Effect of Ionic Liquids on the Chemical Equilibrium of Esterification of Carboxylic Acids with Alcohols

Article abstract of DOI:10.1081/SCC-120027257

The esterification of carboxylic acids with alcohols catalyzed by paratoluene sulfonic acid (pTSA) was studied in different room temperature ionic liquids. The chemical equilibrium of esterification can be tuned by using ionic liquids as solvents.

Full text of DOI:10.1081/SCC-120027257

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Effect of Ionic Liquids on the Chemical Equilibrium of
Esterification of Carboxylic Acids with Alcohols
Tao Jiang a , Yanhong Chang a , Guoying Zhao a , Buxing Han a & Guanying Yang a
a Center for Molecular Science, Institute of Chemistry, The Chinese Academy of Sciences,
Zhongguancun, Beijing, 100080, P.R. China
Published online: 18 Oct 2011.
To cite this article: Tao Jiang , Yanhong Chang , Guoying Zhao , Buxing Han & Guanying Yang (2004): Effect of Ionic
Liquids on the Chemical Equilibrium of Esterification of Carboxylic Acids with Alcohols, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 34:2, 225-230
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SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 2, pp. 225–230, 2004
Effect of Ionic Liquids on the Chemical
Equilibrium of Esterification of
Carboxylic Acids with Alcohols
*
Tao Jiang, Yanhong Chang, Guoying Zhao, Buxing Han,
and Guanying Yang
*
Center for Molecular Science, Institute of Chemistry, The Chinese
Academy of Sciences, Beijing, P.R. China
ABSTRACT
The esterification of carboxylic acids with alcohols catalyzed by para-
toluene sulfonic acid (pTSA) was studied in different room temperature
ionic liquids. The chemical equilibrium of esterification can be tuned by
using ionic liquids as solvents.
Key Words: Carboxylic acid; Alcohol; Paratoluene sulfonic acid; Ionic
liquid; Equilibrium.
*
Correspondence: Buxing Han and Tao Jiang, Center for Molecular Science, Institute
of Chemistry, The Chinese Academy of Sciences, Zhongguancun, Beijing 100080,
P.R. China; Fax: þ86-10-62559373; E-mail: Hanbx@iccas.ac.cn.
225
DOI: 10.1081/SCC-120027257
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

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Jiang et al.
Room temperature ionic liquids (ILs) are organic salts consisting of
cations and anions. They are good solvents for many kinds of organic chemi-
cals. ILs have excellent thermal and chemical stability, almost undetectable
vapor pressure at room temperature, wide electrochemical windows and a
wide liquid range of about 300 K. The physical and chemical properties of ILs
can be tuned by varying cations and anions. These special properties make
them suitable replacement for volatile organic solvents in different chemical
processes. They are regarded as clean solvents. Significant progress has been
made in the application of ILs in catalysis and chemical synthesis in recent
years. Several excellent reviews[1–6] have been published providing com-
prehensive information about the properties, preparation and applications of
ILs in organic and catalytic reactions. In the past decade, a variety of typical
organic reactions have been successfully conducted in ILs with high
selectivity and yields, such as Friedel–Crafts reactions, alkylations, hydro-
genations, hydroformylations, Diels–Alder reactions, Heck reactions, olefin
dimerization and oligomerizations, and oxidations. Several papers have
recently been published on the use of ILs as reaction media or catalyst for
esterifications[7–10] and transesterifications.[11–14]
Tuning equilibrium of reversible reactions is one of the interesting
topics in chemistry. In this work we probe the possibility to tune the
equilibrium of a chemical reaction using an IL. The esterification reactions
of carboxylic acids with alcohols in 1-butyl-3-methylimidazolium hexa-
fluorophosphate ([bmim][PF6]), 1-butyl-3-methylimidazolium tetrafluorobor-
ate ([bmim][BF4]), and 1-ethyl-3-methylimidazolium hexafluorophosphate
([emim][PF6]) are studied, and the reactions are also carried out in the
absence of the ILs. The results indicate that the equilibrium conversion can
be enhanced by using some ILs. This opens up a new way to tune chemical
equilibrium using green solvents.
ILs were synthesized according to the method in literature and dried at
408C in vacuum until its mass was independent of drying time. Reactions were
carried out in a stainless steel reactor of 11 mL with magnetic stirrer. Para-
toluene sulfonic acid (pTSA) was used as catalyst.
The esterification was carried out with and without ILs. Only the
experimental procedures with the IL are described because those in the
absence of the IL are similar and simpler. In a typical experiment, suitable
amount of IL, pTSA, alcohol and acid were added into the reactor after the
reactor was purged with N2. Then the reactor was put into a constant
temperature bath. After a certain time, the reactor was cooled rapidly with
iced-water for about one hour. Then the reactants and products in the IL were
extracted six times with diethyl ether. Experiments indicated that all the
reactants and products could be extracted because they were not detectable in
the solvent after extracting six times. All analysis was carried out with an

