Question

Why is the heat of hydrogenation of benzene less than expected?

Answer 1

Resonance

• Resonance is a way to represent delocalized electrons within molecules or polyatomic ions where the bonding cannot be represented using a single Lewis formula.
• Several resonance structures represent a molecule or ion with such delocalized electrons.
• For example resonance structure of benzene:

Heat of hydrogenation of benzene

• Heat of hydrogenation is the amount of energy released in any unsaturated system when one mole of hydrogen is added.
• Benzene has three double bonds, the heat of hydrogenation should be $-360\mathrm{kJ}{\mathrm{mol}}^{-1}$.
• Its observed hydrogenation heat is low $-208\mathrm{kJ}{\mathrm{mol}}^{-1}$.
• Heat of hydrogenation is reduced by $-152\mathrm{kJ}{\mathrm{mol}}^{-1}$, benzene is more stable than predicted.
• Benzene is stabilized by resonance.
• Thus, the heat of hydrogenation of benzene is lower than predicted.