Structure of Benzene: Resonance and Delocalization
NoteIn the previous article S2.2.11, we briefly covered the resonance structure of benzene.
Definition
Resonance
Resonance arises from the delocalization of electrons, where electrons are not confined to a single bond or atom but are spread across multiple atoms in a molecule.
Resonance and Delocalization in Benzene
- Benzene is best described using resonance theory.
- For benzene, two resonance structures exist where the double bonds shift positions around the ring.
- However, benzene's true structure is better described as a hybrid of these forms where the electrons are delocalized across all six carbon atoms.
- This means the π-electrons from the double bonds are shared equally over the entire ring, rather than being confined to specific bonds.
Experimental Evidence for Benzene's Structure
The concept of delocalization is supported by experimental data:
- Bond Length Uniformity:
- X-ray diffraction reveals all carbon-carbon bonds in benzene have the same length of approximately 0.139 nm, intermediate between typical single (0.154 nm) and double (0.134 nm) bonds.
- Enthalpy of Hydrogenation:
- Hydrogenation of cyclohexene releases around $-120 \text{ kJ mol}^{-1}$.
- If benzene contained three double bonds, its hydrogenation would be expected to release approximately $-360 \text{ kJ mol}^{-1}.
