Study on catalytic property of NaOH-cationic surfactant solutions for efficient, green and selective synthesis of flavanone

Article abstract of DOI:10.1016/j.molliq.2015.02.017

The catalytic activity of aqueous NaOH-quaternary ammonium surfactant (NaOH-QAS) solutions was studied for efficient and selective synthesis of flavanone by cross aldol reaction of benzaldehyde and o-hydroxy acetophenone (as model reaction) under ambient condition (at 30°C). The surfactant's nature (alkyl chain length and head group size) and concentration influence the reaction in NaOH-QAS micellar solutions. The NaOH-QAS systems, with surfactant concentration below critical micelle concentration (CMC), showed high activity giving high conversion and flavanone selectivity. This result was interpreted as substrate induced micellization and favorable interactions of surfactant molecules with reactants and intermediates. The reduction in conversion and selectivity of flavanone was observed with NaOH-QAS solutions having surfactant concentration at CMC or above CMC, which was due to strong solubilization of o-hydroxy acetophenone and intermediates in the micelles. The highest conversion (92% in 24 h) and selectivity of flavanone (98%) were obtained with NaOH-cetylpyridinium chloride (NaOH-C₁₆PQAS) system at surfactant concentration below CMC. The -OH group present in o-hydroxy acetophenone was observed to be responsible for its less reactivity in the aldol reaction. The reusability of NaOH-cationic surfactant solution was examined for three reaction cycles.

Full text of DOI:10.1016/j.molliq.2015.02.017

MOLLIQ-04669; No of Pages 9
Journal of Molecular Liquids xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Study on catalytic property of NaOH-cationic surfactant solutions for
efficient, green and selective synthesis of flavanone
a,b
Manu Vashishtha a, Manish Mishra a, , Dinesh O. Shah
⁎
a
Department of Chemical Engineering & Shah-Schulman Center for Surface Science and Nanotechnology, Faculty of Technology, Dharmsinh Desai University, College Road, Nadiad 387 001,
Gujarat, India
b
Department of Chemical Engineering and Department of Anesthesiology, University of Florida, Gainesville, FL 32608, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The catalytic activity of aqueous NaOH–quaternary ammonium surfactant (NaOH–QAS) solutions was studied
for efficient and selective synthesis of flavanone by cross aldol reaction of benzaldehyde and o-hydroxy
acetophenone (as model reaction) under ambient condition (at 30 °C). The surfactant's nature (alkyl chain length
and head group size) and concentration influence the reaction in NaOH–QAS micellar solutions. The NaOH–QAS
systems, with surfactant concentration below critical micelle concentration (CMC), showed high activity giving
high conversion and flavanone selectivity. This result was interpreted as substrate induced micellization and
favorable interactions of surfactant molecules with reactants and intermediates. The reduction in conversion
and selectivity of flavanone was observed with NaOH-QAS solutions having surfactant concentration at CMC
or above CMC, which was due to strong solubilization of o-hydroxy acetophenone and intermediates in
the micelles. The highest conversion (92% in 24 h) and selectivity of flavanone (98%) were obtained with
NaOH–cetylpyridinium chloride (NaOH–C16PQAS) system at surfactant concentration below CMC. The –OH
group present in o-hydroxy acetophenone was observed to be responsible for its less reactivity in the aldol
reaction. The reusability of NaOH-cationic surfactant solution was examined for three reaction cycles.
© 2015 Published by Elsevier B.V.
Received 15 October 2014
Received in revised form 7 February 2015
Accepted 11 February 2015
Available online xxxx
Keywords:
Claisen–Schmidt condensation
Base catalysis
Flavanone
Chalcone
Cetylpyridinium chloride
Micelles
Premicellar catalysis
1. Introduction
the reactions by providing micelles having confined hydrophobic
environment in water for the solubilization of hydrophobic compounds
The increasing demand to establish ecofriendly methodology for the
synthesis of chemicals has motivated the researcher to use water as safe
and green solvent [1]. In many organic syntheses, water has been
chosen due to its co-catalytic activity in promotion of the reactions [2].
