EDGs vs. EWGs-let's end the confusion...

[SaintJude]

Below is a list of Electron Donating Groups and Electron Withdrawing Groups. I don't understand how to identify this group w/o memorizing them--but memorization wouldn't help anyway b/c I would miss the bigger concept.

All I see are oxygen in both EDGs and the EWGs list. An amine is a strong activator and yet a quaternary ammonium is a strong deactivator??
p.s. ignore arrow on side

Strongly Activating
View attachment 18752
Strongly Deactivating

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[BerkReviewTeach]

Below is a list of Electron Donating Groups and Electron Withdrawing Groups. I don't understand how to identify this group w/o memorizing them--but memorization wouldn't help anyway b/c I would miss the bigger concept.

All I see are oxygen in both EDGs and the EWGs list. An amine is a strong activator and yet a quaternary ammonium is a strong deactivator??

Consider the first atom attached to the pi-system. Does it have a lone pair of electrons? If the answer is yes, then it can be an EDG by resonance. Does it have an atom in a pi-bond that has no electron pair (such as the C of a carbonyl)? if yes, then it can be an EWG by resonance. If it has no lone pair and is not a part of a pi-bond, then you need to consider the inductive effect.

This method is a bit oversimplified, but given that benzene chemistry is NOT tested on the MCAT, you don't need to know about meta-directors and o,p-directors. You need to know how EDGs and EWGs affect acidity, basicicity, nucleophilicity, and electrophilicity. That can be done using the simplified perspective.

 

[MedPR]

How can methyl and halo substituents be grouped together? Aren't alkyl groups EDG and halogens EWG?

 

[SaintJude]

You are right, alkyl-EDG & halogens are considered EWG

Halo groups are the only MCAT EWG groups that are also otho/paro directors (albeit weak). All the other EWgs are meta directors. This list list arranges the substitutes via the the directive effects in an electrophilic aromatic substitution. So the halogen groups sort of overlap into both lists.

 

[SaintJude]

Consider the first atom attached to the pi-system. Does it have a lone pair of electrons? If the answer is yes, then it can be an EDG by resonance. Does it have an atom in a pi-bond that has no electron pair (such as the C of a carbonyl)? if yes, then it can be an EWG by resonance. If it has no lone pair and is not a part of a pi-bond, then you need to consider the inductive effect.

Brilliant.

The ONLY notable exceptions I need to clear up are for the two strongest deactivating group. But according your simplified rule would be EDGs. These are SO3H & NO2. What gives?

 

[BerkReviewTeach]

Brilliant.

The ONLY notable exceptions I need to clear up are for the two strongest deactivating group. But according your simplified rule would be EDGs. These are SO3H & NO2. What gives?

The N in NO2 is positively charged and has a pi-bond to O, so it's a slammin' EWG (by resonance and the inductive effect both). That N has no lone pair of electrons, so it can't be an EDG. The S in SO3H has two pi-bonds to O and no electron pair, so it has double EWG by resonance. It too cannot be an EDG, because it has no lone pair to donate.

And as far as halogens go, they are deactivors (they make the reaction slower) because although they have a lone pair, it's in a 3p, 4p, or 5p orbital which doesn't form a good resonance overlap with the 2p orbital of C. So it has weak EDG tendencies by resonance and EWG tendencies by the inductive effect. The result is as you mentioned.

 

[MedPR]

You are right, alkyl-EDG & halogens are considered EWG

Halo groups are the only MCAT EWG groups that are also otho/paro directors (albeit weak). All the other EWgs are meta directors. This list list arranges the substitutes via the the directive effects in an electrophilic aromatic substitution. So the halogen groups sort of overlap into both lists.

I must be missing something. I thought hydroxyl groups are EWG?

 

[chiddler]

I must be missing something. I thought hydroxyl groups are EWG?

negatory, they can donate their lone pair of electrons into the aromatic.