Sonochemical Synthesis and Characterization of Some Alkoxyl-Functionalized Ionic Liquids Derived from 1-Butoxyl-3-butyl Imidazolium Bromide

Article abstract of DOI:10.1007/s10953-017-0627-6

Non-conventional techniques, such as power ultrasound (US) has been used to promote one-pot synthesis of second generation ionic liquids (ILs), reducing reaction times and improving yields. Because of the emerging importance of the ILs as green materials with wide ranging applications and our general interests in green processes such as sonication, 1-butoxyl-3-butyl imidazolium bromide (alkoxyl-functionalized) and their derivatives were synthesized using a facile and green US assisted procedure. Their structures were characterized by FT–IR, ¹H-NMR, ¹³C-NMR and mass spectroscopy.

Full text of DOI:10.1007/s10953-017-0627-6

J Solution Chem (2017) 46:1068–1076
DOI 10.1007/s10953-017-0627-6
Sonochemical Synthesis and Characterization of Some
Alkoxyl-Functionalized Ionic Liquids Derived from
1-Butoxyl-3-butyl Imidazolium Bromide
Garima Ameta1 Pinki B. Punjabi1
•
Received: 5 September 2016 / Accepted: 23 March 2017 / Published online: 17 May 2017
Ó Springer Science+Business Media New York 2017
Abstract Non-conventional techniques, such as power ultrasound (US) has been used to
promote one-pot synthesis of second generation ionic liquids (ILs), reducing reaction times
and improving yields. Because of the emerging importance of the ILs as green materials
with wide ranging applications and our general interests in green processes such as son-
ication, 1-butoxyl-3-butyl imidazolium bromide (alkoxyl-functionalized) and their
derivatives were synthesized using a facile and green US assisted procedure. Their
1
structures were characterized by FT–IR, H-NMR, 13C-NMR and mass spectroscopy.
Keywords Green synthesis Á Ultrasonic irradiation Á Imidazolium based
ionic liquids Á Functionalization Á Spectroscopy
1 Introduction
A large excess of a conventional volatile solvent is generally required to carry out a
chemical reaction, raising ecological and economic concerns. Ultrasound assisted organic
synthesis is a powerful technique and it is a green approach employed for organic reac-
tions, leading to higher yields, shorter reaction times, milder conditions and high analytical
purity [1–4]. It is considered as a processing aid in terms of energy conservation and waste
minimization compared to other, traditional, methods. Sonochemistry offers synthetic
chemists the advantage of chemical activation through cavitation and provides a source of
energy which can be used to enhance a variety of chemical reactions. The strong accel-
eration of reaction rate by ultrasonic irradiation is based on the cavity effect and hot spot
formation [5, 6].
&
Garima Ameta
garima_ameta@yahoo.co.in
1
Department of Chemistry, University College of Science, Mohanlal Sukhadia University,
Udaipur 313002, Rajasthan, India
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1069
Over the past two decades, the use of ILs as reaction media have attracted considerable
attention as eco-friendly substitutes for volatile organic solvents [7, 8]. These solvents,
commonly defined as a class of salts with low melting points (less than 100 °C) [9], have
attracted intense attention of chemists. ILs present some unique properties, which are often
not available in traditional solvents; these include negligible vapor pressure, high chemical
and electrochemical stabilities, high polarity, easy recyclability, no flammability, and high
ionic conductivity, among others [9–16]. They are widely used as reaction media these
days and new approaches involving ILs have been proposed for aspects of energy
chemistry [17], preparation of materials [18], biomass valorization [19], lignocellulosic
biomass fractionation [20], analytical chemistry [21], microextraction [22], organic and
pharmaceutical chemistry [23], electrodeposition of metals [24–26], food chemical science
[27], the nuclear industry [28] and many others.