There are numerous traditional homogeneous base catalysis processes,
in which NaOH and KOH are being used as base catalysts [3] owing to
the benefits such as the use of mild reaction conditions, a high conver-
sion rate and selectivity. The main disadvantages of these catalysts
are their immiscibility (due to water soluble nature) with the organic
reactants of hydrophobic nature and their difficult separation. The
catalyst separation problem may be overcome if the reactions are
performed under aqueous–organic biphasic condition, in which catalyst
resides in the aqueous phase and so the aqueous phase containing
dissolved catalyst can be separated by decantation. However, the limited
interfacial area in aqueous–organic biphasic reactions (immiscibility
problem) results to very slow reaction rate; which is the major constrain
of water mediated organic synthesis [4].
and ionic surface to accumulate the water soluble ionic species [5–7]. In
addition, the organic reactions are made selective for the formation of
desired product in the aqueous surfactant solutions due to spatial
orientation of reactant molecules in the micelles [5a–c,6c,8]. However,
the surfactant's nature and concentration have remarkable effect over
micellar reactions (reaction rate and selectivity). Recently, we reported
the enhancement in conversion and selectivity of cross aldol products in
NaOH catalyzed cross aldol reactions using surfactant solution at high
concentration (CTAB; N150 mM) [5a,b]. In many micellar reactions,
the small concentration of surfactant (below CMC or premicellar
concentration) was found to be more effective in enhancement of reac-
tion rate [9]. Brinchi et al. [9a–c] reported the acceleration of reactions in
cationic surfactant solutions at or below CMC. In premicellar solutions,
the reactions are accelerated due to either complexation of the substrate
with surfactant monomers or pre-micelles or to substrate induced
formation of micelles; and the decreased reaction rate at high surfactant
concentration has been ascribed to dissolution/strong binding of
reactant molecules in the micelles [9]. Pratik et al. [10] reported the
inhibition of hydrolysis of phenyl salicylate in the cationic surfactant
solutions having surfactant concentration below CMC suggesting a
pre-micellar aggregation between the substrate and surfactant mono-
mers, which protects the ester group from water hydrolysis.
The reactant(s)-water or aqueous catalyst immiscibility problem has
been solved by using surfactants in aqueous solutions, which facilitate
⁎
Corresponding author.
E-mail address: manishorgch@gmail.com (M. Mishra).
http://dx.doi.org/10.1016/j.molliq.2015.02.017
0167-7322/© 2015 Published by Elsevier B.V.
Please cite this article as: M. Vashishtha, et al., Study on catalytic property of NaOH-cationic surfactant solutions for efficient, green and selective
synthesis of flavanone, J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.02.017

2
M. Vashishtha et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx
Scheme 1. Synthesis of flavanone by acid/base catalyzed reactions: (A) cross aldol reaction of 1 and 2 to 3, followed by its cyclization to 4; (B) cyclization of 3 to 4.
The flavanones are important bioactive compounds, which exhibit
2. Materials and experimental methods
numerous biological activities such as neuron protection, anti-tumor,
anti-metastasis, anti-microbial, anti-oxidant, anti-inflammatory, and
anti-viral activities [11]. Generally, a flavanone is synthesized by acid/
base catalyzed Claisen–Schmidt condensation or cross aldol reaction
of an aromatic aldehyde (e.g., benzaldehyde: 1) with o-hydroxy
acetophenone (2) or its derivative to chalcone (3) followed by its
cyclization to flavanone (4) (Scheme 1A). Alternatively, a flavanone
can also be synthesized by acid/base catalyzed cyclization of chalcone
(3) (Scheme 1B). Numerous heterogeneous and homogeneous catalysts
have been reported for the synthesis of flavanones using both the
methods [12]. The single step synthesis of flavanone (Scheme 1A) by
cross aldol reaction of 1 and 2 via chalcone is an appropriate route as
compared to the methods using chalcone as precursor for synthesis of
flavanone (Scheme 1B). In most of the reported synthesis methods,
high temperature/reflux condition and use of an organic solvent
have been employed to achieve good yield of flavanone [12a–e]. The
incomplete conversion of chalcone to flavanone and further aldol con-
densation of flavanone with aromatic aldehyde to a di-condensation
product (5; Scheme 2) are very common, which reduce the selectivity/
yield of the flavanone [12b,d].
An ecofriendly, cost-effective and selective synthesis methodolo-
gy for the synthesis of flavanones involving the use of a greener
solvent medium (water) and ambient reaction condition (room
temperature) would be of great interest. We studied the catalytic
activity of NaOH–quaternary ammonium surfactants (NaOH–QAS)
micellar systems for selective synthesis of 4 by cross aldol reaction
of 1 and 2 (as model reaction) at room temperature. The study
revealed strong influence of surfactant's nature (alkyl chain length
and head group size) and concentration on this reaction. It was
found that the reaction is favored at very low surfactant concentration
(below CMC) giving the highest conversion (~92%) and selectivity of
flavanone (~98%).
