Official MCAT Guide--Orgo Discrete

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SaintJude

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Why do the two methylene protons of the dimethylaminomethyl group of the compound shown give separate copuled signals in the 1H NMR spectrum?

Q8LaW.png


A. Because they are anisotropic.
B. Because they are diasterotopics
C. B/c they are equivalent (NO)
D. B/c every proton in a compound gives a separate signal by high-field NMR (NO)

Answer is : B
 
Just need to know the terms enantiotopic and diastereotopic. Enantiotopic = the same, diastereotopic = different.
 
Ah, I see. But what about A? Does anyone know what anisotropic means in relation to NMR? I looked it up in Wiki but don't understand what they are saying.

About A, AAMC says:

A. The orientations do not affect the equivalency of these methylene protons b/c the dimethylaminomethyl group is freely rotating.

Thanks....?
 
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Ah, I see. But what about A? Does anyone know what anisotropic means in relation to NMR? I looked it up in Wiki but don't understand what they are saying.

I thought it was a made up word.

Wiki says:
In NMR spectroscopy, the orientation of nuclei with respect to the applied magnetic field determines their chemical shift. In this context, anisotropic systems refer to the electron distribution of molecules with abnormally high electron density, like the pi system of benzene. This abnormal electron density affects the applied magnetic field and causes the observed chemical shift to change.

I guess it is describing one compound and not comparing one to another.
 
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