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.
![320_Removable substituents.png](https://www.expertsmind.com/CMSImages/320_Removable%20substituents.png)
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.