HeteroCyclic System

Heterocyclic Compounds
Learning Objective :
1. Heterocyclic System and correlation with Homocyclic system
2. Types of heterocyclic Molecules with Aromaticity .
3. Application in our life .

Introduction :
Cyclic compound having only carbons as the ring members, such as benzene are called carbocyclic compounds. As only carbon forms the backbone of the ring, it is also a homocyclic compound.

In contrast, cyclic compounds having at least one atom other than carbon as ring members, (e.g. Pyridine with nitrogen replacing one of the carbon atom) may be termed as heterocyclic compounds. These atoms are termed as heteroatoms. The structure of benzene and pyridine are provided in ( figure 1)

Heterocyclic Compounds By definition :
heterocyclic compounds are five and six membered ringed compounds, that contain at least one hetero-atom, relatively stable and show aromaticity. (However, other sources claim that aromaticity is not a prerequisite for defining heterocyclic compounds. But, we will stick to the textbook definition ).

So, let us check below the significance of the terms defined above:
1) At least one heteroatom,
2) Five and Six Membered Rings,
3) Relatively Stable and
4) Show Aromaticity.

1) Heteroatom Although any atom (other than carbon) can form the heterocyclic ring, usually N, O, and S are the heteroatoms found in nature, especially in biological macromolecules.
2) Five and Six Membered Ringed Structures Containing a Heteroatom . A ring system can be theoretically made up of 3, 4, 5, 6 or more ring members. Examples of such compounds containing one heteroatom is depicted in the figure 2. All compounds in the first 3 columns have saturated carbon atoms and do not possess a conjugated bond system are not aromatic compounds. The last 3 columns contain compounds that contain unsaturation, however not all of them are aromatic (as defined below).
3) Stability The 3 and 4 membered ring compounds however are not stable due to the strain in the ring system. Hence, by definition of heterocyclic compounds these compounds are not heterocyclic compounds.
4) Aromaticity Aromatic compounds should satisfy the following 4 conditions:
(a) A system of conjugated and delocalized pi bonds.
(b) A planar molecule.

(c) A ring system following Huckle’s rule, which requires( 4n+2 ) pi electrons in the ring system (where n=0,1,2,3…). So, aromaticity normally exists when n=2, 6, 10… pi electrons exist in the ring system. In special cases, a pair of pi electrons may be substituted by non-bonding, “n” electrons. Hence a conjugated bond system with 2, 6, or 10 pi or n electrons is a prerequisite for aromaticity.