NMR Spectroscopy Homework: Proposing Structure from Data

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Discussion Overview

The discussion revolves around proposing a molecular structure based on NMR spectroscopy data for a compound with the formula C15H14O. Participants analyze the significance of the spectral data, particularly focusing on the interpretation of a multiplet peak and its implications for the structure of the compound.

Discussion Character

  • Homework-related
  • Technical explanation
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • One participant notes that the multiplet likely corresponds to hydrogens on an aromatic ring due to the high degree of unsaturation indicated by the molecular formula.
  • Another participant seeks clarification on what constitutes a multiplet and the factors that lead to its formation.
  • A simplified explanation is provided, suggesting that the complexity of the multiplet arises from closely spaced doublets or triplets resulting from aromatic substitution.
  • Examples are requested to aid understanding, including a specific case of a heptet arising from a hydrogen in a propane molecule interacting with neighboring hydrogens.
  • Participants discuss the terminology used for multiplets, noting that "multiplet" is a general term for signals with five or more peaks, while specific terms like "sextet" or "heptet" may be used in high-resolution contexts.
  • One participant expresses confidence in solving the problem after engaging with the discussion, while another shares their personal enjoyment of similar puzzles in organic chemistry.
  • A hint is provided regarding the expected chemical shift range for hydrogens adjacent to a carbonyl group, suggesting a connection to the singlet observed in the data.

Areas of Agreement / Disagreement

Participants generally agree on the interpretation of the multiplet as related to aromatic hydrogens, but there is no consensus on the specifics of terminology or the best approach to visualize and understand multiplets. The discussion remains exploratory with various viewpoints presented.

Contextual Notes

Some participants express uncertainty about the definitions and applications of terms like "multiplet," "sextet," and "heptet," indicating a need for clarity in the context of NMR interpretation. There are also unresolved questions regarding the specifics of peak splitting and resolution in NMR spectroscopy.

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Homework Statement


Propose a structure that is consistent with the data.

C15H14O singlet 2.20 (3H)
singlet 5.08 (1H)
multiplet 7.25 (10H)


Homework Equations



None

The Attempt at a Solution



The multiplet is giving me trouble. I don't know how to interpret this. Please help.
 
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With that many degrees of unsaturation, the multiplet in that range is almost certainly hydrogens on aromatic ring / rings.
 
Could you explain the multiplet part? When do you see a multiplet peak? And what causes it? I don't quiet understand that.
 
The oversimplified version is this, depending on the substitution of the aromatic ring, you can have a few doublets or triplets so close together than they become a large multiplet that is hard to differentiate unless you have very high resolution.
 
Could you give me a hint on how to solve this problem?
 
Could you give me an example of a multiplet? That would help me visualizing it better. Thank you.
 
Take one of the hydrogens on carbon #2 in the center of a propane molecule for an example. It has 6 neighboring hydrogens that will split the NMR signal. This would give a total of 7 peaks, which could be referred to as a heptet or more commonly a multiplet.
 
so does heptet mean multiplet? Why do sometimes you see textbook uses sextet, heptet, ... and not multiplet? This is what confuses me. When do you use sextet, heptet,... and multiplet? Please give an example. This would clarify things a lot. Thank you.
 
The term multiplet isn't specific as far as I know. When you see multiplet, just think 5+ peaks. A specific description, like sextet or heptet, may be used if very high resolution is being used. You generally won't need to know how many peaks are in a multiplet to solve any of the problems you come across in undergrad organic chemistry.
 
  • #10
Thank you very much. Now I think I could figure this problem out.
 
  • #11
People thought I was crazy for it, but puzzles like this were the reason I liked organic chem. I just worked through this one myself now and it took about 10 minutes of drawing possible structures and crossing them out.

Hint: Recall that 2 - 2.5 is the normal range for hydrogens bound to a carbon alpha to a carbonyl group.
 
  • #12
... and the singlet at 5.08 is likely to have a fair amount of line broadening.
 

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