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Chemistry · Glossary

What is Electrophilic aromatic substitution?

Also known as: Wheland intermediate

Definition 22.3 University Chemistry — Year 2 · Chapter 22 — Aromaticity and Electrophilic Aromatic Substitution

An electrophilic aromatic substitution replaces a hydrogen of an aromatic ring by an electrophile EX+\ce{E+}. The electrophile first bonds to one ring carbon, giving a cation in which that carbon is tetrahedral and the positive charge is spread over the other five: the Wheland intermediate (an arenium ion). A base then removes the proton from the tetrahedral carbon.

Energy profile of an electrophilic aromatic substitution, schematic (a model). The first step, which forms the Wheland intermediate, has the highest barrier. Loss of a proton (solid) restores the aromatic ring; addition of a nucleophile (dashed) would leave a product above the reactants.
Energy profile of an electrophilic aromatic substitution, schematic (a model). The first step, which forms the Wheland intermediate, has the highest barrier. Loss of a proton (solid) restores the aromatic ring; addition of a nucleophile (dashed) would leave a product above the reactants.
Nitration of benzene. Sulfuric acid protonates nitric acid, which loses water: the nitronium ion NO2+ is the electrophile. It bonds to one carbon (slow), giving the Wheland intermediate, whose positive charge is spread over the ring; loss of a proton (fast) gives nitrobenzene. Nitration of benzene. Sulfuric acid protonates nitric acid, which loses water: the nitronium ion NO2+ is the electrophile. It bonds to one carbon (slow), giving the Wheland intermediate, whose positive charge is spread over the ring; loss of a proton (fast) gives nitrobenzene.
Nitration of benzene. Sulfuric acid protonates nitric acid, which loses water: the nitronium ion NOX2X+\ce{NO2+} is the electrophile. It bonds to one carbon (slow), giving the Wheland intermediate, whose positive charge is spread over the ring; loss of a proton (fast) gives nitrobenzene.
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