Surface phenomenon in micellar media: An excellent controlling factor for oxidation of fatty aldehyde in aqueous medium

Article abstract of DOI:10.1016/j.molliq.2020.113224

Oxidation of long chain aldehydes is truly tough in aqueous medium. So, an attempt has been made to oxidise octanal, a fatty aldehyde, by Cr(VI) in acidic aqueous media with the assistance of surfactants. The detailed kinetics of this successful micellar oxidation reaction has been studied spectrophotometrically under pseudo1st order condition at 25 °C (±0.1 °C). In promoting the oxidation reaction three hetero-aromatic bases i.e. 1, 10 – phenanthroline (Phen), 2-picolonic acid (PA) and 2, 2′ – bipyridine (Bpy) have been used in combination with the surfactants. Mixture of SDS and Bpy has come out to be the most effective among the others to enhance the oxidation rate maximum. Necessary spectral analysis and thermodynamic facts have also been produced in support of the whole process.

Full text of DOI:10.1016/j.molliq.2020.113224

Journal Pre-proof
Surface phenomenon in micellar media: An excellent controlling
factor for oxidation of fatty aldehyde in aqueous medium
Suman Chowdhury, Atanu Rakshit, Animesh Acharjee,
Kalachand Mahali, Bidyut Saha
PII:
S0167-7322(20)31152-1
DOI:
https://doi.org/10.1016/j.molliq.2020.113224
MOLLIQ 113224
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
23 February 2020
2 April 2020
22 April 2020
Please cite this article as: S. Chowdhury, A. Rakshit, A. Acharjee, et al., Surface
phenomenon in micellar media: An excellent controlling factor for oxidation of fatty
aldehyde in aqueous medium, Journal of Molecular Liquids (2020), https://doi.org/
1
0.1016/j.molliq.2020.113224
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2020 Published by Elsevier.

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Surface phenomenon in micellar media: An excellent
controlling factor for oxidation of fatty aldehyde in aqueous
medium
1
1
1,2
3
Suman Chowdhury , Atanu Rakshit , Animesh Acharjee , Kalachand Mahali and Bidyut
1
*
Saha
1
Homogeneous Catalysis Laboratory, Department of Chemistry, The University of Burdwan,
Burdwan 713104, WB, India
2
Department of Chemistry, Hooghly Mohsin College, Chinsurah, WB, India
3
Department of Chemistry, University of Kalyani, Kalyani, 741235, WB, India
*
9
Corresponding author, E-mail Id: b_saha31@rediffmail.com (B. Saha) Mobile: +91
476341691
Abstract
Oxidation of long chain aldehydes is truly tough in aqueous medium. So, an attempt has been
made to oxidise octanal, a fatty aldehyde, by Cr(VI) in acidic aqueous media with the assistance
of surfactants. The detailed kinetics of this successful micellar oxidation reaction has been
st
studied spectrophotometrically under pseudo1 order condition at 25°C (± 0.1°C). In promoting
the oxidation reaction three hetero-aromatic bases i.e. 1, 10 – phenanthroline (Phen), 2-picolonic
acid (PA) and 2, 2’ – bipyridine (Bpy) have been used in combination with the surfactants.
Mixture of SDS and Bpy has come out to be the most effective among the others to enhance the
oxidation rate maximum. Necessary spectral analysis and thermodynamic facts have also been
produced in support of the whole process.

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Keywords
Fatty aldehyde oxidation; aqueous micellar media; homogeneous catalysis; pi-pi interaction.
Abbreviations
CMC – Critical Micelle Concentration
SDS – Sodium dodecylsulphate
CPC – Cetylpyridinium chloride
Bpy – 2, 2´-Bipyridine
Phen – 1, 10-Phenanthroline
PA – 2-Picolinic acid
rds – Rate Determining Step

