Electrophilic substitution of benzeneEdexcel A-Level Chemistry: Revision notes

Benzene, C₆H₆, has a ring of six carbon atoms with a delocalised π system above and below the plane of the ring. This makes benzene unusually stable and the ring is a region of high electron density, so it is attacked by electrophiles.

Because an addition reaction would destroy the delocalisation, benzene reacts by electrophilic substitution: a hydrogen atom is replaced by another group and the ring is kept.

Benzene burns in air with a smoky flame. Its carbon to hydrogen ratio is high (1 : 1), so combustion is incomplete and particles of carbon (soot) glow in the flame:

2C₆H₆ + 15O₂ → 12CO₂ + 6H₂O (complete combustion)

Every electrophilic substitution of benzene follows the same three-step pattern:

1. The electrophile is generated, usually with a catalyst or a strong acid.
2. A pair of electrons from the delocalised ring attacks the electrophile (E⁺). This forms a positively charged intermediate in which delocalisation is partly broken and the carbon attacked is bonded to both H and E.
3. A base removes H⁺ from that carbon; the electrons return to the ring, restoring delocalisation, and the catalyst is regenerated.

The curly arrow in step 2 starts from the ring, not from an atom, and in step 3 starts from the C–H bond.

Benzene reacts with chlorine or bromine only if a halogen carrier is present, such as FeBr₃, AlBr₃ or AlCl₃ (iron filings can be used, forming FeBr₃ in situ). Conditions are anhydrous and at room temperature.

Generating the electrophile:

Br₂ + FeBr₃ → Br⁺ + FeBr₄⁻ (the carrier accepts a lone pair from Br₂ and polarises it)

The Br⁺ is attacked by the ring, then FeBr₄⁻ removes H⁺:

FeBr₄⁻ + H⁺ → FeBr₃ + HBr

Overall: C₆H₆ + Br₂ → C₆H₅Br + HBr, with FeBr₃ unchanged.

Benzene is nitrated with a mixture of concentrated nitric and concentrated sulfuric acids, kept at about 50 °C.

The sulfuric acid is the stronger acid, so it protonates the nitric acid, which then loses water to form the nitronium ion, NO₂⁺ (the electrophile):

HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻

The ring attacks NO₂⁺, then HSO₄⁻ removes H⁺ and sulfuric acid is regenerated, so it acts as a catalyst:

HSO₄⁻ + H⁺ → H₂SO₄

Product: nitrobenzene, C₆H₅NO₂. Above about 50 °C further substitution gives dinitrobenzene.

A halogenoalkane or an acyl chloride reacts with benzene in the presence of anhydrous aluminium chloride, forming a new C–C bond to the ring.

Alkylation, e.g. with chloromethane:

CH₃Cl + AlCl₃ → CH₃⁺ + AlCl₄⁻, then C₆H₆ + CH₃Cl → C₆H₅CH₃ + HCl

Acylation, e.g. with ethanoyl chloride:

CH₃COCl + AlCl₃ → CH₃CO⁺ + AlCl₄⁻, then C₆H₆ + CH₃COCl → C₆H₅COCH₃ + HCl

The aluminium chloride accepts a lone pair from a chlorine atom (a Lewis acid), and is regenerated: AlCl₄⁻ + H⁺ → AlCl₃ + HCl. Water must be excluded because it reacts with both the catalyst and the acyl chloride.

Phenol reacts with bromine water at room temperature with no catalyst. The orange bromine water is decolourised and a white precipitate of 2,4,6-tribromophenol forms:

C₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBr

Why phenol is more reactive than benzene: a lone pair on the oxygen of the –OH group is delocalised into the π system. This increases the electron density of the ring, especially at positions 2, 4 and 6. The ring polarises Br₂ so strongly that no halogen carrier is needed, and three substitutions occur. Benzene has no such donation, so it needs FeBr₃ and gives only monosubstitution.