Synthesis and dissolving power of 1-Alkyl-3-methylpyridinium-based ionic liquids

Article abstract of DOI:10.1134/S1070363212120158

1-Alkyl-3-methylpyridinium-based ionic liquids with substituents from C₂ to C₁₀ and anions Cl^- and Br^- were synthesized, and their dissolving power toward the cellulose was investigated. The results of quantum-chemical calculations of molecules of ionic liquids are presented.

Full text of DOI:10.1134/S1070363212120158

ISSN 1070-3632, Russian Journal of General Chemistry, 2012, Vol. 82, No. 12, pp. 1994–1998. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © E.S. Sashina, D.A. Kashirskii, M. Zaborski, S. Jankowski, 2012, published in Zhurnal Obshchei Khimii, 2012, Vol. 82, No. 12,
pp. 2040–2045.
Synthesis and Dissolving Power
of 1-Alkyl-3-methylpyridinium-Based Ionic Liquids
E. S. Sashinaa, D. A. Kashirskiia, M. Zaborskib, and S. Jankowskib
a St. Petersburg State University of Technology and Design,
ul. Bolshaya Morskaya 18, St. Petersburg, 191186 Russia
e-mail: organika@sutd.ru
b Technical University of Lodz, Lodz, Poland
Received May 28, 2012
Abstract—1-Alkyl-3-methylpyridinium-based ionic liquids with substituents from C2 to C10 and anions Cl–
and Br– were synthesized, and their dissolving power toward the cellulose was investigated. The results of
quantum-chemical calculations of molecules of ionic liquids are presented.
DOI: 10.1134/S1070363212120158
Ionic liquids defined as organic salts with melting
points up to 100°C and consisting of cation and anion,
in recent years found wide application in various fields
of science and technology. Their unique properties of
thermal stability, ability of regeneration, and low vapor
pressure make them promising for use in processing of
natural polymers, primarily cellulose [1–7], and in the
production of new biodegradable materials, including
films, membranes, and fibers. An important feature of
this new class of solvents is the possibility of varying
the cation–anion pairs, which allows assembling the
ionic liquids with the required properties.
solution was obtained and with the ammonium salts
5% concentration was achieved.
However, such an important issue as the
relationship between the structure of ionic liquid (the
nature of the cation and anion, the number and size of
the substituents), is yet little studied, although some
trends have already been identified. For example, the
efficiency of the solvent is strongly dependent on the
nature of the anion: the dissolving power of 1-alkyl-3-
methylimidazolium-based ionic liquids decreases in
the series of CH3COO– > Cl– >> Br– ≈ SCN– [7].
Equally important is the number and position of
substituents in the ring. For example, pyridinium salts
with one substituent in the ring have lower dissolving
power than those with two alkyl chains [10–12].
According to [13], a series of solvents with decreased
dissolving power that includes allylpyridinium
chlorides and acetates corresponds to the following
positions of methyl substituent: 3–4–2.
A significant number of the solvents dissolving
cellulose is known now belonging to the class of ionic
liquid, salts based on the nitrogen-containing cations
like ammonium, pyridinium, and imidazolium, with
–
anions Cl–, Br–, RCOO–, SCN–, (MeO)2(R)PO2 etc.
[8–10]. The best studied ionic liquids are based on the
imidazolium cation. The pyridinium-containing ionic
liquids were studied comparatively poorly, although
the first report of dissolving cellulose in them relates to
1934 [10]. In [8] the data on the maximum
concentration of dissolved cellulose were presented
achieved with the use of the chloride-based ionic liquid
on the 1-butyl-3-methylimidazolium, 1-butyl-3-methyl-
pyridinium, and benzyldimethyl(tetradecyl)ammonum
clorides. The quaternary pyridinium salt was the most
effective, providing 39% solution of cellulose (DP
286), for comparison, with imidazolium salt 18%
At replacing the allyl group in the position 1 by
propyl or butyl, only the salt with 1,3-position of two
substituents is capable to dissolve cellulose. Thus,
among of the most effective solvents for cellulose may
be pyridinium ionic liquids with two alkyl chains in
the positions 1 and 3. It can be assumed that the size of
the substituents also affects the dissolving power of
ionic liquids, as was shown for the imidazolium-
containing solvents [7].
1994

