General characteristics of arenes

         (1) All arenes have general formula [C_nH_2n - 6y]. Where y is number of benzene rings and n is not less than 6.

         (2) Arenes are planar and cyclic. They undergo replacement rather than addition reactions.

         (3) Aromaticity or aromatic character : The characteristic behaviour of aromatic compounds is known as aromaticity. Aromaticity is because of extensive delocalisation of p-electrons in planar ring system. Huckel (1931) explained aromaticity on the basis of following rule.

         Huckel rule : For aromaticity the atom can be planar, cyclic system having delocalised (4n + 2)Π electrons where n is an integer equal to 0, 1, 2, 3,------.

         Thus, the aromatic molecules have delocalised electron cloud of 2,6,10 or 14 p electrons.

  As like:     4n +2 = 6; 4n = 4; n = 4/4 = 1       

n = 1

               n= 2                                   n = 3

         Similarly tropylium ion or cyclolpentadienyl anion are also aromatic because of containing 6p electrons (n=1).

         Hetrocyclic compounds also have 6p electrons (n = 1).

         Atoms do not suit huckel principle are not aromatic.

4Π electrons                 4Π electrons

       8Π electrons                    4Π electrons                                

   (4) Antiaromaticity : Planar cyclic conjugated substances, less stable than the relating acyclic unsaturated species are called antiaromatic. Molecular orbital computation has given that such compounds have  4nΠ  electrons. In fact these cyclic elements which have  4nΠ  electrons are known as antiaromatic compounds and this characteristic is called antiaromaticity.

         Example : 1,3-Cyclobutadiene, It is extremely unstable antiaromatic compound because it has  4nΠ electrons (n-1) and it is less constant than 1,3 butadiene by about 83.6 KJ mol^-1.

                                                  4n = 4 ; n = 4/4 = 1

   Thus, cyclobutanediene indicates two same contributing structures and it has n = 1.

       4Π electrons                                   4Π electrons

     8Π electrons                            8Π electrons

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