Diphosphorus tetraiodide is an orange crystalline solid with the formula P
2I
4. It has been used as a
reducing agent
In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ).
Examples of substances that are commonly reducing agents include the Earth me ...
in organic chemistry. It is a rare example of a compound with phosphorus in the +2 oxidation state, and can be classified as a
subhalide of phosphorus. It is the most stable of the diphosphorus tetrahalides.
Synthesis and structure
Diphosphorus tetraiodide is easily generated by the
disproportionation
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 ...
of
phosphorus triiodide
Phosphorus triiodide (PI3) is an inorganic compound with the formula PI3. A red solid, it is a common misconception that PI3 is too unstable to be stored; it is, in fact, commercially available. It is widely used in organic chemistry for convertin ...
in dry
ether
In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl or aryl groups. Ethers can again be ...
:
:2 PI
3 → P
2I
4 + I
2
It can also be obtained by treating
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 toxi ...
and
potassium iodide
Potassium iodide is a chemical compound, medication, and dietary supplement. It is a medication used for treating hyperthyroidism, in radiation emergencies, and for protecting the thyroid gland when certain types of radiopharmaceuticals are u ...
in anhydrous conditions.
The compound adopts a centrosymmetric structure with a P-P bond of 2.230 Å.
Reactions
Inorganic chemistry
Diphosphorus tetraiodide reacts with
bromine
Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table ( halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simi ...
to form mixtures PI
3−xBr
x. With sulfur, it is oxidized to P
2S
2I
4, retaining the P-P bond.
[ It reacts with elemental ]phosphorus
Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
and water to make phosphonium iodide
Phosphonium iodide is a chemical compound with the formula . It is an example of a salt containing an unsubstituted phosphonium cation (). Phosphonium iodide is commonly used as storage for phosphine and as a reagent for substituting phosphorus int ...
, which is collected via sublimation at 80 °C.
Organic chemistry
Diphosphorus tetraiodide is used in organic synthesis mainly as a deoxygenating agent. It is used for deprotecting acetal
In organic chemistry, an acetal is a functional group with the connectivity . Here, the R groups can be organic fragments (a carbon atom, with arbitrary other atoms attached to that) or hydrogen, while the R' groups must be organic fragments n ...
s and ketal
In organic chemistry, an acetal is a functional group with the connectivity . Here, the R groups can be organic fragments (a carbon atom, with arbitrary other atoms attached to that) or hydrogen, while the R' groups must be organic fragments no ...
s to aldehyde
In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...
s and ketones, and for converting epoxide
In organic chemistry, an epoxide is a cyclic ether () with a three-atom ring. This ring approximates an equilateral triangle, which makes it strained, and hence highly reactive, more so than other ethers. They are produced on a large scale ...
s into alkene
In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond.
Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
s and aldoxime
In organic chemistry, an oxime is a organic compound belonging to the imines, with the general formula , where R is an organic side-chain and R’ may be hydrogen, forming an aldoxime, or another organic group, forming a ketoxime. O-substitute ...
s into nitriles. It can also cyclize 2-aminoalcohols to aziridine
Aziridine is an organic compound consisting of the three-membered heterocycle . It is a colorless, toxic, volatile liquid that is of significant practical interest. Aziridine was discovered in 1888 by the chemist Siegmund Gabriel. Its derivati ...
s and to convert α,β-unsaturated carboxylic acids to α,β-unsaturated bromides
A bromide ion is the negatively charged form (Br−) of the element bromine, a member of the halogens group on the periodic table. Most bromides are colorless. Bromides have many practical roles, being found in anticonvulsants, flame-retardan ...
.
As foreshadowed by the work of Bertholet in 1855, diphosphorus tetraiodide can convert glycols
A diol is a chemical compound containing two hydroxyl groups ( groups). An aliphatic diol is also called a glycol. This pairing of functional groups is pervasive, and many subcategories have been identified.
The most common industrial diol is e ...
to ''trans'' alkenes
In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond.
Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
.[ This reaction is known as the Kuhn–Winterstein reaction, after the chemists who applied it to the production of ]polyene
In organic chemistry, polyenes are poly- unsaturated, organic compounds that contain at least three alternating double () and single () carbon–carbon bonds. These carbon–carbon double bonds interact in a process known as conjugation, result ...
chromophores.
References
{{Iodides
Phosphorus iodides
Phosphorus(II) compounds
Phosphanes
Reducing agents