SYNTHESIS AND DISSOLVING POWER
1995
R
Fig. 2. Dependence of the interaction energy of the
ion pair on the length of the alkyl chain (R) in 1-alkyl-
3-methylpyridinium chlorides-based ionic liquids.
Fig. 1. Optimized structure of 1-ethyl-3-methylpyridinium
chloride ion pair.
The aim of this work is finding a possible relation-
ship between the structure of ionic liquids based on the
cation of 1-alkyl-3-methylpyridinium and their
dissolving power toward cellulose. The problem can be
solved by comparing the results of quantum-chemical
calculations and experimental study of their physico-
chemical properties. To do this, we synthesized a series
of ionic liquids based on 1-alkyl-3-methylpyridinium.
cation were synthesized (Table 2). As an example,
Figs. 3 and 4 show the H NMR and IR spectra of 1-
1
ethyl-3-methylpyridinium chloride. No peaks of the
starting materials and external contaminants were
detected indicating the acceptability of the chosen
method of synthesis and purification. The water
content in the synthesized ionic liquid does not exceed
0.6–0.7 wt %.
The results of the thermal analysis show that the
lengthening of the N-alkyl chain reduces the melting
temperature of the compounds (Fig. 5). The ionic
liquids with the number of carbon atoms in the
substituent at the nitrogen atom from C4 to C10 melt at
the temperature below 90°C.
Quantum-chemical calculations of the geometric
parameters and electron density distribution on atoms
in the ionic liquids based on 1-alkyl-3-methyl-
pyridinium was performed by the HF/6-31++G(6d,7f)
method with the Gaussian 03 package. An example of
the optimized geometry of the ion-pair model of 1-
ethyl-3-methylpyridinium chloride is shown in Fig. 1.
Table 1. The distance between the ring proton H2 and anion
X, and the charge on the anion (by Mulliken) in ionic liquids
based on 1-alkyl-3-methylpyridinium
Analysis of the results of calculations shows that
with increasing length of the alkyl chain of the
substituent at the nitrogen atom in a series of C2–C10
the absolute value of the interaction energy (Einteract) in
the ion pairs decreases (Fig. 2). At the same time, there
is a tendency to an increase in the distance between the
hydrogen atom in 2 position of the pyridinium ring and
the anion with the increase in the number of carbon
atoms in the N-alkyl chain (Table 1). For the same
cation, the distance H2···Br is longer than H2···Cl. The
distances H2···Cl in ionic liquid with N-allyl and N-
ethyl substituents are comparable. The results of
calculation show that with the lengthening of the N-
alkyl chain the cation and anion become less
associated, therefore, the packing density of the
solvent decreases.
R
Х
H2···X, Å
2.23021
2.24275
2.24370
2.24396
2.24410
2.24416
2.24401
2.24425
2.24413
2.22811
2.32403
2.34980
q, e
C2H5
Cl
–0.778
–0.774
–0.773
–0.772
–0.773
–0.773
–0.773
–0.772
–0.772
–0.767
–0.741
–0.730
C3H7
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Br
Br
C4H9
C5H11
C6H13
C7H15
C8H17
C9H19
C10H21
СН2=СНСН2
C2H5
Twelve ionic liquids with various anions and
diverse alkyl substituents at the nitrogen atoms in the
C4H9
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 12 2012

