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Topic: molecular sieves and triethylamine distillation  (Read 583 times)

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Offline Babcock_Hall

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molecular sieves and triethylamine distillation
« on: March 05, 2019, 11:00:58 AM »
Hello Everyone,

In the past I distilled TEA from calcium hydride.  There is generally a yellow color in the distillant pot.  My question concerns the best choice of molecular sieve to use to store the fractions after distillation.  Perrin's book mentions 4 Å.  I read Williams and Lawton's paper, and I scanned several of the series of papers by Burfield and colleagues, but I did not see TEA mentioned.

Offline TheUnassuming

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Re: molecular sieves and triethylamine distillation
« Reply #1 on: March 05, 2019, 11:39:34 AM »
I've use mol sieves for short'ish storage of various trialkylamines, but after very long they all seem to discolor and I loose my trust in them.  Not sure if its actually some manner of decomposition or just something of the mol sieves leaching into/dissolving in the NEt3. 
When in doubt, avoid the Stille coupling.

Offline OrganicDan96

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Re: molecular sieves and triethylamine distillation
« Reply #2 on: March 05, 2019, 02:23:36 PM »
i was once told that mol sieves are slightly acidic so maybe that is a problem

Offline wildfyr

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Re: molecular sieves and triethylamine distillation
« Reply #3 on: March 05, 2019, 05:09:56 PM »
I thought the yellow color in various commercial amines is trace amine oxides.

Offline wildfyr

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Re: molecular sieves and triethylamine distillation
« Reply #4 on: March 05, 2019, 05:10:49 PM »
Also Dan, you can see on our other current sieve topic that they are basic.

Offline Babcock_Hall

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Re: molecular sieves and triethylamine distillation
« Reply #5 on: March 06, 2019, 11:00:02 AM »
I thought the yellow color in various commercial amines is trace amine oxides.
That has always been my assumption also, but I have not tried to identify the yellow contaminant.  If I am not mistaken, a yellow color develops over time in the absence of sieves.

Any thoughts from anyone on whether 3 Å or 4 Å is better?

Offline pgk

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Re: molecular sieves and triethylamine distillation
« Reply #6 on: March 06, 2019, 11:04:57 AM »
1). The number before Å indicates the apparent diameter of cavity and permits to estimate up to which size, molecules can be trapped therein. Indicatively, molecular sieves 10 Å are used for removing toluene, ethylbenzene, xylenes, mesitylene, etc.
Triethylamine has a Van der Waals diameter longer than 4 Å, which means that triethylamine cannot be trapped therein. But it is more safe to use 3 Å or 3-4 Å for this purpose.
2). The yellow color is due to the trace of triethyalmine N-oxide whose formation in air, is further catalyzed by molecular sieves due to the increase of contact surface area. But if you store under nitrogen, there is no problem.
3). Molecular sieves are basic and not acidic. See the post:
http://www.chemicalforums.com/index.php?topic=98992.0

« Last Edit: March 06, 2019, 11:46:57 AM by pgk »

Offline clarkstill

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Re: molecular sieves and triethylamine distillation
« Reply #7 on: March 07, 2019, 05:55:16 AM »
I've always assumed the colour is due to the N-oxide too, but a quick google indicates it's a colourless solid...

Slightly off topic, but does anyone have any other ideas what actually causes the colour?

Offline pgk

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Re: molecular sieves and triethylamine distillation
« Reply #8 on: March 07, 2019, 11:49:07 AM »
Indeed, TEA-N→O absorbs at 198 nm (in hexane), which means that it transmits in rest of UV plus the overall visible spectrum and consequently, it has colorless appearance.
But dilution in polar solvents (such as TEA) and especially in presence of oxygen, causes absorption blue shift that may reach up to the near UV spectrum, depending on the strength of intermolecular interactions with the solvent.
On the other hand, the clearly visible spectrum is 380-740 nm. However and under certain circumstances, humans can percept wavelengths that are out but near the edges of the visible spectrum; but weakly and unclearly.
Thus, TEA-N→O diluted in TEA absorbs in the near UV due to the absorption blue shift; as a consequence it transmits in the complementary spectrum, which is unclear brown that is the additive color of white, red and black. But in the high dilution and due to the high dilution, TEA-N→O (as well as all amine N-oxides) has an unclear yellow-orange-brown appearance, depending on the storing time.
In daily life, the same phenomenon occurs when washing white clothes, either by hand or in the washing machine. The washing waste water has a weak and unclear grey-brown coloration (complementary transmision color), due the wavelength absorption of dirtiness in the close-near UV spectrum.

Ultraviolet absorption spectra of trimethylamine N-oxide, Journal of Molecular Spectroscopy, 20(3), 226-232, (1966)
https://www.sciencedirect.com/science/article/abs/pii/0022285266900245
The Singlet-triplet Absorption Spectra of Heterocyclic Amine N-Oxides (I), Bulletin of the Chemical Society of Japan, 45(11), 3282-3286, (1972)
https://www.journal.csj.jp/doi/pdf/10.1246/bcsj.45.3282
Human infrared vision is triggered by two-photon chromophore isomerization, PNAS, e-article in early edition, (2014)   
https://www.pnas.org/content/pnas/early/2014/11/25/1410162111.full.pdf
« Last Edit: March 07, 2019, 01:12:19 PM by pgk »

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