In chemistry, melon is a

compound Compound may refer to: Architecture and built environments * Compound (enclosure), a cluster of buildings having a shared purpose, usually inside a fence or wall ** Compound (fortification), a version of the above fortified with defensive struc ...

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

, and

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

of still somewhat uncertain composition, consisting mostly of

heptazine Heptazine, or tri-''s''-triazine or cyamelurine, is a chemical compound with formula , that consist of a planar triangular core group or three fused triazine rings, with three hydrogen atoms at the corners. It is a yellow, weakly fluorescent soli ...

units linked and closed by

amine In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen Hydrogen is the chemical element wi ...

groups and bridges (, , , etc.). It is a pale yellow solid, insoluble in most solvents. A careful 2001 study indicates the formula , that consists of ten

imino In organic chemistry, an imine ( or ) is a functional group or organic compound containing a carbon–nitrogen double bond (). The nitrogen atom can be attached to a hydrogen or an organic group (R). The carbon atom has two additional single bon ...

units connected into a linear chain by amino bridges; that is, . However, other researchers are still proposing different structures. Melon is the oldest known compound with the heptazine 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 In chemistry, photocatalysis is the acceleration of a photoreaction in the presence of a catalyst. In catalyzed photolysis, light is absorbed by an adsorbed substrate. In photogenerated catalysis, the photocatalytic activity depends on the abil ...

and as a possible precursor to carbon nitride.

History

In 1834

Liebig Justus Freiherr von Liebig (12 May 1803 – 20 April 1873) was a German scientist who made major contributions to agricultural and biological chemistry, and is considered one of the principal founders of organic chemistry. As a professor at t ...

described the compounds that he named

melamine Melamine is an organic compound with the formula C3H6N6. This white solid is a trimer of cyanamide, with a 1,3,5-triazine skeleton. Like cyanamide, it contains 67% nitrogen by mass, and its derivatives have fire retardant properties due t ...

melam Melam is a group of Maddalams and other similar percussion instruments' ( Chenda) rhythmic performance. Those who play melam are called 'Melakaar'. In ancient Tamilakam (Tamil country) melam was used for all the occasions in temples (''Kovil Mela ...

, and melon. The compound received little attention for a long time, due to its insolubility. In 1937 Linus Pauling showed by

x-ray crystallography X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

that the structure of melon and related compounds contained fused triazine rings. Melon_structure_unit_Redemann

In 1939, C. E. Redemamm and other proposed a structure consisting of 2-amino-heptazine units connected by amine bridges through carbons 5 and 8. The structure was revised in 2001 by T. Komatsu, who proposed a tautomeric structure.

Preparation

The compound can be extracted from the solid residue of the thermal decomposition of

ammonium thiocyanate Ammonium thiocyanate is an inorganic compound with the formula NH4SCN. It is the salt of the ammonium cation and the thiocyanate anion. Uses Ammonium thiocyanate is used in the manufacture of herbicides, thiourea, and transparent artificial resin ...

at 400 °C. (The thermal decomposition of solid

melem In chemistry, melem is a compound with formula ; specifically, 2,5,8-triamino-heptazine or 2,5,8-triamino-tri-''s''-triazine, whose molecule can be described as that of heptazine with the three hydrogen atoms replaced by amino groups. It is a whit ...

, on the other hand, yields a graphite-like C-N material.)

Structure and properties

According to Komatsu, a characterized form of melon consists of oligomers that can be described as condensations of 10 units of

tautomer with loss of

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...

. 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. The crystal structure of melon is orthorhombic, with estimated lattice constants a = 739.6 pm, b = 2092.4 pm and c= 1295.4 pm.

Polymerization and decomposition

Heated to 700 °C, melon converts to a polymer of high molecular weight, consisting of longer chains with the same motif.

Chlorination

Melon can be converted to 2,5,8-trichloroheptazine, a useful reagent for synthesis or heptazine derivatives.

Applications

Photocatalysis

In 2009, Xinchen Wang and others observed that melon acts as a catalyst for the splitting of water into hydrogen and

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

, or converting back into fuel, using energy from sunlight. It was the first metal-free

, and it was seen to enjoy a number of advantages over previous compounds, including low cost of material, simple synthesis, negligible toxicity, exceptional chemical and thermal stability. The downside is its modest efficiency, which however seems amenable to improvement by doping or nanostructuring.

Carbon nitride precursor

Another wave of interest for melon happened in the 1990s, when theoretical computations suggested that β- — a hypothetical carbon nitride compound structurally analogous to β- —might be harder than

diamond Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the chemically stable form of carbon at room temperature and pressure, ...

. Melon seemed to be a good precursor for another form of the material, "graphitic" carbon nitride or g-.

References

J. Liebig (1834): ''Annalen Pharmacie'', 10, 1. Xinchen Wang, Kazuhiko Maeda,

Arne Thomas Arne Thomas (born 1975) is a German chemist who researches Porous medium, porous and Nanomaterials, nanostructured materials for Catalysis, catalytic applications. Career Thomas studied chemistry at the University of Giessen, the University of M ...

, Kazuhiro Takanabe, Gang Xin, Johan M. Carlsson, Kazunari Domen, and Markus Antonietti (2009): "A metal-free polymeric photocatalyst for hydrogen production from water under visible light", ''Nature Materials'' volume 8, pages 76-80. Dale R. Miller, Dale C. Swenson, and Edward G. Gillan (2004): "Synthesis and Structure of 2,5,8-Triazido-s-Heptazine: An Energetic and Luminescent Precursor to Nitrogen-Rich Carbon Nitrides". ''Journal of the American Chemical Society'', volume 126, issue 17, pages 5372-5373. Elizabeth K. Wilson (2004), "Old Molecule, New Chemistry. Long-mysterious heptazines are beginning to find use in making carbon nitride materials". ''

Chemical & Engineering News ''Chemical & Engineering News'' (''C&EN'') is a weekly news magazine published by the American Chemical Society, providing professional and technical news and analysis in the fields of chemistry and chemical engineering.Online version
accessed on 2009-06-30. Tamikuni Komatsu (2001)> "The First Synthesis and Characterization of Cyameluric High Polymers". ''Macromolecular Chemistry and Physics'', volume 202, issue 1, pages 19-25. Barbara Jürgens, Elisabeth Irran, Jürgen Senker, Peter Kroll, Helen Müller, Wolfgang Schnick (2003): "Melem (2,5,8-Triamino-tri-s-triazine), an Important Intermediate during Condensation of Melamine Rings to Graphitic Carbon Nitride: Synthesis, Structure Determination by X-ray Powder Diffractometry, Solid-State NMR, and Theoretical Studies". ''Journal of the American Chemical Society'', volume 125, issue 34, pages 10288-10300. {{doi, 10.1021/ja0357689 Fabian Karl Keßler (2019),
Structure and Reactivity of s-Triazine-Based Compounds in C/N/H Chemistry
'. Doctoral thesis, Fakultät für Chemie und Pharmazie,

Ludwig-Maximilians-Universität München The Ludwig Maximilian University of Munich (simply University of Munich or LMU; german: Ludwig-Maximilians-Universität München) is a public research university in Munich, Germany. It is Germany's sixth-oldest university in continuous operatio ...

Nitrogen heterocycles Polymers