Melon (chemistry)
From Wikipedia, the free encyclopedia
In chemistry, melon is a compound of carbon, nitrogen, and hydrogen of still somewhat uncertain composition, consisting mostly of heptazine units linked and closed by amine groups and bridges (−NH−, =NH, −NH2, etc.).[2] It is a pale yellow solid, insoluble in most solvents.[1]
A careful 2001 study indicates the formula C60N91H33, that consists of ten imino-heptazine units connected into a linear chain by amino bridges; that is, H(−C6N8H2)−NH−)10(NH2).[1] However, other researchers are still proposing different structures.
Melon is the oldest known compound with the heptazine C6N7 core, having been described in the early 19th century. It has been little studied until recently, when it has been recognized as a notable photocatalyst and as a possible precursor to carbon nitride.[2]
In 1834 Liebig described the compounds that he named melamine, melam, and melon.[3][4]
The compound received little attention for a long time, due to its insolubility. In 1937 Linus Pauling showed by X-ray crystallography that the structure of melon and related compounds contained fused s-triazine rings.[4]
In 1940, C. E. Redemann and Howard J. Lucas proposed a structure consisting of 2-amino-heptazine units connected by amine bridges through carbons 5 and 8.[1][5] The structure was revised in 2001 by T. Komatsu, who proposed an isomeric structure.[1][4]
Preparation
The compound can be extracted from the solid residue of the thermal decomposition of ammonium thiocyanate NH4SCN at 400 °C.[1][6] (The thermal decomposition of solid melem, on the other hand, yields graphitic carbon nitride.[7])
Structure and properties
According to Komatsu, a characterized form of melon consists of oligomers that can be described as condensations of 10 units of melem tautomer with loss of ammonia NH3. In this structure 2-imino-heptazine units are connected by amino bridges, from carbon 8 of one unit to nitrogen 4 of the next unit. X-ray diffraction data and other evidence indicate that the oligomer is planar, and the triangular heptazine cores have alternating orientations.[1]
The crystal structure of melon is orthorhombic, with estimated lattice constants a = 739.6 pm, b = 2092.4 pm and c = 1295.4 pm.[1]
Polymerization and decomposition
Heated to 700 °C, melon converts to a polymer of high molecular weight, consisting of longer chains with the same motif.[1]
Chlorination
Melon can be converted to 2,5,8-trichloroheptazine, a useful reagent for synthesis of heptazine derivatives.[6]