Micellar catalyzed hydroxylation of 1,2,3-trichloro-4,6-dinitrobenzene: Role of cationic head group–π interaction

Article abstract of DOI:10.1016/j.molliq.2019.112429

The micellar catalysis of the hydroxylation of 1,2,3-trichloro-4,6-dinitrobenzene (DNTCB) with aqueous NaOH, to synthesize 2-chloro-4,6-dinitroresorcinol (CDNR), was studied using cationic, anionic and non-ionic surfactants at different concentrations to

Full text of DOI:10.1016/j.molliq.2019.112429

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Micellar
catalyzed
hydroxylation
of
1,2,3-trichloro-4,6-dinitrobenzene: Role of cationic head group–π
interaction
Ankita Ravani, Atindra Shukla, Nandhibatla Vishwanada Sastry,
Dinesh Ochhavlal Shah, Manish Kumar Mishra
PII:
S0167-7322(19)34902-5
DOI:
https://doi.org/10.1016/j.molliq.2019.112429
MOLLIQ 112429
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
2 September 2019
20 December 2019
30 December 2019
Please cite this article as: A. Ravani, A. Shukla, N.V. Sastry, et al., Micellar catalyzed
hydroxylation of 1,2,3-trichloro-4,6-dinitrobenzene: Role of cationic head group–π
interaction,
Journal
of
Molecular
Liquids(2020),
https://doi.org/10.1016/
j.molliq.2019.112429
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Micellar catalyzed hydroxylation of 1,2,3-trichloro-4,6-dinitrobenzene:
Role of cationic head group– interaction
Ankita Ravani,a Atindra Shukla,a Nandhibatla Vishwanada Sastry,b Dinesh Ochhavlal Shah,c
Manish Kumar Mishra*,b
aShah-Schulman Center for Surface Science and Nanotechnology,
Dharmsinh Desai University, College Road, Nadiad – 387 001, Gujarat, India.
bDepartment of Chemistry, Sardar Patel University,
Vallabh Vidyanagar- 388 120, Gujarat, India.
cDepartment of Chemical Engineering and Department of Anesthesiology,
University of Florida, Gainesville, FL, USA 32608.
E-mail: manishorgch@gmail.com; Tel: +91 2692 226856
Abstract
The micellar catalysis of the hydroxylation of 1,2,3-trichloro-4,6-dinitrobenzene (DNTCB)
with aqueous NaOH, to synthesize 2-chloro-4,6-dinitroresorcinol (CDNR), was studied using
cationic, anionic and non-ionic surfactants at different concentrations to promote the reaction
kinetics. The micellar catalysis using cationic surfactants at optimum concentrations was
effective giving highest conversion of DNTCB (92–100%), which was ascribed to high
solubilization capacity of the micellar solutions for DNTCB, surface positive charges of
micelles increasing OH– ions concentration near micellar surface, and the cation- interaction
of surfactant molecules (through cationic head group) with DNTCB molecules. The cation-
interaction plays important role in solubilization process, substrate activation and catalysis of
the reaction. The higher surfactant concentrations retard the reaction owing to strong
solubilization of DNTCB in micelles and reduced OH– ions concentration around the
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micelles. The cationic micellar solutions at optimum surfactant concentrations can be
effective catalytic system for the aromatic nucleophilic substitution reactions.
Keywords: Cationic surfactants; micellar catalysis; cation– interaction; aromatic
nucleophilic substitution; 1,2,3-trichloro-4,6-dinitrobenzene; 2-chloro-4,6-dinitroresorcinol.
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1. Introduction
The 2-chloro-4,6-dinitroresorcinol (CDNR) is an important chemical for the synthesis of 4,6-
diaminoresorcinol (DAR), which is used as monomer to polymerize with bisacids, bisacid
halides, bisesters or bisnitriles to synthesize various poly-p-phenylene-benzobisoxazoles
(PBOs; Scheme 1a) [1]. The PBOs are fibers with high tensile strength and thermal stability,
and are suitable for military, aerospace and high performance material applications [2]. The
DAR monomer is synthesized from CDNR by reduction of its both nitro groups followed by
dechlorination using hydrogen and Pd/C catalyst [3]. Alternatively, DAR has also been
synthesized by reduction of 4,6-dinitroresorcinol obtained from nitration of resorcinol
diacetate [4] and by hydroxylation of 2,4-dinitrochlorobenzene followed by reduction [5].
However, these processes suffer from low yield of DAR due to the formation of byproducts.
The 1,2,3-trichlorobenzene (TCB) is produced in significant amount as by-product in the
benzene chlorination process to synthesize chlorobenzene and dichlorobenzene. TCB has
been considered to be an organic waste creating serious environmental issue for the industries
[6]. It is mainly used as solvent/ dye carrier by textile industries, however, this application is
hazardous to the environment because of its toxic nature [7]. The use of TCB as starting
material for the synthesis of valuable chemicals could be the appropriate way for its
utilization and to solve its serious concerns. The nitration of TCB with nitric acid and sulfuric
acid results to high yield of 4,6-dinitro-l,2,3-trichlorobenzene (DNTCB) [8], which is a
suitable precursor for the synthesis of CDNR by hydroxylation reaction [9] (Scheme 1b). The
synthesis of DAR from CDNR, which can be produced by hydroxylation of DNTCB obtained
from TCB (as shown in Scheme 1), can provide a cost effective methodology for the
synthesis of PBOs. Thus, the use of TCB as starting raw material for the synthesis of CDNR
(from DNTCB) will be promising to utilize an industrial organic waste and to reduce the
production cost of PBOs.
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Cl
HO
OH
HO
OH
O
N
O
N
Polymerization
Reduction
(a)
(b)
n
H2N
NH2
O2N
NO2
with bisacids/
bisacid halides/
CDNR
PBO
DAR
bisesters/ bisnitriles
Cl
Cl
Cl
HO
OH
NO2
Cl
Cl
Cl
Cl
NO2
Hydroxylation
Nitration
O2N
O2N
with NaOH/ KOH
CDNR
TCB
DNTCB
Scheme 1. (a) Synthesis of PBO fibers from DAR, which can be obtained from CDNR, and (b) synthesis of
CDNR from DNTCB, obtained from nitration of TCB.
The synthesis of CDNR from DNTCB by treating with alkali (NaOH or KOH) in an alcohol
(methanol, ethanol, n-propanol, etc.) has been reported [9]. However, the use of large
amounts of expensive and flammable organic solvents (alcohols) with excess amount of
alkali is not practicable at industrial scale. Therefore, there is need of an environmentally
benign process, a Green Chemistry approach for the conversion of DNTCB into CDNR using
optimum amount/ concentration of alkali and avoiding the use of organic solvents.
