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Author Topic: Purifying Hydrophobic Ionic Liquids  (Read 1092 times)

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MangoPaws

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Purifying Hydrophobic Ionic Liquids
« on: November 30, 2017, 11:00:12 AM »

I am attempting to synthesize a hydrophobic N-alkyl-pyridinium based ionic liquid in a one pot reaction in methanol. I've come up against a bit of an issue in that I know from the crude NMR that the reaction has been rather successful with around a 60% yield however I really have very few ideas as to how to get the bloody stuff out of the mixture.
Column chromatography doesn't work as the product adheres to silica and the starting materials are about as apolar as the product.

Does anyone have an idea? Or also any general advice on how to approach such a problem as I'm pretty new to this.

Thank x
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wildfyr

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Re: Purifying Hydrophobic Ionic Liquids
« Reply #1 on: November 30, 2017, 02:08:57 PM »

Recrystallize at low (dry ice/acetone) temperatures?
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clarkstill

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Re: Purifying Hydrophobic Ionic Liquids
« Reply #2 on: November 30, 2017, 11:04:05 PM »

I don't know much about it, but if you have access this link looks like it might be a useful overview of techniques:

https://link.springer.com/chapter/10.1007%2F128_2008_31
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MangoPaws

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Re: Purifying Hydrophobic Ionic Liquids
« Reply #3 on: December 01, 2017, 01:49:29 AM »

theyre liquids so they don't crystalize. Thanks for the link
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rolnor

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Re: Purifying Hydrophobic Ionic Liquids
« Reply #4 on: December 01, 2017, 02:03:54 PM »

Reversed-phase chromatography or ion-exchange chromatography would be better then straight-phase I think.
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wildfyr

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Re: Purifying Hydrophobic Ionic Liquids
« Reply #5 on: December 03, 2017, 03:54:02 PM »

theyre liquids so they don't crystalize. Thanks for the link

Um, ethanol is a liquid at room temp and it crystallizes. Ionic liquids are unique because they are salts that are liquids at room temp, but they have melting points. Often barely below room temp. They are salts with alkyl chains, they probably crystallize nicely in the right conditions.
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phth

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Re: Purifying Hydrophobic Ionic Liquids
« Reply #6 on: December 03, 2017, 06:37:30 PM »

theyre liquids so they don't crystalize. Thanks for the link

Um, ethanol is a liquid at room temp and it crystallizes. Ionic liquids are unique because they are salts that are liquids at room temp, but they have melting points. Often barely below room temp. They are salts with alkyl chains, they probably crystallize nicely in the right conditions.

Wildfyr is right.  A common way to recrystallize things is to do it cold when the melting point is below room temperature. maybe the MP is -10º and putting it in a -40º freezer would do the trick.
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Raphael

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Re: Purifying Hydrophobic Ionic Liquids
« Reply #7 on: January 01, 2018, 10:40:25 AM »

what solvents have you tried to run columns with? Have you tried adding any salts into it such as KPF6 or KNO3 either has water solutions (usually saturated solutions are used) or by dissolving around 0.1% in something like acetonitrile?
I have run columns on a lot of charged molecules and metal complexes and they can be moved on a column once you start thinking about other solvents than just the standard things (ethyl acetate/hexanes, DCM/MeOH, etc) organic chemists usually use.
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kriggy

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Re: Purifying Hydrophobic Ionic Liquids
« Reply #8 on: January 01, 2018, 11:06:55 AM »

Wouldnt alkylpyridinium stick to the start while the alkylating agent run with the front?

MangoPaws

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Re: Purifying Hydrophobic Ionic Liquids
« Reply #9 on: January 09, 2018, 03:11:13 AM »

Yeah badly worded. I meant that presumably due to their asymmetry etc they wont crystalise easily in a temperature range I can reach in a controlled way. I've tried down to -80ish or as low as I can reach with dry ice and no joy.

what solvents have you tried to run columns with? Have you tried adding any salts into it such as KPF6 or KNO3 either has water solutions (usually saturated solutions are used) or by dissolving around 0.1% in something like acetonitrile?
I have run columns on a lot of charged molecules and metal complexes and they can be moved on a column once you start thinking about other solvents than just the standard things (ethyl acetate/hexanes, DCM/MeOH, etc) organic chemists usually use.
To be honest I've only tried with some of the standard organic solvent systems but it seems to be an issue with the interaction to the silica rather than the insolubility of the ion as the ion is soluble in Acetonitrile, DCM, MeOH even petrol ether. Potentially rev phase would work but never done it
Also as they're supposed to be novel solvents/extractants and I need to test their properties after so getting 10mg off a column is not a useful strategy. I haven't attempted a column using the salts you suggested but I'd worry the anions may persist within the product liquid altering its properties.
Thanks for the suggestions though folks x
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Raphael

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Re: Purifying Hydrophobic Ionic Liquids
« Reply #10 on: January 10, 2018, 03:57:59 PM »

Counter ions can change the properties of things for sure, but there are ways to change counter ions to the ones that you like (salt metathesis, ion exchange columns etc). Good luck. Separating charged things on silica can be really hard.
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wildfyr

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Re: Purifying Hydrophobic Ionic Liquids
« Reply #11 on: January 10, 2018, 04:21:23 PM »

How does it partition in ether/water or linear alkane/water? Maybe you can get most of the pdt to go into the water layer, and the starting materials should be more oleophobic and go organic. Then wash the aqueous with ethyl acetate to get the charged product out of the water layer.
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