Benzaldehyde (N99%), o-hydroxy acetophenone (HAP; 99%),
acetophenone (AP; 98%), sodium hydroxide (NaOH; 97%), ethyl acetate
(99%), concentrated hydrochloric acid (HCl; 35%) and sodium chloride
(NaCl; 99%) were purchased from Merck, India. The quaternary ammo-
nium surfactants (QAS) like n-decyl trimethyl ammonium bromide
(C10QAS; 98%) were from Spectrochem, India, n-dodecyl trimethyl am-
monium bromide (C12QAS; 98%), n-tetradecyl trimethyl ammonium
bromide (C14QAS; 98%), n-hexadecyl trimethyl ammonium bromide
(C16QAS; 98%), n-hexadecyl benzyl dimethyl ammonium chloride
(C16BQAS 99%) and cetylpyridinium chloride (C16PQAS; 99%) were
from S.D. Fine Chemicals, India and n-octadecyl trimethyl ammonium
bromide (C18QAS; 99%) and n-hexadecyl ethyl dimethyl ammonium
bromide (C16EQAS; N98%) were procured from Sigma Aldrich. All the
chemicals were used without any further purification. The milli-pore
deionized water was used in all the reactions.
In a 50 mL reaction tube of reaction station (12 Place Heated Carousel
Reaction Station, RR99030, Radleys Discovery Technologies, UK), 5 mL of
QAS aqueous solution (of required concentration) or water (without
surfactant) was taken and a mixture of benzaldehyde (1; 2.5 mmol)
and HAP (2; 2.5 mmol) was added in the solution under stirring
(Scheme 3). The NaOH (2.5 mmol) was dissolved in the solution and
the reaction mixture was stirred at 30 °C for the required period
of time. To monitor the reaction progress (conversion and product
distribution), the reaction mixture was worked up after the completion
of reaction. For work up, the reaction mixture was neutralized with
concentrated HCl and excess of saturated NaCl solution was added to
reduce the surfactant concentration below the CMC. The organic phase
was extracted with ethyl acetate (10 mL) and was analyzed by gas
chromatography (Agilent 5975) having a HP-5 (60 m, 250 μm diameter)
capillary column with a programmed oven temperature from 50 to
280 °C, at 1 mL min−1 flow rate of N2 as carrier gas and FID detector.
Scheme 2. Formation of di-condensation product (5) by acid/base catalyzed aldol reaction of 1 and 4.
Please cite this article as: M. Vashishtha, et al., Study on catalytic property of NaOH-cationic surfactant solutions for efficient, green and selective
synthesis of flavanone, J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.02.017

M. Vashishtha et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx
3
Scheme 3. Base catalyzed reaction (cross aldol reaction followed by cyclization) of 1 and HAP (2) to 4 via 3.
The conversion of HAP was calculated on the basis of its weight
3. Results and discussion
percent as follows,
3.1. Reaction of benzaldehyde and HAP in biphasic condition
(in aqueous NaOH)
Conversionðwt:%Þof HAP
¼ 100 ꢀ ½Initialwt:%of HAP−Finalwt:%of HAPꢁ=Initialwt:%of HAP:
The biphasic (surfactant free) reaction of benzaldehyde with HAP in
aqueous NaOH solution was very slow giving 95% conversion of HAP
and selectivity of 4 (93%) after 96 h (Table 1, entries 1–4). In addition
to 3 (2%), benzoic acid (2%) and di-condensation product (5; 3%) were
by-products in this reaction, which were probably formed by the oxida-
tion of benzaldehyde and aldol condensation of 1 with 4 (Scheme 2), re-
spectively. The reaction with spent NaOH solution (Table 1, entry 5) gave
almost similar conversion of HAP, but with reduced selectivity to 4 (78%),
and substantial amount of benzoic acid (11%), 3 (4%) and 5 (7%) were
formed. In our previous study on cross aldol reaction between benzalde-
hyde and n-heptanal to jasminaldehyde in biphasic condition, formation
of acids (benzoic acid and n-heptanoic acid) was observed in a significant
amount due to oxidation of reactants [5b]. The result of biphasic reaction
indicates that the reaction is very slow and side reactions reduce the
The selectivity of the product (4) was calculated as below,
Selectivityð%Þof 4 ¼ 100
ꢀðGCpeakarea%of product4Þ=∑Totalpeakareaforalltheproducts:
The products and by-products were characterized by GC–MS analysis
and the data were matched with those reported in the literature. GC–MS
analysis was carried out using gas chromatograph mass spectrometer
(Agilent 5975 GC/MSD with 7890A GC system) having HP-5 capillary
column of 60 m length and 250 μm diameter with a programmed oven
temperature from 50 to 280 °C, at 1 mL min−1 flow rate of He as carrier
gas and ion source at 230 °C.