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1
. Introduction
Organic synthesis is one of the most successful segments of chemical science to define the
necessity, applicability, viability and favour of science to mankind. It is inevitable that the
products obtained from organic syntheses have provided us numerous benefits from commercial
to survival purposes. Chronologically, many advancements have been made by the heirs of
researchers keeping parallelism and compatibility with the desire of human beings but in few
cases compatibility with the green requirements of reaction procedures cannot be retained.
Usages of hazardous organic solvents in those synthetic procedures has made them refractory to
eco-friendly green principles.[1-3] So, in the present era the growing concern of environment
has led us find an approach with non-violation of green principles. For some past years
surfactants are in good agreement with those green principles replacing hazardous organic
solvents in organic syntheses. Its amphiphilic nature [4] has gifted us the privilege of
solubilizing many hydrophobic organic species and continue the reaction procedures in
hazardless aqueous medium. Many popular organic reactions e.g. Diels-Alder reaction, Friedel-
Crafts reaction, Suzuki coupling, Epoxidation reaction, Heck coupling, Ether synthesis etc. have
also been revised successfully in micellar media formed by surfactants.[5-7] In many cases
surfactants, above their critical micelle concentration, have also been obtained with some
additional power of catalysing reaction rates.[8-15]
In this paper also the ‘like dissolves like’ mechanism and surface phenomena of
micellar media of reducing interfacial tension [4] has been exploited to solubilize octanal, an
eight carbon saturated fatty aldehyde, in aqueous medium which is an eco-friendly and easily
available solvent. Not only that, but also an considerable catalysing power of the surfactants has
been come across in our experiment of oxidising octanal by hexavalent chromium Cr(VI) and a
detailed kinetics of the octanal oxidation has been studied spectrophotometrically under
st
pseudo1 order condition at 25°C (± 0.1°C). It is truly a great achievement that such a long chain
aldehyde has been oxidised to octanoic acid in aqueous medium though oxidation of another
fatty aldehyde, comparatively of short chain, has been reported previously [16]. Octanoic acid,
also popularly known as caprylic acid, is commercially used in the production of esters which is
used in the preparation of perfumes and dyes. In addition, it has plenty uses as antimicrobial
pesticide in food, dairy, breweries, wineries, beverage processing plants and also as disinfectant,
algaecide, bactericide, fungicide, and herbicide in nurseries, greenhouses. For epilepsy, tremors,
renal failure, digestive disorders also octanoic acid is taken by mouth. [17-21] Therefore,

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preparation of octanoic acid is very important for its so many unavoidable usages. In our
experiment, we have also used some organic hetero-aromatic bases in the micellar media to
reinforce the catalysing power of the surfactants. The organic hetero-aromatic bases i.e. 1, 10 –
phenanthroline (Phen), 2-picolonic acid (PA), 2, 2’ – bipyridine (Bpy) should better be termed as
‘promoter’ due to the formation of Cr(III)-Phen, Cr(III)-PA and Cr(III)-Bpy after the oxidation
reaction is over which is discussed in detail latter. [9-10] At the end of the introductory part it is
essential to mention that using Cr(VI) as an oxidising agent in our experiment no green principle
is violated because after the reaction is over the toxic Cr(VI) is reduced entirely to non-toxic
Cr(III) which is also useful in dietary supplements. [22] There is also no chance of staying
Cr(VI) unreduced in the reaction medium as the fatty aldehyde, octanal, is taken 15 times higher
than the concentration of Cr(VI) which is mentioned latter.
2
. Experimental
2
.1. Materials and reagents
Octanal (99.0 %, AR, SRL, India), K Cr O (99.9 %, AR, BDH, India), H SO (98%, AR,
2
2
7
2
4
Merck, India), sodium dodecylsulfate (SDS) ( > 90.0 %, AR, Merck, India), N-cetylpyridinium
chloride (CPC) (98.0 %, AR, SRL, India), Triton X-100 (98.0 %, AR, HIMEDIA, India), 1, 10 –
phenanthroline (99.5 % AR, Merck, India), 2, 2’ – bipyridine(99.0 %, AR, Spectrochem, India),
Pyridine-2-carboxylic acid (99.0 %, AR, Sigma Aldrich, UK), deuterium oxide (D O) (99.9 %,
2
AR, Sigma-Aldrich, USA), chloroform-d (CDCl ) (99.8%, AR, Sigma-Aldrich, USA). In
3
preparation of all requisite solutions double distilled water has been used. All the chemicals were
used as received without further purification.
2
.2. Procedures and kinetic measurements
The process of this experiment is mainly based upon spectrophotometric analysis where a double
beam UV-visible spectrophotometer [UV-vis 1600 (SHIMADZU)] has been used which is
attached with a temperature controller. In preparation of homogeneous aqueous solution of fatty
aldehyde, octanal, measured volume of octanal has been taken in three different miceller media
(SDS, CPC and TX-100) of different concentrations separately and then the solutions are
acidified with concentrated H SO to keep its acidity 0.3 (M), i.e. pH˂ 1. It is very necessary to
2
4
-4
-
mention here that in every reaction Cr(VI) and octanal has been taken 2.5×10 (M) and 3.75×10
3
st
(M) respectively ([Octanal] = 15 [Cr(VI)]) to keep the whole process go on under pseudo 1
order reaction condition at a temperature 25°C (± 0.1°C). Then for kinetic measurements the
decreasing absorption intensities of reaction solutions have been recorded at λmax = 438nm at a
regular time interval starting from the moment of addition of Cr(VI) solution (aqueous solution
of K Cr O ) to the homogeneous solution of octanal. Some experiments have also been
2
2
7

