Question

Normally no catalyst is required in the electrophilic addition reaction of alkenes, But benzene requires a catalyst. Explain.

Answer 1

In benzene the there 6 $\pi$ electrons which are delocalized over the entire ring of the benzene.

While in alkene the $\pi$ electrons are delocalized over only two carbon atoms. Thus, benzene has a lower charge density than alkene.

So no electrophilic addition reaction can take place between benzene and bromine.

Thus benzene requires a catalyst for the electrophilic addition reaction.