To recover the spent aqueous NaOH (without surfactant) or aqueous
NaOH–QAS solution after the completion of reaction, the reaction mix-
ture was not worked up by previously discussed method. The stirring
of reaction was stopped and the reaction mixture was allowed to
stand for the crystallization of the product. The crystals of product
were separated by filtration and the filtrate (spent aqueous solution)
was reused for the next reaction cycle (1st cycle). In spent aqueous
solution, the mixture of 1 (2.5 mmol) and 2 (2.5 mmol) was added
under stirring and the reaction was further carried out under similar
reaction condition. The reaction mixture was worked up as previously
described and the organic phase was analyzed by gas chromatography
to measure the conversion and products selectivity. For 2nd and subse-
quent cycles reuse study, the spent aqueous solution was obtained
after product crystallization and filtration (i.e., without work up) of the
reaction mixture of previous cycle.
Table 1
Conversion and product selectivity in biphasic reaction of benzaldehyde and HAP in aq.
NaOH.a
Entry
Reaction
time (h)
Conversion
of HAP (wt.%)
Selectivity (%) of
(4)
(3)
Others
1
2
3
4
24
48
72
96
96
51
85
92
95
90
89
92
92
93
78
11
6
5
2
4
–
1b, 1c
1b, 2c
2b, 3c
11b, 7c
5 (reuse)
a
2.5 mmol (0.26 g) benzaldehyde, 2.5 mmol (0.34 g) HAP, 2.5 mmol (0.1 g) NaOH, 5 ml
water, 30 °C.
b
c
Benzoic acid.
Di-condensation product (5).
Scheme 4. Different QASs and their CMC values in water at 25 °C [14] used in aldol reaction
of benzaldehyde and HAP in NaOH–QAS micellar solutions.
Please cite this article as: M. Vashishtha, et al., Study on catalytic property of NaOH-cationic surfactant solutions for efficient, green and selective
synthesis of flavanone, J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.02.017

4
M. Vashishtha et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx
selectivity of flavanone. The slow reaction in biphasic system is due to
immiscibility of water and organic reactants (1 and 2) and less interfacial
area between two phases for interaction between reactants and catalyst
(OH−) taking longer reaction time to achieve the highest conversion.
To make the organic reactants compatible with aqueous solution,
the surfactants were used in the reaction. As the reaction involves
the participation of OH− ions (as catalyst), the quaternary ammonium
surfactants (QASs; cationic surfactants) were chosen to have positively
charged surfactant aggregates (micelles), which can help in the solubi-
lization of the reactants and also concentrate the OH− ions near
positively charged micellar surface. As the total amount of both the
reactants are very high as compared to surfactant concentration, the
addition of reactants in micellar solution would result into a mixed
system comprising of emulsion droplets and micelles having solubilized
reactants [5a]. Menger et al. [13] have also reported that the micellar
catalysis is a combination of both micelles and emulsion droplets.
reaction faster than biphasic reaction. However, the QAS concentration
in the reaction medium was found to be major factor influencing the
reaction (i.e. conversion of HAP and product selectivity). The exciting
result was that below CMC of all the QASs, the highest conversion
(81–92%) and selectivity of 4 (96–98%) were obtained (Fig. 1a and b).
At CMC and above CMC of each QAS, there was a significant decrease
in the conversion of HAP as well as selectivity of 4, showing a decrease
in conversion and selectivity of 4 on increasing the surfactant concen-
tration. It can be seen that the selectivity of 4 at CMC or at above CMC
of ach QAS was reduced due to unconverted 3 and also because of the
formation of 5 (Fig. 1c & d). In comparison to the di-condensation reac-
tion (formation of 5), the incomplete conversion of 3 to 4 (cyclization
reaction) had a major role in the reduction of flavanone selectivity at
high surfactant concentration. The results are contrary to our previous
report, in which a micellar system at higher QAS concentration
(N150 mM) gave the highest conversion and product selectivity in
cross aldol reactions [5a,b]. There was no formation of 5 in the biphasic
reaction. In NaOH–QAS solutions, reactions did not produce acid by-
products showing that the micellar system avoids the oxidation of
benzaldehyde during aldol reaction.
