Micellar Catalysis Strategy of Cross-Condensation Reaction: The Effect of Polar Heads on the Catalytic Properties of Aminoalcohol-Based Surfactants

Article abstract of DOI:10.1007/s10562-019-03045-6

Abstract: The effect of the polar head and the concentration of quaternary ammonium surfactants (C₁₄EtOH, C₁₄iPrOH, C₁₄PrOH, where, 14 = carbon number iPrOH = iso-propanol, EtOH = ethanol, PrOH = propanol) in micellar cata

Full text of DOI:10.1007/s10562-019-03045-6

Catalysis Letters
https://doi.org/10.1007/s10562-019-03045-6
Micellar Catalysis Strategy of Cross‑Condensation Reaction: The Efect
of Polar Heads on the Catalytic Properties of Aminoalcohol‑Based
Surfactants
Zakaria Hafdi1 · Mohamed Ait Taleb2,3 · Abdallah Amedlous4 · Mohammed El Achouri1
Received: 7 September 2019 / Accepted: 15 November 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
The efect of the polar head and the concentration of quaternary ammonium surfactants (C14EtOH, C14iPrOH, C14PrOH,
where, 14=carbon number iPrOH=iso-propanol, EtOH=ethanol, PrOH=propanol) in micellar catalysis for the cross-
condensation reaction (Claisen–Schmidt) was investigated. The reaction in the Micellar-NaOH system with diferent concen-
trations of surfactants above the critical micelle concentration from 15 to 30 mmol between benzaldehyde and acetophenone
was used as a model of reaction for this study. The examination of the efectiveness of the catalytic activity reveals that the
compound C14EtOH has the best yield (80% of the desired product), followed by C14iPrOH and C14PrOH. The results were
interpreted according to the solubilization capacity, droplet size analysis of reagents (benzaldehyde and acetophenone) in
the micellar medium and stability of reaction intermediate (enolate) by the electrostatic interactions generated by positive
charge of N+ quaternary ammonium atom. Therefore, the quantum calculations carried out by DFT method for surfactants
in the aqueous medium, show that electrophilicity degree and the reaction yield% for three cationic surfactants varies in the
same following order: C14PrOH<C14iPrOH<C14EtOH.
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-019-03045-6) contains
supplementary material, which is available to authorized users.
* Mohammed El Achouri
achmedens@yahoo.fr
1
Laboratoire de physico-chimie des matériaux inorganiques
et organiques, Centre des Sciences des Matériaux, Ecole
Normale supérieure-Rabat, Mohammed V University
in Rabat, Rabat, Morocco
2
Equipe de Matériaux, Catalyse & Valorisation des
Ressources Naturelles, Université Ibn Zohr, BP 8106,
80000 Agadir, Morocco
3
Equipe de Chimie Bio-Organique Appliquée, Université Ibn
Zohr, BP 8106, 80000 Agadir, Morocco
4
Laboratoire de Matériaux, Catalyse et Valorisation des
Ressources Naturelles, URAC 24, FST Mohammadia,
Université Hassan II-Casablanca, Casablanca, Morocco
Vol.:(0123456789)
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Z. Hafdi et al.
Graphic Abstract
Keywords Cationic surfactant · Micellar-catalysis · Claisen–Schmidt · Cross condensation · Electrophilicity
than 85% of the total mass utilization of a chemical pro-
cess) and incomplete recovery efciency (50–80%) [12].
In this context, water is the most preferred solvent for
an organic reaction, with several advantages such as its
co-catalytic activity [13–15] nontoxic, abundant, nonfam-
mable, and renewable. Furthermore, the use of water as a
solvent accelerates the rates of the organic reaction and
improves reaction selectivity, even when the reagents are
poorly soluble or insoluble in this medium [16]. The use
of surfactant molecules in an aqueous medium can solve
the problem of the low solubility of organic reagents. The
formation of micelles in aqueous media favors organic
reactions by the concentration of organic compounds in
micelles [17–19]. In micellar systems formed by cationic
surfactants, the base-catalyzed condensation reactions
show their efciency compared to the biphasic system
(absence of surfactants) [20–23]. Thus, surfactants with
diferent types have sometimes been used for aldol con-
densation reaction [20, 24–27], the participation by which
surfactants are involved in catalytic reactions is known as
micellar catalysis [21, 28–30] or phase-transfer catalysis
[31, 32].
1 Introduction
The α, β-unsaturated ketones (Chalcones) are the class of
compounds possessing diferent biological activities: anti-
microbial, antimalarial, anti-infammatory [1], anticancer [1,
2], antifungal, antibacterial [3] and inhibit leukotriene B4
[4]. They constitute an important precursor for the synthesis
of heterocyclic compounds such as pyrimidines, pyrazolines,
isoxazolines, favonoids, isofavonoids [5–8].
The Chalcones are commonly prepared via
Claisen–Schmidt condensation between acetophenones, and
aromatic aldehydes in an organic solvent under acidic or
basic homogeneous conditions, or even using heterogeneous
basic systems: alumina [9], zeolites [10], natural phosphates
alone or activated with an ammonium salt [11]. However,
most of this catalyst has several disadvantages including the
use of toxic solvents, high-temperature reaction, and long
reaction time, which limit their industrial applications.
The ecological procedure for organic synthesis is the
replacement of volatile organic solvents by green reaction
media. Volatile organic solvents are the major contributors
to environmental pollution due to their abundant use (more
1 3

