Reactions of Aromatic Compounds: Electrophilic Aromatic Substitution

Reactions of Aromatic Compounds

Introduction

Aromatic compounds, such as benzene and its derivatives, are characterized by their stability and unique reactivity. The most important class of reactions for aromatic compounds is Electrophilic Aromatic Substitution (EAS), in which an atom, usually hydrogen, attached to the aromatic ring is replaced by an electrophile.

Electrophilic Aromatic Substitution (EAS)

Definition and Overview

• Arene (Ar-H): A generic term for an aromatic hydrocarbon.

• Aryl group (Ar): Derived by removal of a hydrogen atom from an arene.

• Electrophile: A species with a full or partial positive charge that seeks electrons.

• General Reaction: Aromatic compounds undergo EAS, where the aromatic ring acts as a nucleophile and reacts with an electrophile.

Common types of EAS reactions include:

• Halogenation

• Nitration

• Sulfonation

• Friedel-Crafts Alkylation

• Friedel-Crafts Acylation

General Mechanism for Electrophilic Aromatic Substitution

Arenium Ion Intermediates

The EAS mechanism proceeds via the formation of an arenium ion (also called a sigma complex or cyclohexadienyl cation), which is stabilized by resonance.

1. Step 1: Formation of Arenium Ion The aromatic ring uses two of its π electrons to bond with the electrophile, temporarily disrupting aromaticity. $\ce{Ar-H + E^+ -> [Ar-E-H]^+}$

2. Step 2: Restoration of Aromaticity A base removes a proton from the carbon atom bonded to the electrophile, restoring the aromatic system. $\ce{[Ar-E-H]^+ + B^- -> Ar-E + HB}$

Energy Profile:

• The first step (formation of the arenium ion) is highly endothermic and rate-determining due to the loss of aromaticity ($\Delta G^{\ddagger}$ is large).

• The second step (deprotonation) is highly exothermic and restores aromatic stabilization.

Major Electrophilic Aromatic Substitution Reactions

1. Halogenation of Benzene

Halogenation introduces a halogen atom (Cl, Br, I, F) onto the aromatic ring. The reaction requires a Lewis acid catalyst (e.g., FeCl3, FeBr3).

• Chlorination: $\ce{C6H6 + Cl2 \xrightarrow{FeCl3} C6H5Cl + HCl}$

• Bromination: $\ce{C6H6 + Br2 \xrightarrow{FeBr3} C6H5Br + HBr}$

• Fluorination: Occurs too rapidly; requires special apparatus to control.

• Iodination: Iodine is unreactive; alternative methods (e.g., oxidizing agents) are used.

Mechanism of Bromination (Example)

1. Bromine reacts with FeBr3 to generate the electrophilic bromine species ($\ce{Br^+}$).

2. The benzene ring attacks $\ce{Br^+}$, forming an arenium ion.

3. Deprotonation restores aromaticity and regenerates the FeBr3 catalyst.

2. Nitration of Benzene

Nitration introduces a nitro group ($\ce{NO2}$) onto the aromatic ring using a mixture of concentrated nitric and sulfuric acids.

• Electrophile: Nitronium ion ($\ce{NO2^+}$)

• Reaction: $\ce{C6H6 + HNO3 \xrightarrow{H2SO4} C6H5NO2 + H2O}$

1. Sulfuric acid protonates nitric acid, generating the nitronium ion.

2. Benzene attacks the nitronium ion, forming an arenium ion.

3. Deprotonation restores aromaticity, yielding nitrobenzene.

3. Sulfonation of Benzene

Sulfonation introduces a sulfonic acid group ($\ce{SO3H}$) onto the aromatic ring, typically using fuming sulfuric acid (contains $\ce{SO3}$).

• Electrophile: Sulfur trioxide ($\ce{SO3}$)

• Reaction: $\ce{C6H6 + SO3 \xrightarrow{H2SO4} C6H5SO3H}$

1. $\ce{SO3}$ is generated in situ from concentrated sulfuric acid.

2. Benzene attacks $\ce{SO3}$, forming an arenium ion.

3. Deprotonation yields benzenesulfonic acid.

Summary Table: Major EAS Reactions

Reaction	Reagents	Electrophile	Product
Halogenation	Cl2/Br2, FeCl3/FeBr3	Cl+/Br+	Halobenzene
Nitration	HNO3, H2SO4	NO2+	Nitrobenzene
Sulfonation	SO3, H2SO4	SO3	Benzenesulfonic acid

Key Points

• Aromaticity is temporarily lost during EAS but is restored in the final step.

• The rate-determining step is the formation of the arenium ion (first step).

• Different EAS reactions use different electrophiles and conditions, but all proceed via a similar two-step mechanism.

Example: Bromination of Benzene

• Step 1: $\ce{Br2 + FeBr3 -> Br^+ + FeBr4^-}$

• Step 2: $\ce{C6H6 + Br^+ -> [C6H6Br]^+}$ (arenium ion)

• Step 3: $\ce{[C6H6Br]^+ + FeBr4^- -> C6H5Br + HBr + FeBr3}$

Additional info: Further EAS reactions (Friedel-Crafts alkylation/acylation, substituent effects, and synthetic applications) are covered in subsequent sections of the chapter.