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performed using three promoters 1, 10 – phenanthroline (Phen), 2-picolonic acid (PA) and 2, 2’
-
3
-4
–
bipyridine (Bpy) at a concentration range of 10 to 10 (M). Sartorius BSA 224S-CW balance
and Digital Ultrasonic Cleaner CD 4820 Sonicator have been used for the preparation of reaction
1
solutions. DLS study has been done using Horiba nano partica SZ-100 instrument. Various H-
NMR has been recorded by Bruker Ascend 400 MHz spectrometer. For detail study of kinetics
the activation parameters have also been calculated and reported below at three different
temperatures 25°C, 35°C and 40°C.
2
.3. Product analysis and stoichiometry
After recording the absorption intensities of reaction solution till the pale yellow colour goes off,
it has been stored in a safe place for 3 days to assure complete reduction of Cr(VI) by octanal
with the appearance of faint colour solution caused by Cr(III). The bluish solution on solvent
extraction (using diethyl ether) followed by fractional distillation gives two organic liquids of
boiling points 171°C (literature value 170°C corresponds to octanal) and 238°C (literature value
1
2
39°C corresponds to octanoic acid) [23-24]. The H-NMR spectroscopy of this high boiling
liquid proves the product of the oxidation reaction to be octanoic acid. (Fig. S1a, S1b)
UV-Visible spectra of the bluish coloured solution in three different micellar media
show λmax at 581nm which confirms Cr(III) to be produced in the reactions. [25] For the
determination of stoichiometry of the reaction a Cr(VI) solution of known concentration has
been reacted with various concentrations of octanal in micellar media of fixed concentration and
+
at a constant H concentration of 0.3(M). The recorded absorbances of the solutions after 4 days
gives the following stoichiometry of the reaction.
1
H-NMR spectral analysis
1
To record the H-NMR spectrum of the product TMS has been taken as reference and CDCl
3
1
solution of the product has been prepared. The recorded H-NMR spectrum at 400 MHz shows a
peak at 11.837 ppm corresponding to carboxylic acid and other chemical shift values have also
been produced here in support of the product. (Fig. S1a, S1b) (Table 1)
Table 1 Chemical shift values for different protons of Octanoic acid.
Octanoic acid
Atom
Chemical shift (δ ppm)

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1
2
3
11.837 (1H, s)
2.34 (2H, t)
1.633 (2H, m)
1.311-1.299 (8H, m)
0.884 (3H, t)
4
- 7
8
-3
Fig. 1 Scanned absorption spectra of the reaction at regular time interval: [Octanal] = 3.75×10
T
-
4
-3
(
M), [Cr(VI)] =2.5×10 (M), [H SO ] = 0.3(M), [SDS] = 0.03(M), [Bpy] = 1×10 (M)
T 2 4 T T T
Temperature = 25°C.

