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Author: Subject: Drying an impossibly deliquescent iminium salt (how stable are iminium salts?)
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sad.gif posted on 29-6-2022 at 02:19
Drying an impossibly deliquescent iminium salt (how stable are iminium salts?)


Hi,

I have recently attempted to prepare Böhme's salt, which is N,N-dimethylaminomethyleneiminium chloride:
bohme.png - 7kB

Similar to the more well-known Eschenmoser's salt, except the latter has an iodide anion.

However, due to some water that was mysteriously present in my solvent, I ended up with a syrupy solution of the iminium salt, rather than dry crystals.
It is hard to state how deliquescent this stuff is, I thought dimethylamine hydrochloride was bad, but this seems to absorb water impossibly fast, it turns from dry crystals to droplets of solution in a matter of seconds.

I don't know exactly how much water the salt has absorbed, but I'd imagine it is quite a small quantity of water, it doesn't take much water to turn something like this into a liquid, it might only be 10% water...

Anyway, I have placed it in a vacuum desiccator over phosphorus pentoxide and left it there overnight, but it made no difference, the weight of the substance (initially around 2g) has not dropped by even 10mg, and the appearance is exactly the same; a slightly yellow, very viscous liquid, exactly like honey.



Sigma-Aldrich says this substance has a melting point of 146-148c, so one might suppose it is quite stable to heating below this temperature.

The problem with using a desiccant as I am doing is that the desiccant can only remove water from the air surrounding it, thereby pushing forward the equilibrium and allowing water to escape the substance to be dried, but in order for water to escape, the substance has to have a vapour pressure, and I suspect the vapour pressure of this solution is incredibly low, so I imagine it will dry completely in the desiccator, but it will probably take forever!

The solution, then, seems to be to increase the vapour pressure, I can see two ways of doing this:
1) Decrease the pressure
2) Heat the substance

I have only got a ~80% vacuum at the moment, due to the use of the hand-pump, however I could achieve almost 100% with my vacuum aspirator.

I was thinking of using the following apparatus:
An RBF containing the compound to be dried is connected to a 3-way adapter, the top of the adapter is stoppered, and the side-neck (to which you would normally connect the condenser) is attached to a vacuum take-off adapter, the vacuum line of which is connected to the vacuum pump; to the vacuum take-off adapter is attached another RBF which is filled with phosphorus pentoxide drying agent.

The apparatus is then evacuated, and the RBF containing the compound to be dried is heated to whatever might be a safe temperature (say 100c).

This might expedite the drying and bring it into an acceptable time period...

Another idea; If one were to dissolve the damp iminium salt in an anhydrous solvent such as isopropanol, assuming that it would dissolve and not remain biphasic due to the strong affinity of the salt for water, could one then dry the solution with molecular sieves before distilling off the alcohol?

I imagine this might work by breaking the strong ion-dipole attraction between the salt and water, by adding a second solvent; As far as I know, there is no solution from which molecular sieves will not remove the water, but yet this seems too easy, I suspect I must be missing something.


So this thread is basically about two things:
Do you think it is possible to dry something as deliquescent as this, without Hell first freezing over?
How stable do you suppose an iminium salt would be to heating, especially in the presence of water?


Thanks in advance!
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clearly_not_atara
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[*] posted on 29-6-2022 at 07:34


Quote:
such as isopropanol

I don't think this would be an appropriate choice; you probably want something aprotic. I'm not sure exactly what solvents this will dissolve in -- the more popular term is N,N-dimethylmethyleniminium chloride. The literature shows it being used for reactions in solvents as hydrophobic as toluene, so I think that there are probably some reasonable possibilities.

I think the salt is probably stable to heating -- if it's dry, that is. However, it will probably react with water when heated, and since it contains water, you can expect some hydrolysis. Nonetheless, this might be sufficient for your purposes, since it does get rid of the water, and leave most of your salt.

Wild guess: dissolve/suspend in acetonitrile, dehydrate by catalytic nitrile-water reaction with e.g. Raney copper or other copper nanoparticle catalyst. Am not sure how best to remove the salt from MeCN afterwards.




[Edited on 04-20-1969 by clearly_not_atara]
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[*] posted on 29-6-2022 at 08:34


You could try a drop of the syrupy liquid in conc. sulphuric acid ?
I've no idea if the acid would destroy your salt ;)




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[*] posted on 29-6-2022 at 09:44


Could you rig something like this?
https://en.wikipedia.org/wiki/Dean%E2%80%93Stark_apparatus
That way you can sweep the vapour from the hot syrup using something like toluene vapour.
(Assuming the stuff is stable)
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[*] posted on 29-6-2022 at 09:46


Maybe it would be worth changing the anion to something that fits better?



