Recyclability Tests The aqueous layer, containing [BMIM][Br], was extracted from the Diels-Alder reaction mixture and was placed into a rotary evaporator with a 75°C hot water bath until only the viscous ionic liquid remained (approximately 2 hours). The ionic liquid ([BMIM][Br]) was then reused in the Diels-Alder Reaction described. TLC and FT-IR were performed, confirming that the ionic liquid was not changed during the reaction. 

Results and DiscussionIonic Liquid SynthesisMost ionic liquid synthesis procedures require too much time to be used for educational purposes; however, Microwave-Enhanced ionic liquid synthesis reduces the time required from hours to minutes. 3 It was demonstrated that [BMIM][Br] and [BMIM][I] could be synthesized in as little as 2 minutes. Similarly, [MPtolsIM][Cl] was synthesized via 2 minutes of microwave radiation (Scheme 1).

By comparing the IR spectra of the ionic liquids [BMIM][Br] and [MPtolsIM][Cl] with the 1- methylimidazole spectra, it was concluded that the diethyl ether washes were insufficient to remove the remaining 1-methylimidazole, since many of the 1-methylimidazole peaks were also present in the [MPtolsIM][Cl] spectra; however, indications of the ionic liquid product are still present. For example, the [MPtolsIM][Cl] spectra has peaks that are indicative of aromatic C= C bonds (1581.5 and 1455.5 cm -

1). These sharp peaks are observed on the [MPtolsIM][Cl] spectra, but not in the 1-methylimidazole spectra and could not have indicated a contaminant p-toluenesulfonyl chloride. P-toluenesulfonyl chloride’s role as the limiting reagent suggests that all the compound had reacted, and it is quite soluble in diethyl ether. Another indicator of [MPtolsIM][Cl] is the broad peak at 1200 cm -1, which may suggest the simultaneous presence of sulfnyl S=O bond and aromatic nitrogen, both of which have characteristic peaks near 1180 cm-1 .3,4 Also, the S=O peak from the p-toluenesufonyl chloride peak is broad, which would account for the width of the peak on the [MPtolsIM][Cl] spectra.

GREEN CHEMISTRY: MICROWAVE SYNTHESIS OF IONIC LIQUIDS AND THEIR USE AS MEDIUM IN DIELS-ALDER REACTION Anthony J. Lilienthal, Arthur J. Koudelka, Benjamin C. Haenni, and Gholam A. Mirafzal (Mentor)

College of Arts and Sciences, Department of Chemistry, Drake University

AbstractThe effectiveness of the substitution of ionic liquids for traditional aromatic solvents in the Diels-Alder Reaction was investigated. Three methylimidazolium based ionic liquids – 1-butyl-3-methylimdazolium bromide, 1-butyl-3-methylimidazolium iodide and 1-methyl-3-(p-toluene sulfonyl)imidazolium chloride – were synthesized in the microwave and were then used as solvents/catalysts in the Diels-Alder reaction between maleic anhydride and 1,3-cyclohexadiene. The results presented here illustrate how ionic liquids are effective and recyclable solvents for the Diels-Alder reaction.

IntroductionIonic Liquids have been termed “Designer Solvents”1 due to their variable physical properties. Due to their low volatility and recyclability, they are also considered Green Solvents.2 Ionic liquids could offer students the opportunity to perform many chemical synthesis reactions without using harsh, volatile organic solvents. The synthesis of a new ionic liquid and its effectiveness as a solvent for the Diels-Alder reaction between 1,3-cyclohexadiene and maleic anhydride is herein described. In addition, the recyclability of another ionic liquid is also described.

Materials and MethodsMaleic anhydride, 1,3-Cyclohexadiene, 1-iodobutane, 1-bromobutane, 1-methylimidazole, and p-toluenesulfonyl chloride were all purchased from the Sigma Aldrich company and were used without any further purification.

FT-IR were performed neat on a Nicolet 510P Spectrometer. FID-GC were performed on an H6890 series instrument, equipped with an HP-5 (Crosslinked 5% PH ME Siloxane) column with 30m x 0.32 mm x 0.25 μm film thickness. During the analyses of the Diels-Alder Reaction products, the initial column temperature was 60°C (1 minute), which was then increased 10°C/min. to 250°C (1 minute).

Experimental ProcedureIonic Liquid Synthesis (Scheme 1)Butylmethylimidazoluim Halide Ionic Liquid Synthesis:15 mmoles of 1-bromobutane (or 1-iodobutane) and 15 mmoles of 1-methylimidizole (MIM) were added to a small beaker and heated for 2 minutes (8 x 15 s.) at 10 % power in a SHARP Carousel Microwave. The product was washed twice with diethyl ether to remove any possible remaining reactants.

