Proton NMR question

[FeralisExtremum]

I have a question on the following problem (from Qvault):

The answer is the figure drawn on the bottom left. There's two issues I have:

1. Why are there only 4 H-environments shown? I tried selecting all the ones I could find (color coded to distinguish them in the bottom right) and I'm getting 7 total. It seems like they just lumped all the protons from the benzene ring as one environment of H's. Is this standard for proton NMR's as they're given on the DAT or would the protons of a benzene ring (assuming they're not symmetrical) be distinguishable?

2. The peaks on the graph surrounded by the blue rectangle correspond to the same color carbon in the figure on the bottom right, according to the answer key. But from what I can tell there should be 8 peaks present because it has a neighbor (purple rectangle) with 3 H's which gives us a quartet and a neighbor with 1 H (green rectangle) which means a quartet of a doublet. Is this just supposed to represent a multiplet and the actual peak count doesn't matter? At what point do we start disregarding the number of peaks and assume it is just a multiplet?

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[BYU4you]

Haven't taken the dat so can't compare to that. But this NMR just doesn't seem that accurate. The integration on the benzene H's seem pretty low considering they grouped them. As far as the purple rectangle goes it is split by four H's total I wouldn't think it would split into a quartet and then a doublet from that either. As a rule of thumb If it is more than 4 you can stop caring and its a multiplet.

Again not the most accurate NMR... dat destroyers seem to be reasonable and accurate if you want good examples albeit few.

 

[DAT Destroyer]

I have a question on the following problem (from Qvault):

The answer is the figure drawn on the bottom left. There's two issues I have:

1. Why are there only 4 H-environments shown? I tried selecting all the ones I could find (color coded to distinguish them in the bottom right) and I'm getting 7 total. It seems like they just lumped all the protons from the benzene ring as one environment of H's. Is this standard for proton NMR's as they're given on the DAT or would the protons of a benzene ring (assuming they're not symmetrical) be distinguishable?

2. The peaks on the graph surrounded by the blue rectangle correspond to the same color carbon in the figure on the bottom right, according to the answer key. But from what I can tell there should be 8 peaks present because it has a neighbor (purple rectangle) with 3 H's which gives us a quartet and a neighbor with 1 H (green rectangle) which means a quartet of a doublet. Is this just supposed to represent a multiplet and the actual peak count doesn't matter? At what point do we start disregarding the number of peaks and assume it is just a multiplet?

This is a nice graph. I like your question. All the aromatic protons come in very close,,,,,,,thus a single peak appears around 7ppm due to signal overlap. Often a single peak is seen for a mono-substituted benzene ring unless an O or N is attached...then the signal would be split. The blue squared area you depicted would indeed give a doublet of quartets,,,,,but good luck seeing it. Often , peak distances are so close......i.e coupling constants, J......they appear as a multiplet . As a rule.....anything over 4 peaks,,,we call a multiplet.

I hope this helps

Dr. Jim Romano

 

[swindoll]

I have a question on the following problem (from Qvault):

The answer is the figure drawn on the bottom left. There's two issues I have:

1. Why are there only 4 H-environments shown? I tried selecting all the ones I could find (color coded to distinguish them in the bottom right) and I'm getting 7 total. It seems like they just lumped all the protons from the benzene ring as one environment of H's. Is this standard for proton NMR's as they're given on the DAT or would the protons of a benzene ring (assuming they're not symmetrical) be distinguishable?

2. The peaks on the graph surrounded by the blue rectangle correspond to the same color carbon in the figure on the bottom right, according to the answer key. But from what I can tell there should be 8 peaks present because it has a neighbor (purple rectangle) with 3 H's which gives us a quartet and a neighbor with 1 H (green rectangle) which means a quartet of a doublet. Is this just supposed to represent a multiplet and the actual peak count doesn't matter? At what point do we start disregarding the number of peaks and assume it is just a multiplet?

Are you prepping for DAT???

 

[FeralisExtremum]

Are you prepping for DAT???

Yep! I have it coming up next month.

 

[swindoll]

Yep! I have it coming up next month.

For the first time? But you wrote those notes forever ago?..

 

[FeralisExtremum]

For the first time? But you wrote those notes forever ago?..

It will be the first (and hopefully last) time. I did write the notes a while ago the first time I was studying but some personal stuff came up so I never ended up taking the exam, unfortunately.

 

[swindoll]

Ahh I see! Well, I have no doubt you'll ace it!