H NMR of unknown sample (organic chem)

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SUMMARY

The discussion centers on the analysis of an H NMR spectrum for the compound C7H5NO3, identified as 4-nitrobenzaldehyde. A perplexing signal at 7.2 ppm, which does not integrate for any hydrogens, raised questions about its origin. Participants concluded that this signal is likely due to contamination from the solvent CDCl3, which also produces a signal at the same chemical shift, rather than representing any structural feature of the compound.

PREREQUISITES
  • Understanding of H NMR spectroscopy
  • Familiarity with chemical shift values and their interpretations
  • Knowledge of organic compound structures, specifically aromatic compounds
  • Experience with solvent effects in NMR analysis
NEXT STEPS
  • Research the impact of solvent choice on H NMR spectra
  • Study the chemical shift values for aromatic protons in various solvents
  • Learn about the enol and keto forms of aldehydes and their NMR characteristics
  • Explore common contaminants in NMR spectroscopy and how to identify them
USEFUL FOR

Organic chemistry students, researchers analyzing NMR data, and chemists working with aromatic compounds will benefit from this discussion.

chops369
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Homework Statement


Last week in organic chem lab, we were provided with an H NMR spectrum of an unknown compound with the formula C7H5NO3. I'm not terribly good at analyzing these spectra, but I've managed to figure out that I'm looking at 4-nitrobenzaldehyde. The only thing that's bothering me is this tiny signal, which shows up at about 7.2 ppm and is not integrated for any hydrogens. Known chemical shift data gives three possibilities: (1) aromatic proton, (2) conjugated vinylic proton, and (3) phenolic proton. I don't really see how this signal could represent any of those. Would it be possible that the signal is representative of the tiny amount of the enol form of the aldehyde? If not, what do you think it might be?

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The Attempt at a Solution


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Chloroform gives an H-NMR signal around 7.2 ppm. Was your sample dissolved in CDCl3?
 
Ygggdrasil said:
Chloroform gives an H-NMR signal around 7.2 ppm. Was your sample dissolved in CDCl3?
Yes it was. I would assume that means the signal really has nothing to do with the structure of the sample?
 
Yes, it's likely contamination from the solvent. A very common occurrence in NMR.
 

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