A2 Chemistry – All about Benzene ring!

Discovery of Benzene C6H6

                                 

Properties of Benzene
Benzene is a clear, colourless, aromatic hydrocarbon which has a characteristic sickly, sweet odor. It is both volatile and flammable.

Benzene is highly stable, but it does react with electrophiles in substitution reaction.

  • What is special about Benzene?
  1. Benzene has bond angle of 120°, because it has exactly same bond length (whether it is double bond or not). And it has planar shape because that is the only way that the p orbitals can overlap sideways to give the delocalised pi system.
  2. The electrons are delocalised, so Benzene is more stable than normal cycloalkenes.
  3. Benzene is wrtien like this –

because the double bonds keep on swaping.

Kekule Structure
August Kekule was a German scientist who suggested the first sensible structure for benzene. The reason why it has snakes around the structure in the diagram above is because Kekule came up with the ‘ring’ idea after dreaming about snakes going around the elements.

  • Problems with Kekule struture:
  1. Benzene has three double bonds, so the reaction it undergoes should be similar to ethene. But instead of addition reaction, benzene undergoes substitution reaction.
  2. The bond lengths are same in Benzene – but the length of C-C is 0.154nm and C=C is 0.134.
  3. The theoratical enthalpy change of hydrogenating is 360kJ mol-1, but the actual enthalpy change of hydrogenating of Benzene is 208 kJ mol-1. This is less than the enthalpy change of hydrogenating of cyclohex-di-ene.
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