Electrophilic Substitution Reactions of Benzene

Full Notes

The structure of benzene has been covered in detaile here.

Electrophiles (electron pair acceptors) are attracted to the high electron density in benzene and this means benzene reacts with electrophiles.

Benzene won’t react with nucleophiles (they would be repelled by the high electron density of benzene).

Benzene reacts by electrophilic substitution. There are a few reactions you need to know.

Bromination

• Forms bromobenzene
• Requires FeBr₃ or AlBr₃ as a catalyst to polarise the Br₂ molecule.

Nitration

• Forms nitrobenzene
• Uses a nitrating mixture of conc. HNO₃ and H₂SO₄ to form NO₂⁺ electrophile.

Friedel–Crafts Alkylation/Acylation

• Forms alkylbenzene or acylbenzene
• React with halogenoalkanes or acyl chlorides using AlCl₃.

Matt’s exam tip

The Friedel-Crafts acylation and alkylation reactions are very useful and important as they enable a carbon-carbon bond to be made. Pay close attention to this when looking at complex organic exam questions with synthesis routes and pathways.

Electrophilic Substitution Mechanism of Benzene

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, all these reactions the following standard mechanism:

Mechanism Overview:

• Step 1: Electrophilic attack — electrophile accepts a pair of electrons from the π-system in benzene.
• Step 2: Elimination of a proton (H⁺) — restores delocalised ring stability.
• Step 3: Substituted product forms — substitution rather than addition has occurred.

You need to know the mechanism for both nitration and halogenation, including the generating of the electrophile.

Nitration of Benzene Mechanism

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

Note: The H⁺ ion removed from benzene can recombine with HSO₄⁻ to form H₂SO₄. This means the H₂SO₄ can be considered as a catalyst.

Halogenation of Benzene Mechanism

Electrophile: Bromonium ion (Br⁺):
Br₂ + FeBr₃ → Br⁺ + FeBr₄⁻

Note: The Br⁺ ion is actually formed at the same time as the Br₂ bond breaks, however it is often represented in mechanisms as simple Br⁺, and this is allowed by OCR examiners.

Stability and Reactivity

Benzene does not readily undergo addition reactions, unlike alkenes. This is due to the energetic stability of the delocalised π-system, which would be lost in addition.

Predicting Substitution Mechanisms

You should be able to identify steps in unfamiliar electrophilic substitutions, including electrophile generation and the reforming of the benzene ring. Just follow the standard electrophilic substitution mechanism shown above.

Summary

• Benzene undergoes electrophilic substitution because it preserves the stable delocalised ring.
• Key reactions: nitration, halogenation, and Friedel–Crafts alkylation/acylation.
• Mechanism involves electrophile generation, electrophilic attack, and loss of H⁺.
• Nitration electrophile = NO₂⁺, halogenation electrophile = Br⁺.