The carbon-carbon bond lengths in benzene
C-C single bonds and C=C double bonds have different lengths. Years later revealed that all six of carbob-carbon bond lengths in benzene are same length. Evidence Kekule incorrect.
Hydrogenation of benzene
Kekule structure contained three C=C double bonds. Cyclohexa-1,3,5-triene.
- When an alkene reacts with hydrogen enthalpy change called enthalpy of hydrogenation. When cyclohexane, with one C=C bond reacts with hydrogen, enthalpy of hydrogenation is -120 KJmol.
- Kekule's structure, with three C=C bonds, would be expected to have an enthalpy change of hydrogenation of -360 KJmol.
When benzene is hydrogenated enthalpy change is -208 KJmol, 152 KJmol more than expected.
- real structure 152 KJmol more stable than Kekule's structure.
- energy known as delocalisation energy, or resonance energy of benzene.
Helps to explain why benzene is less reactive than alkenes.
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