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Topic: H-NMR chalcone : interpret these peaks  (Read 774 times)

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Offline xshadow

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H-NMR chalcone : interpret these peaks
« on: October 31, 2020, 06:47:11 AM »
Hi
I have this H-NMR spectrum and I have to discuss its peaks:



I try to discuss the spectrum :



But I don't find the doublet for the olefinic HB hydrogen

There two   small doublets at:
7.0
7.8

...without any assignation
Can one of these  be the peak for Hb ?
Or are impurity peaks?

Thanks!!

Offline Babcock_Hall

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Re: H-NMR chalcone : interpret these peaks
« Reply #1 on: October 31, 2020, 09:30:16 AM »
If hydrogen P splits hydrogen Q with a certain magnitude of coupling constant, then hydrogen Q splits hydrogen P with the same magnitude of J.  I still print out my spectra and use a drafting tool to measure which coupling constants have the same magnitude.  I would also give some thought to how the magnitude of J can be used to tell us about the stereochemistry of a molecule.

Offline xshadow

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Re: H-NMR chalcone : interpret these peaks
« Reply #2 on: October 31, 2020, 10:52:19 AM »
If hydrogen P splits hydrogen Q with a certain magnitude of coupling constant, then hydrogen Q splits hydrogen P with the same magnitude of J.  I still print out my spectra and use a drafting tool to measure which coupling constants have the same magnitude.  I would also give some thought to how the magnitude of J can be used to tell us about the stereochemistry of a molecule.
Ok I'll wiat

Thanks

Offline Babcock_Hall

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Re: H-NMR chalcone : interpret these peaks
« Reply #3 on: October 31, 2020, 01:03:16 PM »
Suppose we think about the methylene hydrogens of Ph-CH2OH, just as an example.  What is shift of these two hydrogen atoms, relative to the situation in which there is an alkyl group, not a phenyl group, on the left-hand side of the molecule?  What is the standard explanation for this?  You might look at styrene derivatives also.

Although the numerical integrals on the spectrum are not ideal, you should be able to make some arguments, based on peak areas.  You can rule one one of your own suggestions, for example.
« Last Edit: October 31, 2020, 01:22:55 PM by Babcock_Hall »

Offline xshadow

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Re: H-NMR chalcone : interpret these peaks
« Reply #4 on: October 31, 2020, 06:00:02 PM »
Suppose we think about the methylene hydrogens of Ph-CH2OH, just as an example.  What is shift of these two hydrogen atoms, relative to the situation in which there is an alkyl group, not a phenyl group, on the left-hand side of the molecule?  What is the standard explanation for this?  You might look at styrene derivatives also.

Although the numerical integrals on the spectrum are not ideal, you should be able to make some arguments, based on peak areas.  You can rule one one of your own suggestions, for example.

You mean the chemical shift of the -CH2- group in a molecule like CH3CH2OH??

Its chemical shift should be at around 3.8 ppm .
And the signal is a quartet (n+1,seeing a methyl group)

If I have a PhCH2OH its signal should be  >3.8 pm because it 's also benzylic
But it doesn't couple with anythink (perhaps long range with an aromatic hydrogen)


But sorry don't understand what does it has to do with my NMR ...

Also in my molecule there is at 3.8 ppm the - OCH3 signal...
But why does it seems a triplet instead of a singlet? Those OCH3 shouldn't couple with anything
Or the two band near the main peak  are not important...
(Because for a triplet the relative  intensity band should be  121...here the middle peak is a lot higher)!!

Thanks

Offline AWK

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Re: H-NMR chalcone : interpret these peaks
« Reply #5 on: October 31, 2020, 07:41:56 PM »
Your chemical is not perfectly pure and you have to ignore all weaker signals.
AWK

Offline xshadow

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Re: H-NMR chalcone : interpret these peaks
« Reply #6 on: October 31, 2020, 08:13:19 PM »
Your chemical is not perfectly pure and you have to ignore all weaker signals.

So can I say that  the -OCH3 signal at 3.8  ppm Is a singlet (as expected) and not a triplet ?

Thanks

Offline AWK

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Re: H-NMR chalcone : interpret these peaks
« Reply #7 on: October 31, 2020, 09:36:52 PM »
One of the reactants was methoxy benzaldehyde. Did you check his NMR? I can even see it on your spectrum.
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Offline xshadow

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Re: H-NMR chalcone : interpret these peaks
« Reply #8 on: November 01, 2020, 03:35:35 AM »
One of the reactants was methoxy benzaldehyde. Did you check his NMR? I can even see it on your spectrum.

Yes, surely i can see the aldehydic hydrogen at 9-10.
The only thing,repeat, I don't understand if the signal at 3.8 is a singlet or a triplet

In theory should be a singlet because the -OCH3 can't couple with anything (at least vicinal coupling)...and the other two peaks are very small (a triplet should have 1 2 1 as intensity peaks)


thanks

Offline AWK

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Re: H-NMR chalcone : interpret these peaks
« Reply #9 on: November 01, 2020, 04:51:18 AM »
You just see a methyl group conjugated with two CH2 protons and you want to see 1-2-1 intensities. There is no such possibility.