Preparation of ionic liquids by classical methods involves long reaction times and low
yields [29, 30]. An efficient microwave assisted synthesis of ionic liquids has reduced the
reaction time from several hours to a few minutes [31, 32]. However, as a result of the
exothermic nature of the reaction, continuous microwave heating may result in overheating
that leads to the formation of colored products. To overcome these problems, ionic liquids
have been prepared using ultrasound as an energy source [33–35], which reduces the
reaction time and improves the yield.
Herein, we report the sonochemical synthesis (green synthesis) and characterization of
some new, task specific, alkoxyl functionalized ionic liquids derived from 1-butoxyl-3-
butyl imidazolium bromide.
2 Materials and Methods
Imidazole (99%, Himedia, India), n-BuLi (98%, Sigma–Aldrich, India), 1-bromobutane
(99%, CDH, India), H2O2 (30%, Thomas Baker, India), dichloromethane (99%, Fisher
Scientific, India), sodium tetrafluoroborate (99%), silver sulfate (99%), sodium trifluo-
romethane sulphonate (98%) and p-toluene sulphonic acid (99%) (Himedia) were used as
received. Ultrasonic cleaner 392 (Systronics) was used for ultrasonic irradiation with a
frequency of 40 kHz and a nominal power of 115 W.
3 Experimental
All reactions were carried out in an ultrasonic bath. The synthesis of ionic liquids involves
four steps (Scheme 1).
Step I
Step II
Step III
1-Butylimidazole was synthesized by N-alkylation of imidazole (13.6 g,
0.2 mol) with n-butyl lithium (12.8 g, 0.2 mol) in ethanol (50 mL) under the
influence of ultrasound for 3 h at room temperature. The progress of the
reaction was monitored by TLC.
1-Butylimidazole-3-N-oxide was prepared by the reaction of 1-butylimida-
zole (12.4 g, 0.1 mol) with H2O2 (6.8 mL, 0.2 mol) in acetone (30 mL), in
the presence of ultrasonic waves for 30 min at room temperature. The light
yellow reaction mixture was dried under vacuum.
Reaction of 1-butyl imidazolium 3-N-oxide (14 mL, 0.1 mol) with 1-bro-
mobutane (13.7 mL, 0.1 mol) for 2 h at room temperature under the influence
123

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J Solution Chem (2017) 46:1068–1076
.))), r.t., 3 hr
C2H5OH
Step I
+
BuLi
NH
N
N
N
Bu
Imidazole
1-Butylimidazole
.))), 30 mins, r.t.
H2O2
Step II
Step III
N+
N
N
N
Bu
O-
Bu
1-Butyl imidazole
1-Butylimidazole-3-N-oxide
.))), 2 hrs, r.t.
Br-
N+
N
+
C4H9Br
N+
N
Bu
Neat
1-Bromobutane
Bu
O-
O
Bu
1-Butoxyl-3-butyl imidazolium bromide
1-Butylimidazole-3-N-oxide
Step IV a
-
N+
.))), 2.5 hrs, r.t.
MeOH
Br-
Bu
BF4
N+
Bu
N
NaBF4
N
+
Bu
O
Bu
O
1-Butoxyl-3-butyl imidazolium tetrafluoro borate
1-Butoxyl-3-butyl imidazolium bromide
Step IV b
.))), 4 hrs, r.t.
MeOH
2-
Br-
N+
N+
Ag2SO4
SO4
2
Bu
+
Bu
N
N
Bu
Bu
O
O
2
1-Butoxyl-3-butyl imidazolium bromide
1-Butoxyl-3-butyl imidazolium sulfate
O
F
F
Step IV c
.))), 3 hrs, r.t.
N+
N
Na+
Br-
F
S
O-
Bu
+
MeOH
Bu
O
O
Sodium trifluoromethyl sulphonate
1-Butoxyl-3-butyl imidazolium bromide
O
S
F
F
F
O-
Bu
N+
N
O
O
Bu
1-Butoxyl-3-butyl imidazolium trifluoromethyl sulphonate
O
.))), 3.5 hrs, r.t.
