More EWG/EDG

[MedPR]

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Can someone explain the reasoning behind what makes something an EWG or EDG? I'm trying really hard to not have to know them by rote memorization and I think having a good understanding will be more helpful anyway.

For example can we start with why halogens are weak electron withdrawing groups?

And on the other side, can we start with why an amine (NH2) substituent is a strong electron donating group?

 

[chiddler]

Can someone explain the reasoning behind what makes something an EWG or EDG? I'm trying really hard to not have to know them by rote memorization and I think having a good understanding will be more helpful anyway.

For example can we start with why halogens are weak electron withdrawing groups?

And on the other side, can we start with why an amine (NH2) substituent is a strong electron donating group?

Halogens are weak electron withdrawing groups because they have two competing activities. One, they can donate electrons but very little because they obviously do not fare very well as cations. On the other hand, they have an inductive effect going for them that pulls electrons. Inductive is weak, though. So they are weak electron withdrawing groups.

Amines have a lone pair and, unlike halogens, can exist happy as a clam in a cationic state. Thus they can donate and are not necessarily destabilized while doing so.

 

[Jarudy]

In aromatic chemistry,
Halogens: withdraw electron density through the sigma bond due to electronegativity; donate electron density due to pi electrons (draw resonance forms)
Amines: lone pair of electrons which are more free to move again which is evident if you draw resonance forms

 

[sweet d]

Think in terms of resonance and inductive effect, and how that affects the stability of the intermediate (a cation) in reactions.

Electron donating groups (activating groups) are going to stabilize the intermediate, usually by contributing to resonance. For –NH2, the lone pair on nitrogen adds an additional resonance structure. You’ll notice that most other activating groups (NR2, OH, OR, etc) also have lone pairs to contribute. The exceptions are groups like -R, -Ar and -vinyl, which stabilize the intermediate via hyperconjugation.

Electron withdrawing groups (deactivators) are going to pull electrons away from the ring and are not good resonance contributors. Halogens are a little unusual because they do have lone pairs, but they are deactivators. F, Cl, Br, and I are more electronegative than carbon and pull electrons away from it. Since their lone pairs can contribute to resonance, they are ortho-para directing instead of meta directing like all other deactivating groups. In general, if the atom connected to the ring is bonded to atoms that are more electronegative than carbon or if the atom connected to the ring has positive formal charge then you're looking at an EWG.

Hopefully that makes sense!