Preparation And Characterization Of A Novel Benzimidazolium ...
J. Chem. Sci., Vol. 120, No. 5, September 2008, pp. 481–483. © Indian Academy of Sciences.
Preparation and characterization of a novel benzimidazolium brønsted
acidic ionic liquid and its application in esterifications
SHUAN-HU CHEN*, QIONG ZHAO and XUE-WANG XU
Department of chemistry, Northwest University, Xi’an 710069, China
e-mail: csh318361@163.com
MS received 22 October 2007; revised 21 April 2008
Abstract. A novel brønsted acidic ionic liquid 1-butylbenzimidazolium tetrafluoroborate ([Hbbim]
BF4) based on the benizimidazolium cation was synthesized and characterized. This new ionic liquid
was successfully used as a suitable catalyst for the esterifications of carboxylic acids with aliphatic alco-
hols.
Keywords. Ionic liquids; benzimidazole; esterifications.
1. Introduction
ionic liquid, [Hbbim]BF4 and study its application in
the esterifications of several carboxylic acids with
Ionic liquids (ILs), also known as molten salts with
aliphatic alcohols (scheme 1).
melting point under 100 or 150°C1, have attracted an
increasing attention in the context of green synthesis
2. Experimental
in recent years. Although ionic liquids were initially
introduced as an alternative green media because they
2.1 Materials and reagents
are room temperature molten salts that are nonvolatile,
thermally stable, recyclable, and easy to handle,2–7
All chemicals (AR grade) were commercially avail-
they have marched far beyond showing their signifi-
able and used without further purification unless
cant catalytic activities for many reactions.8–11 Since
otherwise stated. Infrared spectra on KBr pellets were
the first successful use of ionic liquid, dialkylimida-
recorded on a Bruker EQUINOX-55 FT-IR spectro-
zolium chloroaluminate, as a catalyst in Friedel–Crafts
photometer in the range 4000–400 cm–1. Elemental
acylations was reported in 1986,12 a number of ionic
analyses were determined with a Perkin-Elmer model
liquids with unique properties have been developed
240C instrument. 1HNMR spectra in propanone-d6
and applied to catalyse many types of reactions.13
solution using a Varian INOVA-400 spectrometer;
However, just as conventional organic solvents, proton chemical shifts were recorded relative to an
not all ionic liquids are appropriate for a particular internal TMS standard. The differential scanning
reaction, and a single ionic liquid will not always calorimetry (DSC) experiment and the thermal sta-
be the best for every reaction.14 It continues to be bility of each ionic liquid was performed using
worthwhile to synthesize novel ionic liquids for par-
Q1000DSC+LNCS+FACS Q600SDT at a heating rate
ticular chemical processes even though a large num-
5°Cmin–1 with nitrogen as the purge gas. Esterifica-
ber of these new compounds are already known.
tion reaction progress was monitored by GC-MS
Recently,
Dai
et al reported the synthesis of a (AligentGC: 6890N, MS:5793N).
novel brønsted acidic ionic liquid based on benzimi-
dazolium cation, 1-ethylbenzimidazolium tetrafluo-
roborate ([Hebim]BF4) and found that it was an
efficient medium for the preparation of arylic es-
ters.15 In this paper, we describe fully the preparation
and characterization of another benzimidazolium
*For correspondence
Scheme 1. The synthetic process of ionic liquid.
481
482
Shuan-Hu Chen et al
2.2 Synthesis of ionic liquid [Hbbim] BF4
3. Results and discussion
2.2a 1-butyl benzimidazole ([bbim]): The synthesis
3.1 Synthesis of ionic liquid
process was carried out according to the previous
literature.16 A mixture of 11⋅8 g (0⋅1 mol) of ben-
The [Hbbim]BF4 ionic liquid was prepared by sim-
zimidazole, 30 ml of 50% sodium hydroxide and ple acid-base neutralization from the corresponding
8⋅7 ml (0⋅11 mol) of 1-bromobutane were placed in 1-butyllbenzimidazole with tetrafluoroboric acid.
a three-necked flask, which was provided with a Ethanol was used as the solvent because of the high
mechanical stirrer, reflux condenser, and the ther-
solubility involved. The reaction was carried out in
mometer. The reaction mixture was stirred for 3 min.
an ice bath and required little time or labour. The
When the mixture solidified, the temperature of the
molecular structure of the ionic liquid was character-
water bath was raised to 30–40°C and the content ized via FT-IR, 1HNMR and elemental analysis. All
became liquid. After 2 min an exothermic effect was
the data of characterization are in accordance with
observed, and the temperature of the reaction mixture
the expected compositions and structures.
was maintained at 30–40°C for 10 min. The organic
layer was extracted with CHCl3 (3 × 30 ml), washed 3.2 Solubility
with water and dried with anhydrous sodium sulfate.