3.2. Reaction of benzaldehyde and HAP in NaOH–QAS solutions: effect of
surfactant's alkyl chain length and concentration on the reaction
The reaction of benzaldehyde and HAP was carried out using NaOH–
QAS solutions of different QASs (Scheme 4) with varied alkyl chain
length and size of head group to study their effect on catalytic perfor-
mance of the NaOH–QAS systems. The reactions were performed
under three different conditions of surfactant concentrations viz.
at CMC, below CMC (1/4 of CMC) and above CMC (15 times CMC).
Fig. 1a is showing that the presence of QAS in reaction medium makes
On varying the alkyl chain length of QAS from C10 to C18, the conver-
sion of HAP and selectivity of 4 were observed to be increasing (Fig. 1a
and b), which can be attributed to a decrease in QAS concentration
(due to a decrease in CMC) in the solutions (Table 2). The CMCs of
QASs decrease as alkyl chain length of QAS increases from C10 to C18
(Scheme 4 and Table 2). Contrary to our previous results on the effect
of alkyl chain length of QAS on micellar catalyzed aldol reactions [5a],
100
(a)
90
100
90
(b)
80
70
60
50
40
30
20
10
0
80
70
60
50
40
O
30
O
20
(4)
10
0
C16QAS
C10QAS
C12QAS
C
14QAS
C18QAS C16PQAS C16BQAS C16EQAS
C
C16BQAS
16EQAS
C10QAS
C14QAS
C16PQAS
C18QAS
C
12QAS
C
16QAS
QASs
QASs
22
20
18
16
14
12
10
8
(d)
(c)
40
35
30
25
20
15
10
5
O
O
OH
O
(3)
(
5)
6
4
2
0
0
C10QAS
C14QAS
C
C16BQAS
16EQAS
C12QAS
C
16QAS C18QAS
C
16PQAS
C10QAS
C16PQAS C16BQAS
C16EQAS
C
12QAS C14QAS C16QAS C18QAS
QASs
QASs
Fig. 1. (a) Conversion of HAP and selectivity of different products (b) 4, (c) 3 and (d) 5 in the reaction of benzaldehyde and HAP in NaOH–QAS micellar solutions of different QASs and at
different conditions of concentrations viz., at 1/4 of CMC (□), at CMC (○) and at 15 times of CMC (Δ) [reaction condition: 2.5 mmol (0.26 g) benzaldehyde, 2.5 mmol (0.34 g) HAP,
2.5 mmol (0.1 g) NaOH, 5 mL QAS aqueous solution, 30 °C, 24 h].
Please cite this article as: M. Vashishtha, et al., Study on catalytic property of NaOH-cationic surfactant solutions for efficient, green and selective
synthesis of flavanone, J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.02.017

M. Vashishtha et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx
5
Table 2
produce its carbanion or enolate ion, which reacts with benzaldehyde.
The QAS molecules in new micelles will concentrate the OH− ions at
micellar interface to facilitate the formation of enolate ions from HAP.
The QAS molecules can have interactions with molecules of reactants
in the new micelles as shown in Scheme 5i & ii [5a,15]. In dilute surfac-
tant solutions, such interactions between QAS and reactants could be
helpful for the reaction. The quaternary ammonium group of QAS can
interact with benzaldehyde molecules through carbonyl oxygen
(Scheme 5i [5b]) making carbonyl carbon more electrophilic for reac-
tion with enolate ions of HAP. Similarly, the quaternary ammonium
group of QAS can also interact with HAP through carbonyl group [15]
as well as through phenoxide (PhO−) oxygen [10] (Scheme 5ii). The
interaction of carbonyl group of HAP with positively charged head
group of QAS can activate the HAP for enolization [15]. In addition, the
QAS molecules at interface can also stabilize the enolate ions of HAP
(Scheme 5iii) for the reaction with benzaldehyde.
Concentrations of different QASs in aqueous solution at CMC [14], above CMC (15 times of
CMC) and below CMC (1/4 of CMC).
