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Green Chemistry
Mary M. Kirchhoff
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Synthesis of Imidazolium Room-Temperature Ionic Liquids
Exploring Green Chemistry and Click Chemistry Paradigms
in Undergraduate Organic Chemistry Laboratory
Sergei V. Dzyuba,* Katherine D. Kollar, and Salil S. Sabnis
Department of Chemistry, Texas Christian University, Fort Worth, TX 76129; *[email protected]
Ionic liquids or molten salts are compounds that are composed entirely of ions and exist in a liquid state. For example,
inorganic salts, such as NaCl or AlCl3 when heated above their
melting points can be classified as ionic liquids. Obviously, high
melting points of many inorganic salts negate their utility in the
molten state. Combinations of anion–cation pairs leading to
salts with phase transitions at or below room temperature can be
suitable as room-temperature ionic liquids (1). Predominantly,
these have been quaternary nitrogen-containing heterocycles
(Figure 1). Physical properties of these solvents can be moduA
N
R
N
N
A
R
R
A
A
N
R
R3
R1
N
R2
A = anion [PF6, N(SO2CF3)2, N(CN)2, BF4, etc.]
R = alkyl, aralkyl
Figure 1. Structures of some common ionic liquids.
Br
Br
N
N
N
N
H2O
reflux, 1.5 h
[C4-mim]Br
KPF6
H2O, 15 min
PF6
N
N
[C4-mim]PF6
Scheme I. One-pot synthesis of [C4-mim]PF6 ionic liquid.
856
lated by modifications of the anion and cation scaffolds (1).
One of the main advantages of ionic liquids is their low, almost
negligible, vapor pressure compared to volatile and hence hazardous organic solvents. This has prompted the claim that ionic
liquids are environmentally benign, “green” solvents. However,
it should be noted that the widely popularized benign nature of
ionic liquids might understate potential toxicity (2).
The possibility to conduct chemical, biochemical, and
analytical processes in an ionic, low coordinating, and highly
solvating environment over a wide temperature range has contributed to the enormous growth and expansion of the field of
ionic liquids for use primarily as alternative solvents in organic
reactions (1). Controlling the outcome of a particular process
by designing the best available solvent for the desired product
holds great promise and potential for both fundamental and
applied research (3). Unlike conventional molecular solvents,
the structures of ionic liquids can be modulated with ease. Thus,
application of “task-specific” ionic liquids can provide additional
benefits for a variety of processes (4).
Advances in the area of ionic liquids should prompt the
introduction of ionic-liquid experiments and concepts into undergraduate organic chemistry laboratories. The nature of the organic laboratory courses often dictates what types of experiments
can be carried out within an allocated time period. Previously
reported preparations of room-temperature ionic liquids were
not suitable for an undergraduate laboratory experiment, either
due to the time constraints or the availability of the required
equipment and, therefore, are better suited for advanced-level
laboratory courses, while still requiring substantial involvement
of the instructor (5). Here, we report a facile synthesis of an
imidazolium-based ionic liquid (Scheme I), readily adaptable
for the basic organic chemistry laboratory.
The following abbreviation for ionic liquids can be utilized:
1-butyl-3-methylimidazolium bromide can be described as
[C4Źmim]Br (used in this article), where C4 depicts the butyl
group, and mim represents methylimidazole fragment; this
particular style is believed to be the most versatile way to abbreviate lengthy names of ionic liquids. The other commonly used
abbreviation system would depict 1-butyl-3-methylimidazolium
bromide as [bmim]Br; however, this nomenclature has some
limitations. For example, 1-hexyl-3-methylimidazolium and
1-heptyl-3-methylimidazolium cations would either be given
the same abbreviations, [hmim], or would require the addition
of hex and hep.
The simple and robust nature of the protocol is an advantage over other available procedures. The experiment can serve as
an illustration of one-pot processes, heterocyclic chemistry, and
Journal of Chemical Education t Vol. 86 No. 7 July 200 t www.JCE.DivCHED.org t © Division of Chemical Education
In the Laboratory
organic reactions in water. The specific nature of the reported
synthetic protocol for the synthesis of the ionic liquid allows
not only the exploration of various reactions using ionic liquids
as solvents (5, 6), but also the introduction of various concepts
of green chemistry (7) and click chemistry (8).
Green chemistry is based on principles that are designed
to prevent and reduce the waste and hazard associated with the
production of chemicals (7). Any improvements to the existing
processes as well as the design of the new processes that lead
to more efficient, catalytic, environmentally benign and safe
reactions that utilize renewable feedstocks, reuse and recycle
chemicals and solvents constitutes an important and useful area
of modern research.
