Chemical evidence for the delocalized model.
Kekulé proposed alternating single and double bonds. Evidence proves Benzene has a symmetrical, delocalized structure.
All C–C bond lengths are identical (140 pm), intermediate between single (154 pm) and double (134 pm).
Hydrogenation releases −208 kJ/mol, much less than expected −360 kJ/mol (3× cyclohexene). It is 152 kJ/mol more stable due to resonance energy.
Does not undergo addition reactions (does not decolorize bromine water). Undergoes Electrophilic Substitution to preserve ring stability.
Benzene does not decolorise bromine water, but cyclohexene (C₆H₁₀) does. Both are cyclic hydrocarbons. Why the difference in reactivity?
Cyclohexene has a localised C=C double bond that readily undergoes electrophilic addition with Br₂. Benzene's electrons are delocalised across the ring, creating extra thermodynamic stability (152 kJ/mol resonance energy). It resists addition reactions and instead undergoes electrophilic substitution to preserve the stable ring.