Benzene is an alicyclic compound which means that they can form a ring of carbon atoms. The structure of benzene is not easy to deduce. Early scientists had many ideas as to what its structure could be. However, a German scientist, Kekule, realised that benzene has a ring shape. He also suggested that there were alternating double bonds between the carbon atoms.
However, there was a problem. Benzene was much more stable than a an alkene. In a thermochemistry calculation based on a Kekule structure, the enthalpy change calculated would always be wrong. Considering its double bonds, benzene should behave like an alkene, reacting with bromine to form a colourless solution. Unlike alkenes, benzene undergoes substitution reactions instead of addition reactions. Benzene also has a planar structure. The structure which Kekule suggested shows that the bonds have different lengths due to the electron densities around the single and double bonds. It was known that a single bond is longer than a double bond.
Hence, Kekule’s structure is not entirely correct. The solution to this is for the electron densities to be spread out evenly. Thus each carbon atom in Benzene has 1.5 of a bond.
The above image shows that the electrons are evenly spread out. This explains why benzene does not undergo addition reaction easily. It does not have regular pi bonds.