The Concept Of Aromaticity

Aromaticity

         Although the name ‘aromatic’ was originated from the characteristic odour or ‘aroma’ of benzene-like compounds, chemists now have a completely different method of deciding whether a compound is aromatic or not.     Based on the analysis of a number of compounds with unusual resonance stabilization energies, the some characteristics have been accepted as criteria for aromaticity.

Benzene is a cyclic compound which has a planar structure with a delocalized cloud of p electrons above and below the plane of the ring

Criteria for Aromaticity

•         There must be an uninterrupted ring of p orbital-bearing atoms leading to a delocalized p cloud;  For the p cloud to be cyclic, the molecule must be cyclic; For the p cloud to be uninterrupted, every ring atom must have a p orbital; For the p cloud  to form, each p orbital must be able to overlap the p orbital on either side.;  The p cloud must have an odd number of pairs of p electrons, or (2n+1)•2 = 4n+2 p electrons.;   The clouds of π-electrons must therefore contain a total of (4n+2) π-electrons, where n is a positive integer (n=0,1,2,…). Thus an aromatic ring must contain 2,6.10,…π-electrons.

    This requirement is called Hϋckel’s 4n+2 rule and is based on quantum mechanics.

Aromatic, non-aromatic and anti-aromatic compounds

   Molecules which fully obey the criteria for aromaticity as well as Huckel’s rule are said to be aromatic and those which follow those rules partially fall in the category of anti-aromatic and non aromatic compounds.   Non aromatic compounds, are not aromatic due to reasons such as lack of planarity or disruption of  delocalization. They may contain 4n or 4n+2 π electrons.

        Antiaromatic compounds are planar, cyclic, conjugated systems with an even number of pairs of electrons. Such compounds satisfy the first three criteria for aromaticity. i.e. they are planar, cyclic with an uninterrupted ring of p orbital bearing atoms. But they have an even number of pairs of π electrons (4n, n = 1, 2, 3 etc).

The criteria for aromaticity also can be applied to polycyclic hydrocarbons       Naphthalene (5 pairs of p electrons), phenanthrene (7 pairs of p electrons), and chrysene (9 pairs of p electrons) all are aromatic

 Heterocyclic Aromatic Compounds
         Lone pair can’t be in p orbital because p orbital used to build p bond with adjacent carbon(s);       The lone pair on pyridine’s nitrogen is in an sp² hybrid, not part of the 3-pair aromatic p system

    In pyrrole the lone pair could be put into either an sp³ hybrid or a p orbital with bonds in sp² hybrid;     Pyrrole puts the lone pair in a p orbital, making 3 pairs of p electrons (aromatic is more stable)

    In furan and thiophene there are 2 pairs of unshared electrons - one is an sp² hybrid orbital and one pair is in a p orbital, like pyrrole (3 pairs of p electrons, aromatic)

       Quinoline, indole, imidazole, purine, and pyrimidine also are aromatic heterocyclic compounds

REFERENCES

Crocker,Ernest C (1992).Application of the Octet Theory To Single-Ring Aromatic Compounds

Macmurry,John(2007).Organic Chemistry(7th edition).Brooks-Cole

  Organic Chemistry. Marc Loudon, 3^rd edition The Benjamin/Cummings Publishing Company,Inc., California, 1995.

  Organic Chemistry. John MucMurry. 3^rd edition. Brooks/Cole Publishing Company, 1992