Evidence for Delocalisation - Enthalpy of Hydrogenation

Enthalpy changes of hydrogenation give strong numerical evidence that benzene is more stable than the Kekulé structure.Cyclohexene has one C=C bond. Hydrogenating it to cyclohexane releases 120 kJ per mole:C₆H₁₀ + H₂ → C₆H₁₂    ΔH = −120 kJ mol⁻¹Prediction. If benzene were cyclohexa-1,3,5-triene, with three separate C=C bonds, hydrogenating it should release three times as much energy:ΔH (predicted) = 3 × (−120) = −360 kJ mol⁻¹Measurement. The real value for benzene is much smaller:C₆H₆ + 3H₂ → C₆H₁₂    ΔH = −208 kJ mol⁻¹Worked example: both reactions end at the same product, cyclohexane, so the difference shows how much lower in energy benzene is than the Kekulé structure: 360 − 208 = 152 kJ mol⁻¹.Benzene is therefore 152 kJ mol⁻¹ more stable than a molecule with three separate double bonds. This extra stability is called the delocalisation energy (or stabilisation energy). It comes from spreading the six p electrons around the whole ring.This also explains why benzene resists addition reactions: addition would destroy the delocalised system, and the 152 kJ mol⁻¹ of stabilisation would be lost.