NMR Spectroscopy Homework: Proposing Structure from Data

[ephemeral1]

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.

 

[JJMB]

With that many degrees of unsaturation, the multiplet in that range is almost certainly hydrogens on aromatic ring / rings.

 

[ephemeral1]

Could you explain the multiplet part? When do you see a multiplet peak? And what causes it? I don't quiet understand that.

 

[JJMB]

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.

 

[ephemeral1]

Could you give me a hint on how to solve this problem?

 

[ephemeral1]

Could you give me an example of a multiplet? That would help me visualizing it better. Thank you.

 

[JJMB]

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.

 

[ephemeral1]

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.

 

[JJMB]

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.

 

[ephemeral1]

Thank you very much. Now I think I could figure this problem out.

 

[JJMB]

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.

 

[chemisttree]

... and the singlet at 5.08 is likely to have a fair amount of line broadening.