Homotopic vs Enantiotopic vs Diastereotopic Groups

[@VictortheOrganicChemistryTutor]

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[@TheDortmunderJungs]

I work on relatively large complex molecules with around 80 Protons, rotamers and 4-5 chiral centers. I hate all but homotopic

[@VictortheOrganicChemistryTutor]

😂 I hear you! 80 proton HNMR is going to be a mess no matter how you spin it.

[@peybak]

Thanks for the lesson. I've heard of the idea before but this is very useful to go over.

[@VictortheOrganicChemistryTutor]

This is extremely useful concept for spectroscopy. I'm surprised how few instructors emphasize this topic in class.

[@marwansal4175]

Hi Mr. Victor. In the last example (methyl cyclopropanone), you said the two H on C3 are diastereotopic and therefore they show up as two separate signals on H-NMR and they also split each other. in that case do we expect a triplet ( 1H from C3 and 1 H from C2 + 1) for each of the two hydrogens in question? Similarly the H on C2 , does it split as a sextet ? Thank you

[@VictortheOrganicChemistryTutor]

It's actually going to be a bit messier. You'll have complex splitting in cases like that since the J constants are not going to be the same. It's a bit too long for a comment to explain, but I have a video brewing on the topic, so it should be out sometime later this spring.

[@marwansal4175]

@@VictortheOrganicChemistryTutor thank you. I actually thought about that after posting my comment. The fact that J constant are different will yield a messy signal. Currently looking online to see if there's a HNMR spec for this molecule. I look forward to your next post

[@dalhology6735]

Mr. Victor I have in my compound 1 chiral center however, the HNMR and CNMR gave me interpretation as if it is one molecule. But when I have 2 chiral centers (as I added another substituent while the first chiral center remained as it is), the HNMR gave me protons as if it's one molecule, however in the CNMR each peak is splitted as if I have 2 molecules,, so why this occur with 2 chiral centers only and not 1 / and why it shows up only in CNMR not HNMR? THANKS FOR YOUR HELPFUL VIDEOS.

[@VictortheOrganicChemistryTutor]

It depends on the structure, of course. But diastereotopic protons can be very similar to each other (especially, if they are far away from the chiral atom) and in open chains with free rotation often overlap giving you one signal. Typically, in cases like this you could either use a much more powerful spectrometer (>750MHz), use special tuning techniques on the spectrometer that can resolve specific signals while "blurring" the rest (not that many people know how to do that, so most likely not an option for you), or the only other method I can think of is going to be the classic enantiomer resolution by adding another chiral atom.

[@dalhology6735]

@@VictortheOrganicChemistryTutorMr.Victor, thanks a lot for your informative reply 🙏🙏 I want to be sure that I understood what you mean, so you mean that when it's only one chiral center, diasteriomer is present but didn't appear because of the low power of NMR instrument ? And even if there are 2 chiral centers, the resolution didn't occur too in HNMR right? But why when there are 2 chiral centers splitting in the CNMR occurs as if I have 2 molecules , although they are 2 chiral centers and so they should appear as 4 equivalent carbons and protons?

[@VictortheOrganicChemistryTutor]

@dalhology6735 No, this is not what I said. If you need a moro thorough discussion of what's going on in your case, you can book an appointment on my website. I don't think we can have a constructive discussion in comments here without being able to draw molecules and look at them in details.