Micellar Catalysis Strategy of Cross‑Condensation Reaction: The Efect of Polar Heads on the…
The catalyzed acid–base reactions are by far the most
OH
N
N
numerous, and well-studied reaction types [33–36], in which
C14EtOH
cmc= 4.2mmol
cmc= 3.5mmol
cmc= 5.1mmol
Br
the organic functional groups undergo a series of diferent
transformations with nucleophilic reagents in the presence
of acids or bases as catalysts, among these types of reac-
tions: the base-catalyzed (Claisen–Schmidt) condensation
C14iPrOH
Br
OH
(Scheme 2) which generally involves the formation of the
anion of acetophenone followed by its attack on the carbonyl
group of benzaldehyde [37]. According to their required ori-
N
OH
Br
entation in the micellar regions [37–40], the aromatic organic
C14PrOH
reagents (aldehydes and ketones) and the reaction intermedi-
ates (enol) undergo electrostatic interactions with the cationic
polar head of the surfactants which remains among the fac-
tors responsible for the modifcation of the reaction rates [23,
38]. In the base-catalyzed condensation, reaction the posi-
tively charged micellar surface can increase the local pH in
the micellar region because of the Colombian forces, allow-
ing to attract more bases to the micellar interfaces [41, 42].
In this study, the cross-condensation reaction was made
between acetophenone, and benzaldehyde as a model of reac-
tion, in the Micellar catalysis-NaOH system for three new syn-
thesized QAS with diferent polar heads (C14EtOH, C14PrOH,
C14iPrOH). The diference in the efciency presented for the
condensation reaction in the diferent micellar environment
studied was discussed by comparing the surfactant capacity to
solubilize the reagents in the reaction media, as well a micro-
scopic study was introduced to distinguish the diferent texture
of the dispersion (droplet size) of the reagents in the micellar
media at diferent concentrations of surfactants. Furthermore,
the study of the efect of the ionization degree (α) has been
investigated to include the efect of the positive charge of the
micelle to increase the local pH at the micellar interfaces. The
electrostatic afnity between the positive heads N+ of micelles
with the solubilized substances (enolate intermediate, benzalde-
hyde, and acetophenone) in the micellar interface was estimated
by the study of the degree of electrophilicity of the surfactant
molecules in the aqueous medium. The calculation by the func-
tional density theory, method associated with the non-local-Parr
(B3LYP) was made in an aqueous medium for the two possible
forms in which the catalytic support is presented in interaction
with the counter-ion (Br− and OH−), in the reaction medium.
Scheme 1 The three types of synthetic cationic surfactants,
C14EtOH, C14iPrOH, and C14PrOH
chloroform (99%), ethyl acetate (99%), dichloromethane
(99%), hexane (99%) and ethanol absolute were obtained
from Fluka. 3-(dimethylamino)propane-1-ol (> 99%),
1-(dimethylamino)propane-2-ol (>99%) and 2-(dimethyl-
amino)Ethan-1-ol (>99%) were purchased from Fluka, the
1-bromododecane (>97%), acetonitrile, dichloromethane,
methanol, and sodium bromide, were purchased from Sigma
Aldrich. All these chemicals were used without further
purifcation.
2.2 Preparation of Catalytic Support
The series of N-(n-hydroxyalkyl)-N, N-dimethyl n-alkyl ammo-
nium bromide (Scheme 1) referred to as C14EtOH, C14PrOH,
C14iPrOH (n=14 carbons, EtOH=ethanol, PrOH=propanol,
iPrOH=iso-propanol) were synthesized and characterized
according to the procedure well detailed in our previous work
[43]. The CMC values for synthetic cationic surfactants were
determined in aqueous using the electrical conductivity (Fig. 1).
2 Experimental
2.1 Materials and Methods
4-Chlorobenzaldehyde, 4-methoxybenzaldehyde (99%),
4-methylbenzaldehyde (99%), benzaldehyde (99%), aceto-
phenone (98%), sodium hydroxide (NaOH, 97%), Magne-
sium sulfate (MgSO4, 95%), sodium chloride for analysis
(NaCl, 99%), concentrated hydrochloric acid (35% HCl)
were purchased from Sigma Aldrich. Solvents such as
Fig. 1 Positively charged micelles of Cationic Surfactants having sol-
ubilized reactants within the micelles and surface OH−ions
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Z. Hafdi et al.
Scheme 2 Cross-condensation
(Claisen–Schmidt reaction)
of R-benzaldehyde (5 mmol)
and acetophenone (5 mmol)
in Micellar catalysis–NaOH/
Water–NaOH
O
10mL of Surfactant
solution or wayer
without surfactant
O
O
+
R
R
2.5mmol of NaOH
25°C
R-benzaldehyde
(5mmol)
acetophenone
(5mmol)
Chalcone Product
R
= 4-Cl
4-OCH3
4-H
4-CH3
micellar surface. For a surfactant with a single ionic head
group, the degree of bonds is β=1−α [44].
2.3 General Procedure for the Preparation
of Chalcones in Micellar Systems
NaOH‑Surfactant
The value of α, in reality, gives an idea of the size of the
cloud of counter-ions around the charged micellar sphere
which also gives an idea of the degree of electrostatic afnity
between the counter-ion and the charged micellar surface.
The α can be determined by the potentiometric measure-
ments, conductivity, electrophoretic mobility or by NMR
[45].
In a 50 ml fask, we have introduced 10 ml of aqueous solu-
tion surfactant at a required concentration above the (CMC)
from 15 to 30 mmol, or water without surfactant. Then, a
mixture of R-benzaldehyde (5 mmol) and acetophenone
(5 mmol) (Scheme 2) was added. Likewise, an amount of
NaOH (2.5 mmol) was dissolved in the reaction mixture
with stirring at temperature t= 25 °C and for the required
time. The progress of the reaction was monitored by TLC
using a solvent system ethyl acetate: hexane (1:2, by vol), the
spots were visualized under UV light. Once the reaction was
completed, a saturated NaCl solution was added to the reac-
tion mixture to reduce the surfactant concentration below the
CMC and to release the maximum of synthesized products
(Chalcone) from the micelles. The product formed under-
goes neutralization under fltration with a concentrated solu-
tion of HCl. The product obtained was dissolved in 25 ml of
dichloromethane to remove traces of water, using MgSO4
desiccant. As soon as the traces of water are removed,
evaporation under vacuum was carried out to remove the
dichloromethane solvent. The dried product obtained was
recrystallized in a volume of 5 ml of absolute ethanol. The
structure of the Chalcone products after recrystallization was
verifed by 1H and 13C nuclear magnetic resonance (NMR),
recorded by using a Bruker spectrometer at 600 MHz (1H)
and 151 MHz (13C) and Melting point.
In this current study the degree of ionization α is obtained
by the conductivity (Fig. 2) method using the Thermo Sci-
entifc Orion Star Conductor A212 Benchtop Conductivity
Meter of a constant cell K equal to 0.4750 cm−1, the curve of
the specifc conductivity κ in (µS/cm), for cationic surfactant
as a function of its concentration C in mol/L at pH 7 and
t=25 °C, represents two straight lines before, and after the
CMC, the ratio of the slopes of these two lines S1 and S2 are
2.4 Ionization Degree α
The ionization degree α is defned as the fraction of charge
of the counter-ions not bound to the micelle. In other words,
α makes it possible to quantify the proportion of counter-
ions that are completely hydrated away from the micelle,
respecting the set of counter-ions. α also allows determining
the degree of bonds β which represents the fraction of dis-
sociated counter-ions, sites on the micellar surface, which
means that the fraction of the charges got neutralized on the
Fig. 2 The yield (%) of the condensation reaction (Claisen–Schmidt)
between benzaldehyde and acetophenone as a function of surfactant
concentration (C14EtOH, C14PrOH, C14iPrOH) from 15 to 30 mmol
in the Micellar catalysis–NaOH system (0.25 mmol of NaOH and at
t=25 °C)
1 3