S. no.
QAS
~CMC (mM)
of QAS
~Concentration of
QAS at 15 times
of CMC (mM)
~Concentration
of QAS at 1/4
of CMC (mM)
1
2
3
4
5
6
7
8
C
C
C
C
C
C
C
C
10QAS
66.3
14.6
3.7
1
0.3
1
994
225
55.5
15
4.4
15
16.5
3.65
0.93
0.25
0.08
0.25
0.13
0.25
12QAS
14QAS
16QAS
18QAS
16PQAS
16BQAS
16EQAS
0.5
1
7.5
15
we observed the negative effect of increasing alkyl chain length of QASs
on this reaction. For instance, at almost similar concentrations of C10QAS
(at 16.5 mM i.e., below CMC; Table 2) and C16QAS (at 15 mM i.e., above
CMC; Table 2), the C10QAS gave a higher conversion and selectivity of 4
(81% and 96%, respectively) as compared to C16QAS solution (72%
conversion and 86% selectivity of 4). The C16PQAS micellar system
with surfactant concentration below CMC gave a slightly higher conver-
sion of HAP (Fig. 1a) as compared to other QAS micellar systems. In
summary, the NaOH–QAS systems with surfactant concentration
below CMC have the highest activity in the reaction of benzaldehyde
and HAP to flavanone.
The electron donating nature of phenolic –OH or phenoxide group in
HAP can reduce its reactivity in this reaction. This was also evident from
its lesser reactivity in cross aldol reaction with benzaldehyde (in biphasic
condition) than acetophenone (AP) (this part will be discussed in detail
in the next section). The interaction of phenoxide oxygen of HAP with
QAS through quaternary ammonium group may also facilitate this reac-
tion by reducing the electron donating tendency of phenoxide group.
Similarly, in dilute surfactant solutions, the chalcone (3) molecules can
interact through carbonyl and phenoxide groups with quaternary
ammonium group of QAS molecules (Scheme 5iv), which can encourage
the cyclization reaction of 3 to form 4 giving high selectivity of 4. The
interaction between carbonyl group of 3 and head group of QAS will po-
larize the α,β-olefinic C_C bond (which is in conjugation with C_O)
making β-carbon reactive for the nucleophilic attack by phenoxide
group. Thus, the substrate induced micellization and the co-catalytic
activity of QAS were main attributes for the promotion of the reactions
(enhanced conversion and selectivity of 4) in the QAS solutions with
surfactant concentration below CMC. It is to be noted that in biphasic
reaction, there was high selectivity of 4 (89–92%; Table 1, entries 1–4)
since the beginning of the reaction and there was no formation of 5.
This result suggests that the molecules of 3 or its phenoxide ions might
have significant polarity allowing them to stay at oil–water interface
for cyclization to form 4. The molecules of 4, being more hydrophobic
in nature, would migrate from interface to bulk of oil phase avoiding
the formation of 5.
3.3. Molecular mechanism of reaction in NaOH–QAS solutions
3.3.1. Cause of high activity of NaOH–QAS micellar systems with surfactant
concentration below CMC
In the QAS solutions with surfactant concentration below CMC, very
likely there are QAS monomers and submicellar aggregates. The cataly-
sis results suggest that the reaction is catalyzed with the help of QAS
monomers or submicelles. The QAS monomers and submicelles may
form new micellar aggregates with reactant(s) molecules [9a,d,e].
This will create comparatively a higher interfacial area in the reaction
medium than biphasic system and will increase the interactions
between reactants and catalyst (OH− ions) giving high conversion
of HAP. The OH− ion removes a proton from acetyl group of HAP to
Scheme 5. Interactions of (i) benzaldehyde molecule, (ii) HAP molecule, (iii) enolate ion of HAP and (iv) chalcone molecule with quaternary ammonium group of QAS in premicellar
solution.
Please cite this article as: M. Vashishtha, et al., Study on catalytic property of NaOH-cationic surfactant solutions for efficient, green and selective
synthesis of flavanone, J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.02.017

6
M. Vashishtha et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx
Scheme 6. Solubilization and interactions of (i) HAP, (ii) chalcone and (iii) enolate ion of flavanone with quaternary ammonium group of QAS (at high surfactant concentration) in micellar
interface.
3.3.2. Cause of reduced activity of NaOH–QAS systems at CMC/high
surfactant concentration
The benzaldehyde molecules present in micellar phase will react with
these enolate ions to form 5. The hydrophobic nature of 4 and possibility
of its ionic interaction as well as stabilization of its enolate ions by
QAS molecules are the attributes for the localization of 4 in micellar
phase for further reaction with benzaldehyde. The bis-alkylation of γ-
phenylcyclohexanone with benzyl halides in two steps to bis-alkylation
products in a higher amount has been reported in cationic micellar
system suggesting that the hydrophobic nature of intermediate and its
stabilization by micellar aggregates facilitate di-alkylation reactions [16].
