Properties of aromatic compounds

                 These are cyclic and each atom in the ring is a π- centre, uses a ƿ-atomic orbital to form π-type bonds such as sp² or sp.

                     Aromatic ring is flat or nearly so. There is high degree of unsaturation  but these are resistant to addition reaction. An electrophilic reagent replaces hydrogen (usually) attached to the ring. Aromatic compounds are usually stable with π-electrons delocalized above and below the ring. 

π-electron delocalization in benzene 

                 Each carbon in the ring is with a p atomic orbital containing one electron. These orbitals are perpendicular to the ring, but parallel to each other. There atomic orbitals are shown in the figure below. Each p orbital interacts (overlaps) with two neighbors. This gives rise to six π-type orbitals, labelled as Ψ1 to Ψ6. Ψ1 to Ψ3 are bonding orbitals; Ψ4 to Ψ6 are antibonding orbitals. It turns out that Ψ1 is the lowest energy orbital. Ψ2 and Ψ3 are degenerate, they have the same energy and it is higher than the energy of Ψ1 . The electrons in the three occupied bonding orbitals give rise to one electron. This aromatic electronic delocalization results in considerable stabilization more than is observed in the case conjugated aliphatic compounds. 

                Because of the π-electrons, benzene and other aromatic compounds, frequently act as Lewis bases, or nucleophiles, thus they are susceptible to electrophilic attack. Because of the stability associated with the delocalozed electrons, this feature tends to be retained in the products. Consequently these reaction are substitutions, not additions.