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-3
Fig. 2 Scanned absorption spectra of the reaction at regular time interval: [Octanal] = 3.75×10
T
-
4
-3
(
M), [Cr(VI)] =2.5×10 (M), [H SO ] = 0.3(M), [TX-100] = 0.02(M), [Phen] = 5×10 (M)
T 2 4 T T T
Temperature = 25°C.
2
.4. Separation of surfactants
Though surfactants help to solubilize organic substances in aqueous medium, they have own
toxic effects to environment. [26-28] So, to drain out the surfactants in our experiments some
steps have been taken chemically and thermally. When the reaction is over SDS has been
precipitated as Ca(DS) by using CaCl , CPC has been eliminated using KBr and TX-100 has
2
2
been precipitated thermally and the precipitates were filtered off therefrom.[29]
3
. Results and discussion
3
.1. UV-Vis spectral analysis
The kinetics have been studied spectrophotometrically and absorption intensities of the reaction
solutions have been used to calculate the pseudo 1st order rate constants (k ). Actually, the
obs
progress of reactions in micellar media shows a decrease in absorbances at λmax = 438nm at a
regular time interval (Fig. 1, 2, S2a, S2b, S3a, S3b) and using this absorbances graphs of –
ln(A ) with respect to time(minute) have been plotted in form of straight lines from which we
4
38
get easily the slope of the straight lines i.e. k .(Fig. 3) Then using the equation t = 0.693/kobs
obs
1/2
half-lives (t ) of reactions have been calculated. All the experimentally obtained k and t1/2
1
/2
obs

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have been mentioned below in a tabular form in details (Table 2). From the table, it can easily be
seen that in presence of the promoters the reaction rates have geared up considerably. Some
additional changes in the nature of spectrum in presence of those promoters seem to be caused
by the formation of Cr(III)-promoter complex. Some significant UV-vis spectra have been
produced having one isosbestic point each of them (Fig. 1, 2, S2a, S2b, S3a, S3b) which reveal
that in the completion of this whole reaction, chromium seems not to be present in any
intermediate oxidation states e.g. Cr(V) and Cr(IV). [30]
3.5
3.4
3.3
3.2
3.1
3.0
2.9
R = 0.9942
0
10
20
30
40
50
60
Time (minute)
Fig. 3 Representative plot for SDS promoted oxidation path at 25°C for calculation of pseudo
-3
first order rate constant: A438 is the absorbance at 438 nm. [Octanal] = 3.75×10 (M), [Cr(VI)]
T
T
-4
=
2.5×10 (M), [SDS] = 0.03(M), [H SO ] = 0.3(M), Temperature = 25ºC.
T 2 4 T
3
.2. Mechanism of the reaction
In the present study reactions have been carried out in two different ways. Firstly, in unpromoted
micellar media and secondly, in promoted micellar media. Both the cases have been explained in
depth separately in this section.
3
.2.1. Unpromoted micellar oxidation
In this experiment three different types of surfactants, i.e. cationic, anionic and neutral, have
been used above their CMC which can solubilize very small amount hydrophobic octanal. CPC
and TX-100 have been found to be more solubilizing in this experiment than SDS, may be due to
having more hydrophobic parts in CPC and TX-100 than that of SDS. Oxidation of octanal in
micellar media has been seen very much successful with some additional observations of
catalysing power of SDS and TX-100. With increase in concentrations of SDS and TX-100, the

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rate of present oxidation reaction increases gradually. Though in presence of CPC the oxidation
rate is greater than that of SDS and TX-100, on increasing CPC concentration reaction rate
decreases. (Table 2) (Fig. 4) A possible reason of these observations has been proposed
mechanistically. (Scheme 1)
Scheme 1. Mechanistic pathway of oxidation of octanal by Cr(VI).
On dissolving potassium dichromate in acidic aqueous medium an anionic chromium
-
species HCrO is generated though it gets protonated in acidic micellar media favouring the
4
binding of H CrO on the micellar surface at stern layer by hydrogen bonding and ion-dipole
2
4
interaction. [29] Actually, an interaction does exist between the Cr(VI) species and micelles
which is supported by the absorbance spectrum shown below displaying red shifted λmax in case
-
of mixtures of Cr(VI) and surfactants. (Fig. S4) The HCrO in acidic medium plays a vital role
4
producing a neutral chromate ester (E) in reaction with octanal. The chromate ester (E) is the
main intermediate of this oxidation reaction leading to the product (P) by acid catalysed
decomposition.