Besides, look at integration 3.41. Aliphatic protons give very good integrations (aromatic ones sometimes deviate more), so integrations of 3.41, 2.21, and 2.55 come from a mixture of compounds.

I advised you to check the aldehyde spectra - you don't want to do that, so you don't want to understand NMR spectroscopy. Commercial p-methoxy benzaldehyde certainly contains a few percent of the meta-isomer.
And then it's clear - you have two products and some unreacted aldehyde. The signals -OCH3 of these three compounds should be very close to each other.
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Offline xshadow

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Re: H-NMR chalcone : interpret these peaks
« Reply #10 on: November 01, 2020, 05:17:40 AM »
You just see a methyl group conjugated with two CH2 protons and you want to see 1-2-1 intensities. There is no such possibility.

Besides, look at integration 3.41. Aliphatic protons give very good integrations (aromatic ones sometimes deviate more), so integrations of 3.41, 2.21, and 2.55 come from a mixture of compounds.

I advised you to check the aldehyde spectra - you don't want to do that, so you don't want to understand NMR spectroscopy. Commercial p-methoxy benzaldehyde certainly contains a few percent of the meta-isomer.
And then it's clear - you have two products and some unreacted aldehyde. The signals -OCH3 of these three compounds should be very close to each other.

Understooood!
In fact it didn't have sense that the -OCH3 signal  (of this molecule only)was a triplet (the side peaks weres too much smaller in intensity)

I've also understood that the 3 peaks at around 7.5 are 2 doublets (one of the benzylic hdrogen, and the other for the orto hydrogen to C=O) with partial overlapping in the middle peak (and inf fact is also higher...that it was strange before having understood this)
If I compare the coupling constat of these two doublet with the relative one at  6.6 and 6.9 is also confirmed

Thanks!!!

Offline Babcock_Hall

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Re: H-NMR chalcone : interpret these peaks
« Reply #11 on: November 02, 2020, 10:20:52 AM »
Suppose we think about the methylene hydrogens of Ph-CH2OH, just as an example.  What is shift of these two hydrogen atoms, relative to the situation in which there is an alkyl group, not a phenyl group, on the left-hand side of the molecule?  What is the standard explanation for this?  You might look at styrene derivatives also.

Although the numerical integrals on the spectrum are not ideal, you should be able to make some arguments, based on peak areas.  You can rule one one of your own suggestions, for example.

You mean the chemical shift of the -CH2- group in a molecule like CH3CH2OH??

Its chemical shift should be at around 3.8 ppm .
And the signal is a quartet (n+1,seeing a methyl group)

If I have a PhCH2OH its signal should be  >3.8 pm because it 's also benzylic
But it doesn't couple with anythink (perhaps long range with an aromatic hydrogen)


But sorry don't understand what does it has to do with my NMR ...
This point was about chemical shifts, not about coupling constants.  All else held equal, B should be downfield of A, as you implied.  For example in benzyl alcohol, the shift of the CH2 group is about 4.6 ppm.

You have not made much use of coupling constants yet.  What can you say about the magnitude of J for HA, relative to other splittings?

Offline Babcock_Hall

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Re: H-NMR chalcone : interpret these peaks
« Reply #12 on: November 02, 2020, 11:43:28 AM »
When one sees small peaks near a large peak, these may be "spinning side bands."  If they are about 15-20 Hz apart from the main peak, then that is what they probably are (assuming that the sample is being spun at this rate).  They occur when the magnetic field is not perfectly homogeneous.  That having been said, I think that I also detect some impurities in this sample.

Offline xshadow

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Re: H-NMR chalcone : interpret these peaks
« Reply #13 on: November 09, 2020, 09:04:34 AM »
Suppose we think about the methylene hydrogens of Ph-CH2OH, just as an example.  What is shift of these two hydrogen atoms, relative to the situation in which there is an alkyl group, not a phenyl group, on the left-hand side of the molecule?  What is the standard explanation for this?  You might look at styrene derivatives also.

Although the numerical integrals on the spectrum are not ideal, you should be able to make some arguments, based on peak areas.  You can rule one one of your own suggestions, for example.

You mean the chemical shift of the -CH2- group in a molecule like CH3CH2OH??

Its chemical shift should be at around 3.8 ppm .
And the signal is a quartet (n+1,seeing a methyl group)

If I have a PhCH2OH its signal should be  >3.8 pm because it 's also benzylic
But it doesn't couple with anythink (perhaps long range with an aromatic hydrogen)


But sorry don't understand what does it has to do with my NMR ...
This point was about chemical shifts, not about coupling constants.  All else held equal, B should be downfield of A, as you implied.  For example in benzyl alcohol, the shift of the CH2 group is about 4.6 ppm.

You have not made much use of coupling constants yet.  What can you say about the magnitude of J for HA, relative to other splittings?


Offline Babcock_Hall

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Re: H-NMR chalcone : interpret these peaks
« Reply #14 on: November 09, 2020, 10:10:56 AM »
This looks reasonable to me.

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