COOH + Cr(III)-phen
Cr(IV)-phen +
CH3CH2CH2CHO
fast
Scheme 4 Chromic acid oxidation of butanal in the presence of 1,10-phenanthroline
of the solubilization phenomenon at the micellar interface, in between the hydrophilic head
groups and the first few carbon atoms of the hydrophobic fragment (commonly referred to
as the palisade layer) and finally in the core of the micelle. Therefore, the polar –CHO
functional groups of butanal are mainly solubilized in the outer region of the micelle,
whereas the non-polar aliphatic framework is preferentially located in the inner portions of
the micelles (Fig. 9). Most of the micellar mediated reactions occur either inside the Stern
layer or at the interfacial junction region of Stern and Guoy–Chapman layers. In the
presence of cationic CPC micelles, hydrophilic negatively charged ions (HSOÀ4 , SO42À and
anionic reactive species such as HCrOÀ4 , Cr2O27À) will compete for binding to the micelle
by exchanging the bound (chloride) counter ions in the case of normal micelle formation
by CPC, but cationic species (H?; positively charged chromium(VI) species are most
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J Solution Chem (2016) 45:109–125
121
Fig. 7 Variation of particle size distribution of TX-100 micelles/vesicles determined by dynamic light
scattering (DLS). The average particle sizes for only (closed square) TX-100 is 5.6 nm and butanal
incorporated (closed circle) TX-100 is 7.5 nm. [butanal] = 5 9 10-4 molÁdm-3, [TX-100] = 2910-2
molÁdm-3
Fig. 8 TEM image of (a) blank TX-100 micelle in the presence of acid; (b) TX-100 micelle in the presence
of Cr(VI) and acid
KW
[RCHO]W
[RCOOH]W
[Cr(VI)]W
KO
+
+
KS
KM
[Cr(VI)]M
[RCOOH]M
[RCHO]M
Where R= CH3CH2CH2-
W = aqueous medium, M = micellar medium, S = substrate (butanal), O = oxidant
Scheme 5 Portioning of the oxidant and substrate between the aqueous phase and the micellar pseudo
phase
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J Solution Chem (2016) 45:109–125
Fig. 9 Schematic representation
of partitioning of substrate and
active oxidant [AO? = Cr(VI)–
PA complex] in neutral surfactant
O
O
O
R
C
O
O
O
O
O
H
O
O
O
H
C
R
AO+
AO+
O
H
C
RCHO
R
O
O
O
O
O
O
R
C
HO
A
O
O
H
+
C
R
AO+
O
RCH
O
O
O
O
O
O
A
R
C
H
O
O
H
O
C
O
O
R
A
O+
O O
H
O
+
O
O
O
C
O
R
O
likely at the acid concentration used in this work) will also take part in the inter-micellar
aqueous phase, due the to micelle formation by CPC. The enhancement of the rate of
reaction for CPC micelles is low compared to those of the SDS and TX-100 micelles as
repulsion takes place between the cationic CPC micelle and positively charged Cr(VI)
species. SDS micelles have an aggregation number around sixty and are roughly spherical
in shape. TX-100 is larger in size than SDS. The non-ionic surfactant TX-100 has a larger
core than SDS, so the active oxidant Cr(VI)–PA complex interacts better with the TX-100
micelle (Fig. 9). There are also hydrophobic interactions between the active oxidants and
the micelles. On the other hand the negative charges on the SDS micelles are partially
neutralized in the presence of excess H?. So smaller number of active oxidants come in
contact with the sodium dodecylsulfate Stern layer due to columbic attraction. So the steric
factor and hydrophobic interaction play dominant roles and the rate of oxidation is fastest
in the presence of the combination of PA and TX-100.
Therefore, the reaction rate follows the order:
kobsðTX À 100Þ [ kobsðSDSÞ [ kobsðCPCÞ [ kobsðwaterÞ:
5 Interaction of Butanal with the CPC micelles
Butanal interactions with CPC monomers were examined by the chemical shifts in the CPC
protons, particularly those on and near the pyridyl ring. Addition of the CPC above the
CMC (2 9 10-3 molÁdm-3, with butanal (75 9 10-4 molÁdm-3) displayed distinct upfield
shifts in the proton peaks situated near the pyridyl ring, verifying the positioning of butanal
close to the pyridyl ring. Importantly, the chemical shifts in the proton signals of CPC from
normal micelles and that in the presence of butanal are noticeably different. This is an
indication that the CPC micelles are modulated by the presence of butanal, assuming the
interaction of the butanal–CPC complex above the CMC. On the other hand, similar NMR
measurements with butanal and CPC (1.2 mmolÁdm-3, above the CMC of CPC), displayed
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J Solution Chem (2016) 45:109–125
123
Table 3 Chemical shift values
for different protons of CPC
CPC
CPC ? butanal
Aromatic proton
(a, a0) CH protons
(b, b0) CH protons
(C)–CH protons
Aliphatic proton
b-CH2 proton
8.983
8.605
8.132
8.926
8.604
8.146
1.980
1.279
1.931
1.241
Bulk proton
Fig. 10 The 1H-NMR spectrum of CPC and butanal in D2O solvent (Inset The 1H-NMR spectrum of CPC
only in D2O solvent)
Table 4 Chemical shift values
for different protons of TX-100
TX-100
TX-100 ? butanal
T8 CH protons
T7 CH protons
T6 CH protons
T5 CH protons
T4 CH protons
T3 CH protons
T2 CH protons
T1 CH protons
7.173
6.799
3.998
3.724
3.549
1.630
1.258
0.683
7.244
6.865
4.072
3.834
3.623
1.699
1.321
0.882
broader proton peaks, having only small changes in the chemical shifts of the pyridyl
protons [25]. The corresponding proton NMR signals are assigned in Table 3 (Fig. 10).
6 Interaction of Butanal with the TX-100 Micelles
Butanal interactions with TX-100 monomers were examined by the chemical shifts in the
TX-100 protons in D2O. TX-100 has 8 different protons denoted: T1, T2, T3, T4, T5, T6, T7,
and T8. Addition of the TX-100 surfactant, above the CMC (2 9 10-2 molÁdm-3, with
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J Solution Chem (2016) 45:109–125
Fig. 11 The 1H-NMR spectrum of TX-100 and Butanal in D2O solvent (Inset The 1H NMR spectrum of
TX-100 only in D2O solvent)
butanal (75 9 10-4 molÁdm-3) displayed distinct downfield shifts of the 8 protons of TX-
100. Therefore in TX-100 medium the butanal molecule stays at Stern-layer of the micelle
[26]. The corresponding proton NMR signals are assigned in Table 4 (Fig. 11).
7 Conclusion
The electron transfer kinetics of butanal to butanoic acid have an important role in lab
based and industrial transformation reactions. The reaction is accelerated by addition of the
surfactants TX-100, SDS and CPC. The non-ionic micelles of TX-100 are better catalysts
than the anionic SDS micelles and cationic CPC micelles. The combination of PA and TX-
100 is the best choice for chromic acid oxidation of butanal in aqueous media and is
capable of increasing the oxidation rate by up to *14 fold.
Acknowledgments We are grateful to UGC-RGNF and CSIR, New Delhi, for financial support.
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