Electrophilic aromatic substitution

Jul 25, 2022, 16:45 IST

About Electrophilic aromatic substitution

pi and sigma complexes: It might be expected that the first phase of reaction would be interaction between the approaching electrophile and delocalised p orbitals leading to p 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 p 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 p electrons of arene are utilized to form a s bond between a particular C atom of the ring and the electrophile, known as s complex (also called Wheland intermediate).

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