With an objective to have organic solvent free process for the synthesis of CDNR from
DNTCB and to utilize TCB as raw chemical, we attempted hydroxylation of DNTCB with
NaOH using water as solvent. The reaction of DNTCB and aqueous NaOH (under biphasic
condition) was sluggish due to poor solubility of DNTCB (non-polar nature) in water. The
organic-aqueous biphasic reactions have received much interest owing to the use of water as
inexpensive and environmentally benign solvent [10], however, the immiscibility of non-
polar organic compounds in water results to slow rate of reaction [11]. In recent years,
micellar catalysis using aqueous solutions of cationic, anionic and non-ionic surfactants has
received much attention to promote organic reactions (conversion rate as well as product
selectivity) in water [12]. The micelles (aggregates of surfactant molecules) facilitate the
reactions by dissolving non-polar reactant(s) in the core and thus increasing their solubility in
aqueous phase [13]. The ionic surface of micelles concentrates water soluble ionic species
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(reactant/ catalyst; e.g., OH– or H+) near the micellar surface [13]. Thus, huge interface
between aqueous and organic phase is created for interaction of water soluble and water
insoluble reaction components. Furthermore, the disintegration and reformation of micelles
due to extremely small life time of micelles (in msec) significantly contribute in generation of
huge interfacial area during the reaction time interval. In addition, the favorable interactions
between surfactant and substrate/ reactant molecules in micelles have also been observed to
facilitate the reactions by bringing substrate molecules in activated form for the reaction [14].
The micellar catalysis enables convenient isolation of the product and the separation of water
soluble catalysts from the reaction mass for reuse [15]. Therefore, we were interested in the
use of aqueous solution of surfactants (micellar solutions) for hydroxylation of DNTCB with
NaOH to develop an efficient, cost effective and organic solvent free process to synthesize
CDNR utilizing TCB, an industrial organic waste. The reaction in water was studied under
different conditions such as biphasic condition, phase transfer catalysis using a phase transfer
agent, and micellar catalysis using different types of surfactants (cationic/ anionic/ non-ionic)
at varied concentrations. The micellar catalysis using aqueous solution of cationic surfactants
at optimum concentrations was found to be effective to promote the reaction kinetics. This
was ascribed to cation- interaction of surfactant molecules (through cationic head group)
with DNTCB molecules, which played important role in solubilization process, substrate
activation and catalysis of the reaction. To the best of our knowledge, there is no report on
micellar catalysis of aromatic nucleophilic substitution reactions facilitated by cation-
interaction.
2. Experimental
2.1. Chemicals
The 1,2,3-trichloro-4,6-dinitrobenzene (DNTCB; 99%) was synthesized by nitration of 1,2,3-
trichlorobenzene (TCB; 98%, supplied by Kutch Chemicals Industries, Baroda, Gujarat,
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India) with nitric acid using sulfuric acid as catalyst [8]. Sodium hydroxide (NaOH; 98%),
concentrated hydrochloric acid (HCl; 35%), sodium chloride (NaCl; 98%) and ethyl acetate
(99%) were procured from Merck, India. Cetyltrimethylammonium bromide (CTAB; 98%),
cetylpyridinium chloride (CPC; 98%), sodium dodecyl sulfate (SDS; 98%), Triton X-100
(98%), methanol (98%) and toluene (99%) were from S. D. Fine Chemicals, India.
Cetylpyridinium bromide (CPB; 98%) was from Sigma-Aldrich. The phase transfer agent,
benzyltriethylammonium chloride (BTEA), was provided by Kutch Chemical Industries
Limited, Baroda, India. All the chemicals were used without any further purification. All the
aqueous and surfactant micellar solutions were prepared by using milli-pore deionized water.
2.2.General procedures for hydroxylation of DNTCB with NaOH under different
conditions
The hydroxylation of DNTCB with NaOH was carried out under different conditions
(Scheme 2) viz. under solvent free (neat) and biphasic conditions (i.e., reactions in pure
water), phase transfer catalysis (PTC; reactions in aqueous solution of BTEA), and micellar
catalysis (reactions in aqueous solution of CTAB, CPC, CPB, SDS and TX-100). In a
reaction tube of the reaction station (12 Place Heated Carousel Reaction Station, RR99030,
Radleys Discovery Technologies, UK), DNTCB (7.3 mmol) was melted at 90°C and then the
temperature was brought down to the reaction temperature (85°C; ±1°C). The NaOH flakes
(21.9 mmol) were added in the molten DNTCB and heated under stirring (1200 rpm) at 85°C
for the required period of time to carry out the reaction under neat condition. The biphasic
reactions were carried out by adding the aqueous solution of NaOH (21.9 mmol; dissolved in
5 mL water) in the molten DNTCB and stirring (1200 rpm) at 85°C for the required period of
time. In addition, the biphasic reaction was also carried out using toluene (2 mL) as solvent to
dissolve DNTCB. For PTC and micellar catalysis of the reaction, the aqueous solutions of
NaOH–BTEA (21.9 mmol NaOH dissolved in 5 mL aqueous solution of 1 mol% BTEA) and
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NaOH–surfactant (21.9 mmol NaOH dissolved in 5 mL surfactant solution of required
concentration), respectively, were added in the molten DNTCB and the reaction mixture was
kept under stirring (1200 rpm) at 85°C for required period of time. After the completion of
reaction, the reaction mixture was allowed to cool and to get phase separated. The aqueous
phase was collected using separating funnel. The reaction product remains in aqueous phase
and therefore, the aqueous phase was analyzed by using UV spectrophotometer (Agilent,
Carry 5000 spectrometer at room temperature) to quantify the yield of the product. In
addition, the organic phase extracted with ethyl acetate (12 mL), which contains un-reacted
DNTCB, was also analyzed by using UV spectrophotometer to quantify the un-reacted
DNTCB (i.e., conversion of DNTCB). To study the progress of biphasic/ PTC/ micellar
reactions (Figure 1 and Figure 2f), the reactions were carried out in different reaction tubes
(8 tubes) for 0.5 h to 12 h, viz. 0.5, 1, 1.5, 2, 3, 6, 9 and 12 h at 85°C. The reaction tubes were
removed from the reaction station after different time period. The aqueous and organic phases
of reaction mixtures were separated and analyzed by UV-vis spectrophotometer to measure
the conversion of DNTCB. All the conversion values are within ±2% precision. The details of
reaction conditions are also mentioned in footnote of tables and figures.
Cl
Cl
NaOH (21.9 mmol),
85oC
HO
OH
Cl
Cl
Neat reaction/
O2N
NO2
O2N
NO2
Biphasic reaction/
Phase transfer catalysis/
Micellar catalysis
DNTCB
CDNR
(7.3 mmol)
Scheme 2. Reaction of DNTCB with NaOH to synthesize CDNR under different conditions.
For the isolation of product, the aqueous phase was neutralized with HCl (8 mL) and
extracted with ethyl acetate (12 mL). The solvent removal by evaporation under vacuum
resulted to solid product. The DNTCB and the product (CDNR) were characterized (ESI;
13
Figure S1 & S2) by 1H and C NMR spectra recorded on a FT-NMR spectrometer (Bruker
UltraShield 400 MHz) in CDCl3/ DMSO-d6 solutions, FT-IR spectra recorded on a FT-IR
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spectrophotometer (IRPrestige-21, Shimadzu), and LC-MS analysis using a Micromass Q-
TOF microTM. The purity of product was analyzed by High Performance Liquid
Chromatograph (HPLC; Dionex Ultimate 3000) with C18 column (Acclaim 120, 5 m,
120Å, 4.6 mm × 250 mm) and using methanol-water (70:30) as mobile phase, flow rate of 1
mL/ min and 278 nm wave length of photodiode array detector at constant column
temperature of 298 K.
