Phosphorus trioxide is the
chemical compound
A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
with the molecular formula P
4O
6. Although the molecular formula suggests the name tetraphosphorus hexaoxide, the name phosphorus trioxide preceded the knowledge of the compound's molecular structure, and its usage continues today. This colorless solid is structurally related to
adamantane
Adamantane is an organic compound with a formula C10H16 or, more descriptively, (CH)4(CH2)6. Adamantane molecules can be described as the fusion of three cyclohexane rings. The molecule is both rigid and virtually stress-free. Adamantane is the m ...
. It is formally the
anhydride
An organic acid anhydride is an acid anhydride that is an organic compound. An acid anhydride is a compound that has two acyl groups bonded to the same oxygen atom. A common type of organic acid anhydride is a carboxylic anhydride, where the pa ...
of
phosphorous acid
Phosphorous acid (or phosphonic acid (singular)) is the compound described by the formula H3PO3. This acid is diprotic (readily ionizes two protons), not triprotic as might be suggested by this formula. Phosphorous acid is an intermediate in th ...
, H
3PO
3, but cannot be obtained by the dehydration of the acid. A white solid that melts at room temperature, it is waxy, crystalline and highly toxic, with garlic odour.
Preparation
It is obtained by the combustion of phosphorus in a limited supply of air at low temperatures.
:P
4 + 3 O
2 → P
4O
6
By-products include red phosphorus suboxide.
Chemical properties
Phosphorus trioxide reacts with water to form
phosphorous acid
Phosphorous acid (or phosphonic acid (singular)) is the compound described by the formula H3PO3. This acid is diprotic (readily ionizes two protons), not triprotic as might be suggested by this formula. Phosphorous acid is an intermediate in th ...
, reflecting the fact that it is the anhydride of that acid.
: P
4O
6 + 6 H
2O → 4 H
3PO
3
It reacts with
hydrogen chloride
The compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colourless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hydrogen chloride ga ...
to form
H3PO3 and
phosphorus trichloride
Phosphorus trichloride is an inorganic compound with the chemical formula PCl3. A colorless liquid when pure, it is an important industrial chemical, being used for the manufacture of phosphites and other organophosphorus compounds. It is toxic a ...
.
: P
4O
6 + 6 HCl → 2 H
3PO
3 + 2 PCl
3
With chlorine or bromine it forms the corresponding
phosphoryl halide, and it reacts with iodine in a sealed tube to form
diphosphorus tetraiodide
Diphosphorus tetraiodide is an orange crystalline solid with the formula P2I4. It has been used as a reducing agent in organic chemistry. It is a rare example of a compound with phosphorus in the +2 oxidation state, and can be classified as a su ...
.
P
4O
6 reacts with
ozone
Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...
at 195 K to give the unstable compound P
4O
18.
[.]
P
4O
18 decomposes above 238 K in solution with the release of O
2 gas. Decomposition of dry P
4O
18 is explosive.
In a
disproportionation reaction
In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation states. More generally, the term can b ...
, P
4O
6 is converted into the mixed P(III)P(V) species P
4O
8 when heated in a sealed tube at 710 K, with the side product being red phosphorus.
As a ligand
P
4O
6 is a ligand for transition metals, comparable to
phosphite
The general structure of a phosphite ester showing the lone pairs on the P
In organic chemistry, a phosphite ester or organophosphite usually refers to an organophosphorous compound with the formula P(OR)3. They can be considered as esters of a ...
. An illustrative complex is P
4O
6·Fe(CO)
4.
With
BH3, a dimeric adduct is produced:
:
References
{{DEFAULTSORT:Phosphorus Trioxide
Sesquioxides
Adamantane-like molecules
Phosphorus oxides
Phosphorus(III) compounds
Tricyclic compounds