Structure & Bonding in Benzene
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What is the kekule model for benzene?
Hexagonal ring with alternating double bonds
What is the delocalised model of benzene?
Hexagonal ring with a circle in the centre
What are the properties of benzene expected if kekule was right?
Undergo addition reactions (3 x C–C) To react with halogens without a catalyst, like alkenes (localised 𝛑 electrons able to polarise X2) Irregular hexagonal ring with alternating bond lengths (C-C longer than C–C) Enthalpy change of hydrogenation should be 3x more exothermic than for cyclohexane (-129x3 = -360KJmol-1)
What was the experimental evidence that led to the updated model of benzene?
• It undergoes substitution reactions and not addition • When benzene reacts it requires a catalyst = the e- density in benzene ring is unable to polarise halogen molecules. • X-ray diffraction shows all C-C bonds are “intermediate in length” between C-C and C–C = Regular hexagonal shape • Enthalpy change of hydrogenation is less exothermic than predicted from cyclohexane
Describe the bonding in the delocalised model of benzene
Each C atom forms 3 σ (to 2 C and H) and also has an e- in p orbital. The p orbitals overlap forming 𝛑 bonds (they are shared between 2 atoms). The electrons in the 𝛑 bonds become delocalised (no longer shared), leading to a 𝛑-electron cloud above and below the ring