There is a cloud of pi-electrons above and below the plane of benzene molecule. It is these pi-electrons that are responsible for electrophilic substitution reactions of benzene. In these types of reactions, an electrophile attacks the pi-system of benzene to yield a delocalized carboncation known as arenium ion or a u -complex. The electrophile does this by taking two electrons of the pi-system to form a u -bond between it and one carbon of benzene ring. This breaks the cyclic system of pi-electrons, because when arenium ion is formed, one carbon (the carbon that is attacked by electrophile) becomes sp3-hybridized. This causes instability and to overcome this instability the arenium ion loses a proton from the carbon that bears the electrophile. The loss of proton results in the regeneration of double bond which restores the stability of the ring and formation of substitution product.