Aromatic Compounds – Benzene & Its Derivatives
(KEAM – Chemistry – Competitive Exam Focus)
Why this topic?
• All EAS reactions are central to organic synthesis.
• Many exam questions test subtle reactivity differences between benzene, hetero‑aromatics, and polycyclic systems.
• Mastery of aromaticity concepts and substitution patterns reduces conceptual mistakes in derivations and mechanism‐based questions.
1. Aromaticity – The Concept
| Feature | Requirement | Typical Value |
|---|---|---|
| Planarity | 3‑D planar ring | ✓ |
| Delocalised π‑system | Conjugated, cyclic | ✓ |
| Hückel’s rule | 4n+2 π‑electrons (n = 0,1,2…) | ✓ |
| Stabilisation | Extra stability vs. non‑aromatic analogues | ~1 kcal mol⁻¹ per aromatic π‑electron pair |
Hückel’s Rule Derivation (MO Theory)
- For a cyclic conjugated system of N atoms, the allowed π‑MO energies are
[ \varepsilon_k = \alpha + 2\beta\cos\frac{k\pi}{N+1}\quad (k=1\ldots N) ]- For N = 6 (benzene), the lowest 3 orbitals are filled (3 pairs) → 6 π‑electrons.
- The sum of the occupied orbital energies is lower by ~36 kcal mol⁻¹ than the sum for the corresponding cyclohexatriene (non‑aromatic), giving the aromatic stabilization energy.
Clar’s Rule (Polycyclic Aromatic Hydrocarbons)
The most stable resonance structure contains the maximum number of disjoint aromatic sextets.
Use it to predict substitution patterns on naphthalene, anthracene, phenanthrene, etc.
2. Benzene – Structure & Properties
| Property | Value / Observation |
|---|---|
| Formula | C₆H₆ |
| π‑electrons | 6 (4n+2, n = 1) |
| Bond length | 1.39 Å (equal C–C) |
| ΔEₐ (stabilisation) | ≈ 36 kcal mol⁻¹ |
| Ionisation Potential | 9.24 eV (≈ 225 kcal mol⁻¹) |
| Electron Affinity | 0.37 eV (≈ 8.7 kcal mol⁻¹) |
| Reactivity | Electrophilic aromatic substitution (EAS); radical halogenation; reduction (Birch, Birch‑type) |
Key Tricky Point:
Benzene does not undergo SN2 substitutions; electrophilic attack proceeds via a σ‑complex (arenium ion) with partial positive charge delocalised over the ring.
3. Electrophilic Aromatic Substitution (EAS)
| Reaction | Mechanism (Key Steps) | Typical Conditions | Key Observations |
|---|---|---|---|
| Halogenation (Cl₂, Br₂) | 1. E⁺ = X₂ → X⁺ + X⁻ 2. σ‑complex 3. Deprotonation |
H₂O (for Cl₂), |