Aromaticity

Aromaticity (Huckel rule) Aromatic compounds are those, which resemble benzene in chemical behavior. These compounds contain alternate double and single bonds in a cyclic structure. They undergo substitution reactions rather than addition reactions. This characteristic behavior is called aromatic character or aromaticity the criteria for which are as follows:   Contains a cyclic cloud of delocalized  electrons above and below the plane of the molecule.   Electrons cloud must contain a total of (4n+2) a electrons, where  is an integer equal to 0,1,2,3 ...........   This is known as 'Huckel rule' according to which the aromatic compounds have delocalized electron cloud of  electrons of 2 or 6 or 10 or 14 electrons.   For example, benzene (6 electrons), naphthalene (10 electrons) and anthracene (14 electrons) are aromatic compounds.              Benzene             Naphthalene                      Anthracene   6 electrons         10 electrons                   14 electrons   Similarly, cyclopentadienyl anion and cycloheptatrienyl cation (tropylium ion) are also aromatic because these contain 6 electrons.                               Cyclopentadienyl anion                  Cycloheptatrienyl cation     6 electrons                           (Tropylium ion) 6 electrons   Heterocyclic compounds such as pyrrole, furan, thiophene and pyridine also behave as aromatic because all have 6 electrons.      Furan             Thiophene            Pyrrole               Pyridine               Source : tutorvista