H NMR Spectrum - Molecular Structure

In summary, the molecule C8H8O3 has a meta-disubstituted benzene ring with an OH group and a C=O group. The NMR spectrum shows a singlet at ~3.5 ppm, which could be attributed to two coincident singlets or a functional group with similar absorbance. The solvent used in the NMR may provide further clues.
  • #1
edimeo25
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Homework Statement


Give the structure of C8H8O3.

Homework Equations


IR tells us that there is an OH group and a C=O group
Degree of saturation = 5 --> Benzene ring + C=O

H NMR spectrum shows:
Peak at ~7.8ppm integrating for 1H and is a doublet
Peak at ~7.45ppm integrating for 1H and is a triplet
Peak at ~7ppm integrating for 1H and is a doublet
Peak at ~6.85ppm integrating for 1H and is a triplet
Peak at ~3.5ppm integrating for 4H and is a singlet

The Attempt at a Solution


It's hard to show my attempt at a solution, but I believe it will be a meta-disubstituted benzene ring. I just can't figure out how to insert the 3rd oxygen atom and how exactly you can have 4 chemically identical hydrogens with no neighboring hydrogens in this type of molecule. The 3 peaks between 7.8 and 7ppm I believe account for the neighboring hydrogrens of the benzene ring if its substituents are in the meta orientation.

Any help would be greatly appreciated. Thanks for your time.
 
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  • #2
Since you have an OH by IR given to you in this problem, start there. In NMR, the OH group usually shows up as a broad hump or a singlet depending on concentration and solvent. You only have one signal that you identified as a singlet at ~3.5 ppm so that's got to be it, however; it integrates to four protons. That's a problem...
Is it possible that you have two coincident singlets present that integerate to 4 protons? If so, what other functional group has an absorbance of ~3.5 ppm? What solvent is used in the NMR?
 
  • #3


Based on the given information, the structure of C8H8O3 can be determined as meta-hydroxybenzaldehyde. This is supported by the IR spectrum, which shows an OH group and a C=O group, and the H NMR spectrum, which has peaks at ~7.8ppm, ~7.45ppm, and ~7ppm, indicating a meta-disubstituted benzene ring. The peak at ~3.5ppm integrating for 4H and is a singlet can be attributed to the four chemically identical hydrogens of the aldehyde group. The third oxygen atom can be placed on the benzene ring, making it a meta-hydroxybenzaldehyde.
 

FAQ: H NMR Spectrum - Molecular Structure

1. What is an H NMR spectrum?

An H NMR spectrum is a type of analytical technique used in chemistry to determine the molecular structure of a compound. It provides information about the different types of hydrogen atoms present in a molecule and their chemical environments.

2. How does an H NMR spectrum help determine molecular structure?

The H NMR spectrum shows the different chemical shifts of the hydrogen atoms in a molecule, which are influenced by their bonding and neighboring atoms. This information can be used to identify the types of hydrogen atoms present and their connectivity, thus helping to determine the molecular structure.

3. What is the chemical shift in an H NMR spectrum?

The chemical shift in an H NMR spectrum is the distance, measured in parts per million (ppm), from a reference compound to the signal of a hydrogen atom. It is influenced by factors such as the electron density and electronegativity of neighboring atoms.

4. What is meant by "splitting" in an H NMR spectrum?

"Splitting" in an H NMR spectrum refers to the splitting of a signal into multiple peaks, which is caused by the interaction of a hydrogen atom with its neighboring atoms. The number and intensity of the peaks can provide information about the number and type of neighboring atoms.

5. How can the integration values on an H NMR spectrum be used?

The integration values on an H NMR spectrum represent the relative number of hydrogen atoms contributing to a signal. This information can be used to determine the molecular formula and to check for consistency with the proposed molecular structure.

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