5 - Substitution Reactions on Aromatic Compounds

Abstract

Substitution reactions on aromatic compounds are the most important methods for the preparation of aromatic compounds. Via substitution reactions, electrophiles and nucleophiles can be introduced into aromatics. A series of mechanisms is available for this. The electrophilic aromatic substitution via Wheland complexes, or the Ar-SE reaction, is the classical method for functionalizing aromatic compounds. Most Ar-SE reactions are irreversible because they have a sufficiently strong driving force and, at the same time, because they can be carried out under sufficiently mild conditions. The most important reversible Ar-SE reactions are tert-alkylation and sulfonylation. In the rate-determining step of an Ar-SE reaction, monosubstituted benzene and an electrophile can form three isomeric Wheland complexes. By the subsequent elimination of a proton, one Wheland complex gives the ortho-disubstituted, the second Wheland complex gives the meta-disubstituted, and the third Wheland complex gives the para-disubstituted benzene. The reactivity and the regioselectivity of Ar-SE reactions with substituted benzenes can be predicted reliably.