Removable substituents:

It is occasionally useful to have a substituent present that can direct or block a specific substitution, and that can then be removed once the desired substituents have been added. The reactions that are shown in figure are employed to remove substituents from aromatic rings.

Figure: Reactions which remove substituents from aromatic rings.

An instance of how removable substituents can be employed is in the synthesis of 1,3,5-tribromobenzene. This structure cannot be made straight from benzene through bromination. The bromine atoms are in the meta positions with to each other, although bromine atoms direct ortho/para. Furthermore, bromine is a deactivating group and thus it would be hard to introduce three such types of groups directly to benzene.