Hexachlorophosphazene is an inorganic compound

Inorganic compound

Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...

 with the formula (NPCl₂)₃. The molecule has a cyclic backbone consisting of alternating phosphorus

Phosphorus

Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...

 and nitrogen

Nitrogen

Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

 atoms. It can be viewed as a trimer of the hypothetical compound N≡PCl₂. Hexachlorophosphazene together with the related (NPCl₂)₄ are precursors to inorganic polymers called polyphosphazenes.

Synthesis

The reaction of PCl₅ and NH₄Cl

Ammonium chloride

Ammonium chloride NH4Cl is an inorganic compound with the formula NH4Cl. It is a white crystalline salt that is highly soluble in water. Solutions of ammonium chloride are mildly acidic. Sal ammoniac is a name of natural, mineralogical form of ammonium chloride...

 affords substances with the empirical formula

Empirical formula

In chemistry, the empirical formula of a chemical compound is the simplest positive integer ratio of atoms of each element present in a compound. An empirical formula makes no reference to isomerism, structure, or absolute number of atoms. The empirical formula is used as standard for most ionic...

 PNCl₂: Purification by sublimation gives mainly the trimer

Trimer (chemistry)

In chemistry, a trimer is a product derived from three identical precursors. Trimers are typically cyclic. Chemical compounds that often trimerise are aliphatic isocyanates and cyanic acids. Often, trimerization competes with polymerization....

 (PNCl₂)₃ and tetramer

Tetramer

A tetramer is a protein with four subunits . There are homotetramers such as glutathione S-transferase or single-strand binding protein, dimers of hetero-dimers such as hemoglobin , and heterotetramers, where each subunit is different.-Subunit interactions in tetramers:The interactions between...

 (PNCl₂)₄. These rings were described by Liebig in 1832 in his study of the reaction of PCl₅ and NH₃

Ammonia

Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

:

PCl₅ + NH₄Cl → 1/n (NPCl₂)_n + 4 HCl

Typically reactions are conducted in chlorobenzene

Chlorobenzene

Chlorobenzene is an aromatic organic compound with the chemical formula C6H5Cl. This colorless, flammable liquid is a common solvent and a widely used intermediate in the manufacture of other chemicals.-Uses:...

 solution.

Inorganic rings

Chemists have long known of rings containing carbon, e.g. benzene

Benzene

Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....

, pyridine

Pyridine

Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one C-H group replaced by a nitrogen atom...

, and cyclohexane

Cyclohexane

Cyclohexane is a cycloalkane with the molecular formula C6H12. Cyclohexane is used as a nonpolar solvent for the chemical industry, and also as a raw material for the industrial production of adipic acid and caprolactam, both of which being intermediates used in the production of nylon...

. Related cyclic compounds lacking in carbon have also been studied. Hexachlorophosphazene is one such inorganic ring. Other well known inorganic rings include borazine

Borazine

Borazine is an inorganic compound with the chemical formula 33. In this cyclic compound, the three BH units and three NH units alternate. The compound is isoelectronic and isostructural with benzene...

, S₄N₄, and the cyclic siloxane

Siloxane

A siloxane is any chemical compound composed of units of the form R2SiO, where R is a hydrogen atom or a hydrocarbon group. They belong to the wider class of organosilicon compounds....

s.

"Inorganic rubber"

Hexachlorophosphazene is a precursor to poly(dichlorophosphazene)

Poly(dichlorophosphazene)

poly, also called dichlorophosphazine polymer or phosphonitrilechloride polymer, is chemical compound with formula n...

 or "inorganic rubber", whose discovery is attributed to H. N. Stokes in 1896. Upon heating to ca. 250 °C, the trimer undergoes ring-opening polymerization

Ring-opening polymerization

In polymer chemistry, ring-opening polymerization is a form of chain-growth polymerization, in which the terminal end of a polymer acts as a reactive center, where further cyclic monomers join to form a larger polymer chain through ionic propagation...

 to give the linear polymer (PNCl₂)_n. Subsequent replacement of the chloride centers by other groups, especially alkoxide

Alkoxide

An alkoxide is the conjugate base of an alcohol and therefore consists of an organic group bonded to a negatively charged oxygen atom. They can be written as RO−, where R is the organic substituent. Alkoxides are strong bases and, when R is not bulky, good nucleophiles and good ligands...

s, yields many polyphosphazene

Polyphosphazene

Polyphosphazenes are a class of inorganic polymers with the repeat unit . The substituents R,R' are usually alkoxy, amino, , or halogens ....

s, some with commercial uses.