Phosphorus triiodide

Phosphorus triiodide

Names
IUPAC names Phosphorus triiodide Phosphorus(III) iodide
Other names Phosphorus iodide Triiodophosphine
Identifiers
Jmol-3D images	Image
SMILES IP(I)I
Properties
Chemical formula	PI3
Molar mass	411.68717 g/mol
Appearance	Dark red solid
Odor	Acrid (moist air)
Density	4.18 g/cm3
Melting point	61.2 °C (142.2 °F; 334.3 K)
Boiling point	200 °C (392 °F; 473 K) (decomposes)
Solubility in water	Reacts
Solubility	Reacts with alcohols Soluble in benzene, carbon disulfide, hexane, 1,2-dichloroethane Poorly soluble in acetonitrile, sulfur dioxide Insoluble in dichloromethane
Vapor pressure	~0 mmHg
Thermochemistry
Std molar entropy (So298)	192 J·K−1·mol−1
Std enthalpy of formation (ΔfHo298)	-45.6 kJ/mol
Hazards
Safety data sheet	Guidechem
Flash point	Non-flammable
Related compounds
Related compounds	Phosphorus trichloride Phosphorus tribromide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Phosphorus triiodide is chemical compound, a red solid which reacts violently with water, releasing phosphorous acid and hydroiodic acid. It is a powerful reducing agent, with the chemical formula PI₃.

Properties

Chemical

Phosphorus triiodide readily hydrolyzes in water, producing phosphorous acid (H₃PO₃) and hydroiodic acid (HI), along with smaller amounts of phosphine and traces of diphosphanes.

PI₃ + 3 H₂O → 3 HI + H₃PO₃

Phosphorus triiodide will react with alcohols to form alkyl iodides:

3 R-OH + PI₃ → 3 RI + H₃PO₃

PI₃ is also an exceptionally powerful reducing agent and deoxygenating agent, capable of rapidly reducing sulfoxides to sulfides, even at temperatures as low as −78 °C.

Heating a 1-iodobutane solution of PI₃ with red phosphorus causes reduction to P₂I₄.

Physical

Phosphorus triiodide is an unstable dark red solid, with a melting point of 61.2 °C and decomposes when heated to 200 °C. It is soluble in benzene, carbon disulfide and fairly soluble in hexane. Its density is 4.18 g/cm³ at standard conditions.

Availability

Phosphorus triiodide is sold by various chemical entities, though it's almost impossible for the amateur chemist to purchase it.

Legality

Although it's made from two DEA List I chemicals (phosphorus and iodine) and upon standing in air/hydrolysis releases another List I chemical (hydroiodic acid, as well as elemental iodine), phosphorus triiodide is curiously not listed in the DEA List of chemicals. However, PI₃'s status is covered by the same legislation that covers phosphorus halides and individuals normally cannot purchase it.

Preparation

Phosphorus triiodide can be made by reacting elemental phosphorus (red preferably) with iodine, in a P:I ratio of 1:3.

2 P + 3 I₂ → 2 PI₃

The reaction best takes place in a solvent, such as carbon disulfide, carbon tetrachloride, dichloroethane. Since the reaction is exothermic, cooling is required to keep it under control. Due to its low boiling point, carbon disulfide is a good choice, and crystallizing the resulting PI₃ from the solution is safe and gives good yield. If the reaction is done without a solvent, the resulting product is impure and the yield is poor.

Another route involves the reaction of phosphorus trichloride with hydrogen iodide:

PCl₃ + 3 KI → PI₃ + 3 KCl₃

This reaction takes place in glacial acetic acid.

Projects

• Make alkyl iodides
• Diphosphorus tetraiodide synthesis
• Organic reductions

Handling

Safety

Phosphorus triiodide reacts with water to release hydroiodic acid and phosphorous acid, which are corrosive and harmful. Direct contact with skin will cause chemical burns. The reaction will also release small amounts of phosphine which is highly toxic, as well as diphosphanes which can be pyrophoric.

Storage

Phosphorus triiodide can be stored in glass air-tight containers, though it's not recommended to be stored for long periods of time and used as soon as it's made.

Disposal

Phosphorus triiodide can be neutralized with a solution of sodium thiosulfate. This should be done slowly, since PI₃ reacts exothermically with water.

References

Relevant Sciencemadness threads

• Solid Phosphorus Triiodide
• Phosphorus triiodide and DEA List
• Preparation of Iodomethane