Apr 18, 2018
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Hi,
I am unsure how you would determine if a group is electron withdrawing or electron donating by the resonance forms.
My confusion stems from the amide group...I thought from the resonance that the carbonyl group would be electron withdrawing (by getting a partial negative charge) and nitrogen would be electron donating (from its lone pair). But TBR mentioned that the oxygen would actually be electron donating and the most basic site.
If carbonyl in an amide is electron donating by resonance, how is carbonyl in a carboxylic acid electron withdrawing by resonance?

(Also: by inductive effects, the oxygen is more electronegative so would be electron withdrawing, but resonance takes priority over induction...right?)

Sorry this may sound like a dumb question but I realize that I haven't actually understood the foundation of this concept and I can't seem to find a straightforward answer anywhere.

Thanks in advance if someone could clear this confusion...been at this all day :(
 

Lawpy

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Amides can be either electron donating or electron withdrawing. It depends on how the amide is attached to benzene/aromatic group. Same applies for esters.






For more information, see: Activating and Deactivating Groups In Electrophilic Aromatic Substitution — Master Organic Chemistry

Ch12 : Substituent Effects
 
Apr 18, 2018
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So it is generalized then that when oxygen is double bonded (C=O), it is electron withdrawing and want to take the pi-bond electrons, by resonance. And when oxygen is singly bonded to 2 different atoms (C-O-R), it is electron donating and wants to donate its lone pair.

I guess I'm still kind of confused. Oxygen always have 2 lone pairs regardless of being double bonded to one atom, or singly bonded to 2 atoms... so then why does it want to donate its lone electron pair when singly bonded.
I do realize that when double bonded, it is sp2 while single bonded it is sp3...does being sp2 make it electron deficient and thus wanting to withdraw electrons, while sp3 makes it willing to donate its lone pair?

I may be overthinking this...but I still don't understand what actually CAUSES a compound to be electron donating or withdrawing by resonance. I guess I can memorize the list, but I'm hoping there may be an explanation...

Thanks again
 
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Lawpy

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So it is generalized then that when oxygen is double bonded (C=O), it is electron withdrawing and want to take the pi-bond electrons, by resonance. And when oxygen is singly bonded to 2 different atoms (C-O-R), it is electron donating and wants to donate its lone pair.

I guess I'm still kind of confused. Oxygen always have 2 lone pairs regardless of being double bonded to one atom, or singly bonded to 2 atoms... so then why does it want to donate its lone electron pair when singly bonded.
I do realize that when double bonded, it is sp2 while single bonded it is sp3...does being sp2 make it electron deficient and thus wanting to withdraw electrons, while sp3 makes it willing to donate its lone pair?

I may be overthinking this...but I still don't understand what actually CAUSES a compound to be electron donating or withdrawing by resonance. I guess I can memorize the list, but I'm hoping there may be an explanation...

Thanks again

The images I posted above basically answer your question. Oxygen and nitrogen atoms that are singly bonded to aromatic groups have lone pairs of electrons that they can readily donate to create a new pi bond. That's why these groups serve as pi donors (they donate their lone pair of electrons to create pi bonds). And the effect of pi donation is stronger than the effect of sigma acceptance that you might expect from the higher electronegativity of oxygen and nitrogen. That's why single bonded oxygen and nitrogen atoms serve as electron donating groups.

In contrast, carbonyl groups have the carbon atom with a partial positive charge and the oxygen atom with a partial negative charge (this is because oxygen is more electronegative than carbon and attracts the electrons towards it). The carbon atom in carbonyl is electron deficient and wants to attract electrons from the aromatic group. So the carbonyl group serves as an pi acceptor where the carbonyl carbon forms a pi bond with the aromatic group at the expense of the aromatic group losing the electrons and becoming positively charged at one of the carbon atoms (in the process, the oxygen atom becomes negatively charged). That's why carbonyl groups serve as electron withdrawing groups.
 
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Lawpy

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Take a look at a pattern. Electron rich functional groups and negatively charged functional groups have a lot of electrons available, which means they can give the electrons to the aromatic groups and create pi bonds. So they are electron donating groups.

In contrast, electron poor functional groups and positively charged functional groups have a deficiency of electrons and a partial or fully positive charge. They want to attract electrons from aromatic groups to neutralize its positive charge, which means they are electron withdrawing groups.

Halogens serve as electron withdrawing groups because by donating electrons to aromatic groups, the halogens end up with a positive charge which is unstable for them. Oxygen and nitrogen can better handle positive charges than halogens even though they'd like to be neutral (this is also why alkoxides are stronger electron donors than alcohols).
 
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