Shapes of aromatic organic molecules; σ and π bonds — practice questions

(a) The hypothetical cyclohexa-1,3,5-triene has three localised C=C bonds. Predicted enthalpy change = 3 × (enthalpy change for one C=C bond) = 3 × (-120 kJ mol⁻¹) = -360 kJ mol⁻¹.

(b) Delocalisation energy = Theoretical value - Experimental value = |-360 kJ mol⁻¹| - |-208 kJ mol⁻¹| = 360 - 208 = 152 kJ mol⁻¹. (Note: We compare the magnitude of energy released. The difference between the expected release and actual release is the stabilisation energy).

(c) The delocalisation energy of 152 kJ mol⁻¹ indicates that benzene is 152 kJ mol⁻¹ more stable than the hypothetical cyclohexa-1,3,5-triene structure. This significant extra stability is a direct result of the delocalisation of the six π electrons over the entire ring.

1. In phenol, the oxygen atom is bonded to an sp² hybridised carbon of the benzene ring. One of the lone pairs on the oxygen atom is in a p-orbital that is parallel to the p-orbitals of the ring.
2. This allows for overlap between the oxygen's p-orbital and the delocalised π system of the benzene ring.
3. The lone pair is partially delocalised into the ring, giving the C–O bond partial double bond character.
4. In cyclohexanol, the oxygen is bonded to an sp³ hybridised carbon, which has no π system. The C–O bond is a pure σ single bond.
5. A bond with partial double bond character is stronger and shorter than a pure single bond. Therefore, the C–O bond in phenol is shorter than in cyclohexanol.