Electrophilic Substitution
(of Benzene)

Full Notes

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 with electrophiles by electrophilic substitution. There are two key reactions and mechanisms you need to know.

Nitration of Benzene

Reagents: Concentrated HNO₃ (Nitric Acid) + H₂SO₄ (Sulfuric Acid Catalyst)

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

Mechanism:

Note that the H⁺ and HSO₄⁻ can recombine, forming H₂SO₄. This is why the H₂SO₄ is a catalyst.

Friedel-Crafts Acylation

Reagent: Acyl chloride (RCOCl)

Catalyst: AlCl₃ (Aluminium Chloride, halogen carrier). The catalyst is needed to generate the acylium ion electrophile.

Electrophile Formation:

Mechanism:

Note that the H⁺ and [AlCl₄]⁻ can react, reforming AlCl₃ and producing HCl. This is why the AlCl₃ is a catalyst.

Summary

• Benzene reacts with electrophiles by electrophilic substitution due to its high π-electron density.
• Nitration uses concentrated HNO₃ and H₂SO₄; the electrophile is NO₂⁺.
• Friedel–Crafts acylation uses an acyl chloride with AlCl₃ to generate the acylium ion.
• In both mechanisms, the aromatic ring is temporarily disrupted then restored, substituting H for the electrophile.
• The acid catalysts are regenerated (HSO₄⁻/H₂SO₄ in nitration; AlCl₃ in acylation).