The solvent was evaporated in vacuum, and the oil
The solubility property of [Hbbim]BF4 in some
obtained was purified by distillation.
common solvents was studied. It was completely
2.2b 1-butylbenzimidazolium tetrafluoroborate
soluble in water, ethanol, methanol, acetonitrile and
([Hbbim]BF4)]: Tetrafluoroboric acid (40% solution acetone and immiscible with benzene, toluene, cyclo-
in water) was added drop-wise to a pre-cooled (0–5°C) hexane, ethyl acetate and diethyl ether.
ethanol solution of equivalent bbim in a three-
necked flask with a mechanical stirrer. The mixture
3.3 Thermal stability and DSC measurement
was maintained at that temperature and stirred for an
additional period of 3 h. Ethanol and water was re-
The thermogravimetric analysis experiment was
moved under vacuum in a rotary evaporator. The conducted to determine the thermal stabilities of the
viscous residue was dehydrated at 80°C in a vacuum novel ionic liquid. Although the study17 indicated
oven for 2 h. The dehydrated residue solidified on that the new ionic liquid began to decompose at
cooling to a colourless crystalline solid. Yield: 13⋅1 g slightly lower temperature than that of imidazole se-
(100%), m.p. 82–84°C. IR, ν (KBr): 3338, 3148, ries, it also displayed high thermal stability. The
3028, 2962, 2872, 1616, 1551, 1498, 1450, 1382, sample was stable to temperature of 210°C and
1319, 1057, 754 cm–1. 1HNMR (400 Hz, CD3COCD3-
showed a weight loss of 90% between 220 and
d6), δ: 9⋅60 (s, 1H, im-H), 8⋅11–8⋅13 (m, 1H, Ar-H), 270°C. When the temperature was 290°C, the amount
8⋅00–8⋅02 (m, 1H, Ar-H), 7⋅71–7⋅76 (m, 2H, Ar-H), of residue was only 0⋅5%. Thus, it has potential us-
4⋅73 (t, J (H, H) = 7⋅2 Hz, 2H, N–CH2), 2⋅07–2⋅13 age as alternative to conventional organic solvents
(m, 2H), 1⋅44–1⋅54 (m, 2H), 0⋅98 (t, J (H, H) = due to its special solubility and excellent thermal
7⋅4 Hz, 3H). Elemental analysis calcd. (%) for stability.
C11H15N2BF4 (262⋅05) C, 50⋅42; H, 5⋅77; N, 10⋅69.
Found C, 50⋅47; H, 5⋅84; N, 10⋅75.
3.4 Esterifications in the novel ionic liquid
[Hbbim]BF
2.3 Esterifications in novel ionic liquid
4
Ethanol 0⋅46 g (0⋅01 mol), equivalent acetic acid The esterifications of several carboxylic acids with
0⋅06 g (0⋅01 mol) and ionic liquids [Hbbim]BF4 1⋅31 g common aliphatic alcohols were carried out using
(0⋅005 mol) were added in a flask with a reflux con-
the [Hbbim]BF4 as a catalyst. Higher yields were
denser and oil-bath. The reaction mixture was stirred
obtained because of the good solubility of the acids
for 2 h with the oil bath at 80°C. Reaction progress and alcohols in [Hbbim]BF4 while the esters are almost
was monitored by GC-MS. After the reaction, the immiscible with ionic liquid. The benzimidazolium
ester was simply decanted from the ionic liquid, and
novel brønsted acidic ionic liquid which was screened
the ionic liquid [Hbbim]BF4 was reused after removal as a catalyst for esterifications of carboxylic acids
of water under vacuum (0⋅01 Torr) at 80°C for 2 h.
and aliphatic alcohols could be easily reused after
Preparation and characterization of a novel benzimidazolium brønsted acidic ionic liquid
483
Table 1. Results of esterification for different acids and alcohols in [Hbbim]BF4.
Entry
Acid
Alcohol
Time (h)
T (°C) Yield
(%)
1 Acetic
acid
1-butanol
2⋅0 120 96
2 Acetic
acid
iso-butyl alcohol
2⋅0 120 89
3 Acetic
acid
sec-butyl alcohol
2⋅0 120 71
4 Acetic
acid
tert-butyl alcohol
2⋅0 120 58
5 Acetic
acid
Ethanol
2⋅0
80 96
6 Oxalic
acida 1-butanol
8⋅0 120 93
7 Benzoic
acid
Ethanol
8⋅0
80 86
aIn the reaction of oxalic acid, no monoesterification products were detected when the
molar ratio of acid to alcohols was 1 : 2
Table 2. Reuse
of
[Hbbim]BF4 in the synthesis of acetic indicated that the ionic liquid [Hbbim]BF4 showed
acetate.
good catalytic activity to the esterifications and high
Run
Time (h)
T (°C) Yield
(%)
yields were obtained under mild reaction conditions.
After the reaction, the esters could be easily sepa-
1 2⋅0 80 95
rated; the ionic liquids could be recycled at least 6
2 2⋅0 80 95
3 2⋅0 80 94
times without a significant decrease in catalytic per-
4 2⋅0 80 95
formance.
5 2⋅0 80 95
6 2⋅0 80 94
Acknowledgement
removal of water under vacuum. Its catalytic activity
This project was supported by the Natural Science
was still very high after 6 times reuse.
Foundation of Shaanxi Province (No. 03B19), China.
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