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Aromatic Benzene

The unconventional CnHn empirical formula of benzene proved to be a great challenge for chemists. Kekule put forth the correct structure in 1865. According to him,benzene has an aromatic structure. This structure  met with lot of criticism in the beginning and was further refined. The only way in which this could be explained is pi bonds in the aromatic ring.

Benzene is the most common aromatic hydrocarbon. The benzene molecule has six carbon atoms connected in a ring. Each carbon atom has four bonding sites available however in benzene three are used and one is free. The three bonds are covalent the fourth can be shared by all six carbon atoms.

 

Resonance

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  1. It means it consists of a conjugated planar ring system with delocalized pi electron clouds.
  2. These electrons which form the double pi bond keep on 'hopping' between subsequent bonds.This also can be expressed in following sentence.
  3. The electrons for C-C bonding are distributed equally between each of the six carbon atoms.
  4. Average length between the C-C and C=C is at 0.139 nm.
  5. This is called as 'resonance'. Resonance adds to the energy of the structure. As a result benzene is more stable by 150 kJ mol-1than predicted by Kekule because of resonance.
  6. Each carbon atom is attached to one hydrogen atom in addition to two carbon atoms.
  7. The electrons of the pi bonds keep on oscillating forth and back.In this way an the valencies of the atoms stand satisfied,albeit in a unconventional way.
  8. Thus in order to facilitate this,the structure would be planar and not three dimensional.This enhanced stability is the fundamental property of aromatic molecules that differentiates them from non-aromatic molecules.
  9. Because of these pi bonds, benzene undergoes nucleophilic as well as electrophilic addition reactions at one end of any double bond.
Structure of Benzene

The resonance energy of benzene gives it a special character called as aromatic stability and is due to delocalization of $\pi$ electrons. The resonance stabilization of benzene and related aromatic compounds is responsible for the lower reactivity of their $\pi$ bonds in comparison to that of the $\pi$ bonds of alkenes and alkynes.

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