R3.4.13 Reactions between benzene and electrophiles (Higher Level Only)

Electrophilic Substitution Reactions in Benzene

Why Benzene Undergoes Substitution, Not Addition

Benzene is an aromatic hydrocarbon with exceptional stability due to its delocalized π-electron system.
This stability, known as aromatic stability, makes benzene resistant to addition reactions, which would disrupt its delocalized π-electron cloud.
Instead, benzene undergoes substitution reactions, where a hydrogen atom on the ring is replaced by another group, preserving its aromaticity.

Key Features of Benzene:

Delocalized π-electrons: These electrons are evenly distributed across the six carbon atoms, creating a stable ring.
Aromaticity: Benzene follows Hückel’s rule ( $4 n + 2$ π-electrons, where $n = 1$ ), which is a criterion for aromatic stability.
Resistance to Addition: Addition reactions would break the conjugation of π-electrons, destabilizing the aromatic system.

Analogy

Think of benzene as a perfectly balanced spinning top. Any attempt to add something to the top would disrupt its balance, but swapping one part for another keeps it spinning smoothly.

General Mechanism of Electrophilic Substitution in Benzene

Electrophilic substitution in benzene involves three main steps:

1. Generation of the Electrophile

The electrophile ( $E^{+}$ ) is generated outside the benzene molecule.

Example

In the nitration of benzene, the nitronium ion ( $N O_{2}^{+}$ ) is produced by reacting concentrated nitric acid ( $H N O_{3}$ ) with sulfuric acid ( $H_{2} S O_{4}$ ): $H N O_{3} + H_{2} S O_{4} ⇌ H_{2} N O_{3}^{+} + H S O_{4}^{-}$ $H_{2} N O_{3}^{+} \to N O_{2}^{+} + H_{2} O$
The nitronium ion ( $N O_{2}^{+}$ ) acts as the electrophile.

Tip

When generating the electrophile, remember that sulfuric acid acts as a catalyst by protonating nitric acid, making it easier to form $N O_{2}^{+}$ .

2. Electrophilic Attack

The delocalized π-electrons in benzene are attracted to the electrophile, donating a pair of electrons to form a new covalent bond.
This step breaks the aromaticity of the ring, creating a carbocation intermediat e(also known as an arenium ion or sigma complex).
The carbocation intermediate is stabilized by resonance, but it is less stable than the aromatic benzene ring.

3.Restoration of Aromaticity

To restore aromaticity, the carbocation intermediate loses a proton ( $H^{+}$ ) from the carbon atom bonded to the electrophile.
A base (often water or $H S O_{4}^{-}$ ) abstracts this proton, and the π-electron cloud is re-established.

Note

The loss of aromaticity during the intermediate stage is temporary. The system quickly regains stability by restoring the delocalized π-electron cloud.

Example

Nitration of Benzene

Let’s apply the general mechanism to the nitration of benzene, where benzene reacts with a nitronium ion ( $N O_{2}^{+}$ ) to form nitrobenzene ( $C_{6} H_{5} N O_{2}$ ).

Step-by-Step Mechanism:

Generation of the Electrophile:
- The nitronium ion ( $N O_{2}^{+}$ ) is produced in a mixture of concentrated nitric acid and sulfuric acid:
  $H N O_{3} + H_{2} S O_{4} \to N O_{2}^{+} + H_{2} O + H S O_{4}^{-}$
Electrophilic Attack:
- The π-electrons in benzene attack the nitronium ion, forming a bond between the carbon atom and the nitrogen atom of $N O_{2}^{+}$ .
- This creates a carbocation intermediate, disrupting the aromaticity of the ring.
- Restoration of Aromaticity:
- A proton ( $H^{+}$ ) is removed from the carbocation intermediate, restoring the aromatic π-system and yielding nitrobenzene ( $C_{6} H_{5} N O_{2}$ ).

Schematic drawing of nitration of benzene (1).

