[znerd]

I have been confused about the behaviour of the alkoxy group
in the following two conditions:

The acidic strengths of methoxy substituted phenols: Here, according the order of acidic strengths
(meta>ortho>no substitution>para)

We are able to explain the highest acidic strength of meta with the fact that at meta position, the methoxy group acts as a −I group.

This question in a reputable exam:


The answer given is


Here, the ethoxy group act as a donating group, activating the ring, and due to this the carbocation formed is a benzylic one, leading to the substitution of Bromine atom at the benzylic position.

I assumed that methoxy and ethoxy have almost the same behaviour.

Would this be any different if the substituent was methoxy and not ethoxy?

If not, then how can we differentiate the times when methoxy group at meta acts as donating and when it acts as withdrawing?

[rolnor]

The methoxy group is not important here, the vinyl is reactive anyhow, methoxy, ethoxy, chloro etc.

[znerd]

I understand that the vinyl is reactive but among the options, one option is: the substitution of Br takes place away from the benzene group, not at the benzylic position. I want to know why benzylic cation is more stable even though there is a withdrawing group at meta.

[rolnor]

The benzyl cation is just more stable, its not so much affected by ring substituents. The reaction you suggest, have you ever seen this? I have not.

[spirochete]

I understand that the vinyl is reactive but among the options, one option is: the substitution of Br takes place away from the benzene group, not at the benzylic position. I want to know why benzylic cation is more stable even though there is a withdrawing group at meta.

The trend you learned with substituted phenols simply does not work perfectly when applied to significantly different cases. The overall concepts still apply, but the exact final conclusion of those concepts might differ. IIRC: An electrophilic aromatic substitution reaction where phenol/anisole reacts at the meta position actually occurs more quickly than EAS on benzene itself. In other words, the minor meta product still forms rather quickly, but just more slowly than the ortho/para products. So apparently when donating toward carbocations, it seems to have some resonance donating effect even when it is in the "wrong" place to donate via resonance. 

In this case you need to be comparing two very different carbocations: A primary carbocation that lacks any resonance stabilization, and a secondary benzylic carbocation that has lots of resonance stabilization.

I would imagine that a reaction of ortho-nitrostyrene would give a similar outcome: bromine still adds to the benzylic position. Again, because of all the resonance stabilization. In this case the reaction would definitely go slower compared to styrene itself. But it would still give similar regiochemistry.

[rolnor]

Yes, I agree, it would slow the rate but give the "expected" product. Its a good question, good that this came up.