Arenes: structure and aromatic stability

· Arenes = aromatic hydrocarbons containing a benzene ring.

· Benzene, C₆H₆, is planar, cyclic, and each carbon is sp² hybridised.

· Each carbon forms 3 σ bonds: two C–C σ bonds and one C–H σ bond.

· The remaining p orbital on each carbon overlaps sideways to form a delocalised π system above and below the ring.

· The 6 π electrons are delocalised around the whole ring, giving aromatic stabilisation.

· Benzene is usually drawn as a hexagon with a circle to show delocalised electrons, not alternating single and double bonds.

· All C–C bonds in benzene are equal length, intermediate between C–C single and C=C double bonds.

Benzene’s six p orbitals overlap to form a continuous delocalised π electron cloud above and below the ring. This explains why benzene is unusually stable and why it reacts mainly by substitution, not addition. Source

Why arenes undergo substitution rather than addition

· Benzene reacts mainly by electrophilic substitution.

· Addition reactions would destroy the delocalised π system and reduce aromatic stabilisation.

· In substitution, one H atom is replaced by another group, but the aromatic ring is restored at the end.

· Exam phrase: benzene resists addition because addition would break the delocalised π system and lose aromatic stability.

The energy diagram shows that electrophilic substitution allows benzene to regain aromatic stability. This supports the key exam explanation that benzene favours substitution over addition. Source

Electrophilic substitution mechanism in arenes

· General mechanism: electrophile generated → benzene attacks electrophile → σ complex forms → H⁺ lost → aromaticity restored.

· Step 1: an electrophile is formed using a catalyst or acid mixture.

· Step 2: the π electrons of benzene attack the electrophile, forming a C–E bond and a positively charged intermediate.

· Step 3: a base removes H⁺, restoring the delocalised π system.

· Curly arrow rule: arrow starts from the π electron cloud or bond/lone pair, showing movement of an electron pair.

· Key intermediate = arenium ion / σ complex, where aromaticity is temporarily lost.

Nitration of benzene

· Reagents: concentrated HNO₃ + concentrated H₂SO₄.

· Conditions: 25–60°C.

· Product: nitrobenzene, C₆H₅NO₂.

· Overall equation: C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O.

· Electrophile: NO₂⁺, called the nitronium ion.

· Formation of electrophile: HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻.

· Mechanism type: electrophilic substitution.

This diagram shows nitration as an electrophilic substitution reaction. The nitronium ion attacks the benzene ring, followed by loss of H⁺ to restore aromaticity. Source

Halogenation of benzene and methylbenzene

· Reagents: Cl₂ or Br₂.

· Catalyst: AlCl₃ or AlBr₃.

· Products: halogenoarenes / aryl halides.

· Benzene + Br₂, AlBr₃ → bromobenzene + HBr.

· Benzene + Cl₂, AlCl₃ → chlorobenzene + HCl.

· Electrophile in bromination: Br⁺.

· Catalyst role: polarises Br₂/Cl₂ and helps generate a stronger electrophile.

· Mechanism type: electrophilic substitution.

Friedel–Crafts reactions

· Friedel–Crafts alkylation adds an alkyl group to a benzene ring.

· Reagents: CH₃Cl + AlCl₃.

· Conditions: heat.

· Product from benzene: methylbenzene.

· Electrophile: CH₃⁺ or a carbocation-like species generated by AlCl₃.

· Friedel–Crafts acylation adds an acyl group, RCO–, to a benzene ring.

· Reagents: CH₃COCl + AlCl₃.

· Conditions: heat.

· Product from benzene: phenylethanone, C₆H₅COCH₃.

· Electrophile: acylium ion, CH₃CO⁺.

· Both reactions are electrophilic substitution reactions.

Oxidation and hydrogenation of arenes

· Methylbenzene can be oxidised at the side-chain.

· Reagents for side-chain oxidation: hot alkaline KMnO₄, then dilute acid.

· Product: benzoic acid, C₆H₅COOH.

· Key point: the alkyl side-chain is oxidised, but the benzene ring remains intact.

· General outcome: alkylbenzene side-chain → –COOH.

· Benzene ring hydrogenation requires strong conditions.

· Reagents: H₂ + Pt/Ni catalyst + heat.

· Product: cyclohexane ring.

· Hydrogenation is an addition reaction, but requires harsher conditions because it disrupts aromatic stabilisation.

Ring vs side-chain halogenation in methylbenzene

· Cl₂/Br₂ + AlCl₃/AlBr₃ → substitution in the aromatic ring.

· Ring halogenation gives mainly 2- and 4-substituted products for methylbenzene because –CH₃ directs to 2- and 4-positions.

· Cl₂/Br₂ + UV light → substitution in the side-chain.

· Side-chain halogenation is free-radical substitution of a hydrogen on the methyl group.

· Exam shortcut: Lewis acid catalyst = ring substitution; UV light = side-chain substitution.

Directing effects of substituents

· In electrophilic substitution, existing groups on the benzene ring affect where the next substituent goes.

· –NH₂, –OH and –R direct new substituents to the 2- and 4-positions.

· –NO₂, –COOH and –COR direct new substituents to the 3-position.

· 2-position = adjacent to the substituent.

· 3-position = one carbon further away.

· 4-position = opposite the substituent.

· For CIE, directing effects are limited to –NH₂, –OH, –R, –NO₂, –COOH and –COR.

Key reaction summary

· Benzene + Br₂/AlBr₃ → bromobenzene.

· Benzene + Cl₂/AlCl₃ → chlorobenzene.

· Benzene + conc. HNO₃/conc. H₂SO₄, 25–60°C → nitrobenzene.

· Benzene + CH₃Cl/AlCl₃, heat → methylbenzene.

· Benzene + CH₃COCl/AlCl₃, heat → phenylethanone.

· Methylbenzene + hot alkaline KMnO₄, then dilute acid → benzoic acid.

· Benzene + H₂/Pt or Ni, heat → cyclohexane.

Common exam traps

· Do not say benzene has three C=C bonds; say it has a delocalised π system.

· Do not forget the catalyst for halogenation: AlCl₃ or AlBr₃.

· Do not use aqueous bromine as the standard benzene halogenation reagent; benzene needs a halogen carrier catalyst.

· Do not confuse ring substitution with side-chain substitution in methylbenzene.

· Do not write nitration conditions above 60°C for this syllabus point.

· Do not say electrophilic substitution permanently destroys aromaticity; aromaticity is restored after loss of H⁺.