Are there any specific requirements for ipso substitution in aromatic compounds?

In one of the questions in my textbook, (bromine + water) is able to replace the -SO3H group attached to phenol via ipso substitution, but in another question, (bromine + Fe) cannot replace the same group when attached to toluene.


Why does this happen? Is it because the water causes greater activation of the ring in the first case by phenoxide ion formation? Or is it because -OH is a better activating group in general?




In each case you have an electron donating (Me/OH) and withdrawing (SO3H) group playing a tug-of-war in terms of activating or deactivating the ring towards EAS. A methyl is **significantly** less EAS activating than hydroxyl, which can formally donate electron density (anion) into the ring through resonance. Methyl's are just slightly activating by induction so toluene rarely undergoes EAS efficiently (personal experience) without forcing conditions, and this is without a deactivating group making it worse.


The introduction of iron would generate Fe(iii) bromide, as the other commenters have said. This species is usually great for helping out EAS brominations on the electrophile's side (increases polarization of remaining Br2). HOWEVER, even this great electrophile setup cannot overcome how poor the "nucleophile" is, being a weak tolule to start off with, further deactivated by the sulfonic acid.


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