Micellar Catalysis Strategy of Cross‑Condensation Reaction: The Efect of Polar Heads on the…
used to calculate the value of α according to the following
gap energy between ELUMO and EHOMO (ΔE). To complete
relation (Eq. 1) [46].
the vision on the property of these molecules, other descrip-
tors have been calculated as electronegativity (χ), the corre-
sponding hardness (η), softness (σ) and electrophilicity (ω),
of cationic surfactants molecules, are given using equations
(Eq. 2) [51], (Eq. 3) [51, 52] and (Eq. 4). For electrophilic
index (ω) it combines softness and chemical potential given
by equation (Eq. 5) [48].
S2
ꢀ =
(1)
S1
2.5 Absorbance Measurements
[
]
ELUMO + EHOMO
(2)
(3)
(4)
(5)
The absorbance spectra of the benzaldehyde (0.05 mmol)—
water and acetophenone (0.05 mmol)—water mixtures at
diferent surfactant concentrations from 15 to 30 mmol were
performed separately using a Jasco V-630 UV–visible spec-
trophotometer with quartz cups. The change in absorbance
value from 200 to 400 nm in the presence of a surfactant was
determined by using water as a reference.
ꢀ =
2
[
]
E
LUMO − EHOMO
ꢀ =
ꢀ =
2
1
ꢁ
2.6 Microscopic Analysis
ꢁ2
ꢀ =
The dispersed media of the aqueous solutions of the benza-
ldehyde (5 mmol) and acetophenone (5 mmol) in the pres-
ence and absence of surfactants at diferent concentrations
from 15 to 30 mmol were analyzed separately by using an
optical microscope. Images captured for the drops of the
analyzed solutions were visualized at ×10 objective lance
magnifcation.
2ꢂ
3 Results and Discussion
The Claisen–Schmidt reaction was frst carried out between
benzaldehyde and acetophenone (Table 1) at t=25 °C in two
systems: Water–NaOH and in the system Micellar catal-
ysis–NaOH system (surfactants concentration = 15 mmol)
for the three diferent surfactants with varied polar heads
(Scheme 1).
2.7 Quantum Chemical Parameters Calculations
Quantum chemical methods and molecular modeling tech-
niques help to defne a large number of molecular descrip-
tors characterizing the reactivity, the form and binding prop-
erties of a complete molecule as well as molecular fragments
and substituents.
From Table 1, in the biphasic system the absence of sur-
factants (Water–NaOH system) the reaction shows a low
yield value (12%) for long periods (180 min) compared with
the presence of surfactants (Micellar catalysis–NaOH sys-
tem) who shows high efciency between (49 and 63%) dur-
ing a short period of time (55 min).
The use of the theoretical parameters makes it possible
to characterize the compounds and their various fragments
and substituents of their molecular structure and also to
prove the mechanism proposed to be taken into considera-
tion in terms of reactivity of the compounds studied [47].
The computational chemistry [the functional density theory
(DFT)], was used, as a tool to know the availability in each
case of N+ as active sites with respect to the position of
the OH alcoholic function for the three types of synthetic
surfactants (C14iPrOH, C14EtOH, C14PrOH). Calculations
were performed with the non-local exchange–correlation
function B3LYP [48], the solvent was treated with self-con-
sistent reaction feld theory (SCRF) [49], All quantum calcu-
lations were performed by using Gaussian software 09 [50]
for Windows. Then, several quantum chemical parameters
in terms of the descriptors in the aqueous phase have been
determined, namely: Highest occupied molecular orbital
energy (EHOMO), low unoccupied orbital energy (ELUMO),
This necessity of high time for the reaction in the bipha-
sic system can be attributed to the existence of a smaller
interfacial surface between the aqueous, and organic phases
available for an interaction between the reagents (acetophe-
none and benzaldehyde) and the hydroxyl anion OH−, which
causes a low probability of acetophenone to be attacked by
OH− to produce the carbanion (acetophenonate-ion) needed
for the reactivity with benzaldehyde which will slow down
the speed of the reaction in the Water–NaOH system.
The low efciency of the reaction presented for the bipha-
sic system (Water–NaOH system) can be attributed essen-
tially to the formation of by-products during the condensa-
tion reaction, which has already been reported by several
studies, include for example the work of Francesco et al. [53]
in which, the authors show that the condensation reaction
1 3

