SplendidAcylation
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Is triethylamine hygroscopic?
Hi,
Just a quick question...
I have a very old bottle of triethylamine, probably at least 60 years old.
It has been stoppered with its original glass stopper, and indeed it hasn't all evaporated, so the seal must have been fairly good.
I intend to use this amine as a solvent for the acylation of an alcohol, in a manner similar to pyridine; Acetyl chloride and the alcohol are refluxed
in triethylamine, and the amine acts as a solvent, and also removed the hydrogen chloride formed, pushing the reaction forward.
I don't have a lot of acetyl chloride and it took a lot of effort to make, so I really don't want to waste it, consequently I'm worried about the
presence of water in my triethylamine.
So the question is, if improperly sealed, is triethylamine likely to have absorbed water?
And an even more important question:
How can I dry it?
My first idea was to simply use magnesium sulfate, however there is no mention in Vogel, or on the ScienceMadness Wiki, or anywhere, about the use of
magnesium sulfate to dry amines; The drying agents recommended are always calcium oxide or barium oxide, neither of which I have.
Sodium hydroxide is also sometimes recommended, but I'm worried this might dissolve in the amine and remain there even after filtration, necessitating
distillation, which I would rather not do.
Is there any particular reason magnesium sulfate wouldn't work (perhaps it isn't a strong enough desiccant)?
Thanks in advance!
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Dr.Bob
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Triethylamine may be a little hygroscopic, but not so much that a sealed container should be an issue. I have found little problem with TEA, even
moderately old bottles. I'm sure that it does absorb water slowly, but not quickly. However, using more than one equivalent seems like a bad idea,
as that would mean even a trace of water would be exagerated.
But I would never use it as the solvent, it is too hard to remove and a waste. Using DCM is most common, but THF, EtOAc, or other solvents would be
better, using only one or 1.5 eq of TEA. It also might react with some reagents if in excess, forming reactive intermediates and such, many of those
lead to side products. Even using an excess of the alcohol might be better, unless it is also hygroscopic, like ethanol. DCM (or CHCl3) is an ideal
solvent, as it holds very little water, even when not well dried.
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Texium
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If your triethylamine is 60 years old (hell, even if it was 6 years old) you need to distill it regardless. I’m guessing it’s probably discolored?
I’d be less worried about water than oxidation. Dry it with NaOH and distill. Anything less than that is not going to give you sufficiently pure
material.
And as a side note, you don’t have to use triethylamine as your solvent for this kind of reaction. I’d run it in DCM or chloroform or MeCN and
just use like 1.5 equiv of triethylamine.
Edit: I see Dr. Bob ninja’d me but it appears we agree!
[Edited on 3-24-2024 by Texium]
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Texium
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Thread Moved
24-3-2024 at 09:24 |
SplendidAcylation
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Thanks for the replies! 
I feel quite silly for not having thought about using a different solvent!
I have only been considering very small-scale experiments (<1g scale), so I suppose the concept of wasting Et3N didn't occur to me at these scales.
Yes, it is discoloured, a light amber I'd say.
I placed a drop on a watch-glass, and it evaporates leaving a thin greasy layer with a peculiar smell (it reminds me very much of nicotine, when I
followed a procedure on this forum to extract it from tobacco, just for fun).
My alcohol isn't hygroscopic, it is a Mannich base; The literature uses pyridine as the solvent, so I thought maybe this was necessary to facilitate
dissolution of such a compound.
When distilling it, should I distill it over NaOH (with NaOH still in the distilling flask) or should I dry it over NaOH, and decant it into the
distilling flask?
Thanks again for the advice, I'll definitely use a different solvent rather than the amine itself!
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SplendidAcylation
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Oops! I said my alcohol was a Mannich base, but of course, that is nonsense; It is an amino-alcohol hydrochloride, derived from a Mannich base.
I shouldn't post when I'm tired 
Anyway an update: Upon closer examination (the bottle is dark brown so hard to see through), the triethylamine has a large amount of black, viscous
liquid at the bottom.
I have about 500mL of liquid, and I'd say at least 10% is a dark tarry liquid at the bottom of the bottle, the remaining 90% being clear yellow
triethylamine.
I wonder what the tarry stuff is... Some sort of polymerization product?
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clearly_not_atara
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Probably oxidation and polymerization, yes. The first step is probably forming the N-oxide, which undergoes various reactions.
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Texium
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Yep, I’ve seen that before for triethylamine and Hünig’s base. I decanted the bulk of the liquid off from the tarry gunk at the bottom, and
distilled it from calcium hydride. Comes over crystal clear. In lieu of calcium hydride, sodium or potassium hydroxide should work reasonably well.
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SplendidAcylation
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Thanks!
I dried it over NaOH overnight, decanted it off, and then distilled it over some more NaOH just to be sure...
It came over crystal-clear, at 87c 
Aside from the NaOH in the distilling flask there was very little residue, just a small amount of black stuff, so whatever it is, it certainly isn't
present in huge quantities dissolved in the amine.
clearly_not_atara:
Does that mean that, if perfectly sealed, this wouldn't happen, due to the absence of oxygen, or might the N-oxide be a step en route to some sort of
polymer, ultimately only requiring a catalytic amount of oxygen?
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