Similarly, the high selectivity of the di-condensation product in di-aldol
condensation of benzaldehyde and cyclohexanone in NaOH–C16QAS
micellar system at high surfactant concentration was explained as high
solubilization of mono-condensation product and its enolate ions in
micellar interface by hydrophobic and ionic interactions [5a]. Thus, the
high solubilization of the molecules of reactants and intermediates, and
In our previous studies [5a,b], the high concentration of QAS
(C16QAS; N150 mM) was found to be beneficial for the reaction, in
which benzaldehyde was one of the reactants. The high surfactant
concentration in reaction medium promoted the reaction, which was
attributed to generation of huge interface, high OH− ion concentration
at interface, more solubilization of reactants, activation of benzaldehyde
molecules, their localization preferably near the interface and stabiliza-
tion of enolate ions [5a]. This shows that high QAS concentration should
also be favorable for benzaldehyde in flavanone synthesis reaction. The
high QAS concentration might be showing a negative effect on the HAP
in this reaction. At CMC and higher concentration (15 times of CMC),
the strong solubilization of HAP by QAS molecules in the micelles
(Scheme 6i) might be reducing the reaction rate giving less conversion
of HAP. The strong interaction (ionic and/or polar and hydrophobic) of
HAP with QAS molecules (Scheme 6i) as well as compact packing
of QAS molecules at micellar interface will inhibit the approach of
OH− ions present at the micellar surface to form its enolate ion.
At CMC or at high surfactant concentration, the reactions gave lesser
selectivity of 4 (Fig. 1b) as compared to the biphasic reaction or reac-
tions using surfactant concentration below CMC, which was observed
to be due to unconverted 3 and also because of the formation of 5
(Fig. 1c & d). At CMC and higher concentration of QAS in the solutions,
there will be more number of micelles, which can solubilize 3 in the mi-
cellar phase due to strong ionic interaction between phenoxide group of
3 and head group of QAS as well as by hydrophobic interaction
(Scheme 6ii). The strong solubilization of 3 in micellar phase will slow
down its cyclization to form 4, and thus reaction give substantial
amount of 3 at CMC and at high concentration of QAS.
100
Conversion of
Selectivity of
Selectivity of
Selectivity of
2
4
3
5
90
80
70
60
50
20
10
0
The formation of 5 in a significant amount at CMC and high concen-
tration of QAS (Fig. 1d) may be due to the solubilization of 4 in the mi-
cellar phase and its subsequent aldol reaction with benzaldehyde
solubilized in micelles. The more number of micelles in QAS solution
at CMC/higher concentration will increase the solubilization of 4. The
solubilized molecules of 4 will produce their enolate ions with the help
of OH− ions, which will also be stabilized at micellar interface due to
interaction with quaternary ammonium group of QAS (Scheme 6iii).
0.00
0.25
0.50
0.75
1.00
50
100
150
200
Concentration of C 16PQAS (mM)
Fig. 2. Conversion and product selectivity in base catalyzed cross aldol condensation of
benzaldehyde and HAP in NaOH–C16PQAS micellar solutions of different concentrations
[reaction condition: 2.5 mmol (0.26 g) benzaldehyde, 2.5 mmol (0.34 g) HAP, 2.5 mmol
(0.1 g) NaOH, 5 mL C16PQAS aqueous solution, 30 °C, 24 h].
Please cite this article as: M. Vashishtha, et al., Study on catalytic property of NaOH-cationic surfactant solutions for efficient, green and selective
synthesis of flavanone, J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.02.017

M. Vashishtha et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx
7
100
80
60
40
20
0
100
90
80
70
60
50
Conversion of
Selectivity of 4
Selectivity of 3
2
40
30
20
10
0
In water
In 15 mM C16PQAS
1
2
3
4
5
6
0
4
8
12
16
20
24
Reaction time (h)
Reaction time (h)
Fig. 4. Kinetics of the reaction of acetophenone (2′) and benzaldehyde (1) to chalcone
(6) in biphasic condition and in NaOH–C16PQAS solution [reaction condition: acetophenone
(2.5 mmol), benzaldehyde (2.5 mmol), NaOH (2.5 mmol), 5 mL C16PQAS aqueous solution
(15 mM), room temp (30 °C)].
