An electrophile replaces a substituent, commonly hydrogen, on an aromatic system. Initial bond formation can make an arenium ion; loss of a proton then restores aromaticity.
Formation of a carbon–sulfur bond by reaction of an electron-rich carbon centre with electrophilic sulfur, followed where needed by deprotonation.
An electrophile adds at an aromatic carbon already bearing a substituent; rearomatization can expel that substituent instead of the newly added proton.
Aromatic nitration replaces an aromatic hydrogen with a nitro group. In a common pathway, aromatic attack on a nitronium ion forms a Wheland intermediate and deprotonation restores aromaticity.
Aromatic oxazolination introduces an oxazolinyl group at an aromatic C–H position. Activating 2-Oxazolidinone with triflic anhydride supplies an electrophilic carbon that an electron-rich arene can attack.
Friedel-Crafts acylation attaches an acyl group to an aromatic carbon through electrophilic substitution. Lewis-acid activation of an acyl chloride is often represented by an acylium ion or strongly polarized contact pair.
An arenium ion is a positively charged intermediate in which electrophilic addition has interrupted the original aromatic delocalization. Lone-pair donors can stabilize contributors that place positive charge at adjacent positions.
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