Chapter 17: Aromatic Compounds

The fundamental mechanism of electrophilic aromatic substitution involves formation of a carbocation intermediate known as the sigma complex or Wheland intermediate, which then loses a proton to restore aromaticity and complete the transformation. The chapter systematically covers six major classes of electrophilic aromatic substitution reactions: halogenation with halogens and Lewis acids, nitration using nitric acid and sulfuric acid, sulfonation with fuming sulfuric acid, Friedel-Crafts alkylation catalyzed by Lewis acids, and Friedel-Crafts acylation producing aromatic ketones. Each reaction receives detailed mechanistic analysis including how the electrophile is generated and how the reaction coordinate proceeds. A critical conceptual framework presented throughout involves directing effects and reactivity effects of substituents already present on the aromatic ring. Electron-donating groups function as activating groups that accelerate substitution and direct new substituents toward ortho and para positions through resonance stabilization of the resulting intermediate. Conversely, electron-withdrawing groups deactivate the ring and direct incoming electrophiles toward meta positions because these positions avoid placing positive charge on atoms bearing the withdrawing group. The chapter employs resonance and inductive effect analysis to predict both the rate of reaction and the regiochemical outcome for polysubstituted aromatic compounds. Understanding these directing and activating properties enables prediction of product distributions in complex molecules. The material also briefly addresses nucleophilic aromatic substitution as an alternative mechanism occurring under specialized conditions, particularly when strong electron-withdrawing groups activate the aromatic ring toward nucleophilic attack. Practical applications in pharmaceutical synthesis and synthetic dye production demonstrate how these principles guide real-world organic synthesis.