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| Names | |
|---|---|
| IUPAC names Phosphorus triiodide Phosphorus(III) iodide | |
| Other names Triiodophosphine | |
| Identifiers | |
3D model (JSmol) | |
| ChemSpider |
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| ECHA InfoCard | 100.033.302 |
| EC Number |
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| UNII | |
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| Properties | |
| PI3 | |
| Molar mass | 411.68717 g/mol |
| Appearance | dark red solid |
| 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) |
| Decomposes | |
| Structure | |
| Trigonal pyramidal | |
| Hazards | |
| GHS labelling:[1] | |
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| Danger | |
| H314,H335 | |
| P260,P280,P301+P330+P331,P303+P361+P353,P305+P351+P338 | |
| NFPA 704 (fire diamond) | |
| Flash point | non-flammable |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Phosphorus triiodide (PI3) is aninorganic compound with the formula PI3. A red solid, it is too unstable to be stored for long periods of time; it is, nevertheless, commercially available.[2] It is widely used in organic chemistry for convertingalcohols toalkyl iodides and also serves as a powerful reducing agent.
Although PI3 is apyramidal molecule, it has only a small molecular dipole because each P-I bond has almost nobond dipole moment. The P-I bond is also weak; PI3 is much less stable thanPBr3 andPCl3, with astandard enthalpy of formation for PI3 of only −46kJ/mol (solid). The phosphorus atom has anNMR chemical shift of 178 ppm (downfield ofH3PO4).
Phosphorus triiodide reacts vigorously with water, producingphosphorous acid (H3PO3) andhydroiodic acid (HI), along with smaller amounts ofphosphine and various P-P-containing compounds.Alcohols likewise formalkyl iodides, this providing the main use for PI3.
PI3 is also a powerfulreducing agent and deoxygenating agent. It reducessulfoxides tosulfides, even at −78 °C.[3] Meanwhile, heating a1-iodobutane solution of PI3 with redphosphorus causes reduction to P2I4.
The usual method or preparation is by the union of the elements, often by addition ofiodine to a solution ofwhite phosphorus incarbon disulfide:
Alternatively, PCl3 may be converted to PI3 by the action ofhydrogen iodide or certain metaliodides.
Phosphorus triiodide is commonly used in the laboratory for the conversion of primary or secondaryalcohols toalkyl iodides.[4] The alcohol is frequently used as thesolvent, on top of being the reactant. Often the PI3 is madein situ by the reaction of redphosphorus withiodine in the presence of the alcohol; for example, the conversion ofmethanol to giveiodomethane:[5]
These alkyl iodides are useful compounds fornucleophilic substitution reactions, and for the preparation ofGrignard reagents.
