856 Journal of Chemical Education Vol. 86 No. 7 July 200 www.JCE.DivCHED.org © Division of Chemical Education

In the Laboratory

Ionic liquids or molten salts are compounds that are com-posed 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 classi"ed 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 modu-

lated by modi"cations of the anion and cation sca#olds (1). One of the main advantages of ionic liquids is their low, almost negligible, vapor pressure compared to volatile and hence haz-ardous organic solvents. $is 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 con-tributed to the enormous growth and expansion of the "eld 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. $us, application of “task-speci"c” ionic liquids can provide additional bene"ts for a variety of processes (4).

Advances in the area of ionic liquids should prompt the introduction of ionic-liquid experiments and concepts into un-dergraduate organic chemistry laboratories. $e nature of the or-ganic laboratory courses o%en 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.

$e following abbreviation for ionic liquids can be utilized: 1-butyl-3-methylimidazolium bromide can be described as [C4 mim]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 abbre-viate lengthy names of ionic liquids. $e 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.

$e simple and robust nature of the protocol is an advan-tage over other available procedures. $e experiment can serve as an illustration of one-pot processes, heterocyclic chemistry, and

Synthesis of Imidazolium Room-Temperature Ionic Liquids

Exploring Green Chemistry and Click Chemistry Paradigms in Undergraduate Organic Chemistry LaboratorySergei V. Dzyuba,* Katherine D. Kollar, and Salil S. Sabnis

Department of Chemistry, Texas Christian University, Fort Worth, TX 76129; *s.dzyuba@tcu.edu

N NR

NR

A

N

R

A

A

AN

R

R3

R2

R1

A = anion [PF6, N(SO2CF3)2, N(CN)2, BF4, etc.]

R = alkyl, aralkyl

Figure 1. Structures of some common ionic liquids.

N NN N

Br

N N

PF6

Br

H2O

reflux, 1.5 h

KPF6

[C4-mim]Br

[C4-mim]PF6

H2O, 15 min

Scheme I. One-pot synthesis of [C4-mim]PF6 ionic liquid.

Green Chemistry edited by

Mary M. KirchhoffACS Education Division

Washington, DC 20036

© Division of Chemical Education www.JCE.DivCHED.org Vol. 86 No. 7 July 200 Journal of Chemical Education 857

In the Laboratory

organic reactions in water. $e speci"c 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 e&cient, 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 sca#olds, 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. $is reaction has been demonstrated to be a unique, useful, and versatile tool for various areas of chemistry, materials science, engineering, and biology (8b–e). $e under-graduate experiment describing this metal-catalyzed cycloaddi-tion reaction has been reported recently (8f ).

$e goal of the current article is to make students aware of room-temperature ionic liquids and allow them to get a "rst-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.

Hazards

Caution should be exercised when handling all the chemicals, which are considered harmful and irritants. 1-Meth-ylimidazole and potassium hexa'uorophosphate are corrosives; dichloromethane, acetone-d6, and 1-bromobutane are 'am-mable; dichloromethane is a probable human carcinogen. $e 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. Dis-pose all waste according to local, state, and federal regulations.

Results and Discussion

One-pot syntheses of several room-temperature ionic liq-uids were carried out in water or ethanol according to Scheme I in a four-hour laboratory period using a simple re'ux setup (sand bath, condenser, round-bottom 'ask, and a boiling chip). Concentration a#ects e&ciency of [C4 mim]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.) $e reaction can be done under neat con-ditions (9), when magnetic stirring and precise temperature con-trol are available. A boiling chip can also be used and the reaction is still facile (no starting material is observed by 1H NMR a%er 15–20 min at 160–180 °C), and formation of a light brown oil [C4 mim]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%

pure by 1H NMR. Several puri"cation 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 re-sulting ionic liquid does not develop a brown color. $e one-pot procedure is versatile and can be done in a variety of solvents. In particular, the synthesis of [C4 mim]PF6 (Scheme I) is read-ily accomplished using water as the only reaction solvent. $e 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 dichlo-romethane, has to be used to achieve a facile and convenient extraction step in the preparation of [C4 mim]PF6. Other-wise, the high viscosity of the ionic liquid leads to signi"cant losses during the phase separation (ionic liquid water and ionic liquid drying agent) steps. While the synthesis of [C4 mim]PF6 might qualify as green chemistry, the use of dichloromethane during the "nal stages diminishes such claims.

$e overall yield for the synthesis of [C4 mim]PF6 is about 80%. $e formation of the imidazolium moiety is con"rmed by 1H NMR by the characteristic down"eld shi% of the heterocy-clic protons upon quaternization of the 1-methylimidazole. $e chemical shi%s of the imidazolium protons are concentration, solvent, and anion dependent. $e anion exchange is established by characteristic bands in the IR spectra of the isolated room-temperature ionic liquid (11). 19F NMR can also be used to con"rm the identity of the anion of the ionic liquid.

Conclusions

$e experiment allows students to test and learn several concepts of contemporary chemistry. $is process is done in the “environmentally” preferred solvent, such as water (12); the reac-tion also meets the standards of click chemistry: readily available starting materials, fast reaction times, ease of synthesis, as well as isolation and puri"cation of the "nal products. Mechanistically, the quaternization of 1-methylimidazole with 1-bromobutane serves as a good example of an SN2 reaction. $e syntheses of ionic liquids can be presented as a single, stand-alone experi-ment or introduced as a sequence, for example, (i) synthesis of ionic liquids and (ii) application of ionic liquids as solvents for chemical reactions.

Acknowledgment

The authors thank TCU for financial support of this work.

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858 Journal of Chemical Education Vol. 86 No. 7 July 200 www.JCE.DivCHED.org © Division of Chemical Education

In the Laboratory

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