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Topic: Is it possible to observed the proton of secondary amines in the 1H NMR?  (Read 20717 times)

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Offline Kaladiscope

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Hi,

I run proton NMR for my compound which contains a secondary amine. I did in CDCl3 and I can observe one peak that its not matching with the other protons. So I guess, that it belongs to the secondary amine.

Problem is that as far as I remember protons from secondary amines and even amines can not be seen by proton NMR. Or could be seen in case that u do it with DMSO...due to the D-H exchange..


Offline discodermolide

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Hi,

I run proton NMR for my compound which contains a secondary amine. I did in CDCl3 and I can observe one peak that its not matching with the other protons. So I guess, that it belongs to the secondary amine.

Problem is that as far as I remember protons from secondary amines and even amines can not be seen by proton NMR. Or could be seen in case that u do it with DMSO...due to the D-H exchange..



You usually see the amine protons, but the signals can be broad, very broad. As far as I remember due to the quadruple moment of N.
D-H exchange will help you with this as well.
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Offline Babcock_Hall

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Kaleidascope,

What chemical shift do you observe?

Offline Kaladiscope

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I saw like one singlet (not sharp) at 1.9 ppm. Funny thing is that I performed again the NMR and the signal appear at 1.69 using the same solvent...

Offline Babcock_Hall

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My recollection of the relevant theory is fuzzy, but I have two general points (perhaps someone else can expand upon them).  One is that anything that can engage in hydrogen bonding can show changes in chemical shift when something happens to change its ability to form H-bonds.  The other is that there can be small amounts of water present from the NMR tube and the solvent, for example.  If a proton exchanges rapidly with water, what one observes is the weighted average in chemical shift between the the two enviroments.  That statement is not true for slow exchange.

Offline fledarmus

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Yes, you can usually see amine peaks in the NMR, but they do tend to be broader than normal and may change chemical shift depending on the concentration in the solvent or traces of water. The peak broadening is normally caused by hydrogen bonding and proton exchange, so the harder it is for the proton to form hydrogen bonds or to exchange, the sharper the peak will be. Very highly hindered amines tend to show sharp peaks.

My favorite way of determining whether a peak is an amine proton was mentioned by Kaladiscope - run the NMR once to locate the peak, add a couple of drops of D2O and shake thoroughly for several minutes, then run the NMR again. If you are using normal NMR concentrations (~5mg/ml), there will be enough D2O to exchange all of the proton for deuterium, and the peak will disappear in your spectrum. This also works for other freely exchangeable protons like hydroxides and most amides.

Offline synthon

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My guess would be that the peak you're seeing at around 1.6 is water that's found it's way into the deuterated solvent.  The resulting D-H exchange with the amino proton could be causing the signal to vanish, especially since the pH will be high. 

In D2O, I've lowered the pH so that the amine does not exchange in solution, and that has worked.  Maybe an organic acid in the sample would do the trick?

I'd expect the amino proton much more downfield near 5 ppm.  Drying the solvent might help.

Offline AC Prabakar

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I'd expect the amino proton much more downfield near 5 ppm.  Drying the solvent might help.

This may not be necessary for all cases(amine protons) as it depend on the electronic environment.
My guess would be that the peak you're seeing at around 1.6 is water that's found it's way into the deuterated solvent.  The resulting D-H exchange with the amino proton could be causing the signal to vanish, especially since the pH will be high. 


There is a possibility for this as H2O proton will appear at 1.56ppm in CDCl3.But as u mentioned it is varying (1.9 & 1.69).Hence this option also can be ruled out roughly.

I am completely agree with "fledarmus" for the determination of amine proton by exchanging it with D2O as it is a very practised method by chemist widely..

Offline synthon

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I know NH2 signals can bounce around quite a bit, but in my experience they have always shown up between 3-6 ppm.  For a run-of-the-mill secondary amine I'd expect that pattern to continue, but now I'm curious what sort of amines should give a signal in that range?  Any examples?

Offline orgopete

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If you made a compound, got its NMR, and it did not match the "predicted" values, would be spectrum or the predictions be in error? The spectrum is always correct. Sometimes, you may have to adjust your structure though (probably not in this case). Since the chemical shift can vary quite widely, I am reluctant to say, it is now too wide, especially if you can verify its identity by D2O exchange.

Since CDCl3 seems as though it is loaded with HCl (or DCl), the stoichiometry of the acid-base reaction may change depending on the amount of amine being neutralized. If you want to shift everything, run the spectrum in d6-DMSO.
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Offline critzz

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Re: Is it possible to observed the proton of secondary amines in the 1H NMR?
« Reply #10 on: November 24, 2016, 04:00:24 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 vacuum.

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

Does anyone know if its likely for the nitro to partly reduce the nitro to the nitroso?

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