MeOH
Step IV d
N+
Br-
OH
S
.H2O
H C
Bu
3
N
+
O
Bu
O
p-Toluene sulphonic acid monohydrate
1-Butoxyl-3-butyl imidazolium bromide
O
-
N+
Bu
S
H C
O
N
3
O
Bu
O
1-Butoxyl-3-butyl imidazolium p-Toluene sulphonate
Scheme 1 Ultrasound-assisted synthesis of 1-butoxyl-3-butylimidazolium based ionic liquids
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J Solution Chem (2017) 46:1068–1076
1071
of ultrasonic waves yields 1-butoxyl-3-butyl imidazolium bromide (ionic
liquid).
Step IV
In the presence of ultrasound, the reaction of 1-butoxyl-3-butylimidazolium
bromide ([BO-BIm] Br) (5.5 mL, 0.02 mol) was carried out either with
sodium tetrafluoroborate (2.2 g, 0.02 mol), silver sulfate (6 g, 0.02 mol),
sodium trifluoromethyl sulphonate (sodium triflate) (3.4 mL, 0.02 mol) or p-
toluene sulphonic acid (3.4 g, 0.02 mol) to give 1-butoxyl-3-butylimida-
zolium tetrafluoroborate (Step IV a), 1-butoxyl-3-butylimidazolium sulfate
(Step IV b), 1-butoxyl-3-butylimidazolium trifluoromethyl sulphonate (tri-
flate) (Step IV c) and 1-butoxyl-3-butylimidazolium p-toluene sulphonate
(tosylate) (Step IV d) ionic liquids, respectively, in methanol (30 mL).
These ionic liquids were dissolved in dichloromethane, which were further extracted with
ether for purification. The muddy solution was kept at room temperature untill it became
transparent. The solutions were then filtered and washed three times with ether.
The alkylating agents are quantitatively extracted with the ether. After work-up, the
products were dried under 80 °C for 12 h at high vacuum (0.01 mbar) to remove any
volatiles remaining from the preparation. Water may still be present in trace amounts.
Table 1 shows impurities present in ionic liquids.
4 Results and Discussion
A Perkin-Elmer spectrum–2000 Fourier transform IR spectrophotometer (USA) was used
to obtain the IR spectra between 400 and 4000 cm-1. A NaCl cell was used for liquid
samples by dissolving a sample in dichloromethane (DCM). 1H-NMR and 13C-NMR
spectra were recorded using a multinuclear FT–NMR spectrometer (Model Avance-II,
1
Bruker). The instrument was equipped with a cryomagnet of field strength 9.4 T. Its H
frequency was 400 MHz, while for 13C the frequency was 100 MHz. Waters, Q-Tof micro
Mass spectrometer (LC–MS) was used for mass spectral analysis of the synthesized ionic
liquids.
Table 1 Impurities present in the synthesized ionic liquids
S.
no.
Ionic liquid
Reagent used
for preparation
[Na?]
(ICP-MS)
[Br-] (ICP- H2O (Karl-
MS) wt%
Fischer titration)
wt%
1
2
3
4
1-Butoxyl-3-butylimidazolium 1-Bromobutane
bromide
0.097
0.021
1-Butoxyl-3-butylimidazolium Sodium
tetrafluoroborate
0.080 wt% 0.090
0.028
0.072 wt% 0.071
0.031
0.009
0.015
tetrafluoroborate
1-Butoxyl-3-butylimidazolium Silver sulfate
sulfate
–
1-Butoxyl-3-butylimidazolium Sodium
trifluoromethane sulphonate
trifluoromethane
sulphonate
1-butoxyl-3-butylimidazolium p-toluene sulphonic
p-toluene sulphonate acid
5
–
0.069
0.019
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5 Characterizations
5.1 1-Butoxyl-3-Butylimidazolium Bromide ([BO-BIm] Br)
IR (NaCl cell, cm-1): C–H stretching, 2874; CH2 bending, 1464; CH3 bending, 1378; =CH
stretching, 3137; N–C stretching, 1260 and 3080, N–O stretching, 1512 and 1323, C–O
stretching, 1285, C=C stretching, 1645 and C=N stretching, 1632 cm-1
.
