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Homework Help: I'm having trouble reading proton NMR!

  1. Mar 12, 2010 #1
    1. The problem statement, all variables and given/known data
    OK. So i've been given a uni lab report to write up but we were only told yesterday how to read proton NMR, and it's due in on Monday. I'm not terrible at deciphering it, but the compound I've been given the NMR for doesn't make sense (to me). I have to look at the proton NMR for Cinnamic acid, (http://en.wikipedia.org/wiki/Cinnamic_acid) and talk about the coupling, splitting and so on.

    2. Relevant equations
    I know the coupling constant is the change in peak values in ppm x MHz of the machine used.

    3. The attempt at a solution
    Cinnamic acid has 8 protons, 5 are on the aromatic ring, 2 along the alkene and one on the COOH.
    -The COOH proton has 0 adjacent protons and 0 equivalent protons so should have one single peak.
    -The alkene proton on the acid end has 1 adjacent proton and 0 equivalent protons. It should have a doublet.
    -The aromatic end alkene proton has 1 adjacent proton and 0 equivalent protons as well so it should also show up as a doublet
    -The 2 ortho protons on the aromatic are equivalent and have 1 adjacent proton so they should show up as a doublet
    -the 2 meta protons are equivalent and have 2 adjacent protons so should show up as a triplet
    -The para proton has 2 adjacent protons so should show up as a triplet.

    All this said, the NMR doesn't look anything like this. There are 2 triplets at around the 7. mark, which makes sense, but there are some other peaks and I'm not sure whether they're doublets or singles. If they are singles, then the total number of peaks is 6, which works because there are 6 different types of protons. However, for 4 singlet peaks you need 4 protons with no adjacent protons, and this doesn't happen. If the peaks are 2 doublets, then there needs to be 2 protons with 1 adjacent proton, and this doesn't happen either because the COOH proton definately has no adjacents.

    This is really confusing!! Anyone know what I'm doing wrong?

    -Cheers, Pyridine ;)
  2. jcsd
  3. Mar 12, 2010 #2

    Char. Limit

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    Gold Member

    Giving us a picture of the Hydrogen NMR would be useful. I've got my org. chem book open to NMR, and I could really use a picture.
  4. Mar 13, 2010 #3
    hey Char Limit, I can't find one on the internet anywhere and I don't have a scanner :(
  5. Mar 13, 2010 #4
    Could you at least describe the locations of the remaining peaks?
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