Benzene

Benzene

1. Molecular formula= C6H6

2. August Kekule- cyclic molecule with alternating double bonds

   • Evidence against this
     • Compounds with double bonds readily decolorize bromine
       • Reacted only when hot and exposed to UV- presence of H carrier
     • Evidence from X-ray crystallography
       • Bond length between nm for single bond and double bond
     • Enthalpy changes for hydrogenation of cyclohexene and benzene model
       • Cyclohexene was -120KJ mol-1
       • Benzene should be x3 more but was -208KJ mol-1

3. Current Benzene model

   • Delocalised structure
   • Each carbon atom is bonded to 2 C atoms and 1 H atom.
     • Each C atom trig planar shape= 120 degrees.
     • Each C atom has 4 OS electons
       • 3 bond to other 2 C atoms and 1 H atom
       • 3 bonds are sigma bonds
       • 4th OS in a 2p orbital above and below plance of C atom
         • Electrons in P orbital overlap with C electrons on either side
           • Ring of electron density above and below plane

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