Z. Hafdi et al.
Table 1 The isolated yields in (%) and the time in (min) for the condensation reaction between acetophenone and benzaldehyde (as a model
of reaction) in two systems: Micellar catalysis–NaOH (15 mmol of surfactants–0.25 mmol of NaOH and) and in the Water–NaOH system
(0.25 mmol of NaOH) at room temperatures (t=25 °C)
O
O
10mL of Surfactant
solution or wayer
without surfactant
O
+
2.5mmol of NaOH
25°C
benzaldehyde
(5mmol)
acetophenone
(5mmol)
Chalcone Product
Entry
Surfactants
Surfactants concentration
Reaction time (min)
Yield (%)
1
2
3
4
Water only
C14EtOH
C14iPrOH
C14PrOH
Neat
180
55
55
12
63
54
49
15 mmol
15 mmol
15 mmol
55
between benzaldehyde and acetophenone in the aqueous
medium (0.25 mol of NaOH) produces the by-products with
a signifcant percentage of 37% of ketol compared to 8% of
Chalcone dehydrated. Moreover, Vashishtha et al. [23] has
found that the biphasic aldol condensation of benzaldehyde
and n-heptanal in an aqueous solution of NaOH (pH 13.7)
causes a decrease of the pH from 13.7 to 13.3. According
to the authors, this decrease of pH was explained to the
neutralization of NaOH fraction with by-products organic
acids such as benzoic acid and n-heptanoic acid. Which were
formed by oxidation of benzaldehyde and -heptanal, respec-
tively in aqueous media.
from 15 to 30 mmol in the solution for the three catalytic
support (C14EtOH, C14PrOH, C14iPrOH), causes according
to Fig. 1 an improvement in the yield (%) for the reactions.
This is due to the increased number of micelles and also
to an increase in the micellar interface which facilitates
the reactivity of the reagents (acetophenone and benza-
ldehyde) by the high concentration of OH− ion in the
micelle’s polar region (Fig. 1).
The three catalytic supports used (C14EtOH, C14PrOH,
C14iPrOH) show diferent values of the yield (%) of the
cross-condensation reaction. As observed in Table 1 and
Fig. 2 the compound C14EtOH has the best yield (%) of the
condensation reaction in the Micellar catalysis–NaOH sys-
tem at diferent concentrations from 15 to 30 mmol, follow-
ing of C14iPrOH and C14PrOH respectively. This diference
between the catalytic efciency of the three surfactants used
is mainly due to the existence of OH alcohol function in
their polar head with diferent positions, which will cause a
diference of the degree of steric hindrance at the polar head
level, in addition to the existence of OH in the polar part can
be contributed to the intramolecular electrostatic interactions
Charge-Dipole type between the N+ quaternary ammonium
atom and the lone pair of the oxygen atom attached to the
OH alcoholic function, refecting as well the electrophilicity
degree of micelles and the degree of steric hindrance of the
polar head in the aqueous medium, which remains a very
important factors, that are responsible for the increasing,
and the decreasing the OH− ion population in the micellar
region.
On the other hand, the Claisen–Schmidt reaction in the
Micellar catalysis–NaOH system with 15 mmol in surfactant
(Table 1) shows a remarkable yield (%) between 49 and 63%
depending on diferent surfactants (C14EtOH, C14iPrOH,
and C14PrOH). This presence of surfactant in the reaction
medium does not only afect the yield but also the time of
the reaction (55 min).
The rapidity (short duration) and the efciency (high
yield %) of the reaction in the micellar medium compared
with lower yield (%) and longer duration (min) in aqueous
media (Water–NaOH system) due to a large micellar surface
provided by the Micellar catalysis–NaOH system that allows
the reagents to be in interaction.
In the Micellar Catalysis–NaOH system with a high
concentration of quaternary ammonium cationic sur-
factants, The positively charged micelles of the cationic
surfactant molecules make it possible to concentrate the
OH− ions at the micellar interface, thus, increasing the
local pH at the micellar interface [54, 55]. This favors the
reagents according to their location as an aromatic com-
pound in the polar region of the micelles [56] to be closer
to the OH− ion for the formation of acetophenonate-ion.
This is why the increase in the surfactant concentration
The ionization degree α of the micelles can gives an idea
of the free charge N+ which does not contribute to the ionic
bonds (electrostatic interaction) with the original counter-
ion (Br−) in aqueous solution, which gives another idea of
the degree of electrophilicity of the micelles which remains
a very important factor for the attraction of OH− ions to
1 3

Micellar Catalysis Strategy of Cross‑Condensation Reaction: The Efect of Polar Heads on the…
Table 2 The values of ionization degree (α) and bonding degree (β)
with a small α=0.29 value (low electrophilicity of micelles)
is the one that presents an intermediate value of the reaction
efciency among the three cationic surfactants studied.
The α values presented in this work are very close, which
prevents us to really draw a conclusion between the ioni-
zation degree and the efciency of the Claisen–Schmidt
condensation reaction. That is why we have introduced the
comparison of the α values for a series of QAS with difer-
ent alkyl chains from C10 to C16 carbon atoms with their
catalytic efciency via the cross-aldol reaction. According
to Zana [46], the ionization degrees of quaternary ammo-
nium surfactants in aqueous medium decreased from 0.27
to 0.16 depending on the increasing alkyl chain from C10 to
C16. From the work of Manu Vashishtha et al. [22], the con-
version of the cross-aldol reaction increases from C10QAS
to C16QAS which reveals that the ionization degree that’s
mean the degree of electrophilicity of the micelle does not
refect the OH− ion population in the micellar polar region
which does not give an idea about the diference in catalytic
efciency for diferent cationic surfactants.
calculated by equation Eq. 1
Surfactants
C14EtOH
C14iPrOH
C14PrOH
Ionization degree α
Bonding degree β
0.32
0.68
0.29
0.71
0.33
0.67
the micellar region in the reaction medium. According to
the results of α (Table 2) calculated from the slopes of the
conductivity curve Fig. 3 as a function of surfactants con-
centration (mol/L).
It must be noted that the efect of the degree of electrophi-
licity of the micelle (α large) does not refect the diference
between the values of the condensation reaction yield (%) for
the three catalytic supports (C14EtOH, C14iPrOH, C14PrOH).
This is why we observe according to Figs. 2 and 3 that the
C14PrOH which presents the high value of ionization degree
α=0.33 (high electrophilicity of micelles), is the one that
has lower reaction efciency. On the contrary, the C14iPrOH
Fig. 3 The ionic conductivity in (µS/cm) for the three cationic surfactants (C14EtOH, C14IPrOH, C14PrOH) in aqueous media as a function of
their concentration in (mol/l) and the values of their slopes after (red) and before (blue) the CMC at pH 7 and T=297.15 K
1 3

Z. Hafdi et al.
Fig. 4 Optical micrographs of benzaldehyde and acetophenone in water, and cationic surfactants solutions (C14EtOH, C14iPrOH, C14PrOH)
(conc.: 15 mmol; the scale bar equals 200 µm). 298.15 K
This discrepancy between the degree of electrophilicity
of micelles and the yield (%) of the reaction in the Micel-
lar catalysis–NaOH system for C14EtOH, C14iPrOH and
C14PrOH can be attributed to the degree of steric hindrance
of the polar head which prevents OH− to integrate in the
hydration region (Stern layer) for the reactivity with ace-
tophenone. The Steric hindrance in micellar media for sur-
factants with diferent polar heads has been already men-
tioned in several works [20, 57, 58], as an example of the
work of Sen et al. [59] who found that the planar structure of
the pyridinium group of cetylpyridinium chloride provides
less steric hindrance for the hydrolytic cleavage of phenyl
salicylate in the Micellar–NaOH system among the three
surfactants used [tetradecyltrimethylammonium bromide
(TTAB), cetyltrimethylammonium bromide (CTAB) and
cetylpyridinium chloride (CPC)].
reagents (benzaldehyde and acetophenone) in the micellar
media (15 mmol of surfactants studies) was analyzed by
optical microscope.
From Fig. 4 the benzaldehyde–water and acetophe-
none–water mixtures in the presence and the absence of
surfactants (C14EtOH, C14iPrOH, C14PrOH) indicates the
existence of diferent droplet size refecting as well the mis-
cibility of benzaldehyde and acetophenone in water. The
presence and absence of surfactants in the two reagents
mixtures refect the solubilizing and emulsifying capacities
of surfactants, from Fig. 4 the optical micrographs of disper-
sion’s textures of the reagents in the micellar media shows
smaller emulsion droplets compared to those presented in
the reagents-water system.
In micellar-reagent systems, the droplet size decreases
from C14PrOH>C14iPrOH>C14EtOH, the formed micelles
allow the surfactant molecules to solubilize and emulsify
the reagents in the form of droplets with diferent size. The
increase in the surfactant concentration from 15 to 30 mmol
in the micellar-reagent systems for the three catalytic sup-
port (C14EtOH, C14PrOH, C14iPrOH) leads to an increase
in the number of micelles in the solution, thus causing a
decrease in the size of the benzaldehyde and acetophenone
droplets (Fig. 5). Due to the repulsive electrostatic forces
between the positively charged micelles, their size decreases
at a higher concentration of surfactants, thus decreasing the
Furthermore, one of the most important properties of
surfactants that are directly related to micelle formation is
solubilization. Solubilization may be defned as the spon-
taneous dissolving of a substance by reversible interaction
with the micelles of a surfactant in a solvent to form a ther-
modynamically stable isotropic solution with the reduced
thermodynamic activity of the solubilized material. Solu-
bilization is distinguished from emulsifcation and misci-
bility (the dispersion of one liquid phase in another) [44,
60]. For a better understanding of the diference catalytic
performance of the surfactants used, the miscibility of the
1 3