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3.4
3.2
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
SDS
TX-100
CPC
0.020 0.025 0.030 0.035 0.040 0.045 0.050 0.055 0.060
Surfactant Concentration (M)
Fig. 4 Dependence of kobs on [micellar catalyst] for Cr(VI) oxidation of octanal (in absence of
T
−
3
promoter) in aqueous H SO media at 25°C. [Octanal] = 3.75 × 10 (M), [Cr(VI)] = 2.5 ×
2
4
T
T
−
4
1
0 (M), [H SO ] = 0.3(M).
2 4 T
100
TX-100
TX-100 + Octanal
80
60
40
20
0
6
8
10
12
14
16
18
20
22
Hydrodynamic diameter (nm)
Fig. 5 Dynamic Light Scattering (DLS) study of micellar aggregates. The average hydrodynamic
diameter for TX-100 is 9.2 nm and octanal incorporated TX-100 is 15.9 nm. [Cr(VI)] = 2.5 ×
T
−
4
−3
1
0 (M), [TX-100] = 0.02(M) and [Octanal] = 3.75 × 10 (M).
T T
Surfactants form a molecular aggregation, above their CMC, named micelle [4] in
which Cr(VI) resides at the stern layer [9] and the hydrophobic substrate molecule resides in the
core which has been proved by DLS experiment. DLS study of TX-100 and mixture of TX-100

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with octanal shows that there is an increase in hydrodynamic radius of TX-100 (9.2 nm) in
presence of octanal (15.9 nm) (Fig. 5) and this fact proves the octanal to be placed in the core of
the micelles. So, up to formation of the chromate ester (E) in every micellar medium the
oxidation rate seems not to be altering but the next step, acid catalysed decomposition of the said
chromate ester, is anticipated to be the reason of this rate alteration. In fact, the charge of the
peripheral surface of the micelles formed by respective surfactants is the main factor here. In
case of SDS the anionic hydrophilic part, projecting outward at the surface of the micelle,
+
facilitates the approach of positively charged H toward the chromate ester (E) at the core of the
micelle and favours the rds (rate determining step) enhancing k. Similarly, CPC being cationic
+
surfactant forms positively charged micellar surface which inhibits the approach of H to E
lowering k value. So, a neutral surfactant TX-100 having no such coulombic interactions with
+
the H ion shows a rate variation consistently. (Fig. 6) After the disappearance of pale yellow
colour of each micellar catalysed reaction solution the completion curves shows peaks at 581nm
4
4
4
4
and 409 nm corresponding to A (F) → T2g (F) and A (F) → T1g (F) transitions
2
g
2g
respectively. (Fig. S5) [30]
Fig. 6 Schematic representation of unpromoted micellar reactions. (a) SDS catalysed; (b) TX-
1
00 catalysed; (c) CPC uncatalysed.
3
.2.2. Promoted micellar oxidation

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In the promoted path each of the hetero-aromatic bases shows additional enhancement of
oxidation rate in SDS and TX-100 micellar media to a great extent. Among the three promoters
2
, 2’- bipyridine has been found to be the strongest in SDS micellar medium and with increase in
concentration of the promoters in the SDS and TX-100 micellar media the rates also increase
consistently. (Table 2, 1S)
Table 2 kobs and half-life (t ) of the reaction in presence of micellar catalysts and promoters
1/2
(Temperature = 25ºC).
-1
Micellar catalyst
Promoters (M) ×
kobs (min ) ×
Half-life
3
2
(M)
10
10
[t ](mins)
1
/2
SDS
0.03
None
None
None
0.76
1.12
1.50
1.95
0.64
0.86
0.98
1.11
3.27
2.01
1.36
0.90
1.59
2.19
2.99
1.52
3.11
90.12
0
0
0
.04
.05
.06
61.87
46.20
35.60
TX-100
0.02
108.28
80.58
70.71
62.43
21.16
34.35
50.96
77.00
43.31
31.64
23.12
45.59
22.29
0
0
0
.03
.04
.05
CPC
SDS
0.02
0
0
.03
.04
0.03
Phen
1.25
2
3
5
.50
.75
.00
PA
1.25
.50
2

