[Nescafe]

Hi,

Via google I constantly see opposite explanations.

One

"Halogens are very electronegative. This means that inductively they are electron withdrawing. However, because of their ability to donate a lone pair of electrons in resonance forms, they are activators and ortho/para directing. Resonance forms win out in directing. Because they are electron withdrawing, halogens are very weak activators"

Second

Same idea but they end up calling them deactivators.

So what are they activators or deactivators?

Nescafe.

[fledarmus]

Halogens are deactivating groups. The more of them that are present on a benzene ring, the less likely the ring is to undergo electrophilic aromatic substitution.

[camptzak]

If you have chlorobenzene, wont it undergo nucleophilic addition at the ortho and para positions? Making it an activating group.

the carbocation resonates to both meta positions and directly under alpha carbon which is stabilized by resonane with the chlorine

I would imagine that as the chlorines are added to the ortho para positions the rate of reaction would rapidly decrease.

[orgopete]

In the first instance, something may be missing from the statement. I think most textbooks and I compare the reaction rate to benzene. If it reacts faster than benzene, it must have been an activating group. Halogens are deactivating.

If you prefer to discuss regiochemistry of electrophilic aromatic substitution by drawing the resonance structures of the starting materials (I do), then you can draw resonance forms that are consistent with ortho/para directors. Normally, o/p-directors are activating. However, the halogens are not. I consider this the exception to activating o/p-directors. Since halogens are electron withdrawing, these seems plausible.

[fledarmus]

If you have chlorobenzene, wont it undergo nucleophilic addition at the ortho and para positions?

Yes it will

Making it an activating group.

No.

These are two different effects, directing effect and ring activation. In most cases, they are related, because electron releasing groups will stabilize carbocations, making the benzene ring more active to substitution, and will stabilize the ortho and para positions preferentially due to their electron donating effect and resonance effects. However, for the specific case of halogens, the substituents are electron withdrawing groups and will destabilize carbocations, making the ring less active to substitution. But, due to the ability of halogens to contribute their lone pairs of electrons into resonance structures, the ortho and para positions are still more active than the meta position. Halogens are deactivating, because they make it harder to substitute into the benzene ring, but are still ortho/para directing, because of the resonance effects. For typical electron withdrawing groups, there are no available lone pairs to conjugate with the ring, and they are both deactivating and meta directors.

I would imagine that as the chlorines are added to the ortho para positions the rate of reaction would rapidly decrease.

Yes, which means the chlorine is deactivating. If it was an activating group, addition of more chlorines would increase the rate of reaction. This is what makes addition of an activating group to a benzene ring somewhat difficult to control - addition of each activating group makes a product that is more reactive than the starting material, and you tend to get multiple substitutions on the ring.

[orgopete]

If you have chlorobenzene, wont it undergo nucleophilic addition at the ortho and para positions?

Yes it will

Just so no one stumbles in from Google, it should have been electrophilic as in electrophilic aromatic substitution, though the aromatic ring is acting as a nucleophile or electron donor. Nucleophilic aromatic substitution would be no reaction if unactivated or benzyne if strong base, and substitution if activated.