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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?

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    $\begingroup$ I suspect this is because you have a different bromine species present. In the reaction with bromine and iron, the bromine first reacts with iron to give Fe(III) bromide which acts as a Lewis acid, giving a Br+ species as the active intermediate. $\endgroup$
    – Waylander
    Commented Dec 1, 2022 at 13:12
  • $\begingroup$ I still don't understand why it happens. In the first case, one of the double bonds in the ring (namely, the one closest to SO3H) breaks and attacks onto one of the bromine atoms in the Br2 molecule. In the second case, the same thing happens except this time, the bromine is extracted from FeBr4 anion. That doesn't explain the disparity though. $\endgroup$
    – Muskaan S
    Commented Dec 2, 2022 at 13:53

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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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