NMR Technicality

[cfdavid]

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EK Mini MCAT #14 had an NMR passage, and one of the questions tripped me up. The problem is #656

In the problem, it shows what are clearly two chemically equivalent methyl groups at the leftmost end of the molecule. But, the splitting signature/peak on the neighboring carbon atom (labeled b) appears to consider them separate (due to an additional 'split' being called out).

The question to anyone who may or may not have EK Mini MCAT's, is; Can you consider two chemically equivalent hydrogen groups as separate peaks??

I mean, what I know of NMR indicates to consider them as one (chemically equiv.) and that the integral curve will tell you the ratio or numbers of H-atoms. But, is it possible that an NMR data group may show two equivalent groups of hydrogens as two separate groups of H's even if they're chemically equivalent.????(like those in an isopropyl group, where the 2 methyl groups would be considered equivalent, and the other H's attached to the other carbon would be considered quite another).

In EK Orgo (pg 90) it shows what are clearly two chemically equivalent groups of H's (the Hb's) as one group, and thus only one peak.

Any help would be appreciated.

 

[Booyakasha]

They are not chemically equivalent if they are giving two different signals. Is there a chiral center in the molecule or a chiral axis? If so, then they are probably not related by an element of symmetry, and if they have the same connectivity, they are diastereotopic. What is the name of the compound?

 

[QofQuimica]

I agree that they are probably diastereotopic. Even if they are chemically equivalent, they may not necessarily be magnetically equivalent, especially if they are near another group that cannot freely rotate, and there is a chiral center in the molecule. We see this occur with some of the amino acid derivatives we are making. In extreme cases, every set of hydrogens shows up in stereo (i.e., two peaks for each), even the diastereotopic ones. As you can imagine, it makes the spectrum very complex when this happens.

All that being said, I wouldn't worry about it too much, cfdavid. I'd be very surprised if diastereotopic protons showed up as a topic on the MCAT.

 

[cfdavid]

Thanks to both of you for the input. QofQuimica, you're our chemistry guru, so I was hoping you'd respond to this one. I was going to title the thread "Question for QofQuimica"!

The two methl groups that I thought were chemical equivalents are not attached to a chiral center (they themselves make that so since they're both attached to the same C).

Regardless, I agree that a confusing spectrum is not probable on the test. And I'm comfortable with my basic knowledge, so I guess I won't worry about it.

 

[QofQuimica]

I didn't mean that the carbon they were attached to would be chiral; I meant that this could happen if there were a chiral center somewhere else in the molecule. Is there a chiral center anywhere in the molecule? The presence of a chiral center leads to the possibility of having diastereotopic protons, which are chemically equivalent but magnetically unequivalent, and you will then see each of the protons by NMR as you apparently did.

I'm always glad to answer questions, so if anyone has any chem questions in particular, feel free to shoot them my way. 🙂

 

[cfdavid]

Actually, none of the carbons in the molecule are chiral, so who knows.

 

[Booyakasha]

It probably either has a chiral axis or the methyl groups are not magnetically equivalent. I am taking a grad course in NMR spectroscopy this semester and have encountered several molecules like that. Write the mirror image of the compound and see if it is superimposable on the original: this will tell you if it has a chiral axis. Anyways, good luck with everything.