Structure of benzene

Structure of benzene

Kekulé ModelConsists of a ring of carbon atoms containing alternating single and double bonds (3 double bonds and 3 single bonds).

Delocalised ModelConsists of the sideways overlap of p-orbitals of all six of the carbon atoms forming π-bonds (rings of delocalised electrons) above and below the plane of carbon atoms.

Evidence for delocalisation

Benzene's low reactivityAs benzene contains C=C bonds you expect it to react in a similar way to alkenes. However benzene doesn't: react with bromine water, decolourising it.undergo addition reactions, preferring substitution reactions instead.

The carbon-carbon bond lengthsX-ray studies revealed that all six carbon-carbon bond lengths in benzene are the same length.

Hydrogenation of benzeneWhen cyclohexene, with one C=C double bond, is reacted with hydrogen, it's enthalpy change of hydrogenation is -120kJmol   .If benzene had three C=C bonds, i.e. the Kekulé structure, it would be expected to have an enthalpy change of hydrogenation of -360kJmol   . But the experimental change of hydrogenation of benzene is -208kJmol   . This value is 152kJmol    less than the expected enthalpy change of hydrogenation of benzene  

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ConclusionThe real structure of benzene is 152kJmol    more stable than the Kekulé structure or a structure containing C=C double bonds which helps to explain why benzene is less reactive than alkenes.

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