IB Chemistry›Structure 2›2.2›2.2.12
2.2.12
Structure 2.2 HL Extension ⏱ ~4 min revision

Benzene

Kekulé structure vs delocalised model, bond length symmetry, resonance energy, and chemical reactivity.

HL Extension

Benzene Delocalisation & Extra Stability

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:

Benzene: Kekulé Model vs Delocalised Structure

Benzene: Kekulé vs Delocalised Model THEORETICAL MODEL Kekulé Structure Cyclohexa-1,3,5-triene (Alternating Bonds) C=C (134 pm) C-C (154 pm) ✕ Alternating short (134 pm) & long (154 pm) bonds ✕ Predicts rapid alkene addition reactions ✕ Missing ~152 kJ/mol of extra aromatic stability EVIDENCE REFUTES ACCEPTED MODEL Delocalised Ring Model Symmetrical Planar Hexagon with π Ring All 6 C-C: 140 pm Bond order = 1.5 ✓ All 6 C-C bonds equal at 140 pm (intermediate) ✓ Resists addition; favours electrophilic substitution ✓ Thermodynamic resonance energy: 152 kJ/mol
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
Key Model

The Delocalised Pi System in Benzene

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.

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