1996
SASHINA et al.
Table 2. The maximum achieved concentration of cellulose
(D) in chloride- and bromide-containing solvents
provide more concentrated solutions of cellulose
(Table 2). This is consistent with the results of studies
of 1-alkyl-3-methylimidazolium halides [7]. The rise
of the temperature from 110 to 165°C leads to twofold
increase in the maximum concentration of cellulose in
solutions of bromide-containing ionic liquid.
Ionic liquid
Т, °С
140
110
120
120
110
120
110
120
110
120
110
120
110
120
110
120
130
165
110
165
D, wt %
26
1-Ethyl-3-methylpyridinium chloride
1-Propyl-3-methylpyridinium chloride
22
22.5
23
With the increasing length of the N-substituent in
the pyridinium chloride ionic liquids their dissolving
power with respect to cellulose decreases. A similar
trend was found for 1-alkylpyridinium [10–12, 15] and
1-alkyl-3-methylimidazolium [7] halides.
1-Butyl-3-methylpyridinium chloride
1-Pentyl-3-methylpyridinium chloride
21
21
1-Hexyl-3-methylpyridinium chloride
1-Heptyl-3-methylpyridinium chloride
1-Octyl-3-methylpyridinium chloride
1-Nonyl-3-methylpyridinium chloride
1-Decyl-3-methylpyridinium chloride
19
Thus, the results obtained suggest that ionic liquids
based on pyridinium cations are the most effective
solvents of cellulose. For comparison, under identical
conditions the replacement of 1-butyl-3-methylpyri-
dinium by 1-butyl-3-methylimidazolium leads to a
decrease in the dissolving power from 23 to 17 wt %.
This is confirmed by the data of [8, 13].
19
14.5
15
6.5
8
4
In the FTIR spectra of the cellulose regenerated
from the ionic liquid appear two intense bands at
3440 cm–1 and 3484 cm–1, corresponding to the intra-
molecular hydrogen bonds, which indicate a modi-
fication of cellulose II (Fig. 6).
5.5
6
7.5
24
1-Allyl-3-methylpyridinium chloride
1-Ethyl-3-methylpyridinium bromide
1-Butyl-3-methylpyridinium bromide
6.5
3.5
7
Thus, according to the results of quantum-chemical
calculations, with the increasing length of N-alkyl
substituent of 1-alkyl-3-methylpyridinium the inter-
action energy between the cation and anion decreases,
while the distance between them increases. Therewith,
the melting points, as well as dissolving power of ionic
liquids with respect to cellulose diminish. Replacing
the chloride anion by bromide decreases the effective
The study of the solubility of cellulose in the
synthesized ionic liquid showed that for the same
cation the 1-alkyl-3-methylpyridinium bromides show
lower dissolving ability than chlorides, the latter
δ, ppm
Fig. 3. The assignment of the peaks in the 1H NMR spectrum (CDCl3) of 1-ethyl-3-methylpyridinium chloride.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 12 2012

SYNTHESIS AND DISSOLVING POWER
1997
Wavenumber, cm–1
Fig. 4. The assignment of the IR bands of the Fourier spectrum of 1-ethyl-3-methylpyridinium chloride.
charge of the anion and reduces the dissolving power
of the ionic liquid with respect to cellulose.
Pyridinium-containing ionic liquids dissolve cellulose
more efficiently than ionic liquids based on the imida-
zolium cation with the same substituents and anions.
99%), 1-chlorohexane (Fluka,> 99%), 1-chloroheptane
(Aldrich, 99%), 1-chlorooctane (Aldrich, 99%), 1-
chlorononane (Aldrich, 98%), and 1-chlorodecane
(Aldrich, 98%). To determine the dissolving ability of
ionic liquids the Alicell Super pulp (Western Pulp,
Inc., Canada) was used with the degree of poly-
merization 599 and water content 8%.
EXPERIMENTAL
We used the following chemical reagents and
materials: allyl chloride (Fluka, 98%), acetonitrile
(Lab-scan analytical science, 99.9%, Poland), 1-
bromoethane (Aldrich, 98%), 1-bromobutane (Aldrich,
99%), diethyl ether (Aldrich), 3-picoline (Aldrich,
99%), toluene (Aldrich), 1-chloroethane (Aldrich, ≥
99.7%), 1-chloropropane (Aldrich, 98%), 1-chloro-
butane (Aldrich, 99 %), 1-chloropentane (Fluka, ≥
The halide salts of 1-alkyl-3-methylpyridinium
were prepared by the alkylation of 3-picoline [14] with
the corresponding alkyl halide. To a two-neck flask
with a reflux condenser and a magnetic stirrer a solu-
tion of 3-substituted pyridine (0.1665 mol) in toluene
(20 ml) was charged, and to it alkyl halide (0.2000 mol)
was added. The solution was then stirred for 1 h at
room temperature under nitrogen atmosphere. Then the
Absorption
Wavenumber, cm–1
Fig. 6. IR spectrum in the region of 2800–3800 cm–1 of
(1) the initial cellulose and (2) recovered from a 5% solu-
tion in 1-butyl-3-methylpyridinium chloride.
R
Fig. 5. The dependence of the melting point on the length
of the alkyl chain (R) for 1-alkyl-3-methylpyridinium
chlorides-based ionic liquids .
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 12 2012

