. Discuss the Mechanism for Electrophilic Aromatic Substitution
Pie and sigma complexes: It might be expected that the first phase of reaction would be interaction between the approaching electrophile and delocalised pie orbitals leading to pie complexes (a weakly bonded charge-transfer complex which exists in solution only and is formed by the association of an electrophilic species (E+) and one electron-donating species) e.g. toluene form a 1:1 complex with HCl at –78°C, the reaction being readily reversible. DCl also forms pie complex with toluene. This complex on decomposition does not form deuterium substituted toluene. Formation of complex leads to a solution that is a non-conductor of electricity.
When an electrophile reacts with an arene usually in presence of a catalyst, a salt is formed. This salt is composed of an anioin and a complex, resonance stabilized carbenium ion (arenium ion) in which only two of the total pie electrons of arene are utilized to form a sigma bond between a particular C atom of the ring and the electrophile, known as sigma complex (also called Wheland intermediate).