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Topic: TLC of secoindary amine  (Read 5555 times)

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Offline Bakterien

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TLC of secoindary amine
« on: November 16, 2016, 08:18:09 AM »
I have done a reduction of an aliphatic nitro compound to the aliphatic amine. When checking the rxn by TLC (silica) all my starting material is consumed but my product (hopefully) are stuck on the baseline. I have tried different solvent ratios but it won´t move. Is this common, that amines get stuck on the baseline ?

Offline kriggy

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Re: TLC of secoindary amine
« Reply #1 on: November 16, 2016, 09:09:48 AM »
Yes it is quite common because silica is acidic.  Try more polar mobile phase, mabye DCM+MeOH? Or add small amount of triethylamine (1% or so)

Offline Dan

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Re: TLC of secoindary amine
« Reply #2 on: November 16, 2016, 09:18:01 AM »
Agreed, 10% MeOH/DCM with 1% NEt3 will move most things.

Before you run the TLC, dope it with NEt3. To do this, before spotting your sample, soak the entire plate in ~10-20% NEt3/pet. ether and then gently dry off the plate with a heat gun or hotplate. Then use the plate as normal. If you don't do this, when you run your 1% triethylamine eluent you will get a line across the middle of your TLC plate (the solvent front of the NEt3).
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Offline Bakterien

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Re: TLC of secoindary amine
« Reply #3 on: November 16, 2016, 10:22:37 AM »
Thank  you! It worked great. I will take this information with me for the rest of my life !

Offline SynDude

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Re: TLC of secoindary amine
« Reply #4 on: November 16, 2016, 11:30:48 AM »
I always use NH3 in methanol (1N usually suffices) as a co-solvent next to CH2Cl2. It's easier to get rid of at the Rotavap than Et3N.

Offline kriggy

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Re: TLC of secoindary amine
« Reply #5 on: November 16, 2016, 06:52:23 PM »
Sure but NH3 can cause troubles because Et3N is less nucleophilic and NH3 is aq. solution usualy. But I guess, it depends on each specific case

Offline SynDude

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Re: TLC of secoindary amine
« Reply #6 on: November 17, 2016, 02:41:30 AM »
@Kriggy

True. It can cause side reactions (although I've not experienced one thus far).
I'd advise to used 7N NH3 in MeOH (commercially available) rather than aqueous since water will deactivate your silica.

Offline Bakterien

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Re: TLC of secoindary amine
« Reply #7 on: November 17, 2016, 09:00:12 AM »
I have a follow up question. The TLC showed that my starting material was consumed and that I got a new compound with different rf value. When I did an H-NMR I couldn´t find the amine protons. Any suggestions ?

Offline phth

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Re: TLC of secoindary amine
« Reply #8 on: November 17, 2016, 12:03:40 PM »
I have a follow up question. The TLC showed that my starting material was consumed and that I got a new compound with different rf value. When I did an H-NMR I couldn´t find the amine protons. Any suggestions ?
increase the relaxation delay to 5 seconds

Offline Optimist

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Re: TLC of secoindary amine
« Reply #9 on: November 24, 2016, 04:33:08 AM »
Using one or two drop of TEA, NH3 in TLC container with your mobile phase will definitely solve your problem. I am currently working with amine, even acetic acid/formic acid will also useful if TEA/NH3 are not working.
I hope this will be helpful.


Offline kriggy

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Re: TLC of secoindary amine
« Reply #10 on: November 24, 2016, 03:05:06 PM »
I have a follow up question. The TLC showed that my starting material was consumed and that I got a new compound with different rf value. When I did an H-NMR I couldn´t find the amine protons. Any suggestions ?

Its quite common that COOH and NH protons are not seen in spectra. You might try to add little bit of acid

Offline critzz

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Re: TLC of secoindary amine
« Reply #11 on: November 24, 2016, 05:48:57 PM »
I assume you are running the NMR in CDCl3. Maybe you will see the amine peak in DMSO-D6. Als from my own experience, the sensitivity/appearance of the amine peak can be dependend on the concentration of your product in the NMR-tube, or the amount of water present. Try drying the CDCl3 with molsieves and have your product dried on high vacuum.

Are the rest of the peaks in accordance with the expected signals? Is the TLC-spot sensitive to hydrazine (amine stain) staining?

ps. Does anyone know if its possible for the nitro group to partly reduce the to the nitroso?

Offline Optimist

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Re: TLC of secoindary amine
« Reply #12 on: November 25, 2016, 02:00:46 AM »
I have a follow up question. The TLC showed that my starting material was consumed and that I got a new compound with different rf value. When I did an H-NMR I couldn´t find the amine protons. Any suggestions ?

This is due to exchange of amine proton, did you try with DMSO-d6? if not then gave a try with DMSO-d6, i hope you will able too see your peak. Also the intensity of your amine proton will highly depend upon the conc. of your sample. Try to use little more than you just for routine NMR, but not that much otherwise you will not able to see the amine proton peak.

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