[davidenarb]

Activators are electron donating atoms or functional groups, and there is 3 type of activators: strong, moderate, and weak.

Activators are known to be ortho-para directors, and I would like to understand the reason behind that.

In the 1st figure, there is a strong activator, and the reason of ortho-para major products is due to the fact that we have 4 resonance structures, whereas in the meta position we have only3.

In the second figure, we have a weak activator, and the reason of ortho-para major products is the result of the fact that there is, in their resonance structures, a positive charge of the sigma complex that is directly adjacent to the electron-donating alkyl group.

So far, I presented my explanation of why strong and weak activators favor ortho and para products.

In the third figure, we have a benzene with a moderate activator. Can I have an explanation similar to which I provided above on how the ortho and para products are favored over the meta product?

Thank you

[discodermolide]

In the third case move the oxygen lone pair into the ring and then draw the resonance structures out and you will see that a + charge does not lie on the meta position.

[davidenarb]

In the third case move the oxygen lone pair into the ring and then draw the resonance structures out and you will see that a + charge does not lie on the meta position.

If I do so, I'll get similar resonance structures to the figure with strong activator (the first figure). Therefore, I don't understand the difference of impact between strong activator and moderate activator.

[discodermolide]

Is there actually a difference. Perhaps in the rate of reaction, but in the terms you have been discussing there is no difference

[davidenarb]

Is there actually a difference. Perhaps in the rate of reaction, but in the terms you have been discussing there is no difference

Is it because of the fact that strong activators have electron density that is essentially concentrated only within the ring; however, moderate activators have the electron density somewhat distributed outside of the ring?

[phth]

Is it because of the fact that strong activators have electron density that is essentially concentrated only within the ring; however, moderate activators have the electron density somewhat distributed outside of the ring?

The EDG strong activators will have a smaller energy difference because their electrons are higher in energy.  That doesn't mean that a strong activator will have its electron density only within the ring.  If it did, the resonance structure would be different; i.e., aromatic LUMO is lower in energy than the EDG, and even then not all of the electron density will be in the ring.  Quantitatively equilibrium constants of resonance structures can be measured>% in each state at equilibrium(T).

[davidenarb]

Is it because of the fact that strong activators have electron density that is essentially concentrated only within the ring; however, moderate activators have the electron density somewhat distributed outside of the ring?

The EDG strong activators will have a smaller energy difference because their electrons higher in energy.  That doesn't mean that a strong activator will have its density only within the ring.  If it did, the resonance structure would be different; i.e., aromatic LUMO is lower in energy than the EDG, and even then not all of the electron density will be in the ring.  Quantitatively equilibrium constants of resonance structures can be measured>quantitiave % in each state at equilibrium(T).

Definitions from my textbook:

Strong activators are characterized by the presence of a lone pair immediately adjacent to the aromatic
ring, and they have groups exhibit a lone pair that is delocalized into the ring.

Moderate activators exhibit a lone pair that is already delocalized outside of the ring. This effect diminishes the capability of the lone pair to donate electron density into the ring.

[phth]

The strong activators are faster because of the smaller energy difference.  For the moderate activator example that you used, the carbonyl pulls electron density away towards the δ⁺ carbonyl carbon.  That means that the electrons are more stabilized (lower in energy means larger ΔE), so they will spend less time being delocalized by the ring.

[orgopete]

Definitions from my textbook:

Strong activators are characterized by the presence of a lone pair immediately adjacent to the aromatic
ring, and they have groups exhibit a lone pair that is delocalized into the ring.

Moderate activators exhibit a lone pair that is already delocalized outside of the ring. This effect diminishes the capability of the lone pair to donate electron density into the ring.

That is exactly how I understand it. If you have a strong activator (OH or NH2), it's activation can be reduced by pulling electrons away from the nitrogen or oxygen by acylation. How do we know the acylation did not convert the substituent into a meta director as might be predicted by the greater resonance contributor? Look at the products and reactivity. The anilides and esters of phenols are less reactive and give o/p products. That reactivity and products is consistent with the oxygen and nitrogen reacting as electron donors, though of a weakened amount. Although this may seem like circular reasoning, I still find this explanation plausible.