Arenes: Benzene & its Reactions (A2)

The structure of benzene

Benzene $\text{C}_6\text{H}_6$ is a planar ring of six carbons. Each carbon uses three electrons in $\sigma$ bonds; the remaining p electron on each carbon overlaps sideways to form a delocalised $\pi$ system — a ring of electron density above and below the plane. Evidence for delocalisation (rather than alternating double bonds, the Kekulé model):

all C–C bonds are equal length (0.139 nm, between single and double);
benzene is more stable than expected — the enthalpy of hydrogenation is less exothermic than 3× that of cyclohexene (the delocalisation/resonance energy, ~150 kJ mol⁻¹);
it resists addition and does not decolourise bromine water without a catalyst.