2.3. Characterization of micellar solutions
1
The H NMR analysis of DNTCB in micellar solutions and in pure D2O was carried out by
using a Bruker UltraShield 400 MHz. To prepare the DNTCB solutions in D2O or in micellar
media, the molten DNTCB (2.0 g) was stirred with 5 mL of D2O or surfactant solution
prepared in D2O at 85°C for 2 h. The solution was allowed to cool and to get phase separated
to obtain the aqueous phase for the 1H NMR analysis. The number of acquisitions was 32 for
1
each sample. The H chemical shifts are reported in  units (ppm) relative to that of
tetramethylsilane (TMS) as external standard. The UV–vis spectra of DNTCB in water and in
micellar solutions (of CTAB and CPC) were recorded on an Agilent, Carry 5000
spectrometer at room temperature using DNTCB solutions in water or in micellar solution.
For this, the solutions of DNTCB in water and in micellar solutions were prepared by stirring
the molten DNTCB (0.5 g) in 5 mL of water and surfactant solutions, respectively, at 85°C
for 2 h. The solutions were allowed to cool and get phase separated, and the aqueous phase
was analyzed by UV–vis spectrometer. It is to be noted the micellar solutions (5 mL) of
CTAB and CPC treated with 2.0 g DNTCB, which is the condition used for reactions,
resulted to very high dissolution of DNTCB, especially in micellar solutions, giving very
high absorption in UV spectra. Therefore, to keep the absorption below 2.0 in the spectra,
UV–vis analysis was performed by dissolving 0.5 g of DNTCB in micellar solutions (5 mL).
The critical micellar concentrations (CMCs) of surfactants in pure water and in presence of
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NaOH (21.9 mmol dissolved in 5 mL water) were obtained by conductivity method (ESI;
Figure S3 & S4). The DNTCB-surfactant (CTAB and CPC at 0.5 mM) emulsion solutions
were analyzed by a transmitted light optical microscope (Bristol, AS-4, 1687) under 10X
objective lance magnification. A drop of solution was placed on a glass slide without a cover
slip. The images were captured with a digital microscope imager (Celestron, Model No.:
44421) and processed by DDU 3D software. The surface charge on micelles in surfactant
solutions was obtained by measuring zeta potential at 25°C using Zetasizer (Malvern). The
dynamic surface tension (DST) of solutions were measured by using a bubble tensiometer
(Biolin, model # BPA-800P) at 25°C using maximum bubble pressure method as a function
of bubble lifetime.
3. Results and discussion
3.1. Hydroxylation of DNTCB with NaOH in different reaction media
The neat reaction of DNTCB and NaOH at 85°C (Scheme 2) did not give product (CDNR)
even after 24 h of reaction, which may be due to insolubility of NaOH in molten DNTCB. To
show the reactivity, NaOH may require the presence of water to produce OH– ions
(nucleophile) for the reaction. The biphasic reaction of DNTCB with aqueous NaOH solution
was very slow giving ~60% conversion of DNTCB after 12 h (Figure 1a), which can be due
to poor solubility of DNTCB in aqueous phase (3 mM; ESI, Table S1). Interestingly, the use
of toluene, which was used as water immiscible organic solvent and to dissolve DNTCB, in a
biphasic reaction facilitated the reaction giving faster conversion of DNTCB than toluene free
biphasic reaction (Figure 1b). This enhancement in reaction rate was found to be attributed
to increased solubility of DNTCB in water (30 mM; ESI, Table S1) in the presence of
toluene. The DNTCB-toluene solution, being less viscous as compared to molten DNTCB,
was easy to stir with aqueous NaOH solution enhancing the partitioning (solubility) of
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DNTCB molecules in aqueous phase. However, in view of the hazardous effects of toluene,
toluene cannot be considered to be an appropriate solvent for this reaction.
100
80
60
40
20
0
100
80
60
40
20
0
(b)
(a)
Biphasic reactn in toluene
PTC reactn in toluene
CTAB micellar reactn
Biphasic reactn
PTC reactn
CTAB micellar reactn
0
2
4
6
8
10
12
0
2
4
6
8
10
12
Reaction time (h)
Reaction time (h)
Figure 1. Progress of biphasic, PTC and CTAB micellar reactions of DNTCB and NaOH (a) in absence and (b)
in presence of toluene as organic solvent [biphasic reactn (-■-): 7.3 mmol DNTCB, 21.9 mmol NaOH, 85⁰C, 5
mL water; PTC reactn (-●-): 7.3 mmol DNTCB, 21.9 mmol NaOH, 85⁰C, 5 mL aqueous solution of 1 mol%
BTEA; CTAB micellar reactn (--): 7.3 mmol DNTCB, 21.9 mmol NaOH, 85⁰C, 5 mL aqueous solution of
CTAB at 15 mM concentration; 2 mL toluene was used in reactions carried out in the presence of toluene].
The phase-transfer catalyzed (PTC) reaction using BTEA, even without using toluene as
solvent, was observed to be much faster than biphasic reactions (Figure 1a). The phase-
transfer catalyst, being an organic molecule with a quaternary nitrogen center, has ability to
get dissolved in both aqueous and organic phase. The BTEA molecules can enhance reaction
rate by increasing the solubility of DNTCB in aqueous phase (10 mM; ESI, Table S1) by
binding the DNTCB molecules (by hydrophobic interaction) and bringing them into aqueous
phase. In addition, the reaction may also be facilitated by increased solubility of OH– ions in
oil phase (i.e., in the molten DNTCB) by BTEA’s cations, as they can bind OH– ions by ionic
interaction to bring them into the oil phase [16].
From above results, we learned that the conversion of DNTCB into CDNR can be improved
by increasing the solubility of DNTCB in aqueous phase. The attractive features of
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surfactants, particularly amphiphilic nature and ability of their micelles to solubilize the non-
polar/ water insoluble compounds in water, motivated us to carry out this reaction in an
aqueous surfactant solution. As the reaction involves the participation of OH− ions, the
aqueous solution of a cationic surfactant, having a quaternary ammonium head group, can be
used to have positively charged micelles [15]. The hydrophobicity of cationic micelles can
serve the purpose of solubilization of DNTCB in the micelles and the positively charged
micellar surface can concentrate OH− ions around the micelles. Therefore, the CTAB (a
cationic surfactant) micellar solution at 15 mM was employed in the reaction and was found
to be much effective giving fastest conversion of DNTCB (Figure 1a). Unlike toluene
assisted biphasic reaction, the use of toluene in PTC or in micellar reactions was not much
helpful (Figure 1b). It is to be noted that in PTC reaction, 1 mol% of BTEA (which
corresponds to 10 M) was used, whereas in the micellar reaction, very dilute solution of
CTAB (15 mM) was taken. This result clearly demonstrates that the CTAB, a quaternary
ammonium head group containing surface active molecule, does not work like a phase
transfer agent; it promotes the reaction with the help of micelles formed in the aqueous
medium. In the PTC reaction, BTEA molecules bind OH– ions and DNTCB molecules to
bring them into organic and aqueous phase, respectively, for the reaction, and therefore huge
quantity of BTEA, as compared to CTAB, is needed to make the reaction faster. This is also
evident from the result showing increasing conversion of DNTCB (from 40% to 88% in 6 h)
on increasing the amount of BTEA (from 0.25 mol% to 1 mol%; ESI, Figure S5). In the
micellar catalysis, the small amount of CTAB (15 mM) promoted the reaction by producing
numerous micelles in the aqueous medium. The micellar effect on the reaction has been
discussed in detail in the following sections.
