Medical Chemistry Practice Test 2026 - Free Medical Chemistry Practice Questions and Study Guide

Aromatic rings favor reactions that keep the ring’s delocalized pi system intact. When an electrophile approaches, it temporarily bonds to the ring, forming a resonance-stabilized arenium ion (Wheland intermediate). This intermediate then loses a proton to restore aromaticity, yielding a substituted aromatic product. This sequence—electrophilic attack followed by deprotonation to reestablish the aromatic system—explains why electrophilic substitution is the typical pathway for aromatic compounds. Adding across the ring would disrupt aromatic stabilization and is therefore disfavored. Nucleophilic substitution is unlikely because the ring is electron-rich, and free radical substitution is not the usual route under common conditions. Classic examples include nitration, chlorination, sulfonation, and Friedel–Crafts alkylation/acylation, all proceeding via electrophilic substitution.