Identifying an Unknown Compound: Can You Help Solve this NMR Puzzle?

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Discussion Overview

The discussion revolves around identifying an unknown compound based on NMR and IR spectroscopy data. Participants explore the characteristics of the compound, which is suspected to be a carboxylic acid with a specific melting point and color, while debating the compatibility of the provided NMR data with known compounds, particularly cinnamic acid.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant identifies the unknown as a UV active acid with a melting point of 133C-135C, suggesting it could be cinnamic acid based on the sodium fusion test showing no heteroatoms.
  • Another participant questions the incompatibility of the NMR data with cinnamic acid, noting the presence of two singlets which may indicate a different structure.
  • A comparison is made with a known NMR spectrum of trans-cinnamic acid, suggesting some similarities but also differences in peak magnitudes.
  • Participants discuss the possibility of the unknown being cis-cinnamic acid or affected by solvent differences, which could alter the appearance of the NMR signals.
  • There is mention of the potential for dimerization of carboxylic acids affecting NMR signals, particularly the acidic proton.
  • Concerns are raised about the color of the sample, with participants speculating on possible impurities that could cause the observed yellow-brown color while still being classified as pure.
  • One participant emphasizes that color should not be a primary identifier, as small impurities can significantly affect the appearance without being detectable by IR or NMR.
  • Another participant suggests calculating coupling constants from the NMR data to further analyze the compound's structure.

Areas of Agreement / Disagreement

Participants express differing views on the compatibility of the NMR data with cinnamic acid, and there is no consensus on whether the unknown compound is indeed cinnamic acid or another aromatic carboxylic acid. The discussion remains unresolved regarding the identity of the compound.

Contextual Notes

Participants note limitations in the analysis, including the potential for solvent effects in NMR that differ from IR results, and the impact of impurities on the observed color and spectral data.

Who May Find This Useful

This discussion may be useful for students and professionals in chemistry, particularly those interested in spectroscopy, compound identification, and the nuances of interpreting NMR data.

Horseb0x
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[PLAIN]http://img585.imageshack.us/img585/362/nmrs.jpg
I had to identify an unknown in the lab and it was a UV active acid with a melting point of 133C-135C. Sodium fusion test revealed no heteroatoms so the only compound that fits the criteria is cinnamic acid but although the IR matches that of cinnamic acid, the NMR doesn't. Also cinnamic acid is white, this stuff was orangy brown. I never took an actual NMR though, that's just the NMR they gave us to go with the compound so maybe they made a mistake. Any idea what compound this NMR is for? There are 2 singlets so already it can't be cinnamic acid can it?
 
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Why do you feel the NMR is incompatible?
How many NMRs have you seen? Sometimes the aromatic protons accidentally resonate at about the same ppm, so it looks like you have singlets when they're not really. If you compare them to the doublets, they don't look that sharp to me.

(is this the complete scan, or is there more information on the sheet)?
 
Yeah that's a complete scan. Heres an NMR for trans-cinnamic acid:
[PLAIN]http://img526.imageshack.us/img526/7272/imgcgi.gif
from http://riodb01.ibase.aist.go.jp
Actually yeah they do match don't they. Only difference is magnitude of the peak at 12.5.
 
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It could be the cis, yes, alternatively your NMR and the Japanese one may be in different solvents - I don't see the acidic proton in the latter, either.
 
Those acidic protons can disappear or broaden so much they look like a baseline. Looks like something there between 10.5 and 11.7... very broad. What do you know about dimers of carboxylic acids in NMR? Solvent effects?
 
The NMRs could both be of the cis product or trans but one question is why would our sample be yellow brown and not white since online says cinnamic acid is white. If it was saturated with water it would have should up on the IR so what could be added that would turn it that colour and add a singlet to the NMR. (same lab as Horseb0x)
 
The solvent effects going on in the NMR tube are not the same effects you see in IR, right? Was the IR determined using a solution in a deuterated solvent? Probably not.

Colored impurities can be easily seen but not easily discerned with IR or NMR if they don't make up a sizeable fraction of the whole. Color is the last thing you should use to identify something unless you are absolutely sure as to it's purity.

Why do you think the protons have exchanged with water? Extreme broadening happens in pure solvents as well. Think about dimers and how that might affect the acidic proton's signal.
 
See me and Horseb0x done the IR ourselfs in the lab so the IR is of our actual sample. What happened is we received a pure (they told us its pure ) sample of an unknown chemical. We had to test to see what it was. We found through the tests that its a carboxylic acid and the only one with the melting range we got was cinnamic acid. I am happy with the IR and NMR just not happy with the colour due to the fact they said its pure and the melting range was bang on the expected one for that compound. So what I am wondering is, is there anything that is aromatic with a melting range of 132-134 and is a carboxylic acid that has the colour described by HorsebOx or should we just go with the cinnamic acid compound? Thanks for all your help :) really appreciated
 
Like I said, color is the last thing you should rely on. A small (<1%) impurity can cause color. 99% pure is pure enough to be labelled 'pure'. The 1% colored impurity can easily give you a colored product. You won't see that colored product by IR, NMR either.

Relax.
 
  • #10
You didin't gave us the values of chemical shifts. Try to calculate the coupling constants for two doublets available. In cinnamoyl acid the coupling should be ca. 13-18Hz. See: http://orgchem.colorado.edu/hndbksupport/nmrtheory/splitting.html
 

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