Deactivating vs Activating

[nyr201]

Hey guys...I was just thinking abt this concept and had a question...

When an electron withdrawing group is said to deactivate a benzene ring or even on other molecules...it increases its acidity by pulling away electron density from its adjacent atoms thus making the electron deficient atom more susceptible to accepting electrons from elsewhere so it acts as a lewis acid...ok so i clearly get this but why is it said that EWG's deactivate?? bc doesn't pulling away electron density and increasing acidity make the electron deficient species less stable and more reactive????

on the flip side: when a donating group is present its said to activate whatever its on by donating electron density to whatever its on which increases basicity (decreases acidity) bc now the species has electrons to donate (lewis base).


so after these observations i came to the conclusion that donating activates whatever its on to attack another species since its rich with electrons and when an EWG is on its said to deactivate bc it isnt electron rich and cannot donate electrons and attack something else...so is it right to think of it that way??

so if u are vulnerable to an attack ur said to be deactivated (EWG)

and if ur the attacker ur said to be activated (bc ur electron rich due to an EDG).

can someone please corroborate or refute this...id realy appreciate it. Thank u!

---

Members don't see this ad.

 

[Thundercatz]

When you have an EDG or EWG on a benzene and you add something to that benzene, the benzene will be left with a carbocation. The carbocation will be stabilized by an EDG because it makes the electron deficient benzene ring more stable (you are adding electrons to an electron deficient ring).

Conversely, if you have an EWG, it will make the electron deficient benzene ring less stable(you pulling electrons from an electron deficient compound), hence it is called deactivating.

 

[drpoppapump]

EWG prevent the benzene ring from being a stable carbocation because I actively takes away an electron so the benzene is electron defeicent. The EDG helps stabilize the ring structure and thus give ts ortho para orientation.

 

[nwoye]

OMG very well put! Thanks for sharing

 

[K Niner]

Sometimes, to decide whether a rxn will proceed, it is better to look at the intermediates that would form. That is what you do here. Don't think of what allows the benzene ring to be suseptible to attack, but what the intermediate (carbocation) would be if this rxn were to take place.

 

[mtavallaei]

.

 

[Lane138]

Sorry if I'm being dense but I don't understand where the carbocation comes from when benzene has something added to it?

 

[PiBond]

Hey guys...I was just thinking abt this concept and had a question...

When an electron withdrawing group is said to deactivate a benzene ring or even on other molecules...it increases its acidity by pulling away electron density from its adjacent atoms thus making the electron deficient atom more susceptible to accepting electrons from elsewhere so it acts as a lewis acid...ok so i clearly get this but why is it said that EWG's deactivate?? bc doesn't pulling away electron density and increasing acidity make the electron deficient species less stable and more reactive????

on the flip side: when a donating group is present its said to activate whatever its on by donating electron density to whatever its on which increases basicity (decreases acidity) bc now the species has electrons to donate (lewis base).


so after these observations i came to the conclusion that donating activates whatever its on to attack another species since its rich with electrons and when an EWG is on its said to deactivate bc it isnt electron rich and cannot donate electrons and attack something else...so is it right to think of it that way??

so if u are vulnerable to an attack ur said to be deactivated (EWG)

and if ur the attacker ur said to be activated (bc ur electron rich due to an EDG).

can someone please corroborate or refute this...id realy appreciate it. Thank u!

I think of it this way. During electrophilic aromatic substitution who is our nucleophile that is doing the attacking? It's benzene. Thus, if I take way electron density from my ring then I effectively make it more difficult to introduce a new group onto the ring. Therefore we call electron withdrawing groups deactivating. Also, as someone else pointed out, our intermediate in this mechanism is a carbocation. Of course, carbocations are electron deficient and therefore EWGs destabilize this positive charge.

On the flip side, activating groups are electron donating and thus make it easier to introduce a new group. Once again we have the electron deficient carbocation, but an EDG stabilizes this intermediate and thus activates our ring.