S2.2.12 Benzene (Higher Level only) Notes

Structure of Benzene: Resonance and Delocalization

Resonance and Delocalization in Benzene

1. Benzene is best described using resonance theory.
2. For benzene, two resonance structures exist where the double bonds shift positions around the ring.
3. However, benzene's true structure is better described as a hybrid of these forms where the electrons are delocalized across all six carbon atoms.
4. 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:

1. Bond Length Uniformity:
   1. 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.
2. Enthalpy of Hydrogenation:
   1. Hydrogenation of cyclohexene releases around $-120 \text{ kJ mol}^{-1}$.
   2. If benzene contained three double bonds, its hydrogenation would be expected to release approximately $-360 \text{ kJ mol}^{-1}.
   3. However, the actual enthalpy change is around $-208 \text{ kJ mol}^{-1}$, indicating extra stability due to delocalization.
3. Spectroscopy Data:
   1. Benzene exhibits a single peak in the proton NMR spectrum, confirming that all hydrogen atoms are equivalent due to the symmetric electron distribution.

Resonance Energy and Benzene's Relative Unreactivity

What is Resonance Energy?

Benzene’s actual energy is lower than that predicted for a hypothetical molecule with alternating single and double bonds, indicating it is more stable than expected.

How Resonance Energy Reduces Reactivity

1. In reactions involving double bonds, the π-electrons are localized and available for reaction.
2. However, in benzene, the delocalized π-electron cloud is more stable and less reactive.
3. This stability reduces benzene's tendency to undergo reactions typical of alkenes, such as electrophilic addition (covered in Structure 3.4).