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3.2. Micellar catalyzed hydroxylation of DNTCB with NaOH in micellar solutions of
different surfactants: Effect of surfactant’s nature and concentration
The results (Figure 1) clearly reveal that the hydroxylation of DNTCB with aqueous NaOH
can be promoted by micellar catalysis using an aqueous solution of surfactant at nominal
concentration. In addition to CTAB, different types of surfactants namely CPC and CPB
(cationic surfactants with different nature of head group), SDS (anionic surfactant) and TX-
100 (non-ionic surfactant), were also studied at similar concentration (15 mM) and reaction
condition. The results (Table 1) suggest the highest activity of CTAB micellar solution
giving highest conversion of DNTCB.
Table 1. Conversion (%) of DNTCB in different surfactant micellar solutions.
Sr. No. Micellar solution Conversion (%) of DNTCB
1
2
3
4
5
15 mM CTAB
15 mM CPC
15 mM CPB
15 mM SDS
100
63
78
20
30
15 mM TX-100
Reaction condition: 7.3 mmol DNTCB; 21.9 mmol NaOH; 85⁰C; 12 h; 5 mL aqueous solution of surfactant.
As these surfactants (CTAB, CPC, CPB, SDS and TX-100) have different alkyl chain, head
group charge and size, and CMCs (0.97, 0.99, 0.85, 7.2 and 0.25 mM, respectively; ESI;
Figure S3), therefore, 15 mM micellar solutions of these surfactants will have micelles with
different characteristics (hydrophobicity, surface charge, intermolecular distance, etc.). To
find out an appropriate surfactant and concentration providing optimum micro-environment
for faster conversion of DNTCB, the reactions were carried out in the micellar solutions of
these surfactants at different concentrations ranging from 0.125 mM to 150 mM and under
similar reaction condition. The CMCs of CTAB, CPC, SDS and TX-100 in aqueous NaOH
solution were found to be slightly decreased (0.89, 0.85, 0.80, 6.9 and 0.21 mM, respectively;
ESI; Figure S4).
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The results (Figure 2a-e) indicate that the nature of surfactant (cationic/ anionic/ non-ionic,
and head group size) and the concentration significantly influence the reaction (i.e., the
conversion of DNTCB). The increase in surfactant concentrations initially increases the
conversion up to certain concentration (optimum concentration), and further increase in
surfactant concentration (above an optimum concentration) decreases the conversion. The
cationic surfactants (CTAB, CPC and CPB) provide most effective micellar media to
promote the reaction giving highest conversion (92-100%) at their optimum concentrations
(15 mM, 0.5 mM and 1 mM, respectively). It is to be noted that as compared to CTAB, CPC
and CPB provide equally efficient micellar medium at very less concentrations (0.5 mM and
1 mM, respectively) to give highest conversion of DNTCB. The nature of counter anion (Cl–/
Br–) present in cationic surfactants (CPC and CPB) does not show remarkable effect on
activity of their solutions for DNTCB conversion. The TX-100 micellar solution also showed
similar activity (92% conversion of DNTCB), however, a higher concentration of TX-100
(100 mM) was required. The SDS micellar solution was least in activity giving maximum
~37% conversion of DNTCB at 5 mM concentration. The Figure 2f shows the progress of
reaction with time in biphasic and in different surfactants’ micellar solutions at their optimum
concentrations. The four surfactant solutions at their optimum concentrations, viz. 15 mM
CTAB, 0.5 mM CPC, 1 mM CPB and 100 mM TX-100, exhibited almost similar activity
giving complete conversion of DNTCB. However, the cationic surfactant solutions i.e., 15
mM CTAB, 0.5 mM CPC and 1 mM CPB, are more appropriate for micellar catalysis of the
reaction giving faster conversion of DNTCB at low concentrations. In several studies, CPC
has been studied for comparison with CTAB to observe the effect of cationic head group size
on micellar catalysis [17], therefore, CTAB and CPC micellar systems were investigated in
detail. To explain the molecular mechanism for higher catalytic activity of cationic micellar
solutions at their optimum surfactant concentrations, the characteristics of the DNTCB
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solubilized micelles viz. molecular interactions, molecular packing, micellar surface charge,
etc., and the solubilization behavior of micellar solutions were studied.
100
80
60
40
20
0
100
80
60
40
20
0
(b)
(a)
0.5 mM
15 mM
0
5
10 20 40 60 80 100 120 140 160
CTAB concentration (mM)
0
1
2
20 40 60 80 100 120 140 160
CPC concentration (mM)
100
80
60
40
20
0
100
80
(c)
1 mM
(d)
60
5 mM
40
20
0
2
4
20 40 60 80 100 120 140 160
SDS concentration (mM)
0 2 4 6 8 10 20 40 60 80 100120140160
CPB concentration (mM)
100
80
60
40
20
0
(e)
100
80
60
40
20
0
100 mM
(f)
0
2
4
20 40 60 80 100 120 140 160
0
2
4
6
8
10
12
TX-100 concentration (mM)
Time (h)
Figure 2. (a-e) Effect of surfactants’ concentration on DNTCB conversion [7.3 mmol DNTCB, 21.9 mmol
NaOH, 85°C, 6 h, 5 mL aqueous solution of surfactant], and (f) Kinetics of biphasic and micellar reactions
[biphasic reactn (-■-): 7.3 mmol DNTCB, 21.9 mmol NaOH, 85°C, 6 h, 5 mL water; CTAB micellar reactn (--
): 7.3 mmol DNTCB, 21.9 mmol NaOH, 85°C, 6 h, 5 mL aqueous solution of CTAB at 15 mM; CPC micellar
reactn (--): 7.3 mmol DNTCB, 21.9 mmol NaOH, 85°C, 6 h, 5 mL aqueous solution of CPC at 0.5 mM; CPB
micellar reactn (-●-): 7.3 mmol DNTCB, 21.9 mmol NaOH, 85°C, 6 h, 5 mL aqueous solution of CPB at 1 mM;
SDS micellar reactn (-×-): 7.3 mmol DNTCB, 21.9 mmol NaOH, 85°C, 6 h, 5 mL aqueous solution of SDS at 5
mM; TX-100 micellar reactn (--): 7.3 mmol DNTCB, 21.9 mmol NaOH, 85°C, 6 h, 5 mL aqueous solution of
TX-100 at 100 mM].
