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

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.

AnalogyThink 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.

DefinitionAromaticityA 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:

1. Generation of Electrophile
2. Formation of Carbocation Intermediate
3. 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).

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

TipRemember 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 MistakeForgetting 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.

ExampleNitrobenzene 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

AnalogyThink 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.