1998
SASHINA et al.
reaction mixture was stirred at a temperature of 110–
130°C for 24–48 h with reflux without nitrogen flow.
Upon completion, the reaction product obtained
separated in layers, toluene was decanted from the
dense layer of ionic liquid. The product was washed
with toluene. Recrystallization was carried out by
dissolution in acetonitrile followed by precipitation
with excess diethyl ether. Volatile organic compounds
were removed on a rotary evaporator (Rotavapor R-
215, Büchi, Switzerland) for 6–8 h, the pressure
gradually was reduced from atmospheric to 20–
120 mbar, and the temperature was raised from 60°C
to 90–140°C.
ACKNOWLEDGMENTS
This work was financially supported by the Ministry
of Education and Science of the Russian Federation.
REFERENCES
1. Swatloski, R.P., Rogers, R.D., and Holbrey, J.D., WO
Patent 03/029329 A3, 2003.
2. Murugesan, S. and Linhard, R.J., Curr. Org. Synth.
2005, no. 2, p. 437.
3. Phillips, D.M., Drummy, L.F., Conrady, D.G., Fox, D.M.,
Naik, R.R., Stone, M.O., Trulove, P.C., De Long, H.C.,
and Mantz, R.A., J. Am. Chem. Soc., 2004, vol. 126,
no. 44, p. 14350.
1
4. Phillips, D.M., Drummy, L.F., Naik, R.R., De Long, H.C.,
Fox, D.M., Trulove, P.C., and Mantz, R.A., J. Mater.
Chem., 2005, no. 75, p. 4206.
The obtained salts were identified by H NMR
spectroscopy (spectrometer Bruker Avance II Plus
700 MHz, USA), infrared spectroscopy (FT spectro-
meter Nicolet 6700 FT-IR, USA), and chromato-
graphy–mass spectrometry (spectrometer GCMS-
QP2010 Plus, Shimadzu, Japan). Water content was
determined by thermogravimetric analysis (system
STARe TGA/DSC 1, Mettler Toledo, Germany).
5. Xie, H., Li, S., and Zhang, S., Green Chem., 2005, vol. 7,
no. 14, p. 606.
6. Novoselov, N.P., Sashina, E.S., Kuz’mina, O.G., and
Troshenkova, S.V., Russ. J. Gen. Chem., 2007, vol. 77,
no. 8, p. 1395.
7. Sashina, E.S. and Novoselov, N.P., Russ. J. Gen. Chem.,
2009, vol. 79, no. 6, p. 1057.
8. Heinze, T., Schwikal, K., and Barthel, S., Macromol.
Biosci., 2005, no. 5, p. 520.
The melting point of ionic liquids were determined
by thermal analysis using differential scanning calo-
rimetry (calorimeter NETZSCH-Gerätebau GmbH
Thermal Analysis DSC 204, Germany).
9. Fukaya, Y., Hayashi, K., Wada, M., and Ohno, H.,
Green Chem., 2008, no. 10, p. 44.
Dissolving power of the synthesized ionic liquids
was determined as follows. Cellulose was added to the
solvent by portions after the complete dissolution of
the previous portion. For dissolution the mixture was
stirred continuously for at least 12 h with temperature
control. The dissolved cellulose con-centration D was
calculated by the formula:
10. Graenacher, C., USA Patent 1943176, 1934.
11. Kokorin, A., Ionic Liquids: Applications and Perspec-
tives, Rijeka: In Tech., 2011, p. 529.
12. Park, T.-J., Murugesan, S., and Linhardt, R.J., ACS
Symposium Series, 2009, vol. 1017, p. 133.
13. Basa, M.L.T.N., Ionic Liquids: Solvation Charac-
teristics and Cellulose Dissolution, Toledo: The
University of Toledo, 2010, p. 179.
D = [m1/(m1 + m2)](100 – cw),
14. Bentivoglio, G., Roder, T., Fasching, M., Buchberger, M.,
Schottenberger, H., and Sixta, H., Lenzinger Ber., 2006,
no. 86, p. 154.
where D is the cellulose concentration in the ionic
liquid, m1 and m2 are weights of cellulose and ionic
liquid, respectively, g; cw is the water content in the
pulp, %.
15. Harjani, J.R., Singer, R.D., Garcia, M.T., and Scam-
mells, P.J., Green Chem., 2009, no. 11, p. 83.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 12 2012