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3.2.1. Molecular interactions and packing in the DNTCB solubilized micellar system
1
The H NMR studies have been used to investigate the location, orientation and molecular
interactions of organic molecules solubilized in the micelles by monitoring the changes in the
1
chemical shifts of the surfactant and organic molecules [18]. The H NMR analysis of
DNTCB in micellar solutions of CTAB, CPC, SDS and TX-100 prepared in D2O at different
concentrations were carried out to investigate molecular interactions and packing in DNTCB
solubilized micelles. There is an aromatic proton in DNTCB, which exhibits a chemical shift
at 8.58 ppm in D2O (without surfactant). The chemical shift of DNTCB (aromatic proton)
1
1
was monitored in H NMR studies of DNTCB-micellar solutions. The H NMR spectra of
DNTCB in the CTAB micellar solutions at different concentrations (Figure 3a) can be seen
to exhibit two signals in the aromatic region, which are very distinct at 15 mM and above
concentrations, indicating the presence of DNTCB in two different forms in CTAB micelles.
The signal “A” represents DNTCB molecules (structure I; Scheme 3), which are involved in
cation- interaction with cationic head group of CTAB molecules in micelles. The signal “B”
was assigned to DNTCB molecules (structure II; Scheme 3), which are hydrophobically
solubilized in micelles, and are not involved in cation- interaction. The cation -interaction
of DNTCB with cationic head group of CTAB will reduce the electron density over aromatic
nucleus of DNTCB showing downfield shifting of its aromatic proton “A” as compared to
aromatic proton of DNTCB (8.58 ppm) in pure D2O. Ikeda and Sbinka [19] have shown two
1
sets of signals in H NMR spectra of 1,3-Alternate Calix[4]arenes-Ag+ and 1,3-Alternate
Calix[4]arenes-K+ systems representing free 1,3-alternate Calix[4]arenes molecules and 1,3-
Alternate Calix[4]arenes-metal complexes having cation- interaction.
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X
X
X
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
X
Cl
X
NO
2
NO
NO
O N
O N
2
2
2
2
O N
2
A
B
H
H
(I)
(II)
X
= Br/ Cl
Scheme 3. DNTCB molecules in cation- interaction with surfactant’s cationic head group (structure I), and
hydrophobically solubilized (structure II) in micellar phase of cationic (CTAB/ CPC) micelles.
The gradual downfield shifting of the signal “A” with CTAB concentration (Figure 3a)
represents increasing cation- interaction (strength of interaction) between solubilized
DNTCB molecules (I) and CTAB molecules in micelles. The downfield chemical shift for
the aromatic proton of II (signal “B”) in 15 mM to 150 mM CTAB solutions as compared to
the aromatic proton of DNTCB in pure D2O (at 8.58 ppm) indicates that II reside in more
1
ionic environment (than water; i.e., in Stern layer) of micelles. Thus, from H NMR study,
the solubilization of DNTCB as I and II in the Stern layer of CTAB micelles is clearly
1
evident. Similarly, the H NMR spectra of DNTCB in CPC (Figure 3b) as well as in CPB
(ESI; Figure S6) micellar solutions also exhibited “A” and “B” signals showing the cation -
interaction between DNTCB and head group and the existence of both types of DNTCB
molecules (I and II) in the Stern layer of micelles.
16

A
(a)
150 mM
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B
Py-H2
(b)
Py-H3
Py-H4
A
B
100 mM
B
150 mM
100 mM
A
B
A
75 mM
50 mM
25 mM
15 mM
10 mM
5 mM
B
B
A
B
B
50 mM
B
A
A
25 mM
10 mM
A
B
A
A
A
1 mM
B
A
0.5 mM
0 mM
1 mM
0 mM
*
*
9.0
8.8
8.6
8.4
9.0 8.8 8.6 8.4 8.2 8.0
Chemical shift (ppm)
Chemical shift (ppm)
f
Py-H2
(d)
Py-H3
H
(c)
H
Py-H4
d' c' b'
a'
H
NCH2CH2(CH2)13CH3
Cl
b
H
H
Py-H2
Py-H3
b'
a
Py-H2
c
a'
Py-H3
Py-H4
c'
e
d'
d
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
9
8
4
2
0
Chemical shift (ppm)
Chemical shift (ppm)
(e)
150 mM
(f)
100 mM
150 mM
75 mM
50 mM
25 mM
100 mM
50 mM
25 mM
10 mM
15 mM
10 mM
5 mM
1 mM
1 mM
b'
a'
d'
c'
b
f
a
e
0.5 mM
c
d
5
2.0 1.5 1.0 0.5 0.0
Chemical shift (ppm)
3
1
0
Chemical shift (ppm)
1
Figure 3. H NMR spectra of (a & b) DNTCB in CTAB and CPC micellar solutions, respectively, at different
surfactant concentrations showing chemical shift of aromatic proton, (c & d) CTAB and CPC micellar solutions,
respectively, at different concentrations in presence of DNTCB, and (e & f) 0.5 mM CTAB and 0.5 mM CPC
micellar solutions, respectively, in absence of DNTCB [*: signal for aromatic proton of DNTCB in D2O; A:
signal for aromatic proton of DNTCB molecules (I) involved in cation- interaction with cationic head group
(quaternary ammonium group) of cationic surfactants in micelles; B: signal for aromatic proton of DNTCB
molecules (II) solubilized in micelles of cationic surfactants by hydrophobic interaction; Py-H2, Py-H3 and Py-
H4: signals for aromatic protons at 2nd, 3rd and 4th carbon atoms, respectively, of pyridinium group of CPC].
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1
Figure 3c and 3d are H NMR spectra of CTAB and CPC solutions, respectively, (without
DNTCB) at 0.5 mM concentration, i.e., below their CMC and therefore, the spectra are
presenting chemical shifts of surfactant molecules existing as monomers and submicellar
aggregates. The changes in chemical shifts of protons of CTAB with increase in
concentration and in the presence of DNTCB can be clearly seen (Figure 3e). On increasing
the CTAB concentration, we noticed the increase in amount of solubilization of DNTCB in
the CTAB micellar solution, which has been discussed in the following section. Therefore,
the spectra (Figure 3e) are showing effect of surfactant as well as DNTCB concentrations on
chemical shifts of CTAB. The downfield shifting of chemical shifts for various protons
(which is much significant for “f” protons) of CTAB with increase in surfactant concentration
and DNTCB solubilization (Figure 3e) indicates the location of solubilized DNTCB near the
head groups of CTAB in micelles, i.e., in the Stern layer. The downfield shifting of various
protons of CTAB with surfactant and DNTCB concentration is indication of slight increase in
the intermolecular distance between the molecules after inclusion of new molecules of
surfactant and DNTCB in micelles. The increasing number of surfactant molecules and
solubilization of DNTCB in Stern layer of CTAB micelles may elongate micelles orienting
CTAB molecules parallel to each other showing increased intermolecular distance and
downfield shifting of protons of CTAB. This shows that CTAB micelles get elongated with
increase in concentrations and in the presence of DNTCB. In a previous work [18a], we
noticed the downfield shifting for CTAB’s protons above 75 mM (in absence of any
solubilized organic molecules), which was ascribed to increased intermolecular distance
between surfactant molecules due to transformation of spherical micelles to elongated
micelles. On the contrary, the CPC molecules get tightly packed in DNTCB solubilized
micelles on increasing CPC concentration. The slight upfield shifting of protons of CPC
including aromatic ring protons (Py-H2, Py-H3 and Py-H4; Figure 3b & 3f) with
18

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concentration and in the presence of DNTCB is clear evidence of increasing closeness of
CPC molecules in micelles.
