🟣 This is Higher Level (HL) content.
Kekulé Model vs Reality
Kekulé proposed benzene as a cyclohexatriene with alternating C–C and C=C bonds. However, three lines of evidence utterly refute this model:
| Evidence Type | What Kekulé Predicts | What We Observe |
|---|---|---|
| Physical (X-ray) | Alternating bond lengths (154 pm / 134 pm) | All 6 C–C bonds identical at 140 pm – intermediate between single and double |
| Thermodynamic | ΔH hydrogenation ≈ 3× cyclohexene | ΔH hydrogenation is significantly less exothermic – benzene is more stable than predicted (resonance energy) |
| Chemical | Should readily undergo addition reactions (like alkenes) | Overwhelmingly favours substitution – preserves the stable aromatic ring |
🔑 The Delocalised Model
Each carbon is sp² hybridised. The unhybridised p-orbital on each carbon overlaps laterally with its neighbours, creating a continuous ring of delocalised π electrons above and below the plane. This is represented as a circle inside the hexagon.
The delocalisation lowers the potential energy of the molecule, giving benzene its remarkable thermodynamic stability and its resistance to addition reactions.