Micellar Catalysis Strategy of Cross‑Condensation Reaction: The Efect of Polar Heads on the…
Fig. 5 Optical micrographs of benzaldehyde and acetophenone in water, and cationic surfactants solutions (C14EtOH, C14iPrOH, C14PrOH)
(conc.: 15 mmol to 30 mmol; the scale bar equals 200 µm)
size of the droplet of the reagents dispersed in aqueous solu-
tions of surfactants with high concentrations [61].
and in diferent micellar solutions of surfactants (C14EtOH,
C14iPrOH, and C14PrOH).
For each surfactant, in the micellar media with difer-
ent concentrations from 15 to 30 mmol, the droplet sizes
of micelle-reagents vary according to the degree of misci-
bility between reagents and the aqueous environment from
C14EtOH to C14PrOH thus refecting the diference of mis-
cibility and solubility capacities for each surfactant.
The UV absorption spectrum of benzaldehyde and ace-
tophenone in water (Fig. 6) shows the maximum wave-
length λmax at 248 nm and 244 nm respectively, assigned
to the transition π → π*. The absorbances bands λmax of
reagents in the presence of surfactants, do not present any
hypsochromic or bathochromic ofset. On the other hand,
a hyperchromic shift with respect to the micellar environ-
ment was observed. In surfactant’s solutions (15 mmol)
the absorbance intensities of the reagents increase progres-
sively as a function of the micellar environment which
To prove this diference in solubility of the reactants
with respect to the diferent micellar environments, the
UV absorption characteristics of the reagents (benzalde-
hyde and acetophenone) were studied separately in water
1 3

Z. Hafdi et al.
Fig. 6 UV absorption spectra of a benzaldehyde and b acetophenone in water and in aqueous solutions of diferent surfactants (C14EtOH,
C14iPrOH, C14PrOH) at 15 mmol of surfactants
indicates the increase of the solubilizing capacity of cati-
onic surfactants from C14PrOH to C14EtOH.
and stabilize reagents (acetophenone and R-benzaldehyde)
and intermediates such as enolate ions at the micellar inter-
face, thus promoting the condensation reaction [62, 63].
However, the electrostatic and hydrophobic interaction
between the reagents and the cationic surfactants will be
discussed below, which represents a responsibility for the
modifcation of the reaction rates [64].
In concentrated aqueous solutions of surfactants, although
the shape of the micelles may be very diferent from that of a
dilute solution, the solubilization site for a particular type of
solubilizer appears to be similar to that of a dilute solution;
that is, the polar molecules are mainly solubilized in the
outer regions of the micellar structures, whereas the nonpo-
lar solubilizers are contained in the inner portions.
The UV absorptions of benzaldehyde and acetophe-
none in the micellar solutions of C14EtOH, C14iPrOH and
C14PrOH at different concentrations from 15 mmol to
30 mmol were also studied according to Fig. 7.
The electrostatic interactions between the N+ quaternary
ammonium group and the carbonyl benzaldehyde group
(Scheme 3a) could force the molecule of R-benzaldehyde
to be located at the Stern layer [22], in the other hand the N+
quaternary ammonium group can also participate in interac-
tion with acetophenone (Scheme 3b) causing a decrease of
electron density at the level of the carbonyl carbon to make
it more electrophilic which favors the attack of OH− anion.
For the enolate intermediate is a delocalized system, with
a negative charge on both C and O atoms (oxyanion and car-
banion) [65] its stability remains a very important factor for
the formation of C–C and C–O bonds [62], the higher elec-
trostatic afnity of the enolate with the quaternary ammo-
nium group (Scheme 3c) justify the greatest reactivity in the
presence of surfactants [66–68].
The intensity of the maximum absorption bands shows
an increase, as a function of the surfactant concentration
in solution from 15 to 30 mmol. Thus, it’s refecting the
increased solubility, which favors the cross-condensation
reaction in the micellar environment in the following order:
C14PrOH<C14iPrOH<C14EtOH.
Several studies [38–40, 56], have been demonstrated
that the polar group for substituted aromatic molecules is
solubilized in a micelle with an orientation such that the
polar group is directed to the aqueous phase. Vashishtha
et al. [22] show using 1H NMR that the benzaldehyde mol-
ecule appears to be located in the hydrophilic zone micelle
(Stern layer) of cationic surfactants. The polarizability of
the π-electron cloud of the aromatic nucleus and its ability
to interact with the positively charged quaternary ammonium
heads at the micelle-water interface may favor the adsorp-
tion of these aromatic hydrocarbons in the hydrophilic zone
micelle.
All interactions in which cationic surfactant molecules
present (Scheme 3) are the attractive interactions between
the positive charge carried by the N+ quaternary ammonium
group and the nucleophilic substances (enolate, benzalde-
hyde, and acetophenone).
For this reason, we have decided to study the elec-
tronic distribution for the three catalytic supports studied
(C14EtOH, C14PrOH, C14iPrOH) by the calculation DFT/
B3LYP 6-31G (d) in the aqueous medium and according to
their two possible states (Fig. 8): the cases of the formation
of a pair of ions with Br− anion ([cationic surfactant] [Br−])
In fact, the micellar surface positively charged by the
polar heads of the cationic surfactants can help to populate
1 3

