Benzene and Resonance HL Only

Full Notes:

Structure of Benzene

Benzene is an unsaturated hydrocarbon with the formula C₆H₆ and a planar hexagonal structure. It was originally described using Kekulé structures:

However, experimental evidence shows that all C–C bonds in benzene are identical in length. Bond length is intermediate between a single and double bond (≈ 0.139 nm). This means benzene’s structure cannot be represented accurately by alternating single and double bonds as in the theoretical Kekulé model.

The actual structure is better described as a resonance hybrid, shown as a hexagon with a ring in the middle representing delocalised π electrons:

Resonance in Benzene

Benzene’s delocalised system of π electrons is formed by sideways overlap of unbonded p orbitals from each carbon atom.

These six π electrons form a continuous cloud above and below the carbon ring, leading to uniform bond lengths and a more stable structure.

This delocalisation makes all C–C bonds equivalent and explains benzene’s unusual stability (resonance energy).

Physical Evidence Supporting Resonance

• Bond length:
  All C–C bonds in benzene are the same length (≈ 0.139 nm), confirmed by X-ray diffraction.
• Stability:
  Benzene is more stable than expected for a molecule with three C=C bonds.
• Planarity:
  All atoms lie in the same plane, allowing effective p orbital overlap.

Enthalpies of hydrogenation show benzene is more stable than expected, especially when compared to the theoretical cyclohexa-1,3,5-triene structure. Less energy is released when benzene is hydrogenated to cyclohexane than predicted for Kekulé benzene.

• Hydrogenation of cyclohexene (one C=C bond): ΔH = –120 kJ mol⁻¹
• Expected hydrogenation of Kekulé model (three C=C bonds): ΔH = –360 kJ mol⁻¹
• Actual hydrogenation of benzene: ΔH = –208 kJ mol⁻¹

This less exothermic enthalpy change for benzene indicates it is more stable than expected due to delocalisation energy.

Matt’s exam tip

Make sure you can explain why we now propose the delocalised electron model of benzene. Use the evidence of equal bond lengths, hydrogenation enthalpies, and its tendency for substitution rather than addition. Remember the theoretical Kekulé model (cyclo-1,3,5-triene) does not actually exist.

Chemical Evidence

Unlike alkenes, benzene does not readily undergo addition reactions (e.g. with bromine). Instead, it undergoes electrophilic substitution reactions:

This behaviour supports the idea of a stable, delocalised structure rather than localised double bonds.

Summary

• Benzene has a resonance hybrid structure with delocalised electrons.
• All C–C bonds are equal in length and intermediate between single and double bonds.
• Delocalisation explains benzene’s stability and unusual chemical behaviour.
• Physical and chemical evidence supports the delocalised model over Kekulé’s alternating double bond model.