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ASIAN JOURNAL OF
SCIENCE AND TECHNOLOGY
ISSN: 0976-3376
Asian Journal of Science and Technology
Vol. 2, Issue, 2, pp.001-003, February, 2011
3
RESEARCH ARTICLE
A FACILE GREEN SYNTHESIS OF 1-PROPYLBENZOATE PROMOTED BY IONIC
LIQUID UNDER SOLVENTLESS CONDITION
*1Rajendran, A., 2Ramu, S. and 1Karthikeyan, C.
1Department
of Chemistry, Sir Theagaraya College, Chennai-600 021, Tamil Nadu, India.
and Development Centre, Bharathiar University, Coimbatore, India.
2Research
Received 12th December, 2010; Received in revised from; 1st January, 2011; Accepted 25th January, 2011; Published online 15thFebruary, 2011
An esterification reaction was carried out to prepare 1-Propylbenzoate using silica sulphuric acid as an ionic liquid
under solvent less condition. The catalytic activity and the amount of the target product 1-propylbenzoate were
determined. The optimal reaction conditions were fixed and the product was characterized by FT-IR, 1H-NMR and
13
C-NMR studies. The catalytic behavior of silica sulphuric acid was compared with that of sulphuric acid, a
standard commercial catalyst. The ionic liquid was recovered and reused for more than half a dozen times without
the potential loss in the activity. There is also the possibility of avoiding the use of any reaction medium as solvent
less reaction, which is the main theme of this article. The present scope and potential economic impact of the
reaction are demonstrated by the synthesis of 1-propylbenzoate. Remaining challenges and future perspectives of
the new transformation are discussed.
Key words: Green synthesis, Solventless reaction, Ionic liquid, esterification reaction, 1-propylbenzoate
© Copy Right, AJST, 2011, Academic Journals. All rights reserved.
INTRODUCTION
The development of environmentally improved
new synthetic routes, which are as much direct as
possible and resort to the use of safe and non-toxic
starting materials, is a major target of modern
chemistry of organic synthesis (Rajendran, 2010).
In recent years, the interest in room temperature
ionic liquids is increasing as green reaction media
for synthetic organic chemistry. Researchers have
recently found that such ionic liquids are very
useful as solvents as well as catalysts for several
organic and inorganic syntheses (Sheldon, 2001).
Ionic liquids are good candidates for replacement
of toxic and volatile organic compounds because of
their lower vapor pressures and lack of
flammability (Rajendran, 2010).
*Corresponding author: [email protected]
The synthesis of these ionic liquids (ILs), their
characterization and possible applications have
been developing progressively as the properties of
this class of organic salts with melting points
below the boiling point of water have gained
intensive attention in nearly all fields of chemistry
(Kubisa, 2004). Furthermore, through the
incorporation of functional groups, the synthesis of
task specific ionic liquids (TSILs) has been a focus
of research, leading to tailor-made substances for
desired applications. Traditional chemical synthesis
focused on optimizing yields, with little regard to a
chemical’s impact on the environment and its long
term viability (Welton, 1999). There is now a
realization that more benign chemical synthesis is
required, as an integral part of developing
sustainable technologies. Optimizing the yield is
important but other issues need to be addressed,
including minimizing the number of steps,
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Asian Journal of Science and Technology, Vol. 2, Issue, 2, pp.001.003, February, 2011
simplicity, waste, atom efficiency, energy usage,
safety
and
whether
the
chemistry is
environmentally acceptable (Dupont, et al., 2002).
In the present work, Fischer esterification was
carried out to synthesize 1-Propylbenzoate using
benzoic acid and 1-Propylalcohol in the presence
of silica-sulphuric acid as the ionic liquid catalyst.
Generally the equilibrium constants for
esterification with most of the primary alcohols are
near unity. For an instance, the equilibrium
constant for the esterification of acetic acid with
ethyl alcohol is 3.38. The reaction is driven to
completion by using the reactant alcohol as the
solvent. Because the alcohol is present in large
excess, the equilibrium is driven toward the ester
product. This is an example of Le Chatlier’s
principle (Smith, and March, 2001).
MATERIALS AND METHODS
Chemicals
All the chemicals were of AR grade and used
without further purification unless otherwise stated.
These chemicals were obtained from S.D. fine
chemicals, Mumbai. Silica gel was obtained from
Lancaster (UK) and was used as received.
Synthesis of Ionic liquid
The synthesis of silica-sulphuric acid has been
carried-out using a similar method reported in the
literature (Salehi, et al., 2004). A 500ml suction
flask equipped with a constant-pressure dropping
funnel and a gas inlet tube for conducting vapors
over an absorbing solution (water) was used.