Click chemistry can be described as any facile, reliable,
modular, and easy-to-perform paradigm related to preparation
of various scaffolds, starting with readily accessible starting
materials. One of the premier examples of a click chemistry is
the copper(I)-catalyzed azide–alkyne cycloaddition yielding
1,2,3-triazoles. This reaction has been demonstrated to be a
unique, useful, and versatile tool for various areas of chemistry,
materials science, engineering, and biology (8b–e). The undergraduate experiment describing this metal-catalyzed cycloaddition reaction has been reported recently (8f ).
The goal of the current article is to make students aware
of room-temperature ionic liquids and allow them to get a
first-hand experience by synthesizing these materials. In view of
the facile and robust nature of the reported protocol, a number
of imidazolium-based ionic liquids can be prepared by simply
varying the structure of the halide. Subsequent application of the
prepared ionic liquids as a reaction medium can be explored.
pure by 1H NMR. Several purification protocols can be utilized
to remove colored impurities either at the stage of the bromide
or later (10), if spectroscopic grade ionic liquids are desired.
Carrying out the reaction in a solvent ensures that the resulting ionic liquid does not develop a brown color. The one-pot
procedure is versatile and can be done in a variety of solvents. In
particular, the synthesis of [C4Źmim]PF6 (Scheme I) is readily accomplished using water as the only reaction solvent. The
synthesis relies on the fact that this ionic liquid is immiscible
with water and, therefore, is easily isolated by using a separatory
funnel or removing the upper aqueous layer with a pipet. In
case of small-scale reactions, an organic solvent, such as dichloromethane, has to be used to achieve a facile and convenient
extraction step in the preparation of [C4Źmim]PF6. Otherwise, the high viscosity of the ionic liquid leads to significant
losses during the phase separation (ionic liquidžwater and ionic
liquidždrying agent) steps. While the synthesis of [C4Źmim]PF6
might qualify as green chemistry, the use of dichloromethane
during the final stages diminishes such claims.
The overall yield for the synthesis of [C4Źmim]PF6 is about
80%. The formation of the imidazolium moiety is confirmed by
1H NMR by the characteristic downfield shift of the heterocyclic protons upon quaternization of the 1-methylimidazole. The
chemical shifts of the imidazolium protons are concentration,
solvent, and anion dependent. The anion exchange is established
by characteristic bands in the IR spectra of the isolated roomtemperature ionic liquid (11). 19F NMR can also be used to
confirm the identity of the anion of the ionic liquid.
Hazards
The experiment allows students to test and learn several
concepts of contemporary chemistry. This process is done in the
“environmentally” preferred solvent, such as water (12); the reaction also meets the standards of click chemistry: readily available
starting materials, fast reaction times, ease of synthesis, as well as
isolation and purification of the final products. Mechanistically,
the quaternization of 1-methylimidazole with 1-bromobutane
serves as a good example of an SN2 reaction. The syntheses of
ionic liquids can be presented as a single, stand-alone experiment or introduced as a sequence, for example, (i) synthesis of
ionic liquids and (ii) application of ionic liquids as solvents for
chemical reactions.
Caution should be exercised when handling all the
chemicals, which are considered harmful and irritants. 1-Methylimidazole and potassium hexafluorophosphate are corrosives;
dichloromethane, acetone-d6, and 1-bromobutane are flammable; dichloromethane is a probable human carcinogen. The
hazards associated with ionic liquids are not fully explored,
therefore contact with skin, eyes, and clothes should be avoided.
Gloves should be worn at all times during the experiment. Dispose all waste according to local, state, and federal regulations.
Results and Discussion
One-pot syntheses of several room-temperature ionic liquids were carried out in water or ethanol according to Scheme I
in a four-hour laboratory period using a simple reflux setup
(sand bath, condenser, round-bottom flask, and a boiling chip).
Concentration affects efficiency of [C4Źmim]Br formation. For
example, a 12.5 M reaction in water is completed within 1.5 h,
whereas a 1.3 M reaction in water requires 10–12 h. (It should
be noted that equimolar amounts of the starting materials are
utilized throughout.) The reaction can be done under neat conditions (9), when magnetic stirring and precise temperature control are available. A boiling chip can also be used and the reaction
is still facile (no starting material is observed by 1H NMR after
15–20 min at 160–180 °C), and formation of a light brown oil
[C4Źmim]Br is achieved. Under these conditions, the reaction
requires careful monitoring, as prolonged heating leads to the
formation of a darker brown oil, which is still, however, >99%
Conclusions
Acknowledgment
The authors thank TCU for financial support of this
work.
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Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2009/Jul/abs856.html
Abstract and keywords
Full text (PDF) with links to cited URLs and JCE articles
Supplement
Instructions for the students
Notes for the instructor
Sample 1H NMR, 19F NMR, and IR spectra
Journal of Chemical Education t Vol. 86 No. 7 July 200 t www.JCE.DivCHED.org t © Division of Chemical Education