9
8
1H-NMR (CDCl3, 400 MHz): d ppm = 0.92 (t, 10CH), 1.47 (m, CH), 1.50 (m, CH),
7
5
4
2
3.45 (t, CH), 7.50 (t, CH), 7.86 (t, CH), 8.84 (s, CH), 3.87 (t, 11CH), 1.91 (m, 12CH),
1.22 (m, 13CH), 0.86 (t, 14CH).
13C-NMR (CDCl3, 100 MHz) : (d ppm): 13.98 (10CH), 17.49 (9CH), 31.23 (8CH), 64.46
(7CH), 122.84 (4CH), 122.65 (5CH), 135.29 (2CH), 45.91 (11CH), 32.56 (12CH), 20.04
(13CH) and 13.86 (14CH).
Mass peaks: 197.3 (C11H21N2O), 140.1 (C7H12N2O), 124.1 (C7H12N2).
5.2 1-Butoxyl-3-Butylimidazolium Tetrafluoroborate ([BO-BIm] BF4)
IR (NaCl cell, cm-1): C–H stretching, 2850; CH2 bending, 1446; CH3 bending, 1368; =CH
stretching, 3061; N–C stretching, 1287 and 3095, N–O stretching, 1534 and 1326, C–O
stretching, 1213, C=C stretching, 1653; C=N stretching, 1664 and B–F stretching, 1453,
712 and 481 cm-1
.
9
8
1H-NMR (CDCl3, 400 MHz): d ppm = 0.93 (t, 10CH), 1.40 (m, CH), 1.48 (m, CH),
7
5
4
2
3.39 (t, CH), 7.74 (t, CH), 7.87 (t, CH), 8.79 (s, CH), 3.91 (t, 11CH), 1.94 (m, 12CH),
1.21 (m, 13CH), 0.88 (t, 14CH).
13C-NMR (CDCl3, 100 MHz): (d ppm) : 13.88 (10CH), 19.19 (9CH), 31.32 (8CH), 63.37
(7CH), 122.53 (4CH), 122.10 (5CH), 136.13 (2CH), 46.88 (11CH), 32.51 (12CH), 19.93
(13CH) and 13.04 (14CH).
Mass peaks: 197.3 (C11H21N2O), 140.1 (C7H12N2O), 124.1 (C7H12N2), 67.8 (BF3).
5.3 1-Butoxyl-3-Butylimidazolium Sulfate ([BO-BIm] SO4)
IR (NaCl cell, cm-1): C–H stretching, 2868; CH2 bending, 1462; CH3 bending, 1380; =CH
stretching, 3133; N–C stretching, 1272 and 3051, N–O stretching, 1545 and 1365, C–O
stretching, 1145; C=C stretching, 1646; C=N stretching, 1639; S=O stretching, 1167 and
S–O stretching, 949 cm-1
.
9
8
1H-NMR (CDCl3, 400 MHz): d ppm = 0.93 (t, 10CH), 1.36 (m, CH), 1.50 (m, CH),
7
5
4
2
3.39 (t, CH), 7.68 (t, CH), 7.97 (t, CH), 8.85 (s, CH), 4.01 (t, 11CH), 1.93 (m, 12CH),
1.24 (m, 13CH), 0.84 (t, 14CH).
13C-NMR (CDCl3, 100 MHz): d ppm = 13.80 (10CH), 17.56 (9CH), 28.98 (8CH), 63.82
(7CH), 123.12 (4CH), 120.97 (5CH), 138.14 (2CH), 47.21 (11CH), 32.47 (12CH), 20.11
(13CH) and 13.54 (14CH).
