Difference between revisions of "Phosphorus triiodide"

Revision as of 16:41, 6 February 2017

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. It has the chemical formula PI₃.

Properties

Chemical

Phosphorus triiodide will react with alcohols to form alkyl iodides:

3 R-OH + PI₃ → 3 RI + H₃PO₃

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), 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. 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.^[1][2]

Projects

• Make alkyl iodides
• Diphosphorus tetraiodide synthesis
• Organic reductions

Handling

Safety

Phosphorus triiodide reacts with water to release hydoiodic acid and phosphorous acid, which are corrosive. The reaction will also release small amounts of phosphine which is highly toxic.

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.

References

1. ↑ Ritter, H.; Liebigs Annalen der Chemie; vol. 95; (1855); p. 208 - 211
2. ↑ Germann, F. E. E.; Traxler, R. N.; Journal of the American Chemical Society; vol. 49; (1927); p. 307 - 312

Relevant Sciencemadness threads

• Preparation of Iodomethane