In the context of EAS, the formation of an arenium ion occurs when an electrophile attacks the π-electrons of an aromatic ring. This interaction forms a sigma complex, also known as a Wheland intermediate or arenium ion. For example, in the nitration of benzene, a nitronium ion (NO2+) acts as the electrophile and attacks the benzene ring, leading to the formation of a nitro-arenium ion.