The change in UV-vis spectra of DNTCB in CTAB and CPC micellar solutions at different
concentrations as compared to the spectra of DNTCB in water (ESI; Figure S7) also
revealed solubilization of DNTCB in micellar phase and the cation- interaction of DNTCB
molecules with surfactant’s cationic head group showing blue shift of the characteristic band
of DNTCB. The spectrum of DNTCB in water showed a weak band in the range of 300-350
nm, which was suppressed in the spectrum of DNTCB-CTAB and DNTCB-CPC micellar
solutions, and a band at lower wavelength (< 300 nm) appeared. The band < 300 nm was
ascribed to DNTCB molecules solubilized in micelles. The intensity of this band was
observed to be increasing with increase in surfactant concentration from 5 mM to 75 mM due
to the increasing concentration of DNTCB in micelles.
The 1H NMR spectra of DNTCB in SDS micellar solutions at 1 mM to 150 mM showed the
solubilization of DNTCB in the SDS micelles preferably in the Stern layer exhibiting slight
downfield shifting for aromatic proton of DNTCB as compared to that in D2O (Figure S8a).
1
The solubilization of DNTCB in TX-100 micellar solutions is also evident from H NMR
spectra (Figure S8b). At 5 mM to 50 mM concentrations of TX-100, the DNTCB molecules
were observed to be located in polar region (in shell) of micelles showing downfield shifting
of DNTCB aromatic proton’s signal. At higher concentrations of TX-100 (75 mM to 150
mM), DNTCB molecules seem to be residing slightly deeper in the shell towards the core of
1
micelles exhibiting upfield shifting of aromatic proton of DNTCB. It has been shown in H
NMR experiments that at higher level of solubilization of cinnamic acid in TX-100 micellar
solutions, the majority of cinnamic acid molecules reside at core–shell interface of TX-100
micelles [20]. The broadness of NMR peaks of various protons of TX-100 with increase in its
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concentration and in the presence of DNTCB was observed (not shown in the spectra), which
indicated the growth of micelles [20].
3.2.2. Solubilization behavior of micellar solutions for DNTCB
The solubilization capacity of four surfactant (CTAB, CPC, SDS and TX-100) micellar
solutions at different concentrations (0.125 mM to 150 mM) for DNTCB was measured to
study the solubilization behavior of micellar media for DNTCB (see ESI). The solubilization
capacity of micellar solutions of all four surfactants was found to be increasing with increase
in surfactant concentration (Figure 4a). The cationic (CTAB and CPC) surfactants’ micellar
solutions exhibited much higher solubilization capacity for DNTCB as compared to anionic
(SDS) and non-ionic (TX-100) surfactants. The increasing intensity of yellow color of
cationic micellar solutions with surfactant concentration indicates the increasing solubility of
DNTCB (Figure 4b-d). The higher solubility of DNTCB in CTAB and CPC micellar
solutions than SDS and TX-100 solutions is also evident from the intense yellow color of
DNTCB-CTAB and DNTCB-CPC solutions at 15 mM and 100 mM surfactant concentrations
(Figure 4c & 4d).
The higher solubilization capacity of CTAB and CPC cationic surfactants than SDS and TX-
100 can be ascribed to their tendency to make cation- interaction, through cationic head
group, with DNTCB molecules. The DNTCB-cationic surfactant -complexed species will
provide more hydrophobic environment to micelles for more solubilization of DNTCB. The
smaller head group of CTAB as compared to that of CPC [21] may allow inclusion of more
DNTCB molecules in the micelles showing higher solubilization capacity than CPC. In
addition, the tendency of CTAB micelles to get elongated at higher concentrations, as evident
1
from H NMR study, can also increase the solubilization of DNTCB. On the contrary, the
tightly packed CPC molecules in micelles and their bulky head group may hinder the further
inclusion/ solubilization of DNTCB on increasing the CPC concentration and therefore, there
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is not much enhancement in the solubilization of DNTCB with CPC concentration above its
CMC (Figure 4a).
(a) 1000
CTAB,
CPC,
SDS,
TX-100
800
ii
i
600
400
200
(b)
(c)
50
40
30
20
10
0
i
i
iii
iv
iv
ii
ii
iii
0.0 0.5 1.0 25 50 75 100 125 150
Surfactant concentration (mM)
(d)
(f)
(e)
Figure 4. (a) Solubility of DNTCB in different micellar solutions, (b-d) Color of DNTCB solubilized micellar
solutions [(b) 0.5 mM, (c) 15 mM, (d) 100 mM; i: CTAB, ii: CPC, iii: SDS, iv: TX-100], (e & f) optical
microscope images of DNTCB-0.5 mM CPC and DNTCB-0.5 mM CTAB emulsion solutions, respectively.
It is to be noted that there is significant solubility of DNTCB in CTAB and CPC solutions
even at below CMC (i.e., < 1.0 mM). For instance, at 0.5 mM concentration, the solubility of
DNTCB in CTAB and CPC solutions is 96 mM and 166 mM, respectively (Figure 4a). In the
solutions below CMC (also known as premicellar concentration; pre-CMC), very likely there
are monomers and submicellar aggregates. The significant solubilization of DNTCB by both
cationic surfactants at below CMC may be due to free surfactant monomers and the surfactant
molecules released by disintegration of premicellar aggregates forming emulsified droplets of
DNTCB. The presence of emulsion droplets of DNTCB in DNTCB-0.5 mM CTAB and
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DNTCB-0.5 mM CPC solutions is evident from the turbidity of both the solutions (Figure
4b) and from their optical microscope images (Figure 4e & 4f). However, the optical
micrograph of DNTCB-0.5 mM CPC solution shows the presence of significant amount of
smaller droplets along with few large size droplets. The reason for the formation of smaller
emulsion droplets of DNTCB in 0.5 mM CPC was investigated (ESI; Figure S9) and was
found to be attributed to the DNTCB-CPC -complex species formed in the solution by
interaction (cationic head group- interaction) of CPC and DNTCB molecules. The aromatic
nature of pyridinium head group of CPC may provide better interaction (cation-) with
DNTCB molecules than CTAB molecules forming more number of DNTCB-CPC -complex
species. The DNTCB-CPC -complex species, being more hydrophobic and surface active
than CPC monomers, will efficiently emulsify DNTCB producing more number of smaller
emulsion droplets and also solubilizing more amount of DNTCB than CTAB.
