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Aromatic Compounds

It is always refreshing to smell "aroma" of a beautiful flower or a perfume. In the early 19th century chemists came across compounds having distinctive fragrances. Then in 1825, Michael Faraday isolated a hydrocarbon compound called "benzene", with molecular formula C6H6. Such, hydrocarbon compounds having a benzene, found to have a distinctive fragrances hence, this benzene family compounds with distinctive aroma are called "Aromatic compunds".

Aromatic compounds.



In benzene(C6H6), each carbon atom uses three valence electrons to bond to one hydrogen and to two adjacent carbon atoms. Carbon actually has four valence electrons to share with any atom it is supossed to bond but here we saw in benzene, carbon atom just used three of its four valence electrons. So, the question is where this last electron got used; scientists first thought this last electron is used to form a double bond with an adjacent carbon atom. This dilemma among experts was solved by August Kekule in 1865, when he proposed that benzene structure is a flat ring in one plane with alternating single and double bonds between the six carbon atoms. 


Resonance In benzene.

Here in the figure above, as per proposed idea by August Kekule, we can see two possible structures. There are three double bonds in both the possible structures but with different locations.

Today it is well-known fact that in benzene six carbon share six valence electrons equally. But you must be thinking out of this two structures with different positions of double and single bonds which one is the Final structure of benzene. The answer to this question lies in resonance character of benzene. At any time this two structures keeps converting into one another, i.e first form to second and again, second form to first. This change is so fast that at any moment, the final structure is a hexagonal hybrid structure with ring in the centre.


Aromatic compound-benzene


Thus, all six carbon-carbon bonds in benzene are identical and equally share six valence electrons. And further, this resonance characteristic makes benzene especially stable. 


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