Electrophilic substitution of benzeneEdexcel International A Level Chemistry: Revision notes

Benzene has a ring of delocalised π electrons above and below the plane, so it is attacked by electrophiles (species that accept a pair of electrons). Addition would destroy the delocalised ring and lose its stability, so benzene reacts by electrophilic substitution: one hydrogen atom is replaced by another atom or group and the ring is kept.

All the electrophilic substitution reactions of benzene follow the same steps, using E⁺ for the electrophile:

1. A pair of electrons from the delocalised ring forms a bond to E⁺ (curly arrow from the ring to E⁺).
2. A positively charged intermediate forms. Delocalisation is partly broken and the carbon attacked has both H and E attached.
3. The pair of electrons in the C–H bond returns to the ring (curly arrow from the C–H bond into the ring), releasing H⁺ and restoring the delocalised ring.
4. H⁺ reacts with the negative ion from the catalyst to regenerate it.

Benzene burns in air with a smoky flame: C₆H₆ + 7½O₂ → 6CO₂ + 3H₂O. Its high proportion of carbon means combustion in air is incomplete and glowing carbon (soot) particles form.

Bromination uses Br₂ with anhydrous FeBr₃ (or AlBr₃) as catalyst. The delocalised ring cannot polarise Br₂ enough, so the catalyst generates a stronger electrophile:

Br₂ + FeBr₃ → Br⁺ + FeBr₄⁻

C₆H₆ + Br₂ → C₆H₅Br + HBr

The catalyst is regenerated: H⁺ + FeBr₄⁻ → HBr + FeBr₃.

Benzene reacts with concentrated nitric acid and concentrated sulfuric acid at about 50 °C to give nitrobenzene. Sulfuric acid, the stronger acid, protonates nitric acid to form the nitronium ion:

HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻

C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O

The temperature is kept at or below 55 °C because at higher temperatures dinitrobenzene forms.

Benzene reacts with fuming sulfuric acid (concentrated sulfuric acid containing dissolved SO₃) to form benzenesulfonic acid:

C₆H₆ + SO₃ → C₆H₅SO₃H

SO₃ is the electrophile, because its sulfur atom is strongly δ+.

Both reactions need anhydrous aluminium chloride, a halogen carrier that forms the electrophile. Water must be excluded because it reacts with AlCl₃.

Alkylation with a halogenoalkane gives an alkylbenzene:

CH₃Cl + AlCl₃ → CH₃⁺ + AlCl₄⁻

C₆H₆ + CH₃Cl → C₆H₅CH₃ + HCl

Acylation with an acyl chloride, under reflux, gives a phenyl ketone:

CH₃COCl + AlCl₃ → CH₃CO⁺ + AlCl₄⁻

C₆H₆ + CH₃COCl → C₆H₅COCH₃ + HCl

In both, H⁺ + AlCl₄⁻ → HCl + AlCl₃ regenerates the catalyst.