3.2.3. Molecular mechanism for efficient micellar catalysis in cationic surfactants’
solutions
The higher efficiency of cationic micellar solutions (of CTAB and CPC at their optimum
concentrations) in the hydroxylation of DNTCB than TX-100 and SDS micellar solutions
can be related to higher solubilization capacity of CTAB and CPC micelles for DNTCB. The
positive values of zeta potential for micelles of cationic surfactants in their aqueous solutions
(e.g., +14.9 mV and +20.5 mV, respectively, for CTAB and CPC at 15 mM; without
DNTCB) indicate the positively charged surface of micelles. The positively charged micellar
surface will increase the local surface pH, as compared to the pH of bulk solution, by
attracting OH− ions near micellar surface to facilitate the reaction [22]. With the help of
micelles or emulsion droplets (in the case of 0.5 mM CPC), the cationic surfactants create
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huge interface between organic (DNTCB) and aqueous (NaOH) phase and provide better
interaction between DNTCB and OH− ions to react with faster rate giving high conversion.
X
X
OH
OH
OH
X
OH
Cl
X
X
Cl
Cl
X
X
X
X
X
Cl
Cl
O2N
Cl
NO2
NO2
O2N
DNTCB
NaOH
NaCl
X
= Br/ Cl
X
X
OH
Cl
X
OH
X
HO
O2N
OH
Cl
OH
X
HO
NO2
CDNR
NO2
O2N
Scheme 4. Nucleophilic attack of OH– ions on DNTCB molecule, interacting (by cation- interaction) with
cationic head group of a surfactant molecule in micellar phase of the micelle.
The cation- interaction of cationic surfactant and DNTCB can reduce the electron density
over aromatic nucleus of DNTCB molecules, which will make them more reactive towards
nucleophilic attack of OH− ions to replace Cl−atom (Scheme 4). In addition to creating high
pH (OH− ions/ nucleophile concentration) at micellar surface, the promotion of the reaction
by cationic micelles can be greatly attributed to cation- interaction, which helps in
solubilization of large amount of DNTCB and activate them for aromatic nucleophilic
substitution reaction. The major role of cation- interaction in micellar catalysis was proved
by carrying out a reaction of DNTCB with NH3 (Scheme 5); in this reaction NH3, a neutral
nucleophile, will not be involved in electrostatic interaction with cationic micelles. The
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biphasic reaction of DNTCB with aqueous NH3 was very slow giving 75% conversion into a
mono-amine derivative (DCDNA) after 48 h of reaction. The biphasic reaction (without
surfactant) was observed to be fast in the presence of a base (0.5 mol% Na2CO3) giving 99%
conversion of DNTCB after 5 h. In 15 mM CTAB and 0.5 mM CPC micellar catalyzed
reactions, the highest conversion of DNTCB (99% and 75%, respectively) into the desired
product was achieved within 5 h without use of the base. This result clearly demonstrates that
the cationic head group of surfactant at optimum concentration plays important role through
cation- interaction in solubilization and in the catalysis of hydroxylation reaction (and
aromatic nucleophilic substitution reactions).
Cl
Cl
Aq. NH3
(30%; 0.11 mole)
Cl
Cl
Cl
NH2
NO2
40oC
O2N
NO2
O2N
DNTCB
2,3-dichloro-4,6-dinitro-aniline
(7.3 mmole)
Reaction condition
Conversion (%)
Biphasic, 48 h
75
Biphasic, 0.5 mol% Na2CO3, 5 h 91
15 mM CTAB, 5 h
0.5 mM CPC, 5 h
5 mM SDS
99
75
15
Scheme 5. Reaction of DNTCB with aqueous NH3 under different conditions.
The negligible solubilization of CDNR in cationic micellar solutions measured by UV-vis
analysis showed that the product (CDNR) was insoluble in micelles. Like DNTCB, CDNR
molecules did not show any interaction (cation-/ ionic/ hydrophobic) with surfactant
1
molecules in the micelles, which was confirmed by HNMR study. The CDNR, being
insoluble in the micelles, comes out from micelles into aqueous medium, and gets
crystallized upon cooling the reaction mixture.
The poor performance of SDS (Figure 2d) can be due to its poor solubilization capacity for
DNTCB (Figure 4a) and the negatively charged micelles (zeta potential of SDS at 5 mM was
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-21.0 mV) repelling the OH– ions and reducing the pH (OH– ion concentration) near micellar
surface. Furthermore, unlike cationic micelles, there is no favorable interaction (like cation-
interaction) between SDS and DNTCB in micelles to facilitate the reaction. At lower
concentrations (<100 mM), TX-100 solutions gave less conversion of DNTCB (Figure 2e),
but at 100 mM concentration, there was highest conversion of DNTCB, which may be due to
the formation of more number of micelles, at this concentration, solubilizing significant
amount of DNTCB and populating sufficient number of OH– ions (by hydrogen bonding with
–OH groups of TX-100) at the micellar surface. The zeta potential of 100 mM TX-100
solution was -0.03 mV, which was found to be -30.7 mV in presence of NaOH (mmol)
indicating the binding of OH– ions with micelles, by hydrogen bonding through –OH groups
of TX-100 molecules. At higher concentrations (>100 mM) of TX-100, again conversion of
DNTCB is observed to be decreased (Figure 2e). This can be attributed to deeply resided
DNTCB molecules in micelles at higher concentrations, which are not easily accessible by
OH– ions for reaction, and also to the dilution of OH– ions around the micelles due to
increased number of micelles. The study shows that 100 mM TX-100 solution provides
sufficient number of micelles with optimum microenvironment for the solubilization and the
reaction of DNTCB.
3.2.4. Efficiency of dilute CPC micellar solution in micellar catalysis
The better performance of CPC than CTAB for the hydroxylation of DNTCB is evident from
the highest conversion of DNTCB (92%) obtained in a dilute solution of CPC (at 0.5 mM),
which was very less (8% conversion of DNTCB) with 0.5 mM CTAB solution. The higher
activity of CPC at 0.5 mM can be attributed to comparatively higher solubilization of
DNTCB (166 mM; which was 96 mM in 0.5 mM CTAB) and the formation of smaller
droplets of DNTCB (Figure 4e & 4f) providing huge interface for the reaction of DNTCB
and OH– ions. The chemical shifts for various protons of CPC molecules in 0.5 mM CPC
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solution (in absence of DNTCB) were compared with chemical shifts of CPC in the presence
of DNTCB, i.e., in DNTCB-0.5 mM CPC emulsion solution (Table 2). The negative values
of Δδ (downfield shifting for all chemical shifts) indicate increased intermolecular distance or
loose molecular packing of CPC molecules at oil-water interface (i.e., at the surface of
emulsified DNTCB droplets; Scheme 6a) as compared to the CPC molecules existing as
monomers and sub-micelles in 0.5 mM CPC solution. The loose molecular packing of CPC
molecules at interface of emulsion droplet will enhance the accessibility of DNTCB
molecules and OH– ions to each other for reaction giving faster conversion of DNTCB.