The overall reaction is:
$C_{6} H_{6} + N O_{2}^{+} \to C_{6} H_{5} N O_{2} + H^{+}$

Example

Industrial nitration of benzene is used to produce nitrobenzene, a key precursor in the manufacture of aniline, which is essential for dyes, pharmaceuticals, and explosives like TNT.

Schematic drawing of nitration of benzene (2).

Common Mistakes to Avoid

Common Mistake

Forgetting to show the incomplete dashed circle in the carbocation intermediate. This indicates the temporary loss of aromaticity.

Common Mistake

Misplacing the curly arrow when showing proton loss. The arrow must originate from the C-H bond and point toward the benzene ring.

Reflection

Self review

Why does benzene undergo substitution reactions instead of addition reactions?
What is the role of sulfuric acid in the nitration of benzene?

Theory of Knowledge

How does the use of arrows in chemistry to represent electron movement compare to their use in other disciplines, such as physics or philosophy?

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Question 1

In the electrophilic substitution mechanism of benzene, why is the restoration of aromaticity considered a crucial step?

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Note

Introduction to Electrophilic Substitution Reactions

Benzene is a unique chemical compound that undergoes a specific type of reaction called electrophilic substitution. This is because of its stable ring of electrons.
In these reactions, an electrophile (a positively charged species) replaces a hydrogen atom in the benzene ring.

Analogy

Think of benzene like a roundabout with six exits. Instead of adding a new exit (which would disrupt the roundabout), you replace one of the existing exits with a new one.

Why Benzene Prefers Substitution

Aromatic stability is the key reason benzene prefers substitution over addition.
The delocalized π-electrons form a stable ring that would be disrupted by addition reactions.
Substitution reactions preserve this aromatic stability.

Definition

Aromaticity

A property of cyclic, planar structures with delocalized π-electrons that follow Hückel's rule (4n + 2 π-electrons).

The Electrophilic Substitution Mechanism

The mechanism of electrophilic substitution in benzene can be broken down into three main steps:

Generation of Electrophile
Formation of Carbocation Intermediate
Restoration of Aromaticity

Generation of Electrophile

The electrophile is generated before it reacts with benzene.
In nitration, the electrophile is the nitronium ion (NO₂⁺).
This is formed by mixing concentrated nitric acid (HNO₃) and sulfuric acid (H₂SO₄).

$HNO_3 + H_2SO_4 \rightarrow NO_2^+ + HSO_4^- + H_2O$

Tip

Remember that sulfuric acid acts as a catalyst in this reaction.

Formation of Carbocation Intermediate

The electrophile attacks the benzene ring, forming a temporary carbocation intermediate.
This intermediate is less stable because it loses its aromaticity.
The positive charge is delocalized over the ring.

Common Mistake

Forgetting to show the delocalization of the positive charge in the carbocation intermediate.

Restoration of Aromaticity

The final step is the loss of a proton (H⁺) from the carbocation intermediate.
This restores the aromatic π-electron system.
A base (e.g., HSO₄⁻) usually removes the proton.

Example: Nitration of Benzene

Overall Reaction

$C_6H_6 + HNO_3 \rightarrow C_6H_5NO_2 + H_2O$

The product is nitrobenzene (C₆H₅NO₂).
This reaction is highly important in industrial chemistry.

Example

Nitrobenzene is used in the production of aniline, which is a precursor for dyes and pharmaceuticals.

Other Electrophilic Substitution Reactions

Halogenation: Benzene + Cl₂/FeCl₃ → Chlorobenzene
Friedel-Crafts Alkylation: Benzene + RCl/AlCl₃ → Alkylbenzene
Friedel-Crafts Acylation: Benzene + RCOCl/AlCl₃ → Acylbenzene

Analogy

Think of these reactions like swapping different toppings on a pizza base (the benzene ring) while keeping the base intact.

Applications of Electrophilic Substitution

Pharmaceuticals: Many drugs contain aromatic rings with substituted groups.
Dyes: Azo dyes are synthesized through nitration and reduction of benzene derivatives.
Explosives: TNT is made through multiple nitration steps of toluene.

The discovery of electrophilic substitution reactions in benzene led to the development of synthetic dyes, revolutionizing the textile industry.

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