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3
.75
4.43
5.34
3.55
12.22
21.67
34.66
1.72
2.42
3.40
4.00
1.35
1.50
1.65
1.77
1.23
2.06
3.04
3.56
2.17
2.64
3.50
4.31
3.89
4.71
15.60
12.98
19.52
5.67
5
.00
Bpy
Phen
PA
0.25
0
0
1
.50
.75
.00
3.19
1.99
TX-100
0.02
1.25
40.29
28.64
20.38
17.31
51.33
46.20
42.00
39.15
56.34
33.64
22.79
19.44
31.94
26.25
19.80
16.07
17.81
14.71
2
3
5
.50
.75
.00
1.25
2
3
5
.50
.75
.00
Bpy
Phen
PA
1.25
2
3
5
.50
.75
.00
CPC
0.03
1.25
2
3
5
.50
.75
.00
1.25
.50
2

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3
.75
5.77
6. 89
2.81
3.46
4.50
5.70
12.01
10.05
24.66
20.02
15.40
12.16
5
.00
Bpy
1.25
2
3
5
.50
.75
.00
In promoted micellar oxidation, the mechanism is proposed to be quite different with
the formation of Cr(III)-promoter complexes by the hetero aromatic bases, as said before. Some
+
-
different species (OX ) plays the role of oxidant in place of HCrO . On addition of promoters
4
(Phen, Bpy and PA) initially in the acidic reaction solution the Cr(VI) gets captured to form the
+
oxidant species OX which then oxidise the octanal to its corresponding product. (Scheme 1, 2
and 3) Interaction between Cr(VI) and those promoters has been confirmed by comparing the
UV-Vis spectra of only Cr(VI) solution (aqueous solution of K Cr O ) with the mixtures of
2
2
7
Cr(VI) and promoters. In the spectra Cr(VI) shows peaks near 440 nm and 350 nm whereas, in
most cases of [Cr(VI) + promoters] absence of the peak at 350 nm is very common. (Fig. 7) In
addition, NMR study and fluorometric measurements also proved previously the same
interference of complex formation between Cr(VI) and promoters. [9]
-4
Fig. 7 Absorption spectrum of [Cr(VI)] =2.5×10 (M), [H SO ] = 0.3(M), [PA] = 0.025(M)
T
2
4
T
[
Bpy] = 0.025(M); [Phen] = 0.025(M), Temperature = 25°C.
T T