Table 2. Chemical shifts of various protons of CPC (0.5 mM) and CTAB (0.5 mM) in the absence and the
presence of DNTCB.
d'
b'
2
Py-H
H
2
C
H
2
C
Br
Py-H
H
a'
CH
3
H
H
3
N
C
H
13
2
4
H
Py-H
c'
H
Surfactant solution
Chemical shifts (δ; ppm) of different protons
g′
f′
e′
d′
c′
b′
a′
Pure CPC (0.5 mM)
DNTCB-CPC (0.5 mM) 8.56 8.10
8.41 7.93
8.73
8.88
4.49
4.62
1.86
1.96
1.07
1.09
0.67
0.69
Δδ (ppm)
-0.15 -0.17 -0.15 -0.13 -0.10 -0.02 -0.02
c
e
f
a
CH
H
H
C
2
2
Br
H C
C
3
3
N
C
H
d
C
H
H C
2
2
12
3
CH
3
b
Surfactant solution
Chemical shifts (δ; ppm) of different protons
f
e
d
c
b
a
Pure CTAB (0.5 mM)
DNTCB-CTAB (0.5 mM)
Δδ (ppm)
2.99 3.21 1.64 1.23 1.16 0.74
2.94 3.11 1.59 1.18 1.10 0.70
0.05 0.10 0.05 0.05 0.06 0.04
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Br
(a)
(b)
OH
Br
OH
Br
Cl
Br
Br
Br
OH
Br
Br
OH
Cl
OH
Cl
OH
OH
Cl
Br
OH
OH
Br
OH
OH
Br
Br
Br
Cl
Cl
Cl
OH
Cl
OH
Br
OH
Br
OH
Br
OH
Cl
OH
OH
Br
Br
OH
Br
Br
OH
Br
Br
Cl
OH
Br
OH
Br
Br
Br
Scheme 6. Molecular packing of (a) CPC and (b) CTAB molecules at oil-water interface of an emulsified
droplet of DNTCB at 0.5 mM surfactant concentration.
On the contrary, the emulsion droplets of DNTCB in 0.5 mM CTAB solution have close
molecular packing of CTAB molecules at interface (Scheme 6b), which is evident from the
upfield shifting (positive values of Δδ; Table 2) of all the chemical shifts of CTAB molecules
as compared to chemical shifts of CTAB (existing as monomers and sub-micelles) in 0.5 mM
CTAB solution. The 0.5 mM CTAB micellar solution has significant solubilization of
DNTCB (96 mM), however, the solution showed very poor activity (8% conversion). This
can be ascribed to the formation of larger emulsion droplet size of DNTCB in 0.5 mM CTAB
solution creating lesser interfacial area for the interaction of DNTCB and OH– ions. In
addition, the close molecular packing of CTAB molecules at oil-water interface of emulsion
droplets will also inhibit the approach of OH– ions to react with DNTCB slowing down the
reaction. So, the higher efficiency of a dilute CPC micellar solution was because of its higher
solubilization capacity for DNTCB, formation of smaller emulsion droplets of DNTCB and
loose molecular packing of surfactant molecules at interface. The DST of pure surfactants
(CTAB and CPC) and surfactant-DNTCB solutions at 0.5 mM concentration of surfactant
were measured (ESI; Figure S9), which showed the presence of less number of free
surfactant monomers in DNTCB-0.5 mM CTAB solution as compared to DNTCB-0.5 mM
CPC solution. This indicates the involvement of more number of CTAB molecules in
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emulsification of DNTCB droplets resulting into close packing of CTAB molecules at
interface. The micellar catalytic behavior of CPC differs from CTAB due to planner structure
and large size of its head group [17a-c,23]. In many reactions, especially where anionic
substrates are involved in the reaction and form substrate-induced micelles, the CPC micellar
solution performs better than CTAB solution at pre-CMC concentration [17a-c]. This has
been attributed to loose molecular packing/ large intermolecular gaping of CPC molecules at
micellar interface due to bigger size of pyridinium head group and less steric hindrance by
planner pyridinium group in the reaction.
3.2.5. Reduced activity of cationic micellar solutions at higher (above optimum)
concentrations
It was observed that the increase in CTAB concentration in micellar solution increases the
solubility of DNTCB (Figure 4a), however, the conversion of DNTCB increases only up to
optimum concentration (15 mM to 25 mM) and further increase in CTAB concentration
decreases the conversion of DNTCB (Figure 2a). The increasing concentration of CTAB (up
to optimum concentrations) in the solution increases the number of micelles, which increases
the conversion of DNTCB by solubilizing more number of DNTCB molecules and providing
huge oil-aqueous interface, high local pH effect near micellar surface and favorable (cation-
) interaction with DNTCB. The decreasing conversion with increase in CTAB concentration
above 25 mM (Figure 2a) may be attributed to strong binding of DNTCB in the micelles by
strong cation- interaction and hydrophobicity of the micelles slowing down the reaction.
The gradual downfield shifting of the signal “A” with CTAB concentration (Figure 3a)
indicate the increasing cation- interaction of solubilized DNTCB molecules (I) showing
strong binding/ solubilization of DNTCB molecules in the micelles.
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We also observed that the solubilization of DNTCB in CTAB micelles reduces the surface
charge density of CTAB micelles due to neutralization of charges by cation- interaction
with DNTCB. The zeta potential for CTAB micelles at 150 mM was found to be +24.8 mV,
which greatly decreased to -0.005 mV for DNTCB solubilized CTAB micelles. The decrease
in surface charge for DNTAB solubilized CTAB micelles at 15 mM was also noticed; the
zeta potential values of CTAB micelles before and after solubilization of DNTCB was +14.9
mV and +0.25 mV, respectively. The significant decrease in surface charge density of CTAB
micelles at high concentrations after solubilization of DNTCB will reduce the concentration
of OH– ions around micelles. In addition, at higher surfactant concentrations, the increased
number of micelles in unit volume will also reduce the OH– ions concentration per micelle.
Thus, the strong solubilization of DNTCB and the reduced OH– ions concentration around the
micelles at high CTAB concentrations are possible causes for slow reaction rate giving less
conversion of DNTCB. The study shows that the CTAB micelles in the concentration range
of 15 mM to 25 mM provide optimum characteristics/ microenvironment (strength of cation-
 interaction with DNTCB, surface charge density and surface OH– ions concentration) for
the reaction giving highest conversion of DNTCB.
Similarly, the effect of CPC concentration on micellar catalysis was also noticed; however, at
much lower concentration (1.0 mM), the conversion of DNTCB was significantly reduced
(74%; Figure 2b). At this concentration, the CPC solution will have micelles to solubilize
good amount of the DNTCB molecules (Figure 4a). However, the bulky head group of CPC
than CTAB [21] and close packing of CPC molecules in the DNTCB solubilized micelles
may slow down the reaction by hindering the approach of OH– ions to react with solubilized
DNTCB molecules and, therefore, the conversion was reduced at much lower concentration
(1.0 mM). Further increase in concentration of CPC (>1.0 mM) greatly reduced the
conversion of DNTCB, which can be ascribed to strong solubilization of DNTCB molecules
29