Room
SiO2 - OH + H2SO4
SiO2 – OSO3 + H2O
Temperature
Scheme 1: Synthesis of ionic liquid: silica-sulphuric acid
Synthesis of 1-Propylbenzoate using ionic liquid
1-Propanol (0.01 mol), equivalent amount of
benzoic acid (0.01 mol) and ionic liquid (SiO2OSO3H) (0.005 mol) were taken in a round-bottom
flask fitted with a reflux condenser. The contents
were heated on an oil-bath for 30 min at 60 ⁰C. The
RB flask is then fitted with Young’s still head (4
bulbs), and the contents were fractionally distilled
using water condenser first (unreacted ethanol
distills over). Reaction progress was monitored by
TLC. After the completion of the reaction the
ester was simply decanted from the ionic liquid
(Kilivelu and Yatimah, 2007). The Product 1Propylbenzoate was characterized by using 1HNMR in CDCl3 with TMS as reference (300 MHz)
and by FT-IR on a Shimadzu (Model 8201 pc)
spectrophotometer using KBr pellets in the desired
frequency range and by a double beam UV-Vis.
Spectrophotometer [Elico make Model SL-191].
The ionic liquid in the filtrate was separated from
unreacted starting materials by decantation or by
filtration and was then taken out from the RB
flask and water was removed under vacuum
(0.01Torr) at 100◦C for 1 hr was reused. The same
reaction was carried out under the same set of
conditions without ionic liquid (but with only
H2SO4).
RESULTS AND DISCUSSIONS
Silica-sulphuric acid has been used in small and
long scale reactions as an acid catalyst. In many
Table 1. Synthesis of 1-Propylbenzoate with and without ionic liquid
Description
% yield of the producta
Silica sulphuric acid (with ionic liquid)
Sulphuric acid (without ionic liquid)
98
89
Time (h)
0.5
2
By-product
Nil
Nil
Temperature, oC
60
80
a
Isolated yield based upon the starting carboxylic acid
It was charged with silica gel and sulphuric acid
was added drop wise over a period of 30 minutes at
room temperature. After the addition was
completed the mixture was shaken well for 30
minutes, a white solid of silica sulphuric acid was
obtained.
synthetic reports SiO2.OSO3H has been used
(Salehi, et al., 2004). The silica supported
sulphuric acid is bench top reagent which is easy to
handle with better accessibility of the reactants to
the active sites. Exposure of high surface area
Rajendran et al., A facile green synthesis of 1-propylbenzoate promoted by ionic liquid under solvent less condition
oxides such as Si-O-Si to H2SO4 at room
temperature results in irreversible adsorption. The
resulting surface possess surface species such as
Si-O2-SO3-H+.
This
esterification
reaction
promoted by silica sulphuric acid is a substitution
of –OH at the carbonyl group of the acid by the
oxygen of the alcohol. Thus, it is an example of
substitution at the carbonyl carbon. The mechanism
of acid-catalyzed esterification reaction is very
important because it serves as a model for the
mechanism of several acid catalyzed reactions of
carboxylic acids and their derivatives. In the
mechanism that follows, the formation of 1Propylbenzoate in the solvent 1-Propyl alcohol
(one of the reactants) is shown for concertness. The
ionic liquid silica-sulphuric acid being a superior
proton than sulphuric acid itself, more readily
donates its proton to 1-Propanol to form protonated
alcohol as a conjugated acid of the solvent which in
turn attacks the carbonyl oxygen of carboxyl group
of benzoic acid as the catalyst.
The results in
Table 1 vividly demonstrate that the ionic liquid
(SiO2-OSO3H) used in this Fisher esterification
process more effectively catalyses than sulphuric
acid itself without the consumption of any
environmentally hazardous solvent. Not only that,
the ionic liquid silica sulphuric acid is completely
soluble in water and immiscible with organic
solvents such as ether, chloroform, etc., so the
esterification reaction proceeded smoothly for
completion even without simultaneous removal of
the water produced during the course of the
reaction. The ionic liquid could be easily recovered
(Chen et al., 2007). After the reaction, the ionic
liquid was simply separated and regenerated by
removal of water in vacuum. It was reused
repeatedly for more than half a dozen times for the
same reaction and it was found that the results
were found as almost same as that in Table-1. The
product, 1-Propylbenzoate was identified by its 1H
-NMR in CDCl3 with TMS as reference (300
MHz) and by FT-IR on a Shimadzu (Model 8201
pc) spectrophotometer using KBr pellets by a
double beam UV-Vis. Spectrophotometer [Elico
make Model SL-191] and it was confirmed by
product analysis.
Conclusion
Fischer esterification reaction was carried out to
synthesize 1-Propylbenzoate using the ionic liquid
003
silica sulphuric acid as a catalyst. This method was
found to be a very efficient and a selective protocol
for the esterification in the presence of a reusable
and environmentally benign catalyst. Simple
workup procedure, including washing the mixture
followed by evaporation of the solvent, is another
advantage of this method. Additionally, the ionic
liquid was successfully reused for over 6 times
without significant loss of activity which may
make significant break through in the field of
organic synthesis with special reference to ionic
liquids.
Acknowledgement
The authors immensely thank the UGC, New Delhi
for the financial support rendered for the major
research project [F. No. 35-147 / 2008 (SR)]. The
authors also thank the principal and the
management of Sir Theagaraya College, Chennai21, and Tamil Nadu for their constant
encouragement and support given.
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