Electrophilic Substitution Reactions of Benzene HL Only

Specification Reference R3.4.13

Quick Notes

Benzene (C₆H₆) is highly unsaturated but does not undergo addition reactions easily.
Delocalised π electron system gives benzene extra stability (resonance energy).
Benzene reacts by electrophilic substitution.
Electrophile (E⁺) attacks the ring, replacing one hydrogen.
Example: nitration of benzene using HNO₃ and H₂SO₄, generating NO₂⁺.
In this mixture, HNO₃ acts as a base, and H₂SO₄ acts as a stronger acid.

Full Notes:

Recap - Structure and Stability of Benzene

Benzene has a hexagonal ring of 6 carbon atoms.

IB Chemistry benzene hexagonal ring structure showing 6 carbon atoms in a planar ring.

Each carbon is sp² hybridized, with one unhybridized p-orbital.

IB Chemistry benzene carbon sp2 hybridisation with unhybridized p-orbital.

These p-orbitals overlap sideways, forming a delocalized π system above and below the plane of carbon atoms.

IB Chemistry benzene delocalisation diagram showing π electron cloud above and below the ring.

The electrons are shared equally across all 6 carbon atoms. This delocalization gives benzene unusual stability, known as resonance energy.

Electrophilic Substitution of Benzene Mechanism

Unlike alkenes, benzene undergoes a substitution reaction with electrophiles rather than addition.

This is because the ring of delocalised electrons gets reformed during the mechanism.

Mechanism Overview:

IB Chemistry general mechanism of electrophilic substitution in benzene with electrophile attack, carbocation intermediate, and loss of H+.

Step 1: Electrophilic attack
Electrophile accepts a pair of electrons from π system in benzene ring.

Step 2: Elimination of a proton (H⁺)
The ring loses H⁺ to restore delocalised electrons and regain stability.

Step 3: Formation of substituted product
Substitution occurs rather than addition.

Matt’s exam tip

The general mechanism for electrophilic substitution shown above is the only one you need to know for benzene. Regardless of the electrophile being used, it follows this standard mechanism.

Example: Nitration of Benzene

Reagents: Concentrated HNO₃ + H₂SO₄ (catalyst)

IB Chemistry nitration of benzene equation showing reaction with nitric acid and sulfuric acid catalyst to form nitrobenzene.

Electrophile: Nitronium ion (NO₂⁺), generated in situ:
HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄⁻ + H₂O

Mechanism:

Note: H⁺ and HSO₄⁻ can recombine to regenerate H₂SO₄, showing it acts as a catalyst.

Summary

Benzene is aromatic with a stable delocalized π electron system.
It resists addition reactions but undergoes electrophilic substitution.
Mechanism involves electrophile attack, carbocation intermediate, and loss of H⁺ to restore aromaticity.
Nitration is a classic example, requiring NO₂⁺ formed by HNO₃ and H₂SO₄.

Linked Course Questions

Structure 2.2 — Linked Course Question

Why Doesn’t Benzene Undergo Addition Reactions Easily?

Benzene resists addition reactions because they would disrupt its stable delocalised π system. Breaking this delocalisation would cause a loss of stabilisation energy. Substitution reactions allow the ring to retain aromatic stability.

Reactivity 3.1 — Linked Course Question

How Can the Acid–Base Behaviour of HNO₃ Be Described in Benzene Nitration?

In nitration, HNO₃ acts as a Brønsted–Lowry base, while H₂SO₄ acts as a stronger acid. H₂SO₄ donates a proton to HNO₃, which then decomposes into the nitronium ion (NO₂⁺) that reacts with benzene.