Mass peaks: 197.3 (C11H21N2O), 140.1 (C7H12N2O), 124.1 (C7H12N2).
5.4 1-Butoxyl-3-Butylimidazolium Trifluoromethane Sulphonate ([BO-BIm]
TfO)
IR (NaCl cell, cm-1): C–H stretching, 2939; CH2 bending, 1466; CH3 bending, 1371; =CH
stretching, 3112; N–C stretching, 1256 and 2966; N–O stretching, 1556 and 1351; C–O
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J Solution Chem (2017) 46:1068–1076
1073
stretching, 1166; C=C stretching, 1639; C=N stretching, 1632; S=O, 1371; S–O, 851 and
C–F stretching (trifloromethyl- 2 strong broad bands), 1166 and 1256 cm-1
.
9
8
1H-NMR (CDCl3, 400 MHz): d ppm = 0.91 (t, 10CH), 1.37 (m, CH), 1.54 (m, CH),
3.42 (t, CH), 7.71 (t, CH), 7.78 (t, CH), 8.95 (s, CH), 4.02 (t, 11CH), 1.94 (m, 12CH),
7
5
4
2
1.21 (m, 13CH), 0.85 (t, 14CH).
13C-NMR (CDCl3, 100 MHz): (d ppm) : 13.64 (10CH), 18.21 (9CH), 31.35 (8CH), 64.96
(7CH), 122.68 (5CH), 122.09 (4CH), 136.66 (2CH), 46.56 (11CH), 32.32 (12CH), 20.02
(13CH) and 13.32 (14CH) and 135.02 (aCH).
Mass peaks: 197.3 (C11H21N2O), 140.1 (C7H12N2O), 124.1 (C7H12N2), 149 (CF3O3S).
5.5 1-Butoxyl-3-Butylimidazolium p-Toluene Sulphonate ([BO-BIm] Tos)
IR (NaCl cell, cm-1): C–H stretching, 2875; CH2 bending, 1461; CH3 bending, 1383; =CH
stretching, 3109; N–C stretching, 1333 and 2963; N–O stretching, 1543 and 1326; C–O
stretching, 1185; C=C stretching,1601; C=N stretching, 1719; C=C stretching (conjuga-
tion), 1629; C–H in plane, 1399; C–H bend (para), 817; S=O stretching, 1383 and S–O
stretching, 753 cm-1
1H-NMR (CDCl3, 400 MHz): d ppm = 0.94 (t, 10CH), 1.42 (m, CH), 1.50 (m, CH),
.
9
8
7
5
4
2
3.45 (t, CH), 7.69 (t, CH), 7.88 (t, CH), 8.85(s, CH), 3.99 (t, 11CH), 1.95 (m, 12CH),
1.23 (m, 13CH), 0.87 (t, 14CH) 2.39 (eCH), 7.41 (cCH), 7.69 (bCH).
13C-NMR (CDCl3, 100 MHz): (d ppm) : 14.28 (10CH), 19.15 (9CH), 30.41 (8CH), 64.52
(7CH), 122.76 (4CH), 122.55 (5CH), 135.83 (2CH), 47.12 (11CH), 32.78 (12CH), 19.47
(13CH) and 13.45 (14CH) 21.32 (eCH), 138.06 (dCH), 129.07 (cCH), 126.83 (bCH) and
140.93 (aCH).
Mass peaks: 197.3 (C11H21N2O), 140.1 (C7H12N2O), 124.1 (C7H12N2), 171.2
(C7H7O3S), 156.1 (C6H4O3S), 15 (CH3), 91 (C7H7).
5.6 Viscosity
Ionic liquids are viscous liquids as compared to the conventional organic solvents, being 1–3
orders of magnitude more viscous. Alkyl chain lengthening or fluorination makes the salt
more viscous, due to an increase in Van der Waals interactions and hydrogen bonds (Fig. 1).
