Melem

A heptazine, or tri-s-triazine or cyamelurine, is a type of chemical compound that consist of a planar triangular core group, C₆N₇, or three fused triazine rings, with three substituents at the corners of the triangle.

The general form is 1,3,4,6,7,9,9b-heptaazaphenalene. The parent compound C₆N₇H₃, where the three substituents are hydrogens, is called 1,3,4,6,7,9-hexaazacyc1[3.3.3]azine or tri-s-triazine proper.

Heptazines were discovered in the 19th century but their study has long been hampered by their general insolubility. They are used as flame retardants and have been the object of interest recently for potential applications in electronics materials, explosives, and more.

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 unsubstituted heptazine C₆N₇H₃ was synthesized by Ramachandra S. Hosmane and others from the group of N. Leonard in the early 1980s. The structure of Berzelius's melon was confirmed only in 2001.

The parent substance C₆N₇H₃ is a yellow, weakly fluorescent solid with melting point over 300 °C. The compound is generally stable and soluble in organic solvents such as acetonitrile, but is decomposed by water. It has a peculiar crystal structure, whose cell spans 16 molecules in asymmetric positions and orientations.

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. Another 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.

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