H NMR of unknown sample (organic chem)

In summary, the conversation discusses the analysis of an H NMR spectrum of an unknown compound with the formula C7H5NO3 in organic chem lab. The compound is identified as 4-nitrobenzaldehyde, but there is a small signal at 7.2 ppm which does not fit with any known chemical shifts. The possibility of it representing the enol form of the aldehyde is mentioned, but it is more likely contamination from the solvent, which is a common occurrence in NMR.
  • #1
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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  • #2
Chloroform gives an H-NMR signal around 7.2 ppm. Was your sample dissolved in CDCl3?
 
  • #3
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?
 
  • #4
Yes, it's likely contamination from the solvent. A very common occurrence in NMR.
 
  • #5


As a scientist, it is important to always consider all possibilities when analyzing data. In this case, the unknown compound has been identified as 4-nitrobenzaldehyde, but there is a small signal at approximately 7.2 ppm that does not fit with any of the known chemical shifts for aromatic, conjugated vinylic, or phenolic protons. One possibility that could explain this signal is the presence of the enol form of the aldehyde. However, it is important to consider other factors such as the concentration and stability of the enol form in the sample. It is also possible that the signal could be due to impurities or experimental error. Further analysis and experimentation may be necessary to accurately determine the cause of this signal. As a scientist, it is important to always critically evaluate data and consider all possibilities before drawing conclusions.
 

FAQ: H NMR of unknown sample (organic chem)

1. What is the purpose of performing an H NMR on an unknown organic sample?

The purpose of performing an H NMR on an unknown organic sample is to determine the molecular structure of the sample. H NMR (hydrogen nuclear magnetic resonance) spectroscopy is a technique that uses the magnetic properties of hydrogen atoms to identify the types and number of hydrogen atoms in a molecule. By comparing the NMR spectrum of the unknown sample to known compounds, the structure of the sample can be determined.

2. How does H NMR work?

H NMR works by exposing the sample to a strong magnetic field, which causes the hydrogen atoms to align in either a parallel or anti-parallel orientation. The instrument then sends a radiofrequency pulse through the sample, causing the hydrogen atoms to flip their orientation. As the atoms return to their original state, they emit a specific frequency of radiation, which is recorded by the instrument. The resulting NMR spectrum shows peaks corresponding to the different types of hydrogen atoms in the molecule.

3. What information can be obtained from an H NMR spectrum?

An H NMR spectrum provides information about the types and number of hydrogen atoms in a molecule. The position of the peaks on the spectrum indicates the chemical shift, or the environment of the hydrogen atoms in the molecule. The relative heights of the peaks correspond to the number of hydrogen atoms in each environment. Additionally, the splitting of the peaks reveals the number of adjacent hydrogen atoms, providing information about the connectivity of the molecule.

4. What factors can affect an H NMR spectrum?

Several factors can affect an H NMR spectrum, including impurities in the sample, temperature, and solvent used. Impurities can cause additional peaks or distort the spectrum, making it difficult to interpret. Temperature can also affect the chemical shift of the peaks, so it is important to keep the instrument at a constant temperature during the analysis. The choice of solvent can also impact the spectrum, as different solvents can cause shifts in the peaks or produce additional peaks.

5. How can an H NMR spectrum be used to confirm the identity of a compound?

An H NMR spectrum can be compared to reference spectra of known compounds to confirm the identity of a compound. By matching the chemical shifts and peak patterns, the structure of the unknown compound can be determined. Additionally, the integration values of the peaks can be used to calculate the molecular formula of the compound, providing further evidence for its identity.

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