Benzene undergoes electrophilic substitution because substitution restores its stable, delocalized electron system, whereas addition would permanently destroy this stabilization. Under normal conditions, benzene therefore replaces a hydrogen atom with an electrophile rather than adding atoms across a carbon bonded to carbon bond.
Why Substitution Is Favoured
Benzene contains six electrons in a delocalized π system extending around the ring. This makes benzene unusually stable, a property called aromatic stability.
Although its π electrons can attract an electrophile, benzene is less reactive than an alkene because its electron density is delocalized rather than concentrated in one localized double bond. A sufficiently strong electrophile is therefore usually generated using a catalyst.
For example, during bromination, the Lewis acid catalyst polarizes , producing a strongly electrophilic bromine species. The overall reaction is: