What evidence is there for delocalisation in benzene?
X-ray diffraction shows all carbon-carbon bonds in benzene are the same length (140 pm), between the lengths of a single (147 pm) and double bond (135 pm). This suggests that Kekule's alternating single and double bond model cannot be right as the bonds in the molecule are all the same.
Further evidence for the delocalisation model comes from enthalpy change data. The enthalpy of hydrogenation for cyclohexene, containing one double bond, is -120 kJmol^-1. If Kekule's model was right then we would expect the enthalpy of hydrogenation for benzene to be three times this, -360 kJmol^-1. However, the actual enthalpy of hydrogenation for benzene is -208 kJmol^-1, far less than expected. This lower enthalpy value is due to the delocalised ring of electrons in which electron density is spread over more atoms, lowering the energy of the molecule making it more stable.
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