sir,can you explain the mechanism of electrophilic substitution of benzene?
The first stage
Suppose the electrophile is a positive ion X+.
Two of the electrons in the delocalised system are attracted towards the X+ and form a bond with it. This has the effect of breaking the delocalisation, although not completely.
The second stage
A lone pair of electrons on Y- forms a bond with the hydrogen atom at the top of the ring. That means that the pair of electrons joining the hydrogen onto the ring aren't needed any more. These then move down to plug the gap in the delocalised electrons, so restoring the delocalised ring of electrons which originally gave the benzene its special stability.