Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Organic Spectroscopy => Topic started by: Rutherford on June 25, 2016, 01:12:23 PM
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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.
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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.
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But how to conclude exact compound in question?
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I dont think it is possible to conclude exact structure just from 1H and 13C NMR
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Could NOESY help?
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Maybe (=if you see the intearcions between the side chains then yes)
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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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Thanks for clarifying.