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[*] posted on 29-6-2022 at 11:07


I did the same RBF - vacuum type drying. I had some syrupy mass of CuBr2 (which is also deliquescent) that was impossible to dry on it's own, dessicator (atmospheric pressure) was not able to remove the water.

I dumped it all to RBF, attached a hose adapter and connected with aspirator pump. Then RBF was placed in heating mantle, applied some heat and I got dry crystals in 20 minutes.


[Edited on 29-6-2022 by Oxy]
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[*] posted on 30-6-2022 at 14:20


Crap.
Azeotropic distillation with toluene, in a dean-stark, could fix this?

But I would honestly restart all over again.

Anyways, I attached a picture for building a dean-stark from ground glass joint parts, maybe you got everything on hand for this?
It stems from here: https://erowid.org/archive/rhodium/chemistry/equipment/simpl...
Good luck! :)

fancy dean stark.jpg - 29kB




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[*] posted on 1-7-2022 at 02:14


Quote: Originally posted by clearly_not_atara  
Quote:
such as isopropanol

I don't think this would be an appropriate choice; you probably want something aprotic. I'm not sure exactly what solvents this will dissolve in -- the more popular term is N,N-dimethylmethyleniminium chloride. The literature shows it being used for reactions in solvents as hydrophobic as toluene, so I think that there are probably some reasonable possibilities.

I think the salt is probably stable to heating -- if it's dry, that is. However, it will probably react with water when heated, and since it contains water, you can expect some hydrolysis. Nonetheless, this might be sufficient for your purposes, since it does get rid of the water, and leave most of your salt.

Wild guess: dissolve/suspend in acetonitrile, dehydrate by catalytic nitrile-water reaction with e.g. Raney copper or other copper nanoparticle catalyst. Am not sure how best to remove the salt from MeCN afterwards.



Thanks, I didn't know that it usually went by its formula name, I see a few more articles about it now that I use that name :D
It does indeed dissolve in isopropanol, even when wet (well I don't have any dry stuff to try), but I haven't tried drying the solution yet.


That's encouraging, I'm not too worried about a little bit of hydrolysis, it does seem to smell slightly of formaldehyde so maybe that is evidence of hydrolysis, or maybe it is just residual formaldehyde from the preparation.

Well I need it in MeCN anyway as the reaction solvent, so that would be okay, but I don't have any Raney copper.



Quote: Originally posted by Sulaiman  
You could try a drop of the syrupy liquid in conc. sulphuric acid ?
I've no idea if the acid would destroy your salt ;)


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Quote: Originally posted by DraconicAcid  
Maybe it would be worth changing the anion to something that fits better?



That's an interesting idea, not sure how that would be done though; This salt is a like quaternary ammonium compound (I don't know if there is an analogous term for imines), it is permanently charged and doesn't have any protons to donate, so as far as I know it isn't an acid so you couldn't convert it to a free-base and then react it with a different acid.


Quote: Originally posted by Oxy  
I did the same RBF - vacuum type drying. I had some syrupy mass of CuBr2 (which is also deliquescent) that was impossible to dry on it's own, dessicator (atmospheric pressure) was not able to remove the water.

I dumped it all to RBF, attached a hose adapter and connected with aspirator pump. Then RBF was placed in heating mantle, applied some heat and I got dry crystals in 20 minutes.


[Edited on 29-6-2022 by Oxy]



This is encouraging! I have spent the last two days trying to get the ground-glass joints in my glassware to hold a vacuum, I think I have finally done it by using thick grease (the joints are a little wobbly), this is definitely what I will be trying first so I will let you all know how it goes


Quote: Originally posted by unionised  
Could you rig something like this?
https://en.wikipedia.org/wiki/Dean%E2%80%93Stark_apparatus
That way you can sweep the vapour from the hot syrup using something like toluene vapour.
(Assuming the stuff is stable)



Quote: Originally posted by karlos³  
Crap.
Azeotropic distillation with toluene, in a dean-stark, could fix this?

But I would honestly restart all over again.

