Heterocyclic Compounds

Compounds classified in heterocyclic probably constitute the largest and most varied family of an organic compound. After all, every carbocyclic compound, regardless of structure and functionality, may in principle be converted into a group of heterocyclic analogs by replacing one or more of the ring carbon atom with a differ element. Even if we control our consideration to oxygen, nitrogen and sulfur, the  arrangement and combinations of such a replacement are numerous.

Nomenclature

The aromatic heterocycles have been grouped into those with six-membered rings and those with five-membered rings. The name of six-membered aromatic heterocycles that having nitrogen generally end in 'ine' , though note that 'purine' is the name for a very important bicyclic system which has both a six- and a five-membered nitrogen having heterocycle. Five-membered heterocycles having nitrogen general end with 'ole'. Note the use of italic 'H' in a name such as '9H-purine' to designate the location of an N-hydrogen in a system in which, by tautomerism, the hydrogen could reside on another nitrogen (exe., N-7 in the case of purine). Name such 'pyridine' , 'pyrrole' , 'thiophene' , originally trivial, are now the standard, systematic names for these hetrocycles; name such as '1,2,4-triazine' for a six - membered ring with three nitrogens located by the numbers, are more logically systematic.

Five-membered aromatic heterocycles

A detailed discussion of the systematic rules for naming a polycyclic system in which several aromatic or heteroaromatic rings are fused together is beyond the scope of this book, however, a simple example will serve to illustrate the principle. In the name 'pyrrolo[2,3-b]pyridine' , the numbers signify the positions of the first-named heterocycle, numbered as if it were a separated entity, which are the points of ring fusion; ring is fused, the lettering deriving from the side of the second- named heterocycle as a separate entity, i.e. side a is between atoms 1 and 2, side b is between atoms 2 and 3, etc. Actually, this particular heterocycle is more often referred to as '7-azaindole' - note the use of the prefix 'aza' to denote replacement of a ring carbon by nitrogen, i.e. of C-7-H of indole by N.