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Topic: Difficult C and H NMR problem 2  (Read 4713 times)

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Offline Rutherford

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Difficult C and H NMR problem 2
« on: June 25, 2016, 01:12:23 PM »
I attempted this problem: http://webspectra.chem.ucla.edu/cgi-bin/webspectra.cgi?Problem=jp7. While I managed to determine all the fragments (2 AcO-, -OCH3, Br, and tetrasubstituted benzene) I wasn't able to solve the substitution pattern. How to conclude how the groups are arranged in the way they are? I mean, why do the two acetyl groups need to be para? Without NOESY, I don't see a way.

Offline kriggy

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Re: Difficult C and H NMR problem 2
« Reply #1 on: June 27, 2016, 08:27:34 AM »
WEll you might get some  hints from coupling constants of those aromatic hydrogens. Its 3Hz so that tells you that they are meta to each other. Then you know that the molecule is not symmetrical (otherwise you would see only one aromatic hydrogen) and that even further limits the possibilities, but it still leaves you with 3 possible isomers.

Offline Rutherford

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Re: Difficult C and H NMR problem 2
« Reply #2 on: June 27, 2016, 03:55:46 PM »
But how to conclude exact compound in question?

Offline kriggy

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Re: Difficult C and H NMR problem 2
« Reply #3 on: June 28, 2016, 02:22:51 AM »
I dont think it is possible to conclude exact structure just from 1H and 13C NMR

Offline Rutherford

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Re: Difficult C and H NMR problem 2
« Reply #4 on: June 28, 2016, 12:00:30 PM »
Could NOESY help?

Offline kriggy

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Re: Difficult C and H NMR problem 2
« Reply #5 on: June 28, 2016, 03:24:33 PM »
Maybe (=if you see the intearcions between the side chains then yes)

Offline Dan

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Re: Difficult C and H NMR problem 2
« Reply #6 on: June 30, 2016, 03:57:33 AM »
But how to conclude exact compound in question?

Very difficult to do with confidence. You know the Hs are meta, and no two substituents are equivalent, which gives you 5 possibilities by my count. You could then reasonably conclude that the aromatic 1H signal at 6.7 ppm is adjacent to two O-substituents (given the relatively upfield shift), and the one at 7.0 ppm is not (i.e. it must be adjacent to O and Br). This still doesn't arrive unambiguously at the answer provided - it narrows it down to 3 structures, but from there I would not be confident in choosing one structure.

NOESY should solve it though.
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Offline Rutherford

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Re: Difficult C and H NMR problem 2
« Reply #7 on: July 01, 2016, 04:39:36 PM »
Thanks for clarifying.

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