I'm having trouble reading proton NMR

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Discussion Overview

The discussion revolves around the interpretation of proton NMR for cinnamic acid, focusing on the identification of peaks, coupling, and splitting patterns. Participants are exploring the complexities of the NMR spectrum as part of a homework assignment.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • The original poster describes the expected splitting patterns for the protons in cinnamic acid based on their environments, noting that the COOH proton should appear as a singlet, alkene protons as doublets, and aromatic protons with varying splitting.
  • The original poster expresses confusion over the actual NMR spectrum, which does not match their predictions, particularly regarding the number and type of peaks observed.
  • One participant suggests that a visual representation of the NMR spectrum would aid in understanding the discrepancies.
  • Another participant requests a description of the locations of the remaining peaks to facilitate discussion.

Areas of Agreement / Disagreement

Participants do not seem to reach a consensus on the interpretation of the NMR spectrum, with ongoing confusion and requests for additional information to clarify the situation.

Contextual Notes

The discussion lacks a visual reference for the NMR spectrum, which may limit the ability to accurately analyze the peaks and their corresponding proton environments. There are also unresolved assumptions about the equivalence of protons and their adjacent relationships.

pyridine
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Homework Statement


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.

Homework Equations


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


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 definitely has no adjacents.

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


-Cheers, Pyridine ;)
 
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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.
 
hey Char Limit, I can't find one on the internet anywhere and I don't have a scanner :(
 
Could you at least describe the locations of the remaining peaks?
 

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