The two models, delocalised and Kekulé, to show the structure of benzene.
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Created by Philip Holley
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451767
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2016-02-19T07:11:07Z
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Structure of benzene
Kekulé Model
Consists of a ring of carbon atoms containing alternating single and double bonds (3 double bonds and 3 single bonds).
Consists of a ring of carbon atoms containing alternating single and double bonds (3 double bonds and 3 single bonds).
Delocalised Model
Consists 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.
Consists 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 reactivity
As benzene contains C=C bonds you expect it to react in a similar way to alkenes. However benzene doesn't:
As 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 lengths
X-ray studies revealed that all six carbon-carbon bond lengths in benzene are the same length.
X-ray studies revealed that all six carbon-carbon bond lengths in benzene are the same length.
Hydrogenation of benzene
When 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
When 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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Conclusion
The 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.
The 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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