Heptazine

A heptazine, or tri-s-triazine or cyamelurine, is a chemical compound that consist of a triangular core group, C₆N₇, or three fused triazine rings.

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The general form is 1,3,4,6,7,9,9b-Heptaazaphenalene. The specific compound with three amino substituents is called melem. When heptazine is polymerized with the tri-s-triazine units linked through an amine (NH) link, it is called melon. The other melamine homologue, but not a heptazine, is the dimer melam, the fused product of 2,4-diamino-6-chloro-s-triazine with melamine. Melem, melon, and melam are effective flame retardant compounds. The compounds have in common that they melt or decompose at very high temperatures and that they are insoluble in any solvent. This makes characterisation difficult.

Jöns Jakob Berzelius first mentioned the heptazines in the 1830s when he discovered a polymeric substance after mercury thiocyanate ignition. Justus von Liebig named the polymer melon. Much later in 1937 Linus Pauling showed by x-ray crystallography that heptazines are in fact fused triazines.

The heptazine derivate with three hydroxyl substituents is called cyameluric acid and has many possible tautomeric structures. Of the seventeen tautomeric forms, calculations showed that of the tri-oxo forms was found to be the most stable ¹. Therefore this compound is not a hydroxide but an amide. The first person to suggest a structure for cyameluric acid was Josef Loschmidt, as far back as 1861. His structure was in fact a meta-cyclophane, but this at a time when cyclic compounds of any type were not widely recognised.

The heptazine derivate with an azide substituent and two hydroxyl groups is called Linus Pauling's mystery molecule. It is the last molecule he drew on his chalkboard (preserved for posterity) before he died in 1994. Two theories attempt to lift the mystery. It is suggested that Pauling failed to solve the double helix structure of DNA before Watson and Crick because he viewed uracil as an amide and not as the tautomeric hydroxy compound. The other theory suggests that Pauling intended to use the compound as a potential spectroscopic label for binding to DNA.

The tri-azido derivatives are investigated for their use as high-energy-density materials (explosives).

It is believed that the graphite modification of carbon nitride is built up of linked heptazines. Heptazines could be a precursor molecule to the diamond-like beta carbon nitride.

References

• ¹ Theoretical study of cyameluric acid and related compounds Ibon Alkorta, Nadine Jagerovic, and José Elguero Arkivoc (EM-917K) 2004 Article