Micellar Catalysis Strategy of Cross‑Condensation Reaction: The Efect of Polar Heads on the…
Fig. 7 UV absorption spectra of benzaldehyde and acetophenone in aqueous solutions of diferent surfactants (C14EtOH, C14iPrOH, C14PrOH)
with various concentrations from 15 to 30 mmol
Scheme 3 The electrostatic
afnity between the cationic
heads N+ of the surfactants
(R=ethanol, propanol and
iso-propanol) with a R-benzal-
dehyde, b enolate intermediate
and c acetophenone
and with the hydroxyl anion OH− ([cationic surfactant]
[OH−]) coming from the NaOH molecule.
The location of the alcoholic function OH with respect to
the N+ quaternary ammonium can infuence the electrophi-
licity of the molecule, which plays an important role in the
1 3

Z. Hafdi et al.
Fig. 8 Optimized geometric
structures by DFT method for
C14EtOH, C14PrOH, C14iPrOH
according to their two possible
form ([cationic surfactant]
[Br−], [cationic surfactant]
[OH−]) in aqueous media
obtained by DFT/B3LYP 6-31G
(d) calculations
Table 3 Electronic parameters
of studied surfactants C14EtOH,
C14PrOH and C14iPrOH
Surfactants
EHOMO (eV)
ELUMO (eV)
η (eV)
σ (eV)
ΔE (eV)
ω (eV)
χ (eV)
[C14EtOH][Br−]
[C14iPrOH][Br−]
[C14PrOH][Br−]
[C14EtOH][OH−]
[C14iPrOH][OH−]
[C14PrOH][OH−]
−5.808
−5.777
−5.800
−4.298
−4.262
−4.267
1.239
1.481
1.548
1.563
1.700
1.881
3.524
3.629
3.674
2.930
2.980
3.074
0.283
0.275
0.272
0.341
0.335
0.325
7.048
7.259
7.348
5.862
5.962
6.149
0.74
2.284
2.148
2.125
1.368
1.281
1.193
according to their two possible
form ([cationic surfactant]
[Br−], [cationic surfactant]
[OH−]) in aqueous media
obtained by DFT/B3LYP
6-31G(d) calculations
0.635
0.615
0.3191
0.2752
0.2315
electrostatic afnity force between the diferent surfactants
used and substances with nucleophilic character (enolate,
benzaldehyde, and acetophenone) present in the reaction
medium. Many calculated chemical descriptors are going to
be the subject of the discussion above to correlate the cross-
condensation reaction efciency (yield %) and the molecular
structure of surfactants.
two forms ([C14EtOH] [Br−], [C14PrOH] [Br−], [C14iPrOH]
[Br−], [C14EtOH] [OH−], [C14PrOH] [OH−] and [C14iPrOH]
[OH−].
The highest occupied molecular orbital energy (EHOMO
)
)
and the lowest unoccupied molecular orbital energy (ELUMO
are very popular quantum chemical parameters. According
to the theory of molecular orbitals [69, 70], the formation of
a transition state is due to an interaction between the border
orbitals (HOMO and LUMO) of the reagents [71].
The energy of the highest occupied molecular orbital
(EHOMO), the energy of the lowest unoccupied molecular
orbital (ELUMO), the gap energy (ΔE), hardness (η), softness
(σ), electrophilicity (ω) and electronegativity (χ), param-
eters are given in Table 3, for the three catalytic supports in
Therefore, the EHOMO (higher energy) is often associated
with the ability of the molecule to donate the electron and
1 3