Alkyl chain ramification also increases the viscosity due to reduced rotational freedom. The
viscosity measurement was carried out with Brookfield DV-I (II) viscometer (1% accuracy,
0.2% repeatability) from 25 to 95 °C at an interval of 5 °C under nitrogen gas.
The calibration was made by comparison with reference oils (Canon S20, S60 and
Paragon N10). The estimated overall uncertainty of the temperature readings was 0.5 °C.
Three sets of each measurement were performed, having deviations from the mean smaller
than 0.1%. It has been observed that on increase in temperature, viscosity of an ionic
liquid decreases. Viscosities of the synthesized ionic liquids at 25 °C are given in Table 2.
Br-
5
4
b
c
O
S
9
F
14
O
S
-
7
12
e
O-
H3C
N+
BF4
N
a
d
-
a
10
CH3
F
H C
3
O
1
3
O
2
8
13
11
2-
6
O
O
F
c
b
SO4
Fig. 1 The chemical structure of the 1-butoxyl-3-butylimidazolium cation [BO-BIm]? with different
anions
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Table 2 Properties and reaction data for 1-butoxyl-3-butylimidazolium based ionic liquids
S.
no.
Structural formula
Reaction time
in ultrasound
(h)
Yield Viscosity
g/cp at
25 °C
Conductivity
(SÁm-1) at
27 °C
(%)
1
2
2.0
2.5
91
551
305
0.31
0.25
N+
N
Br-
Bu
Bu
O
1-Butoxyl-3-butyl imidazolium bromide
[BO-BIm] Br
80
75
-
N+
Bu
BF4
N
Bu
O
1-Butoxyl-3-butyl imidazolium tetrafluoro borate
[BO-BIm] BF4
3
4.0
151
0.09
2-
N+
Bu
SO4
N
Bu
O
2
1-Butoxyl-3-butyl imidazolium sulfate
[BO-BIm] SO4
4
5
3.0
3.5
91
86
85
73
0.39
0.41
O
F
F
F
S
O-
Bu
N+
N
O
O
Bu
1-Butoxyl-3-butyl imidazolium trifluoromethyl sulphonate
[BO-BIm] TfO
O
-
N+
S
Bu
H
C
O
N
3
O
Bu
O
1-Butoxyl-3-butyl imidazolium p-Toluene sulphonate
[BO-BIm] Tos
5.7 Conductivity
Ionic liquids are composed of only ions; therefore, they show very high ionic conductivity,
thermal stability and nonflammability. Nonflammable liquids with high ionic conductivity
are practical materials for use in electrochemistry. Conductivity measurements were car-
ried out using conductivity meter (Systronics Model 305). The instrument conductivity
measuring range is 0 to 2000 lS and 0 to 200.0 mS. The instrument has a full scale
resolution of 0.5% and an accuracy of 1%. Before and after measurements, the instru-
ment was calibrated with aqueous KCl solution. The temperature of the sample was kept at
27
0.1 °C. Conductivities of the synthesized ionic liquids are also reported in Table 2.
6 Conclusion
It is clear from these observations that a large number of ionic liquids can be synthesized
by simple combination of different cations and anions, which can be used for a particular
task. Ultrasound assisted synthesis of ionic liquids has the advantages over classical
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methods (conventional, thermal, chemical, etc.) of being ecofriendly, simplying product
isolation, increasing yield and shortening reaction times. Therefore, introduction of such
efficient synthetic protocols should lower the cost of ionic liquids; thus, encouraging a
wider use of these neoteric solvents.
Acknowledgements One of the authors (Dr. Garima Ameta; UGC Award Letter No. F.15-1/2011-12/
PDFWM-2011-12-GE-RAJ-9578(SA-II) dated 01-Nov-2013) is thankful to University Grants Commission,
New Delhi for providing financial assistance in the form of Post Doctoral Fellowship. We are thankful to
Prof. Suresh C. Ameta for giving valuable suggestions during the progress of the work. We are also thankful
to the Director, SAIF and CIL, Chandigarh, India for providing spectral data.
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