Anyways, I attached a picture for building a dean-stark from ground glass joint parts, maybe you got everything on hand for this?
It stems from here: https://erowid.org/archive/rhodium/chemistry/equipment/simpl...
Good luck! :)



Thanks for the suggestions, unfortunately I don't have a Claisen adapter, for some reason! Haha

Yeah, I will start all over if all else fails, but the whole problem really arose because I ran out of ether and so I took someone's advice to use acetone ;)
I had quite nice off-white crystals smelling of benzoyl chloride, but, upon the addition of the acetone, the whole lot melted into syrupy sludge, apparently my acetone contained water inexplicably!

This salt seems to be cursed, everyone who makes it runs out of ether :P

To be honest it's insanely deliquescent, I don't think it's possible to have it truly anhydrous unless you somehow dry it in-situ as I am about to attempt, just how much of a problem is water in the Mannich reaction, one wonders.
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[*] posted on 1-7-2022 at 03:28


If the volume of water is small, and not being created over a longer amount of time like in a reaction, you don't need the Dean Stark. You can just add some toluene, xylene, hexane, cyclohexane, isopropanol etc. and evaporate.

Toluene forms an azeotrope with 20% water, so not much would be needed.
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[*] posted on 1-7-2022 at 04:54


Do you have lots of acetonitrile?
If so, you can start by sacrificing something really valuable to the Gods of chemistry (you are going to need their help) and azeotropically remove the water.
In principle, if you add ACN to your syrup then distill out the solvent you should take out some of the water.
You can dry the distillate with something cheap + easy like CaCl2, redistill it and then repeat the process.
Eventually you will strip all the water out of your product.

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[*] posted on 1-7-2022 at 11:48


Second the Dean-Stark. Just toluene would likely do it. If the decomp is a problem, do it under vacuum.



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[*] posted on 2-7-2022 at 02:05


Thanks, everyone!

I eventually managed to get the flasks vacuum-tight but even at 76mmHg and heating the salt to 60c, with phosphorus pentoxide in the other flask, it still hasn't crystallized, although it seems even more viscous than before.

I haven't weighed it yet, but if it doesn't work soon I shall try the azeotropic distillation.
Is there any reason I would want to use toluene rather than, say, isopropanol, both of which form an azeotrope consisting of ~20% water?
Obviously, it's easier to separate the water from the toluene in the distillate, but I don't have a lot of toluene as it isn't easily available here.

Do you suppose it's likely that this might be a problem, with the salt holding on to the water and breaking the azeotrope?

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[*] posted on 2-7-2022 at 03:14


IIRC both benzene and xylene forms similar azeotropes.



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[*] posted on 2-7-2022 at 05:28


Toluene is notorious for being hard to remove completely after being used as a solvent. I would first try something like hexane,cyclohexane or isopropanol as the temperature required to do the distillation is much lower. Just be sure to use something your salt is not soluble in.

Sure the tendency of your salt will influence the azeotrope, but everything is in equilibrium so as long as your salt doesn't evaporate you will get out the water at some point.

[Edited on 2-7-2022 by Tsjerk]
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[*] posted on 3-7-2022 at 01:57


Alrighty, I will try this!

There appears to be some oily organic compound in the salt as well, so hopefully this will get carried away in the vapour along with the water.


What would happen if I did use a solvent in which the salt is soluble?
I don't really have much data as to what it may or may not be soluble in, except I know it is insoluble in ether, but ether would hardly take away any water, only 1.3%
As Atara said, it may be soluble in non-polar solvents too.

Also, it does seem to be soluble in isopropanol in its syrupy aqueous state, but it may not still be soluble when it is dry.

Anyway, I shall try it with isopropanol first and see what happens.

[Edited on 3-7-2022 by SplendidAcylation]
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[*] posted on 3-7-2022 at 03:04


Truth be told you shouldn't need any azeotrope to get the Dean Stark to work. Any immiscible solvent combo should work, just look at steam distillation. And solubility isn't a problem either as long as it's not volatile.



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[*] posted on 8-7-2022 at 18:56


Quote: Originally posted by SplendidAcylation  

This salt seems to be cursed, everyone who makes it runs out of ether :P

True :D

Quote: Originally posted by SplendidAcylation  

To be honest it's insanely deliquescent, I don't think it's possible to have it truly anhydrous unless you somehow dry it in-situ as I am about to attempt, just how much of a problem is water in the Mannich reaction, one wonders.

Well, normally not really.
But in that "specific" case?

We're talking about a difference of maybe 2% or +40% yield, because of the annoying steric hindrance.
The eschenmoser/böhme salt is the reactive intermediate anyways, but in presence of water it exist in an equilibrium or something like that(sorry can't recount properly), with the result being a reaction rate that is much slower in presence of water.
Like, 12h reflux for 2% yield, opposed to 45min with the eschenmoser salt for +40% yield.


