M. Vashishtha et al. / Applied Catalysis A: General 466 (2013) 38–44
39
appreciated in order to have environmentally friendly alkali based
processes.
mixture was neutralized with concentrated HCl. The organic phase
was extracted with ethyl acetate (10 mL) and was analyzed by
gas chromatography (Agilent 7890A) having a HP-5 (60 m, 250 m
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. The conversion of n-heptanal was calculated on the
basis of its weight percent as follows,
The cross aldol condensation, also known as Claisen Schmidt
reaction, is an important class of organic reactions for the synthesis
of ␣,-unsaturated carbonyls (cross aldol products). The cross aldol
condensation between two different carbonyls usually results to
formation of significant amount of self aldol product, which reduces
the yield of cross aldol product. It is of great interest to establish the
synthesis methodology for selective synthesis of cross product in
cross aldol reactions. Jasminaldehyde (␣-pentylcinnamaldehyde) is
by-product during the reaction by self condensation of n-heptanal,
which reduces the yield of jasminaldehyde. The highest selectivity
of jasminaldehyde is usually achieved by using high molar amount
of benzaldehyde [26–29]. From the industrial point of view, one
of the fundamental challenges and ultimate goals for this reac-
tion is the selective synthesis of jasminaldehyde using equimolar
amount of benzaldehyde and n-heptanal under ambient condi-
tions. In continuation of our work to maximize the selectivity of
cross aldol product in aldol condensation reactions, we studied the
cross condensation of benzaldehyde and n-heptanal to jasminalde-
hyde (cross aldol product). The inorganic base-cationic micellar
system has been used in organic synthesis to promote the reactions
[30–34], however, its applications to several synthetically useful
reactions have been much less explored. For the first time, we have
been able to achieve the highest selectivity (∼90%) of jasminalde-
hyde at equimolar aldol reaction of benzaldehyde and n-heptanal
in aqueous NaOH-CTAB micellar solution at room temperature. Our
novel finding is that the use of cationic micelles with homogeneous
base (e.g., NaOH) not only improves the efficiency of catalytic sys-
tem for selective reaction in aqueous medium but it also makes
the homogeneous catalyst reusable. This work also discusses the
effects of surfactant concentration on cross aldol condensation.
Conversion (wt.%) ofn-heptanal = 100
[Initial wt.% ofn-heptanal − Final wt.% ofn-heptanal]
×
.
Initial wt.% ofn-heptanal
The selectivity of the jasminaldehyde was calculated as below,
Selectivity (%) of jasminaldehyde = 100
(GC peak area% of jasminaldehyde)
ꢀ
×
.
Total peak area for all the products
The products formed in the reactions 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 chromato-
graph mass spectrometer (Agilent 5975 GC/MSD with 7890A GC
system) having HP-5 capillary column of 60 m length and 250 m
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.
In order to recover the spent NaOH solution and to examine the
reusability in next reaction cycle, the biphasic reaction was car-
ried out in another set under similar condition for 72 h. After the
completion of reaction, the stirring was stopped and the reaction
mixture was allowed to stand for 10 min for phase separation. The
aqueous phase (the bottom phase) was completely separated by
using a separating funnel and the obtained spent NaOH solution
was reused in next reaction cycle. In the spent NaOH solution, the
mixture of benzaldehyde (5 mmol) and n-heptanal (5 mmol) was
added under stirring (700 rpm) and the reaction was carried out at
30 ◦C for 72 h to test the reusability. After completion of reaction,
the reaction mixture was worked up and analyzed as previously
described.
2. Experimental
2.1. Materials
Benzaldehyde (>99%), n-heptanal (98%), sodium hydroxide
(NaOH; 97%), ethyl acetate (99%), n-hexane (99%), concentrated
hydrochloric acid (HCl; 35%) and sodium chloride (NaCl; 99%)
were purchased from Merck, India. The quaternary ammonium
surfactants (QASs) like decyl trimethyl ammonium bromide (98%)
was from Spectrochem, India, dodecyl trimethyl ammonium bro-
mide (98%), tetradecyl trimethyl ammonium bromide (98%) and
hexadecyl trimethyl ammonium bromide (98%) were from s.d.
Fine Chemicals, India and octadecyl trimethyl ammonium bromide
(99%) was procured from Sigma Aldich. All the chemicals were used
without any further purification. The milli-pore deionized water
was used in all the reactions.
The aldol condensation of benzaldehyde (1) and n-heptanal (2)
in NaOH-surfactant aqueous solution was carried out under similar
reaction condition (Scheme 2). In the reaction tube, 10 mL of sur-
factant aqueous solution was taken and a mixture of benzaldehyde
(5 mmol) and n-heptanal (5 mmol) was added in the solution under
stirring. The NaOH (5 mmol) was dissolved in the solution and the
reaction mixture was stirred at 30 ◦C for the required period of time
(0.5–8 h). After the completion of reaction, 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.
2.2. Procedure for biphasic aldol condensation of benzaldehyde
and n-heptanal in aqueous NaOH solution
To recover the surfactant-NaOH aqueous solution after the com-
pletion of reaction, the stirring of reaction mixture was stopped
and the solution was allowed to stand for 10 min for phase sep-
aration. The micellar solution (from the bottom) was separated
by using a separating funnel and the surfactant-NaOH solution
was reused for 1st cycle. In spent surfactant-NaOH solution, the
mixture of benzaldehyde (5 mmol) and n-heptanal (5 mmol) was
added under stirring (700 rpm) and the reaction was further car-
ried out at 30 ◦C for 4 h. The reaction mixture was worked up as
previously described. For 2nd and subsequent reaction cycles, the
The biphasic aldol reaction of benzaldehyde (1) and n-heptanal
(2) was carried out in aqueous NaOH solution (Scheme 1). The
10 mL deionized water was taken in a 50 mL reaction tube of reac-
tion station (12 Place Heated Carousel Reaction Station, RR99030,
Radleys Discovery Technologies, UK) and a mixture of benzalde-
hyde (5 mmol) and n-heptanal (5 mmol) was added under stirring
(at 700 rpm). The NaOH (5 mmol) was dissolved in the biphasic mix-
ture and the reaction mixture was stirred at 30 ◦C for the required
period of time (4–72 h). After completion of reaction, the reaction