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The promoted micellar reaction rates are controlled firstly, by the coulombic
+
interaction between oxidant species (OX ) with surface charge of the micelles and secondly, by
the pi electron densities of the promoters. The oxidant species of both Phen and Bpy being di-
positive can approach the anionic micellar surface of SDS more easily than the mono-positive
oxidant species of PA. (Fig. S12a) So, k (Bpy) is expected to be quite equal to k (Phen) and
obs
obs
kobs(PA) is expected to be the lowest. SDS has no pi electrons so there is no question of pi-pi
interaction with promoters but the coulombic repulsive interaction of pi electrons of those
promoters with the anionic surface of the SDS micelle can never be ignored. PA in its oxidant
+
species (OX ) having least pi electron density among the other oxidant species undergoes less
repulsive interaction to approach the substrate (octanal) in the core of the micelle whereas, Phen
has the highest pi electron density and Bpy has the second highest pi electron density. So, on the
basis of this pi electron repulsion with anionic micellar surface of SDS the rate order should be
k (PA)˃ k (Bpy)˃ k (Phen). But, due to the concomitant effect of present and previous
obs
obs
obs
reason the reaction rate order becomes k (Bpy)˃ k (PA)˃ k (Phen) which fully supports our
obs
obs
obs
results. (Fig. 8) A very relevant and essential point is to be noticed here that Bpy is far better a
promoter only in its 10 times lower concentration and the reaction just gets completed in a
twinkling of an eye at slightly more concentration. (Table 2, 1S) In case of neutral TX-100
+
micellar surface there is no chance of coulombic interaction with oxidant species (OX ) as
before (like SDS) (Fig. S12b) but pi-pi interaction between the promoters in oxidant species and
aromatic ring in neutral surfactant molecule controls the reaction rates making the rate order
k (Phen) ˃ k (Bpy)˃ k (PA). In case of CPC, due to bi-positive charge the oxidant species
obs
obs
obs
+
(
OX ) in Bpy experiences more coulombic repulsion than that of PA with mono-positive charge.
So, PA is expected a better promoter than Bpy and the oxidant species of Phen also having di-
positive charge and being bulkier than other two promoters experiences more steric hindrance
making the rate order k (PA) ˃ k (Bpy) ˃ k (Phen). (Fig. S12c, 8)
obs
obs
obs

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Fig. 8 Representative bar diagram of kobs with maximum concentration of surfactants and
−3
−4
promoters. [Octanal] = 3.75 × 10 (M), [Cr(VI)] = 2.5 × 10 (M), [H SO ] = 0.3(M),
T
T
2
4
Temperature = 25°C.

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Scheme 2. Mechanistic pathway of oxidation of octanal by Cr(VI) in presence of PA promoter.
3
.3. Interaction between octanal and surfactants
In the present experiment surfactants have a great role as a solubilising agent and catalyst in the
oxidation of the fatty aldehyde (octanal) in aqueous medium. There are interactions among the
surfactants and the substrate molecules, octanal. The fatty aldehyde has been predicted to be
resided at the core of micelles formed by those surfactants and in support of the said interactions
1
among the chemical species H-NMR spectra of the individual surfactants and mixture of
surfactants with octanal have been recorded (Fig. S6, S7, S8, S9, S10 and S11) and the chemical
shift values (δ) have been produced here in tabular form. (Table 3, 4 and 5)
From the table it can be very clearly seen that in each and every cases there is a
prominent up-field shift in δ (ppm) values. That means on mixing surfactants to the substrate
molecule there must arise some induced type interactions among the hydrophobic tails of

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surfactants (SDS, TX-100 and CPC) and –CHO group in addition to the long chain hydrocarbon
of octanal molecules which increase the electron densities around the protons of the hydrophobic
tails showing up-field chemical shift (δ) in every cases.
Scheme 3. Mechanistic pathway of oxidation of octanal by Cr(VI) in presence of Phen promoter.

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Scheme 4. Mechanistic pathway of oxidation of octanal by Cr(VI) in presence of Bpy promoter.
1
Table 3 Chemical shift (ppm) values of head group protons of SDS from H-NMR spectrum.
Bulk
H’s
Hα
Hβ
Hδ
SDS
3.982
3.861
1.634
1.486
1.25
0.85
SDS + Octanal
1.149
0.739

