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Topic: NMR Spectroscopy problems  (Read 10569 times)

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Offline alansary

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NMR Spectroscopy problems
« on: September 22, 2012, 10:45:27 PM »
Hi chemists ,
i've got four question and i've answered them all. i just want to double check if my answer is correct or not.. and if its not please explain to me.
I've put the answers in separate pic to be clear and readable.
also requested to show the correct stereochemistry where appropriate , but don't know how to do it. need help with that.

thanks in advanced.
« Last Edit: September 22, 2012, 11:48:33 PM by alansary »

Offline discodermolide

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Re: NMR Spectroscopy problems
« Reply #1 on: September 22, 2012, 10:55:05 PM »
I'm sorry but I cannot make out your scribblings on these pictures.
Please re-scan them and make them more readable. Thanks.
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Offline alansary

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Re: NMR Spectroscopy problems
« Reply #2 on: September 22, 2012, 11:50:33 PM »
I have rescan the questions , and this is my answers. what do u think ?

Offline discodermolide

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Re: NMR Spectroscopy problems
« Reply #3 on: September 23, 2012, 12:18:13 AM »
Molecule number 4 looks OK, but what is the double bond stereochemistry, cis or trans.
Molecule number 6, where is the aldehyde signal? It should be around 9.5. Also the signal at 3.9 is of a carbon directly attached to an oxygen atom, for example a methoxy. I think you will need to think again here.
Molecule 7, form the spectra it does not seem to have a carbonyl group in it. These signals are usually around 160-200 ppm in the 13C spectra.
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Offline alansary

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Re: NMR Spectroscopy problems
« Reply #4 on: September 23, 2012, 12:27:11 AM »
ok, Ill have another look at it . but what about no.5 ?

Offline discodermolide

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Re: NMR Spectroscopy problems
« Reply #5 on: September 23, 2012, 12:36:45 AM »
It does not look correct to me, I miss the 4H singlet for the 2 CH2Cl groups.
Your suggestion shows a symmetrical molecule so for your structure you might expect only 2 signals in the 1H-NMR.
You need to re-examine this one.
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Offline alansary

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Re: NMR Spectroscopy problems
« Reply #6 on: September 23, 2012, 01:15:38 AM »

Molecule 4 could be cis since the J value is between 6 to 9. in trance the coupling constant has value (15)

I have attached Molecule 6.

Ill attache the rest tomorrow 

thanks for your help. 

Offline alansary

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Re: NMR Spectroscopy problems
« Reply #7 on: September 23, 2012, 01:22:17 AM »
hi again,

here Molecule number 5


Offline discodermolide

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Re: NMR Spectroscopy problems
« Reply #8 on: September 23, 2012, 01:31:01 AM »
molecule 6 must be a para disubstituted aromatic ring.
molecule 5 as well must be para disubstitued.
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Offline Babcock_Hall

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Re: NMR Spectroscopy problems
« Reply #9 on: September 23, 2012, 10:48:45 AM »
I must be missing something in 4, but I seem to see a signal at 2.1 ppm that integrates to 2H.  What portion of the molecule is that?

Offline discodermolide

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Re: NMR Spectroscopy problems
« Reply #10 on: September 23, 2012, 11:05:29 AM »
Solvent?
The structure looks OK
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Offline Dan

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Re: NMR Spectroscopy problems
« Reply #11 on: September 23, 2012, 12:20:44 PM »
Solvent?
The structure looks OK

I disagree. The splitting patterns are not consistent with but-2-enal. The terminal Me group is a triplet, so it must be adjacent to a CH2 - and that CH2 is the peak at 2.1

Regarding double bond geometry - the coupling constants cannot be calculated from the ppm difference without being given the frequency of the spectrometer.

Quote
molecule 6 must be a para disubstituted aromatic ring.
molecule 5 as well must be para disubstitued.

Agreed.

For 7, this is definitely not 2-nonanone (lacking Me group triplet).
« Last Edit: September 23, 2012, 12:31:30 PM by Dan »
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Offline discodermolide

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Re: NMR Spectroscopy problems
« Reply #12 on: September 23, 2012, 12:28:02 PM »
Then surely the molecular formula given must be wrong. It needs to be C4H8O.
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Offline Dan

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Re: NMR Spectroscopy problems
« Reply #13 on: September 23, 2012, 12:31:03 PM »
I believe so. The spectrum given matches the spectrum for pent-2-enal on the Sigma-Aldrich website exactly.
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Re: NMR Spectroscopy problems
« Reply #14 on: September 23, 2012, 12:44:03 PM »
There seem to be a lot of errors recently in printed material handed out students. Or is this just coincidence?
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