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Aromatic Chemistry Chapter 16: Electrophilic Aromatic Substitution

Chapter 16: Electrophilic Aromatic Substitution

Preserving Aromaticity

Estimated reading time: 1 min

In this chapter

Introduction

Unlike ordinary alkenes, aromatic compounds generally avoid addition reactions.

Addition would destroy aromatic stabilization.

Instead, aromatic compounds typically undergo substitution reactions that preserve the aromatic ring.

These reactions are collectively known as electrophilic aromatic substitution.

The General Pattern

Electrophilic aromatic substitution follows a recurring sequence:

  1. Formation of an electrophile.
  2. Attack by the aromatic ring.
  3. Temporary loss of aromaticity.
  4. Restoration of aromatic stabilization.

Preservation of aromaticity is the driving theme behind these reactions.

Figure 16.1. The arenium-ion (Wheland intermediate) mechanism: an electrophile (E⁺) attacks the ring, temporarily breaking aromaticity and delocalizing the positive charge over three carbons, then loss of H⁺ restores the aromatic ring with E in place of one H. ("Arenium ion mechanism" by Su-no-G, public domain, via Wikimedia Commons.)
Figure 16.1. The arenium-ion (Wheland intermediate) mechanism: an electrophile (E⁺) attacks the ring, temporarily breaking aromaticity and delocalizing the positive charge over three carbons, then loss of H⁺ restores the aromatic ring with E in place of one H. ("Arenium ion mechanism" by Su-no-G, public domain, via Wikimedia Commons.)

Common Reactions

Halogenation

Introduction of halogens.

Nitration

Introduction of nitro groups.

Sulfonation

Introduction of sulfonic acids.

Friedel-Crafts Reactions

Formation of carbon-carbon bonds.

Thinking About Aromatic Reactions

Helpful questions include:

  • Which species is the electrophile?
  • How is aromaticity temporarily disrupted?
  • How is aromaticity restored?

Self-Assessment

I can:

☐ Explain why aromatic rings undergo substitution rather than addition.

☐ Walk through the general EAS mechanism: electrophile formation, ring attack, temporary loss and restoration of aromaticity.

☐ Identify halogenation, nitration, sulfonation, and Friedel-Crafts reactions as EAS.

☐ Recognize that identifying the electrophile is the first step in any EAS problem.

Looking Ahead

Not all aromatic compounds react identically.

Substituents already attached to the ring influence both reactivity and orientation.

Common Mistake — Treating Aromatic Reactions Like Alkene Reactions

Better approach: Recognize the importance of preserving aromatic stabilization.

Common Mistake — Forgetting to Identify the Electrophile First

Better approach: Electrophilic aromatic substitution begins with formation of an activated electrophile, which is often generated in a preparatory step from the reagent. The identity and source of that electrophile determines what type of EAS reaction is occurring — halogenation, nitration, sulfonation, or Friedel-Crafts.