Chemistry
Why Aromatic Electrophilic Substitution Proceeds Through a Sigma Complex Intermediate
Quick fact
The sigma complex intermediate is a high-energy carbocation that forms when an electrophile attacks an aromatic ring, temporarily disrupting its delocalized π system. The reaction proceeds through this intermediate because losing a proton afterward restores aromaticity, which provides the thermodynamic driving force that favors substitution over addition.
Why this is interesting
You might expect a benzene ring to behave like an alkene and add an electrophile across its double bonds. But it doesn't—it substitutes instead. Why?