NMR Question

[ratatat]

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I did the 4th EK Orgo Chem 30 min exams and I'm confused about one thing.

http://i25.tinypic.com/34rc8zo.jpg

IMO, the chemical shift at 4.20 ppm should be a triplet, not a singlet, because each hydrogen on that aromatic compound "sees" two other equivalent hydrogens, right?

I also believe one of the doublets should be a triplet too.

Correct me if I'm wrong please and thank you.

 

[sleepy425]

I did the 4th EK Orgo Chem 30 min exams and I'm confused about one thing.

http://i25.tinypic.com/34rc8zo.jpg

IMO, the chemical shift at 4.20 ppm should be a triplet, not a singlet, because each hydrogen on that aromatic compound "sees" two other equivalent hydrogens, right?

I also believe one of the doublets should be a triplet too.

Correct me if I'm wrong please and thank you.

I'm just going to go through the assignments first just to make sure it's clear.

The 2.30 ppm singlet is the CH3.
The 4.20 ppm singlet integrating to 5 protons is the top (unsubstituted) cyclopentadienyl ligand.
The 4.50 ppm doublet integrating to 2 protons represents two of the protons from the substituted cyclopentadienyl ligand at the bottom. These two protons are the ones farthest away from the ketone group.
The 4.80 ppm doublet integrating to 2 protons represents two of the protons from the substituted cyclopentadienyl ligand at the bottom. These two protons are the ones closest to the ketone group.

Ok, spin-spin coupling which produces peak splitting only occurs between non-identical nuclei. In the 4.20 ppm singlet, each of the 5 protons is adjacent to two protons, but since those two protons are identical, they do not exhibit spin-spin coupling.

On the other hand, each of the protons of the 4.50 ppm doublet is adjacent to one identical proton and one nonidentical proton. Since there's only one nonidentical proton, you only get splitting from that proton, so it's a doublet.

The 4.80 ppm doublet protons are adjacent to one nonidentical proton, so you get coupling there to produce the doublet.

 

[ratatat]

I'm just going to go through the assignments first just to make sure it's clear.

The 2.30 ppm singlet is the CH3.
The 4.20 ppm singlet integrating to 5 protons is the top (unsubstituted) cyclopentadienyl ligand.
The 4.50 ppm doublet integrating to 2 protons represents two of the protons from the substituted cyclopentadienyl ligand at the bottom. These two protons are the ones farthest away from the ketone group.
The 4.80 ppm doublet integrating to 2 protons represents two of the protons from the substituted cyclopentadienyl ligand at the bottom. These two protons are the ones closest to the ketone group.

Ok, spin-spin coupling which produces peak splitting only occurs between non-identical nuclei. In the 4.20 ppm singlet, each of the 5 protons is adjacent to two protons, but since those two protons are identical, they do not exhibit spin-spin coupling.

On the other hand, each of the protons of the 4.50 ppm doublet is adjacent to one identical proton and one nonidentical proton. Since there's only one nonidentical proton, you only get splitting from that proton, so it's a doublet.

The 4.80 ppm doublet protons are adjacent to one nonidentical proton, so you get coupling there to produce the doublet.

Thank you for that comprehensive answer.