5.4 Formation of Carbon-Halogen Bonds and Other Reactions
5.4.1 Metallation
Aromatic compounds react with divalent mercury salt under dry and air free conditions to give mercury derivatives which find useful applications. For example, the synthesis of o -iodophenol can be conveniently accomplished via mercuration of phenol with Hg(OAc)2 followed by transformation of o-acetoxymercury derivative into o -chloromercury derivative and treatment with iodine.
Similarly, the transformation of furan to 2-bromo- and 2-iodofuran can be accomplished.
Thallium(III) salts contains similar applications. For example, the reaction of p -xylene with thallium(III) trifluoroacetate gives the thallium derivative that, with KI at 0°C, provides the iodo-compound in good yield.
5.4.2 Displacement of Groups
There are electrophilic substitutions in which atoms and groups other than hydrogen are displaced from the aromatic ring ( ipso- substitution). Some examples follow:
5.4.2.1 Decarboxylation
In aromatic compounds, activating substituents lead to displacement of carboxyl group. For example, 2,4,6-trihydroxybenzoic acid on heating leads to decarboxylation with an internal electrophilic substitution by hydrogen.
5.4.2.2 Desulfonation
Acids displace sulfonic acid group from aromatic rings. For example, phenol-2,4-disulfonic acid could be converted into picric acid in the presence of nitric acid.
5.4.2.3 Dehalogenation
Dehalogenation takes place when the halogen atom is adjacent to two very bulky substituents. For example, 2,4,6- tri-t -butylbromobenzene undergoes debromination with strong acid.