Quote:

There appears to be some oily organic compound in the salt as well, so hopefully this will get carried away in the vapour along with the water.

Mhmm... might be some 4-(dimethylamino)-2-butanone, the reaction product of the salt with acetone... or the dimethylbenzamide...

I need to remember where my container with the salt is, I wanted to put it to use again anyways.
Will be interesting to find out if the content is still useable, and also, dry.




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[*] posted on 9-7-2022 at 13:50


Quote: Originally posted by Fulmen  
Truth be told you shouldn't need any azeotrope to get the Dean Stark to work. Any immiscible solvent combo should work....

I'm pretty sure that any pair of immiscible solvents will form an azeotrope.
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[*] posted on 9-7-2022 at 14:11


Wait what? Any immiscible liquids? I know the vapor pressures will add up, but forming an azeotrope?



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[*] posted on 10-7-2022 at 04:55


How could they fail?
Consider (well stirred ) water and toluene for example.
You heat them up and at some point, a little below 100C, the combined vapour pressure exceeds atmospheric.
And, at that point the mixture boils.
You are then effectively steam distilling toluene.
And the quantities of both that come over are in proportion to the vapour pressures at that temperature.
So it will be a constant concentration of each (for a given pressure).

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[*] posted on 10-7-2022 at 09:45


I see your point. I'm not sure if it fits the definition of an azeotrope 100%, but you're right in that it should form a constant boiling point.



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[*] posted on 15-7-2022 at 02:15


Ummm. I like to use 100% Ethanol as a desiccant. Just add it to your soupy-salt, and distill off some Ethanol, it takes water with it, as it departs. Will this work in your case.... I don't know.
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[*] posted on 16-7-2022 at 02:29


Update:

I tried distilling off isopropanol, but that didn't seem to work, I would have tried ethanol but I don't have any azeotropic ethanol.
There is some good news, however; I discovered that the phosphorus pentoxide desiccant I was using was no longer dry.

I had taken a small quantity of the desiccant, weighed it, left it in the open air, and thus discovered that it absorbed 40% w/w of water from the air within a day, this lead me to believe that it still contained phosphorus pentoxide (it is mixed with an inert binder, so you can't tell just by looking at it), however I had neglected to consider that phosphoric acid(s) are also hygroscopic, so it turned out there was probably no phosphorus pentoxide remaining in the desiccant!

I have since replaced it with new desiccant, and the salt has been drying over the fresh desiccant in a vacuum desiccator for about a week, and it is beginning to crystallize!

I have lost quite a lot of the salt in innumerable transfers from one container to another, and it is contaminated with a lot of crap (such as the grease I used to seal the joints when I tried the RBF technique mentioned earlier), so I'll probably have to make some more anyway, but at least it is possible to dry it, which is encouraging.


@karlos
Yes, let me know if it is still dry!


@unionised
This azeotrope thing is intriguing, your point seems irrefutable yet I'm still skeptical, will have to find out for sure!
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[*] posted on 16-7-2022 at 03:32


Quote: Originally posted by unionised  
Quote: Originally posted by Fulmen  
Truth be told you shouldn't need any azeotrope to get the Dean Stark to work. Any immiscible solvent combo should work....

I'm pretty sure that any pair of immiscible solvents will form an azeotrope.

Azeotropes form because one or both of the components deviates significantly from the ideal gas laws, e.g. by forming hydrogen bonds; their miscibility is irrelevant.

[Edited on 16-7-2022 by Lionel Spanner]




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[*] posted on 16-7-2022 at 04:25


Quote: Originally posted by Lionel Spanner  
Quote: Originally posted by unionised  
Quote: Originally posted by Fulmen  
Truth be told you shouldn't need any azeotrope to get the Dean Stark to work. Any immiscible solvent combo should work....

I'm pretty sure that any pair of immiscible solvents will form an azeotrope.

Azeotropes form because one or both of the components deviates significantly from the ideal gas laws, e.g. by forming hydrogen bonds; their miscibility is irrelevant.

[Edited on 16-7-2022 by Lionel Spanner]

The reason that they are immiscible is because their properties differ greatly from the ideal. For example, water will not mix with toluene because, to do so, it would have to break the hydrogen bonds in the water.
In reality, immiscible liquids actually form azeotropes whether you like it or not.
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