Diazine

A diazine is an aromatic six-membered heterocyclic compound whose parent ring contains four carbon atoms and two nitrogen atoms. The three unsubstituted parent arrangements share formula ${\mathrm{C}}_4{\mathrm{H}}_4{\mathrm{N}}_2$ but differ in connectivity: pyridazine is the $1,2$ arrangement, pyrimidine the $1,3$ arrangement, and pyrazine the $1,4$ arrangement.^[ref]

Why only three? Fix one nitrogen at position $1$ on a six-membered ring. The other can be adjacent, separated by one intervening ring atom, or opposite. Reflection identifies the remaining apparent alternatives: positions $6$ and $2$ describe the same adjacent arrangement, while $5$ and $3$ describe the same next-nearest arrangement. Thus the distinct shortest ring separations are one, two and three bonds.

The three diazines are constitutional isomers, not different orientations of one molecule. Rotating a drawing changes its appearance without changing its atom-to-atom connections. Moving a ring nitrogen to another connectivity position instead changes which chemical compound the drawing represents. This makes the diazine family a compact example of why a molecular formula alone does not specify chemical identity.

Pyridazine is the $1,2$-diazine: an aromatic six-membered heterocyclic compound with adjacent ring nitrogen atoms and formula ${\mathrm{C}}_4{\mathrm{H}}_4{\mathrm{N}}_2$. The systematic ring position labels refer to connectivity, not to two added substituents on an unchanged hydrocarbon ring.^[ref]

Tracing once around the ring gives the atom sequence $\mathrm{N}-\mathrm{N}-\mathrm{C}-\mathrm{C}-\mathrm{C}-\mathrm{C}$. In pyrimidine, the shortest path between the two nitrogen atoms includes one carbon; in pyrazine, it includes two. Those are different constitutional isomers, although all three have the same elemental formula.^[ref]

For a structure-reading exercise, start with a hexagon marked with two adjacent $\mathrm{N}$ labels and rotate the entire page. It remains pyridazine. Move one $\mathrm{N}$ label across a ring vertex instead, while keeping the ring connectivity interpretation, and the identity can become pyrimidine. The useful distinction is between moving a picture and changing a molecule.