about electron withdrawing/donating groups and NMR

[abcxyz0123]

EK says that electron withdrawing groups move a hydrogen signal to the left, and leaves it at that. What happens if there are electron donating groups near the hydrogen you are looking at? Does the signal move to the right, or do electron donating groups not affect NMR?

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[deleted106503]

Hmm...this part confused me as well. I think I got confused on the O, M, P e- withdrawing and donating groups for benzene by relating it to H NMR. This was incorrect on my part, right? Just because something is e- withdrawing on benzene doesn't make it e- withdrawing in H NMR, right? I think for benzene, just an oxygen is electron donating and ring activating, but a carbonyl on H NMR/C NMR are withdrawing.

 

[ADeadLois]

EK says that electron withdrawing groups move a hydrogen signal to the left, and leaves it at that. What happens if there are electron donating groups near the hydrogen you are looking at? Does the signal move to the right, or do electron donating groups not affect NMR?

Electron withdrawing groups move electron density away from the proton. This is called "deshielding" and the signal moves downfield (to the left). The opposite is true for electron donating groups, which shield the protons moving the shift upfield (to the right). EK probably doesn't mention electron donating groups because you're unlikely to see a question about their effect on NMR on the MCAT. Not because it's less common in practice, but electron donation makes the NMR output ambiguous and difficult to interpret on its own. That is, it's easier to see the effects of electron withdrawal than donation even though the effect is the same just in opposite directions. Carbonyl groups are common electron-withdrawing groups that are very recognizable as they occur very far downfield. Aromatic protons are also very distuingishable.

 

[ADeadLois]

Hmm...this part confused me as well. I think I got confused on the O, M, P e- withdrawing and donating groups for benzene by relating it to H NMR. This was incorrect on my part, right? Just because something is e- withdrawing on benzene doesn't make it e- withdrawing in H NMR, right? I think for benzene, just an oxygen is electron donating and ring activating, but a carbonyl on H NMR/C NMR are withdrawing.

I think a lot of that is beyond the scope of the MCAT. Aromatic substitution effects on NMR gets very complex.