In simple terms, electrophilic aromatic substitution proceeds in two steps.
Initially, the electrophile E' adds to a carbon atom of the benzene ring
in the same manner in which it would react with an alkene, but here the
n-electron cloud is disrupted in the process. However, in the second step
the resultant carbocation eliminates a proton to regenerate the aromatic
system (Scheme 2.1). The combined processes of addition and elimination
result in overall substitution.