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Author Topic: Separation of secondary and tertiary amines  (Read 941 times)

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Richard Jeong

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Separation of secondary and tertiary amines
« on: December 27, 2018, 02:45:12 PM »

Long time no see.
I am currently working as an undergraduate researcher, synthesizing nitrogen ligands for metal.
Well, now I am trying with N-methyl piperazine(which have one hydrogen left for nitrogen) and some alkyl bromide to run an Sn2 reaction.
The reaction was successful, but purification is the problem.
The product should not have hydrogen attached to nitrogen, however, it appeared as a blunt peak in H-NMR.
Is there any solvent that can wash out secondary amine, and leaving tertiary amine behind?
I and my supervisor are thinking of THF and other polar aprotic solvents.
What would you recommend? Thank you very much.
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wildfyr

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Re: Separation of secondary and tertiary amines
« Reply #1 on: December 27, 2018, 05:00:37 PM »

I would first try to recrystallize it, but this is a sometimes intractable problem. It's better to run reactions that minimize the amount of undesired amine via the setup and reagents.
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rolnor

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Re: Separation of secondary and tertiary amines
« Reply #2 on: December 28, 2018, 03:27:58 AM »

TLC with EtOAc 1-10% TEA can separate this type of compounds, run flash.
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wildfyr

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Re: Separation of secondary and tertiary amines
« Reply #3 on: December 28, 2018, 03:44:34 AM »

Ah, I figured column wasn't an option from the way OP phrased it. Yes, definitely a column with triethylamine is the first option.

Another option, is it low weight enough for one or both to be fractionally distilled? Even easier than a column.
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Richard Jeong

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Re: Separation of secondary and tertiary amines
« Reply #4 on: December 30, 2018, 06:49:56 PM »

Thank you for the replies :)
But, I figured out that N-Methylpiperazine has bp of 110~120 C, so I just dried it in the vacuum.
And, as you stated to separate in basic condition, I had another trial with more base. It worked really well.
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zarhym

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Re: Separation of secondary and tertiary amines
« Reply #5 on: January 01, 2019, 09:26:04 PM »

In my experience, there is one trick you can try to separate secondary and tertiary amines chemically, using reductive amination.

This trick will not only add chromophore to secondary amine, but also reduce the polarity of the molecule significantly. Both of them will help the seperation in column chromatography.

Since you have the problem solved, you may save this trick for next time.
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Richard Jeong

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Re: Separation of secondary and tertiary amines
« Reply #6 on: January 04, 2019, 02:56:03 PM »

In my experience, there is one trick you can try to separate secondary and tertiary amines chemically, using reductive amination.

This trick will not only add chromophore to secondary amine, but also reduce the polarity of the molecule significantly. Both of them will help the seperation in column chromatography.

Since you have the problem solved, you may save this trick for next time.

Well, thank you for the trick! But, as I mentioned in the post, I am preparing ligands for metals. I would save it for other topics.
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