Micellar Catalysis Strategy of Cross‑Condensation Reaction: The Efect of Polar Heads on the…
Table 4 The isolated yields in (%) and the time in (min) for the condensation reaction between acetophenone and various substituted benzalde-
hyde (4-Cl, 4-OCH3, 4-CH3) in Micellar catalysis–NaOH systems (30 mmol of C14EtOH) at 25 °C
O
O
O
10mL of C14EtOH at
30mmol
+
R
R
2.5mmol of NaOH
25°C
R-benzaldehyde
(5mmol)
acetophenone
(5mmol)
Chalcone Product
Entry
1
R
Time (min)
Yield (%)
Chalcones pictures
4-Cl
35
45
75
91
86
75
2
3
4-OCH3
4-CH3
the ELUMO (lowest energy) indicates the more probable the
molecule would accept electron [72].
with the nucleophilic compounds (enolate, benzaldehyde,
and acetophenone) following the C14iPrOH and C14PrOH
respectively.
In Table 4 it is observed that the [cationic surfactant]
[Br−] form, has lower values of ELUMO compared to the val-
ues presented by the [cationic surfactant] [OH−] form, for the
three surfactants studied (C14EtOH, C14iPrOH, C14PrOH).
In both forms [cationic surfactant] [Br−] and [cationic Sur-
factant] [OH−], the HOMO energy has negative values for
the three cationic surfactants studied, which indicates that
these catalytic supports do not have the ability to donate
electrons, likewise our interest it is to verify and examine
their ability to accept the electrons earlier than give them,
that is why we will examine the energy of LUMO which
reveals that ELUMO decreases from C14EtOH to C14PrOH
for both forms.
The Hard–Soft–Acid–Base theory (HSAB) classifies
reactive species as relatively “hard” or “Soft”, based on
polarizability, that is the ease with which electronic density
can be displaced or delocalized to form new covalent bonds
[75], the hard molecules are less reactive than the soft mol-
ecules because they require a higher energy for molecular
excitation [76]. From Table 3, for two forms it was observed
that the chemical hardness (η) decreases in the order of
C14PrOH>C14iPrOH>C14EtOH, the opposite trend is also
observed for the chemical softness (σ) which explains the
highest catalytic proprieties efciency of C14EtOH.
For other descriptors such as electrophilicity (ω) and
electronegativity (χ), these are the indices allows to predict
the reactivity of species to give or accept an electron, there-
fore the molecules have the lowest electronegativity values
having the ability to easily give electrons and vice versa to
molecules with high values of electronegativity [70].
As can be seen from Table 3 the two descriptors (ω)
and (χ) present high values for the [cationic surfactant]
[Br−] form more than the form [cationic surfactant] [OH−],
moreover in both forms, the C14EtOH remains the most spe-
cies with the ability to accept a negative charge among the
three surfactants studied because of these high values of
The energy of the gap ΔE=ELUMO−EHOMO refects the
polarizability and the reactivity of a molecule [73, 74], this
parameter ΔE gap increases in order, C14EtOH < C14iP-
rOH<C14PrOH, which indicated that the acceptance of the
negative charge was easily made from C14EtOH to C14PrOH
for both forms [cationic surfactant] [Br−] and [cationic sur-
factant] [OH−], which is in agreement with the results of
the condensation reaction yield (%) shown in Table 1, there-
fore, the compound C14EtOH has the best reaction efciency,
which means that C14EtOH has a high capacity to accept a
negative charge thus refecting the electrostatic afnity force
1 3

Z. Hafdi et al.
Fig. 9 The proposed mechanism
of cross-condensation (Claisen–
Schmidt reaction) in Micellar
catalysis–NaOH/Water–NaOH
the donor and attractor group attached to the benzaldehyde
molecule has an efect on the yields (%) and the time (min)
of the condensation reaction (Table 1). This efect is due to
the degree of nucleophilicity of the R group in the following
order [79]: 4-Cl>4-OCH3 >4-CH3 which causes an infu-
ence on the electron density of the aldehyde carbon in order
to make it more electrophile thus encouraging more the attack
of the acetophenonate-ion. On the other hand, the reaction in
the Micellar catalysis-NaOH system (15 mmol in surfactant)
shows (Table 1 and Fig. 1) a remarkable yield (%) depending
on the R-benzaldehyde (R=4-Cl, 4-OCH3, 4-CH3) between
75 and 91%.
electronegativity (χ) thus following C14iPrOH and C14PrOH
respectively.
This height acceptance capacity of negative charge for the
species C14EtOH in both forms ([cationic surfactant] [Br−]
and [cationic surfactant] [OH−]) following respectively of
C14iPrOH and C14PrOH, indicates the order of the electro-
static afnity (C14PrOH>C14iPrOH>C14EtOH) between the
positive charge carried by the quaternary ammonium atom N+
and the substance with nucleophilic character present in the
reaction medium (enolate, benzaldehyde and acetophenone),
this also justifes the same order in which the cross-conden-
sation reaction efciency (yield(%)) with diferent substituted
benzaldehyde increases from C14PrOH to C14EtOH.
In this context, and with all these information’s concerning
the orientation of the reagents (acetophenone and R-benzal-
dehyde) and their reactivity in the Micellar catalysis–NaOH
system we propose the following mechanism (Fig. 9).
with diferent R-benzaldehyde (R=4-Cl, 4-OCH3, 4-CH3)
the Claisen–Schmidt reaction was carried out in Micellar
catalysis–NaOH of C14EtOH surfactants at 30 mmol (Table 3).
According to the mechanism proposed by Nayak and Rout
[77, 78] in which the methyl group of acetophenone under-
goes a nucleophilic attack by OH− result an abstract of the
proton thus forming the acetophenonate-ion this anion formed
to attack the carbon atom of the carbonyl group in benzalde-
hyde, this step will be more favorable each time the carbon of
the aldehyde is more electrophilic (poorer in electron). That
is why it can be observed from the results that the efect of
4 Conclusions
The catalytic efciency for the three cationic surfactants
with diferent polar heads has been studied for the condensa-
tion reaction (Claisen–Schmidt) between benzaldehyde and
acetophenone as a model of reaction. The comparison of the
1 3

Micellar Catalysis Strategy of Cross‑Condensation Reaction: The Efect of Polar Heads on the…
17. Akram M, Yousuf S, Sarwar T, Kabir-ud-Din (2014) Colloids Surf
yield (%) in Micellar catalysis–NaOH system for (C14EtOH,
C14PrOH, C14iPrOH) reveals the following conclusions:
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•
As we have already mentioned above the positively
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ity of the micelles shows a no-correlation between the
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•
•
The solubilization and the emulsifcation capacity for
the various surfactants show, based on the spectroscopic
UV–Vis and optical microscopic methods, that the mis-
cibility of the benzaldehyde and acetophenone reagents
has a signifcative efect on the yield (%) of the reaction.
According to their positive charge, the cationic heads of
the surfactants are contributed to the electrostatic inter-
action with the nucleophilic substance (benzaldehyde,
acetophenone, and enolate). In the aqueous medium, the
theoretical study with DFT shows that the degree of elec-
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