Hydrocarbons

🔷 Introduction to Arenes

Arenes are aromatic hydrocarbons, typically based on the benzene ring (C₆H₆). Benzene and its derivatives exhibit unique aromatic stability due to a delocalised π-electron system over the six carbon atoms.

• Structure of Benzene:
  • Planar, hexagonal ring.
  • Each carbon is sp² hybridised.
  • All C–C bond lengths are equal (~0.139 nm).
  • Delocalised electrons form a π-system above and below the ring.
  • Aromatic stability makes benzene less reactive than alkenes toward addition reactions.

🔶 1. Reactions of Benzene and Methylbenzene

(a) Halogenation (Substitution) with Cl₂ or Br₂

• Conditions: Presence of AlCl₃ / AlBr₃ (Lewis acid catalyst).
• Example:
  Benzene + Cl₂ → Chlorobenzene + HCl
  Methylbenzene + Br₂ → Bromomethylbenzene (if side-chain) or Bromotoluene (ring)
• Mechanism: Electrophilic substitution (see section 2).

(b) Nitration

• Reagents: Conc. HNO₃ + H₂SO₄
• Temperature: 25–60 °C
• Reaction:
  Benzene + HNO₃ → Nitrobenzene + H₂O
  Methylbenzene → Nitrotoluene (mixture of ortho and para isomers)
• Electrophile: NO₂⁺ (nitronium ion) generated in situ.

(c) Friedel–Crafts Alkylation

• Reagents: CH₃Cl + AlCl₃, heat
• Reaction:
  Benzene + CH₃Cl → Methylbenzene (Toluene)
• Mechanism: Electrophilic substitution with CH₃⁺ as the electrophile.

(d) Friedel–Crafts Acylation

• Reagents: CH₃COCl + AlCl₃, heat
• Reaction:
  Benzene + CH₃COCl → Phenyl ethanone (Acetophenone)
• Electrophile: CH₃CO⁺

(e) Complete Oxidation of Side Chain

• Reagents: Hot alkaline KMnO₄, then dilute HCl
• Reaction:
  Any side chain (e.g., methyl, ethyl) → –COOH group
  Methylbenzene → Benzoic acid

(f) Hydrogenation of the Benzene Ring

• Reagents: H₂ + Ni/Pt, heat
• Reaction:
  Benzene → Cyclohexane
• Requires high temperature and pressure due to aromatic stability.

🔶 2. Mechanism of Electrophilic Substitution

(a) Example: Nitration and Bromination

• Step 1: Generation of the electrophile (e.g., NO₂⁺, Br⁺)
• Step 2: Attack by benzene’s π electrons on the electrophile
• Step 3: Formation of a carbocation intermediate (resonance-stabilised)
• Step 4: Loss of H⁺, restoring aromaticity

(b) Why Substitution Over Addition?

• Addition would disrupt the aromatic π-system and lose delocalisation energy.
• Substitution allows retention of aromatic stability.

🔶 3. Prediction of Halogenation Site

• Side-chain halogenation: Occurs in UV light (free-radical substitution)
  • e.g., Methylbenzene + Cl₂ (UV) → Benzyl chloride
• Ring halogenation: Occurs with AlCl₃/AlBr₃ in dark
  • Electrophilic substitution on aromatic ring

Key:

• Light → Side-chain
• Catalyst → Aromatic ring

🔶 4. Directive Effects of Substituents on Aromatic Ring

Substituents on a benzene ring direct further substitution to specific positions.

• Electron-donating groups (EDG): Activate ring → direct ortho/para
• Electron-withdrawing groups (EWG): Deactivate ring → direct meta

🔷 Summary Table of Key Reactions

🔍 Additional Notes

• Stability of Benzene: Explained by resonance hybrid model.
• Benzene is less reactive than alkenes due to delocalisation energy (~150 kJ/mol).
• Arenes undergo electrophilic substitution, not addition, to preserve aromaticity.

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