Aromaticity

The word aromaticity comes from the word ‘aroma’ which means fragrance or odor. Since most of the aromatic compounds are derivatives of benzene and benzene gives distinct odor so, the compounds were named as aromatic compounds. Although presently many non – benzene aromatic compounds have been discovered which do not have any odor. The term aromatic was first used by August Wilhelm Hofmann in 1855.

What is Aromaticity? 

In organic chemistry, the term aromaticity is used to describe a property of a cyclic, planar molecule with a ring of resonance bonds that exhibits more stability than other geometric or connective arrangements with the same set of atoms. Aromatic compounds possess the property of aromaticity. Aromatic molecules do not react easily with other compounds and remain stable. An aromatic functional group is called the aryl group. aromatic compounds possess following properties –

  • Aromatic compounds have extremely high resonance energy.

  • Aromatic compounds undergo substitution reactions rather than additional reactions.

  • These have delocalized pi-electrons. 

Aromaticity Examples

Following are examples of aromatic compounds which possess aromaticity –

  • Benzene 

In the 19th century chemists were struggling with the structure of benzene as benzene is unreactive towards addition reactions being a highly unsaturated compound. Thus, it shows exceptional stability. The cyclohexatriene structure for benzene was 1st August Kekule in 1865. After Kekule many other scientists also proposed the structure of benzene. The quantum mechanical origins of stability or aromaticity was 1st modelled by Huckel in 1931. 

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  • Naphthalene 

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  • Non-Benzene Aromatic Compound – Furan 

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We see many aromatic compounds around us and use them in daily use. Aromatic compounds are highly important for industries as well as these act as key compounds for the synthesis of many compounds. Double ringed bases in RNA and DNA are also examples of aromatic compounds. 

Aromaticity Rules 

To be an aromatic compound or possess aromaticity, compounds must fulfil following all four conditions –

  • The molecule must be cyclic. 

Example – Benzene, pyrrole are aromatic in nature while acyclic compound C4H5NH2 is a non-aromatic compound. Structures are given below –

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  • Every atom in the cyclic ring must be conjugated. As it will provide the cyclic ring delocalized pi-electron system. Thus, we can say every atom in the cyclic ring must have an empty p orbital and must be capable of participating in resonance. Examples are given below –

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  • All compounds must obey Huckel’s Rule i.e. molecules must have [4n+2] pi-electrons. Benzene has 6 pi-electrons and [(4×1)+2] = 6, thus it obeys Huckel’s Rule while cyclooctatetraene has 8 pi-electrons [4n+2]8, thus it does not follow Huckel’s Rule. So, benzene is aromatic and cyclooctatetraene is a non-aromatic compound. 

  • The molecule should be planar or flat. Those compounds which follow the above 4 rules of aromaticity, they are generally flat as in that condition they possess large enough potential energy.  

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