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Carboxylic Acids with Alcohols
227
Figure 1. Dependence of ethanol conversion on reaction time for the reaction of
ethanol and acetic acid at 608C (ethanol : acetic acid : pTSA ¼ 1 : 1 : 0.01 in molar
ratio, ethanol 13 mmol).
Agilent 4890D GC equipped with a FID and an Innowax capillary column
(30 m  0.252 mm  0.25 mm).
Effect of reaction time on conversion was first investigated. Figure 1
illustrates the conversion of the reaction of ethanol and acetic acid as a
function of reaction time without solvent, which demonstrates that chemical
equilibrium can be reached in 5 h. The effect of reaction time on conversion of
ethanol in [bmim][PF6] is shown in Table 1. The data indicate that reaction
equilibrium can be reached after 20 h at 608C, which is longer than that in the
absence of the IL. The main reason is that the viscosity of ILs is larger than
ordinary organic liquids.[15] It is easy to understand that the higher the
viscosity, the longer the time is needed for reaching equilibrium.
The equilibrium conversions of the reactions are presented in Table 2.
Entries 1–6 show that the equilibrium conversion of the reaction of ethanol
Table 1. Conversion at different reaction times in
[bmim][PF6].a
No.
Reaction time/h
Ethanol conv./%
1
2
3
16
20
24
69.2
73.1
73.2
aEthanol : acetic acid : pTSA ¼ 1 : 1 : 0.01 in molar
ratio; ethanol 13 mmol; [bmim][PF6] 3.3 g; tempera-
ture 608C.

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Jiang et al.

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Carboxylic Acids with Alcohols
229
and acetic acid in the absence of the ILs is 63%, and increases with the amount
of the IL in the reactor and then keeps constant after the amount of IL is more
than 3 g. The equilibrium conversion can reach 73% as 3.3 g IL is used. In
other words, the equilibrium conversion can be enhanced by using the IL as a
reaction medium.
The data of entries 1–8 indicate that the equilibrium conversion of
esterification of ethanol with acetic acid in [emim][PF6] is considerably higher
than that in [bmim][PF6], suggesting that the properties of the ILs affect the
reaction considerably. Esterification of acetic acid with iso-pentanol or n-
butanol (entries 9–13) is performed in [bmim][PF6] and [emim][PF6],
respectively. Conversion of iso-pentanol or n-butanol can also be enhanced
significantly. Esterification of methanol with acrylic acid is conducted in
[bmim][PF6] and [emim][PF6] (entries 14–16). The increase in conversion is
more pronounced for the acid containing C55C double bond. Results of
esterification of alcohols and acids with longer carbon chains are shown by
entries 17–19, which show that the reactions have high conversion in
[bmim][PF6]. Entries 1, 14, 20 and 21 demonstrate that [bmim][BF4] reduces
the conversion of the reactions. One of the possible reasons is that
[bmim][BF4] is miscible with water, while [bmim][PF6] can dissolve little
amount of water. Therefore, some water produced in the reaction comes out
from the solvent as [bmim][PF6] is used, which shifts the chemical
equilibrium to higher conversion.
In summary, equilibrium conversion of the esterification of alcohols with
carboxylic acids can be tailored by using ionic liquids as solvents.
ACKNOWLEDGMENT
This work was financially supported with National Key Basic Research
Project of China (G2000048010), National Natural Science Foundation of
China (20133030, 20332030).
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Received in Japan May 27, 2003

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