Benzene - (Evidence and Models)

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  • Benzene
    • Models of Benzene
      • Kekule
        • Alternating single and double bonds
          • Has 3 C=C and 3 C-C bonds. Represented as a symmetrical molecule.
        • Suggested that benzene had 2 forms, differing only by the positions of the double bonds
        • Chemical name is Cyclohexa-1,3,5-triene
      • Delocalised
        • Cyclic hydrocarbon with six carbon atoms and six hydrogen atoms.
        • 6 carbon atoms arranged in a planar hexagonal ring. Each carbon atom is bonded to 2 other carbon atoms and 1 hydrogen atom.
        • Shape arund each carbon atom is trigonal planar with a bond angle of 120
        • Each carbon atom has 4 outer shell electrons. 3 bond to 2 other carbon atoms and 1 hydrogen atoms. Three bonds(sigma) 4th outer shell electron in a 2p orbital above and below the plane of carbon atoms.
        • Electron in a p-orbital of a carbon atom overlaps with the electrons in the p-orbitals of the carbon atoms on either side, results in electron density above and below.
        • Overlap produces a system of pi bonds which spread over all 6 C atoms. Electrons are now delocalised.
    • Evidence of Delocalised model
      • X ray studies
        • Six carbon carbon bond lengths are the same: 0.139 nm, between the C-C (0.153) and C=C (0.134)
      • Halogenation of Benzene
        • One C=C bond has ECOH : -120kj mol
          • 3 C=C bonds would be expected to have a ECOH of -360 kj mol
        • When hydrogenated, actual experimental value is -208 kj mol, 152 kj mol less than expected value.
          • 3 C=C bonds would be expected to have a ECOH of -360 kj mol
        • Real structure of benzene is more stable than Kekule.
      • Does not react with bromine water and decolourise it, doesn't take part in electrophilic addition reactions.

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