Question

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?

Answer 1

The difference in reactivity between phenol and toluene with bromine and water or bromine and Fe is due to the presence of the -OH group on phenol, which acts as an activating group, making the ring more nucleophilic. This allows bromine to substitute the -SO3H group in phenol. In the case of toluene, which does not have an -OH group directly attached to the ring, the substitution reaction does not occur.

Step-by-step explanation:

The reason why (bromine + water) is able to replace the -SO3H group attached to phenol but (bromine + Fe) cannot replace the same group attached to toluene is due to the differences in the reactivity and stability of the aromatic compounds involved.

In the case of phenol, the -OH group attached to the benzene ring acts as an activating group, making the ring more nucleophilic and facilitating the substitution reaction. On the other hand, toluene does not have an -OH group directly attached to the ring, so it is less reactive and the substitution reaction does not occur.