Heterocyclic Compounds
   •   Heterocycles form the largest class of organic compounds. In fact, many 
       natural products and most drugs contain heterocyclic rings. The colors of 
       flowers and plants, antibiotics known to all as penicillins, compounds that 
       transport the oxygen we breathe to our vital organs, and the components of 
       DNA responsible  for  the  genetic  code  are  all  heterocyclic  compounds. 
       From an organic chemist’s viewpoint, heteroatoms are atoms other than 
       carbon or hydrogen that may be present in organic compounds. 
   •   The  most  common  heteroatoms  are  oxygen,  nitrogen,  and  sulfur.  In 
       heterocyclic compounds, one or more of these heteroatoms replaces carbon 
       in a ring.
               The Criteria for Aromaticity and  Hückel’s Rule
   •   Aromatic heterocyclic  compounds are those  have  a  heteroatom  in  a  ring  and 
       behave in a manner similar to benzene in some of their properties (i.e. react by 
       electrophilic aromatic substitution). Further more, these compounds comply the 
       criteria for Aromaticity and with the general rule proposed by Hückel.
   The Criteria for Aromaticity and  Hückel’s Rule:
   1. The compound must be cyclic (a ring of atoms).
   2. The molecule is planar (all atoms in the molecule lie in the same plane) so that 
       there is continuous or nearly continuous overlap of all p orbitals.
   3. The molecule is fully conjugated (p orbital at every atom in the ring)
   (for C atoms to have p orbitals they must have double bonds, or bear a positive or 
       negative charge i.e. ions)
   4. The molecule has a closed loop of (4n+2)  electrons in the cyclic arrangement of 
       p orbitals, where n= 0, 1, 2, 3----integral number
        Aromatic heterocycles
 • Erich Hückel, a German physical chemist recognized in the early 1930s through MO 
 calculations that cyclic planar molecules with a closed loop of 2,6,10,14,18,22……p-
 electrons in a fully conjugated system should be aromatic this finding is called the 
 (4n+2)  -electron rule.  
 •Conversely, monocyclic planar molecules which fulfill all criteria except for Huckel’s 
 rule; with 4n -electrons are classified as antiaromatic, e.g. cyclobutadiene
 •The compounds which doesn’t fulfil criteria for aromaticity are described as
        non aromatic, e.g. cyclopentadiene
           sp2 Hybridization of C, N, and O atoms
 •  The  atomic  number  of  C  is  6-electrons  thus  the  electronic  configuration  is: 
     2 2  2                         2
    1s 2s 2p  with 4 valence electrons, thus the sp  hybridization will  be as shown
 •                                                  2 2 3 
    Similarly, N has 7-electrons thus the electronic configuration is : 1s 2s 2p with 5 
    valence electrons thus the sp2 hybridization will lead to presence of one lone pair 
    of electrons occupies p-orbital
                                              2 2 4 
   O has 8-electrons thus the electronic configuration is : 1s 2s 2p With 6 valence 
                   2
   electrons  thus  the  sp   hybridization  will  lead  to  presence  of  two  lone  pairs  of 
   electrons; one in p-orbital  and the other in sp2 orbital
           Practice problem -1
                                        2
   Q1. Write down the electronic configuration of sp  hybridized sulfur atom and 
   conclude the number of lone pairs of electrons it posseses.