Methyl(p-toluenesulfonyl)imidazolium Halide Ionic Liquid Synthesis:5 mmoles of p-toluenesulfonyl chloride and 20 mmoles 1-methylimidazole (MIM) were mixed (20 s.) into a slurry in a small beaker. The slurry was heated for 2 minutes (8 x 15 s.) at 10% power in a SHARP Carousel Microwave. The product was washed three times (4 mL each) with diethyl ether to remove any possible remaining reactants. Diels-Alder Reaction1.2 mmoles of maleic anhydride and a magnetic stirbar were added to a 10 mL round bottomed flask. To this 6.0 mmoles of cyclohexadiene was added. Immediately following the addition of the cyclohexadiene, 1.00 mL of the ionic liquid (1-butyl-3-methylimdiazolium bromide ([BMIM][Br]), 1-butyl-3-methylimidazolium iodide ([BMIM][I]), or 1-methyl-3-(p-toluenesulfonyl)imidazolium chloride ([MPtsIM][Cl])) was added and the reaction was mixed at room temperature for 45 minutes. The Diels-Alder product was then separated from the ionic liquid by the addition of equal volumes diethyl ether and water. This organic layer was analyzed via FID-GC to determine the %conversion of maleic anhydride into the Diels-Alder Product (Scheme 2).

Diels-Alder Reaction with [PtolsMIM][Cl]The effectiveness of 1-methy-3-(p-toluenesulfonone)imidazolium chloride ([PtolsMIM][Cl]) and freshly synthesized [BMIM][Br] and [BMIM][I] was compared. The % conversion of the maleic anhydride from the [PtolsMIM][Cl] solvated reaction was 65.819% while the % conversion for [BMIM][Br] and [BMIM][I] were 83.854% and 97.887% , respectively (Figure 1). The GC spectra of the organic layers from each of the Diels-Alder reactions show corresponds with the previous results from the previous presentation, where maleic acid (converted from excess maleic anhydride during the extraction) has a retention time of 9.324 minutes while the product has a retention time of 9.741 minutes. Even with it’s decrease % conversion, since the ionic liquid-solvated Diels-Alder reaction was performed at room temperature, it is still a more effective solvent than the volatile molecular solvents.

Recyclability TestsSince, in some situations, it is more efficient for the ionic liquids to be prepared before the students attend the lab, a recyclable solvent would allow more ionic liquid to be conserved. It was concluded through TLC and FT-IR analysis that the removal of water from aqueous [BMIM][Br] is sufficient to recycle bromide ionic liquid. [BMIM][Br] was able to solvate three repetitions of the Diels-Alder reactions (Figure 2) using this recyclability procedure; moreover, the percent maleic anhydride converted during the Diels-Alder reaction solvated by [BMIM][Br] actually increased as the repetitions were performed. It is predicted that [BMIM][I] will have similar recyclability properties, since the dehydration of aqueous layers containing [BMIM][I] also purified the compound and [BMIM][I] has similar properties to [BMIM][Br]; however, more extensive experiments must be performed in order more accurately describe the recyclability butylmethylimidazolium halide ionic liquids as solvents for the Diels-Alder reaction. 

ConclusionIonic liquids offer educators the opportunity to teach students without using hazardous, volatile organic solvents. Three methylimidazolium halide ionic liquids were successfully synthesized in 2 minutes via microwave-enhanced synthesis; however, the 1-methyl-3-(p-toluenesulfonyl)imidazolium chloride was unsuccessfully purified, which, at least partially, accounted for its low maleic anhydride conversion rate during the Diels-Alder reaction. Even with its decreased conversion rate, [PtolsMIM][Cl] was still a more effective solvent for the Diels-Alder reaction, since classic solvents would require hours at high temperature for the reaction to go to completion. Finally, the butylmethylimidazolium bromide ionic liquid were shown to be easily recyclable; multiple Diels-Alder reactions could be performed using the same ionic liquid, as long as the aqueous layer was evaporated prior to the repetition of the experiment.

Future WorkWe are currently experimenting to replace 1,3-cyclohexadiene with less volatile dienes, such as anthracene. Also, the effect of ionic liquid solvation in other reactions will be investigated. Further tests will be performed on the [PtolsMIM][Cl] to further purify it. After which, effectiveness in the Diels-Alder reaction will be reassessed.

References1Yau, Hon Mau et al. “Ionic Liquids: Just Molten Salts After All?” Molecules. 2009, 14 (7) 2521-34.2Varma, Rajender S. "Expeditious Synthesis of Ionic Liquids using Ultrasound and Microwave Irradiation," Ionic Liquids as Green Solvents: Progress and Prospects. Ed. Robin D. Rogers and Kenneth R. Seddon. Boston: American Chemical Society. (2003) 82-92.3 Abdel-Wahab. M. F. et al, “Infrared Study of New Sulfonylurea Derivatives and Sulfonamides.” Analytical Chemistry. 1966, 38 (3) 508.4Chatwal, Gurdeep R., “Application of Infrared Spectroscopy to Organic Compounds.” Spectroscopy. Himalaya Publishing House. (2009) 2.70.

Ionic Liquid

% Maleic Anhydride Converted

[BMIM][Br] 83.854

[BMIM][I] 97.887

[MPtolsIM] [Cl]

65.819

% Maleic Anhydride Converted

Ionic Liquid DA Reaction 1 DA Reaction 2 DA Reaction 3

[BMIM][Br] 83.854 97.482 98.885

Scheme 1: The General Equation of Microwave-Enhanced Synthesis of methylimidazolium halide ionic liquids.

a) [BMIM][Br] R = Butyl X = bromideb) [BMIM][I] R = Butyl X = iodidec) [MPtolsIM][Cl] R = P-toluenesulfonyl X = chloride

Scheme 2

Figure 2

Figure 1