Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Organic Spectroscopy => Topic started by: janapix on April 06, 2012, 04:14:38 AM
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I'm just having some issues with this spectrum.
I've attached the work I've done on it so far... could you help me assign the rest of the peaks?
spectrum: http://i317.photobucket.com/albums/mm395/MunsterMunster/University%20stuff/knorrhnmrspechelp.png
ps: This is for my Knorr synthesis lab - I've also had an issue with the final cyclization step... so just in case anyone would like to save me further hours of pain, I've attached that too =D
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fi317.photobucket.com%2Falbums%2Fmm395%2FMunsterMunster%2FUniversity%2520stuff%2Ffinalknorr.png&hash=d6c39df5aa7c76b448246bd6f21ac5d5335373e5)
mechanism:
http://i317.photobucket.com/albums/mm395/MunsterMunster/University%20stuff/finalknorr.png
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This is a little bit tricky to see straight away because you've got some overlapping peaks.
A: NH - I agree
B: chloroform (from the NMR solvent).
That leaves C, D and E.
You've called E a quartet, and assigned this as the methyl groups 8 and 11. This signal is for Me groups 8 and 11, but it is not a quartet, it is two separate signals. Me 8 and 11 are chemically inequivalent, and so will give two separate signals. Each of them couples to an adjacent CH2, so we would expect to see two separate 1:2:1 triplets for methyl groups 8 and 11. Now, in this case these two signals are very close together and overlap slightly.
I will try to explain, you have two triplets: 1:2:1 1:2:1
But they overlap like this: 1:2:1
1:2:1
So it looks like a 1:3:3:1 quartet, but in reality is not.
The overlapping is nothing other than bad luck.
Hint: Consider this kind of situation when looking at signal C
What splitting patterns would you expect from the H at positions 7, 10, 12 and 13?
Hint: For signal D, you have measured a coupling constant of 21 Hz, which implies you think it's a doublet. 21 Hz is too high for a H-H coupling constant (0-15 is the usual range)...
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Try running the spectrum again in C6D6 you will get an aromatic induced shift which, with a bit of luck, will separate your signals for you.
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Absolutely thrilled to wake up to an answer that helped me assign all my peaks =)
Thanks very much Dan!
As for running a new spectrum, I'm still looking forward to running an NMR at all (sigh, undergrad degree...)