I graduated with a master's degree this year and am currently working as a researcher specializing in ionic liquid synthesis.
I am currently synthesizing EMIM TFSI, and after reviewing 40 papers, I have established a procedure and am conducting the experiment as follows:
(Weighing reagents was performed inside a glove box, and the reaction was carried out in an ambient environment using parafilm)
- Dissolve EMIM Br (12 mmol, 1 equiv) in 3 mL of water to prepare an aqueous solution.
- Dissolve Li TFSI (13 mmol, 1.08 equiv) in 2 mL of water to prepare an aqueous solution.
- Add the Li TFSI aqueous solution slowly dropwise (by my hand) to the EMIM Br aqueous solution.
- Stir vigorously at room temperature for 24 hours.
- Transfer the reaction mixture to a separatory funnel and extract with 5 mL of DCM (repeat 5 times).
- Wash the extracted organic layer with 5 mL of water (repeat 5 times, checking for Br ions with AgNO₃).
- Dry the mixture using a rotary evaporator at 65°C for 1 hour.
- Place it in a vacuum oven at 80°C for 48 hours to remove trace amounts of moisture.
I uploaded two NMR results: one from the synthesized EMIM TFSI and the other from commercially available EMIM TFSI for comparison.
Upon examining the NMR, I can confirm that the reaction has proceeded, as the EMIM TFSI peak is visible. However, there are still peaks from unreacted EMIM+ cations present
(Looking at the integration, the peaks show that EMIM TFSI cation, an additional EMIM cation, and water are present in a 2:1:1 ratio).
I thought this result is due to the hygroscopic nature of EMIM Br and Li TFSI, which leads to hydration, reducing reactivity and forming a water shell that prevents the reaction. To address this, I performed weighing in a glove box and repeated the experiment.
Nevertheless, the NMR results were similar to those obtained when the glove box was not used.
Therefore, I began to think that I might have been misunderstanding the cause all along, and I’m seeking advice by asking this question.
If there’s anything I might be overlooking, please let me know. I would also be incredibly grateful if you could share any insights or experiences you have regarding NMR results appearing in this form.
Thank you once again for taking the time to read through this lengthy message.
Hi Minseok Oh
Doesn't seem necessary to stir the aqueous mixture for 24 hours, or even for 24 minutes. This is a simple ion-pair exchange reaction, so a few seconds (or a few minutes to be on the "safe" side) of vigorous stirring should be enough. This will not only save you time. TFSI anions undergo slow hydrolysis in aqueous solutions (see Zhou & al., J Power Sources 577, 1 Sept 2023), so you may be losing some of your starting material there - probably not a lot but why take a chance?
For similar reasons I would suggest to minimize the time spent water-washing your DCM extract and the volume of water used in each wash. The concern is not only some extra hydrolysis but also the fact that EMI TFSI solubility in water is not negligible.
Good luck!
Emanuel Cooper
Hello Pedro Nakasu
First of all, thank you so much for your response.
The second image I uploaded is the NMR spectrum I stacked of the substance I synthesized and that of commercially available EMIM TFSI, while the first image with only the blue spectrum is the NMR of the substance I synthesized.
As you mentioned, I will arrange for additional analysis using DMSO-d6 and will get back to you after that.
Hi Emanuel Cooper
Thank you so much for your response.
I have been optimizing my procedure by referring to more than 40 papers so far. About half of them suggested overnight stirring, while the other half recommended stirring for 30 minutes to 2 hours. I've been curious about why the reaction times differ like this. But after reading your response, I feel like I can plan my next experiment more effectively.
Until now, I had never considered the possibility of hydrolysis, as I assumed the TFSI anion to be stable. Moreover, since EMIM TFSI is a hydrophobic ionic liquid that separates into layers with water, I never realized that some dissolution might occur.
Based on your advice, I will proceed with the experiment using the minimum amount of water for separation.
Thank you for your guidance!
Hi Pedro Nakasu
Thank you for your advice.
Following your advice, I analyzed NMR using DMSO.
As a result, I was able to obtain a clean spectrum, as shown in the picture.
When analyzing the same sample in CDCl₃, there is a noticeable difference compared to DMSO. I am curious about the reason for this.
I also want to understand why I am seeing different results, even though other references analyzed the same compound dissolved in CDCl₃.
Thank you for your attention and response. If you have any insights into this issue, I would greatly appreciate it if you could share them with me.
Best regards,
Minseok Oh
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