Why is 1,4-cyclohexadiene stabilized? Using qualitative perturbation theory to teach conjugation
Six-carbon polyenes display quite regular properties, when one compares their stabilities using hydrogenation enthalpies. Each double bond in a conjugated system is worth about 10 kJ/mol in resonance energy, and non-conjugated dienes show no resonance energy. Cyclohexadienes are puzzling: 1,3-cyclohexadiene is less stable, and 1,4-cyclohexadiene is more stable, than corresponding open-chain dienes. Qualitative molecular orbital perturbation theory can be used to explain these anomalies.