AAMC Ochem question

[orangeman25]

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I was doing some AAMC organic chemistry discretes (off the official guide book) and came across a question where I had to know the difference between anisotropic and diastereotropic. Can someone explain this? It has to do with H NMR.

 

[polyatomic]

By doing some quick googling, it seems like ansiotropic has to do with how the molecule aligns (either horizontally or vertically) due to the magnetic signal. As a reminder, deshielding is essentially stripping electrons off the hydrogen.

With double bonds, the alkene aligns perpendicular to the induced magnetic field. This makes it so the magnetic field is aligned in a way that strips the electrons off the hydrogens, causing double bonds to shift downfield and have the very high shifts of ~6ppm.

In triple bonds, for whatever reason it's better if it aligns parallel to the magnetic field. This disallows the stripping of electrons from the hydrogens, and therefore alkynes have a much lower shift (2-3ppm).

Diastereotopic is an instance where two protons on the same carbon appear as if they are completely different protons. This will make them show up as separate signals on an NMR. This can happen if the molecule has at least one chiral center, however it doesn't always occur.

That's my 2 cents. Most of my knowledge comes from my course I took specifically in Spectroscopy. To me it seems like the question is rather trivial. It is asking you to compare two entirely different phenomenons. If you were comparing enantiotopic vs. diastereotopic, that'd be a different story.

Let me know if any of this was confusing or if you have further questions!

 

[orangeman25]

By doing some quick googling, it seems like ansiotropic has to do with how the molecule aligns (either horizontally or vertically) due to the magnetic signal. As a reminder, deshielding is essentially stripping electrons off the hydrogen.
View attachment 177894
With double bonds, the alkene aligns perpendicular to the induced magnetic field. This makes it so the magnetic field is aligned in a way that strips the electrons off the hydrogens, causing double bonds to shift downfield and have the very high shifts of ~6ppm.
View attachment 177895
In triple bonds, for whatever reason it's better if it aligns parallel to the magnetic field. This disallows the stripping of electrons from the hydrogens, and therefore alkynes have a much lower shift (2-3ppm).

Diastereotopic is an instance where two protons on the same carbon appear as if they are completely different protons. This will make them show up as separate signals on an NMR. This can happen if the molecule has at least one chiral center, however it doesn't always occur.

That's my 2 cents. Most of my knowledge comes from my course I took specifically in Spectroscopy. To me it seems like the question is rather trivial. It is asking you to compare two entirely different phenomenons. If you were comparing enantiotopic vs. diastereotopic, that'd be a different story.

Let me know if any of this was confusing or if you have further questions!

Amazing answer. Cleared up a lot but basically what I learned was that not only did I not have to know specifics about either term, I just had to know which term related to proton splitting in H NMR. Another instance of AAMC trying to make an easy question look difficult. Luckily I just took my MCAT and had learned that lesson beforehand!

 

[polyatomic]

Amazing answer. Cleared up a lot but basically what I learned was that not only did I not have to know specifics about either term, I just had to know which term related to proton splitting in H NMR. Another instance of AAMC trying to make an easy question look difficult. Luckily I just took my MCAT and had learned that lesson beforehand!

Yeah, AAMC is really good at that.

Congrats on finishing the MCAT!