There are three possible ways in which two nitrogen atoms may be combined with four carbon atoms to form the system of six-membered heterocyclic rings known as diazines. The most familiar class of this system is that in which the two nitrogen atoms are meta (viz. 1,3) with respect to each other, namely the pyrimidines.
These are of enormous importance because of their occurrence in such diverse natural products as the purines (caffeine, uric acid, etc.); vitamins (thiamin, riboflavin); the synthetic barbiturates; and the nucleic acids which are the governing chemical agents of life.
The ortho-diazines (viz. 1,2) named the pyridazines, are not as well known as their other two counterparts.
Their chemical significance has surfaced mainly in the last twenty years, particularly in the area of synthetic medicinal chemistry.
The final class of diazines are the pyrazines, in which the ring nitrogens are situated para to each other (viz. 1,4).
