NMR question

[deleted647690]

There is a question/answer in BR that says that 3,3-dichloropentane would show a 2H quartet in H NMR

It says that this compound has two isolated 2H quartets. Looking at the compound, it looks symmetrical. I would think this would show up as a 4H septet.

Is that not the case when there are symmetrical protons like that? Why are they considered isolated?

Or I guess it would show up as 2H, in the smallest ratio possible. And just as a quartet

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[aldol16]

It says that this compound has two isolated 2H quartets. Looking at the compound, it looks symmetrical. I would think this would show up as a 4H septet.

Where are you getting the protons that split the methylene protons into a septet? The protons on either side of the 3-carbon are chemically equivalent. Therefore, corresponding protons (e.g. both sets of methylene protons) are going to have the same chemical shift. So draw out the molecule. The methylene protons on the left side are going to be split into a quartet by the methyl protons on the left. The methylene protons on the right side are going to be split into a quartet by the methyl protons on the right. Because all the methylene protons are equivalent, they will all have the same chemical shift, i.e. exactly overlap each other. Therefore, you're going to get a quartet that corresponds to those four protons.

 

[deleted647690]

Where are you getting the protons that split the methylene protons into a septet? The protons on either side of the 3-carbon are chemically equivalent. Therefore, corresponding protons (e.g. both sets of methylene protons) are going to have the same chemical shift. So draw out the molecule. The methylene protons on the left side are going to be split into a quartet by the methyl protons on the left. The methylene protons on the right side are going to be split into a quartet by the methyl protons on the right. Because all the methylene protons are equivalent, they will all have the same chemical shift, i.e. exactly overlap each other. Therefore, you're going to get a quartet that corresponds to those four protons.

I think I misunderstand chemical equivalence. I thought that chemical equivalence would amplify the signal, like make it show up as 4H rather than 2H. If you were just looking at the NMR reading, how would you know that a 2H quartet corresponded to two equivalent groups like that? I guess you would need to see the formula to know that there are additional hydrogens that are not necessarily accounted for completely in the integrations listed. By that, I am talking about how the integration would show up in a ratio of 3:2, but you would have to see that it actually translated to 6 protons : 4 protons in the structure

BTW, thanks for the quick reply

 

[aldol16]

I think I misunderstand chemical equivalence. I thought that chemical equivalence would amplify the signal, like make it show up as 4H rather than 2H. If you were just looking at the NMR reading, how would you know that a 2H quartet corresponded to two equivalent groups like that? I guess you would need to see the formula to know that there are additional hydrogens that are not necessarily accounted for completely in the integrations listed. By that, I am talking about how the integration would show up in a ratio of 3:2, but you would have to see that it actually translated to 6 protons : 4 protons in the structure

Yes, so with NMR, you can't figure out the structure well if there are planes of symmetry about the molecule. Here's why and this reconciles your knowledge.

Yes, chemical equivalent does amplify the signal. But here's the problem. Specifically, with NMR, we look at integrations to figure out how many protons correspond to a signal. So a signal with an integration of 3 would correspond to a methyl group. But do you see the problem? What is the integration calculated relative to? In practice, we set one peak that we can assign as the reference and when we integrate, that's set as the reference, or "1." So with symmetrical molecules, the signal would be amplified if you compared it to the corresponding non-symmetrical molecule but you would never know because your frame of reference is different. In other words, you have to realize that with symmetry, the signal you set to "1" may actually be 2 or 3 or 4 or whatever.

So NMR is good for confirming structure but not so good for de novo structure prediction. For practice, try assigning di-isopropyl ether.

 

[deleted647690]

Yes, so with NMR, you can't figure out the structure well if there are planes of symmetry about the molecule. Here's why and this reconciles your knowledge.

Yes, chemical equivalent does amplify the signal. But here's the problem. Specifically, with NMR, we look at integrations to figure out how many protons correspond to a signal. So a signal with an integration of 3 would correspond to a methyl group. But do you see the problem? What is the integration calculated relative to? In practice, we set one peak that we can assign as the reference and when we integrate, that's set as the reference, or "1." So with symmetrical molecules, the signal would be amplified if you compared it to the corresponding non-symmetrical molecule but you would never know because your frame of reference is different. In other words, you have to realize that with symmetry, the signal you set to "1" may actually be 2 or 3 or 4 or whatever.

So NMR is good for confirming structure but not so good for de novo structure prediction. For practice, try assigning di-isopropyl ether.

Thanks

Would that give a 3H doublet (corresponds to 4 equivalent methyl groups) and a 1H septet (corresponds to 2 equivalent methine groups)?

 

[aldol16]

Would that give a 3H doublet (corresponds to 4 equivalent methyl groups) and a 1H septet (corresponds to 2 equivalent methine groups)?

Keep track of the protons. Good job with the splitting, but remember the integration. There are 12 total methyl protons (they are all equivalent) and 2 total methine protons (also equivalent to each other). Therefore, the ratio of methyl to methine protons is always going to be 12:2, or 6:1. So you would get a doublet with an integration of 6 and a septet with an integration of 1.

Here you go: http://sdbs.db.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi

 

[deleted647690]

Keep track of the protons. Good job with the splitting, but remember the integration. There are 12 total methyl protons (they are all equivalent) and 2 total methine protons (also equivalent to each other). Therefore, the ratio of methyl to methine protons is always going to be 12:2, or 6:1. So you would get a doublet with an integration of 6 and a septet with an integration of 1.

Here you go: http://sdbs.db.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi

Thanks!