Fig. 3. Kinetics of the reaction of benzaldehyde and HAP to chalcone and flavanone
[reaction condition: 2.5 mmol (0.26 g) benzaldehyde, 2.5 mmol (0.34 g) HAP, 2.5 mmol
(0.1 g) NaOH, 5 mL C16PQAS aqueous solution (0.25 mM), room temp (30 °C)].
their strong interactions with QAS molecules in micellar phase slow
down the reaction giving less conversion and selectivity of 4.
concentrations (up to 200 mM) to find out the optimum concentration
of C16PQAS giving the highest activity in the flavanone synthesis. The
result (Fig. 2) indicates that 0.25 mM (i.e., premicellar concentration)
is the optimum concentration giving maximum conversion of HAP
(92%) and selectivity of 4 (98%). Further increase in surfactant concen-
tration from 0.25 mM to 200 mM significantly lowers the conversion
of HAP and also the selectivity of 4. Fig. 2 also indicates that the incom-
plete conversion of chalcone to flavanone (cyclization reaction) and
formation of di-condensation by-product (5) in reactions at high
surfactant concentration decrease the selectivity of 4 and incomplete
conversion of chalcone to flavanone has more contribution in reduction
of the flavanone selectivity. The kinetic study (Fig. 3) of the reaction in
3.4. Effect of surfactant's head group size on the reaction
The C16PQAS micellar system with surfactant concentration below
CMC gave a slightly higher conversion of HAP (Fig. 1a) as compared to
other QAS micellar systems having C16 chain length (C16QAS and
C16EQAS) and almost equal CMC (1 mM). The slightly higher conversion
with C16PQAS can also be seen even at CMC and at above CMC than
other C16 surfactants (C16QAS, C16BQAS and C16EQAS) at their CMCs
and above CMCs (Fig. 1a). The comparatively higher conversion of
HAP with C16PQAS micellar system may be probably due to loose
packing of C16PQAS molecules in micelles and less hindrance of planner
pyridinium group in the reaction. The bigger size of head group
(pyridinium group) of C16PQAS molecules [17,18] may be responsible
for large intermolecular gaping between surfactant molecules in
micelles. The intermolecular gape/space can be beneficial in the accom-
modation of reactant molecules at interface for reaction and also for
better accessibility of OH− ions to HAP molecules present in micellar
interface. Pratik et al. [10] demonstrated lesser interference of
the C16PQAS molecules than other QASs (C14 and C16 surfactants) at
interface in the hydrolysis of phenyl acetophenone because of planner
pyridinium head group showing less hindrance in approach of water
molecules for nucleophilic attack at ester group. This shows that as com-
pared to other QASs, C16PQAS micellar system (at 0.25 mM) possesses
optimum properties and/or microenvironment (hydrophobicity, inter-
molecular packing, etc.) for this reaction giving the highest conversion
and selectivity of 4.
C
16PQAS pre-micellar (0.25 mM) solution revealed that the reaction
took around 24 h to achieve ~92% conversion and the chalcone is pro-
duced during the reaction as intermediate, which undergoes cyclization
and is finally converted into flavanone (98%).
3.5. Cross aldol reaction of benzaldehyde and acetophenone to chalcone in
biphasic condition and in NaOH–C16PQAS micellar system
The cross aldol reaction of 1 with acetophenone (AP; 2′) to chalcone
(3′) in NaOH–C16PQAS micellar system (Scheme 7) was studied to
investigate the effect of –OH group present in aromatic ketone
(i.e., HAP) on its reactivity for the aldol reaction. The reaction of 1
with 2′ in 15 mM C16PQAS solution was found to be very fast giving
~82% conversion of 2′ under biphasic condition and almost 100% con-
version within 6 h (Fig. 4). It can be seen that AP could give considerably
high conversion (~80%) within 30 min in C16PQAS micellar solution
even at high surfactant concentration (15 mM). The result clearly
shows that AP is more reactive than HAP for cross aldol reaction. The
lesser reactivity of HAP than AP in the cross aldol reaction can be due
The reactions were also performed at different concentrations
of C16PQAS ranging from its premicellar concentrations to higher
Scheme 7. Base catalyzed cross aldol reaction of benzaldehyde (1) and acetophenone (2′) to chalcone (3′) in water and in NaOH–C16PQAS solution.
Please cite this article as: M. Vashishtha, et al., Study on catalytic property of NaOH-cationic surfactant solutions for efficient, green and selective
synthesis of flavanone, J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.02.017

8
M. Vashishtha et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx
Conversion of HAP
Selectivity of 4
Selectivity of 3
Acknowledgment
100
80
60
40
20
0
Authors are thankful to Dr. H.M. Desai, Vice-Chancellor, DDU for
providing necessary facilities to pursue the research work.
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