HL Only
2.2.12 Benzene (C₆H₆)
Chemical evidence for the delocalized model.
Kekulé vs Reality
Kekulé proposed alternating single and double bonds. Evidence proves Benzene has a symmetrical, delocalized structure.
Real Structure: Electrons smeared in a ring above/below.
1. Bond Lengths
All C-C bond lengths are identical (140 pm), intermediate between single (154 pm) and double (134 pm).
2. Enthalpy
Hydrogenation of benzene releases -208 kJ/mol, much less than the expected -360 kJ/mol (3x cyclohexene). It is 152 kJ/mol more stable due to resonance energy.
3. Reactivity
Does not undergo addition reactions (does not decolorize bromine water). Undergoes Electrophilic Substitution to preserve the ring stability.