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They can either activate or de-activate the ring based on whatever substituent you have. i will try to explain what this means. Let's take a look at this for starters; we have the methoxy group that's called O-CH3 |dw:1440104338830:dw| see those lone pairs on the oxygen? well we can move those into the ring, and create a resonance structure. what happens is that you have net negative charge on the ring and we can move this negative charge around, that's what resonance is, the movement of pi electrons around a molecule; we're not changing anything about that molecule itself, not breaking bonds, just moving electrons around to give us a better picture of how the electrons can be distributed. this effect makes the ring more electron rich. (if I remember correctly in electrophilic aromatic substitution the benzene ring acts like the nucleophile and attacks the electrophile) if this is so, we can see that if we have an electron donating group like the methoxy group it's going to be a much better nucleophile, why? because it's more electron rich, and that resonance structure we drew helps us to understand this. |dw:1440104445573:dw| now below, (If I remember exactly) the ring attacks the electrophile, the more electron rich it is, the faster this will be, b/c more - charge on that ring. and you break aromaticity, those three double bonds in your cyclic compound. Remember aromaticity is favorable and breaking aromaticity is unfavorable, but the presence of the electron donating group methoxy allowed us to do this. you can see in that structure and that positive charge can be moved around by resonance and is stabilized by those lone pair of electrons on that oxygen. now we have a case where that B i drew was a base. what that base does is it helps us to restore aromaticity by plucking off that hydrogen and putting dumping it's electrons to the carbon (I think). then we get our final product back. |dw:1440104838070:dw| you might ask why I put the E group there, ill explain what that is also We have established that this guy (the O-CH3) is our electron donating group and we said why. now let's look at it this way. In the textbook it tells us that electron donating groups are ortho, para directors, which means that when we do a reaction our group we want to put on our ring will be added at either carbon 2, adjacent to our electron donating group, ortho, or para. |dw:1440105299289:dw|
|dw:1440105560515:dw| you could also get the group above as well, but remember, the bigger your group, ( you would have to take into account something called serics, repulsion of electron clouds. say if we had this: you can clearly see how big and hideous that group is, and putting another group at the ortho position would not be as favorable, I guess you could get maybe this as a (very minor product) but the para-position is clearly better since it's farther away. |dw:1440105689877:dw|
|dw:1440105929493:dw| UNRELATED but To show you this, when we have just cyclohexane substituted, no aromaticity business, we can make a chair, and with this you can see that one group is up the other is in an (equatorial) meaning out, position. If the two group(s) are axial you'll have something called (1,3) di axial interactions and the ring will try to flip to get out of that conformation to a more stable one if it can. .