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1
Table 4 Chemical shift (ppm) values of head group protons of TX 100 from H-NMR spectrum.
Ha
Hb
Hc
1.287 7.189 6.821 4.031 3.746 3.638-3.719
3.59 3.358 Up-field
Hd
He
Hf
Hg
Hh
TX-100
0.711
1.66
TX-100 + Octanal 0.267 1.219 0.832 6.763 6.386
1
Table 5 Chemical shift (ppm) values of head group protons of CPC from H-NMR spectrum.
Hα
Hβ
Ha,a’
Hb,b’
Hc
Bulk H’s
Hδ
CPC
4.699 1.98 8.976 8.114 8.585 1.10-1.29 0.703
CPC + Octanal 4.355 1.655 8.642 7.792 8.262 0.82-0.98 0.423
3
.4. Calculation of activation parameters
‡
Study of thermodynamic properties, basically enthalpy of activation (ΔH ) and entropy of
‡
activation (ΔS ), is essential in kinetics measurement of any reaction. Here, in this experiment
‡
‡
also ΔH and ΔS values have been calculated by (Table 6) determining k (k in Eq. 1) of
obs
requisite reactions at three different temperatures (25°C, 35°C and 40°C) (Table 1, 2S) and using
Eyring equation (Eq. 1). Actually, -ln(kh/k T) vs 1/T graphs have been plotted for each of the
B
‡
reactions which give straight line and from the slope we get ΔH and from the intercept we get
‡
‡
ΔS .(Fig. 9) In this experiment, all the promoted micellar reactions show greater ΔH than that
of the non-promoted micellar reactions which signifies the lower energy of the transition states
in case of promoted reactions and therefore fast reactions. The combination of SDS and Bpy
‡
gives the lowest ΔH value because it has the highest reaction rate. The combinations showing

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‡
greater - ΔS values correspond to the more ordered transition states in comparison to the ground
states of the reaction.
‡
‡
ln(kh/k T) = - ΔH /RT + ΔS /R
….(Eq.
B
1
)
(
Where, k = observed rate constant, h = Planck’s constant, k = Boltzmann constant, T =
B
absolute temperature, R = universal gas constant)
Table 6 Representation of activation parameters.
3
‡
-1
‡
-1
-1
Micellar catalyst (M)
Promoters (M) × 10
ΔH (kJmol ) -ΔS (JK mol )
SDS
TX-100
CPC
0.03
0.02
0.02
None
None
None
50.38
59.00
116.79
89.21
32.76
38.59
163.68
144.53
Phen
PA
2.5
2.5
26.93
22.38
31.14
40.58
35.92
183.49
191.38
171.43
143.74
156.80
SDS
0.03
0.02
Bpy
Phen
PA
0.25
2.5
2.5
TX-100
Bpy
2.5
Phen
PA
2.5
2.5
2.5
30.46
19.29
23.51
169.34
201.86
190.56
CPC
0.02
Bpy

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34.4
34.2
34.0
33.8
33.6
33.4
R = 0.9987
0.00320
0.00325
0.00330
0.00335
1
/T
−
3
Fig. 9 Representative plot for evaluation of activation parameters. [Octanal] = 3.75 × 10 (M),
T
−
4
[
Cr(VI)] = 2.5 × 10 (M), [H SO ] = 0.3(M), [SDS] = 0.03(M), temperature 25°C, 35°C,
T
2
4
T
4
0°C.
4
. Conclusion
After the elongated discussion on the kinetics of our said reaction, there are no other alternatives
other than saying that it is really a novel achievement of oxidising a fatty aldehyde with Cr(VI)
in an aqueous medium with the help of amphiphilic property of surfactants and at the very
moment catalysing the same reaction with the same amphiphilic species. The surface phenomena
of those amphiphilic surfactants have truly a lot of contribution in attaining this novelty in this
reaction. Many aromatic and aliphatic aldehydes were oxidised previously using higher valent
metal oxidants in micellar medium but, no one is as exemplary as this one. [16, 32-35] It is also
very factual that Bpy, among the used promoters, has shown the best performance in
combination with SDS, the anionic surfactant, in our attempt. So, it will help researchers go
ahead in the field of micellar catalysis and they can show the dare to untie the bunch of research
works on fatty molecules in a hygienic way.
Acknowledgement
We are very thankful to Dr. Kalachand Mahali, Dept of Chemistry, The University of Kalyani,
India and Ms. Kuheli Dome, Dept of Chemistry, IIT(ISM), Dhanbad, India for instrumental
assistance.

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Author Statement
The highlights of the manuscript (MOLLIQ_2020_1096_R1) have been amended with necessary corrections
mentioning core findings of this experiment briefly according to the comment of the respected editor and
reviewer. It is seemed that this manuscript needs no further revision to be published in The Journal of
Molecular Liquids.

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Declaration of interest
No financial or others competing interests are known to the best the knowledge of the authors in
performing this experimental work.

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Graphical Abstract