Phosphorus triiodide
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| Phosphorus triiodide | |
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| IUPAC name | Phosphorus triiodide Phosphorus(III) iodide |
| Other names | Triiodophosphine |
| Identifiers | |
| CAS number | [13455-01-1] |
| Properties | |
| Molecular formula | PI3 |
| Molar mass | 411.58 g/mol |
| Appearance | dark red solid |
| Density | 4.18 g/cm3, solid |
| Melting point |
61.2 °C |
| Boiling point |
Decomposes |
| Solubility in water | Decomposes |
| Structure | |
| Molecular shape | Trigonal pyramidal |
| Hazards | |
| MSDS | External MSDS |
| EU classification | not listed |
| Flash point | non-flammable |
| Supplementary data page | |
| Structure and properties |
n, εr, etc. |
| Thermodynamic data |
Phase behaviour Solid, liquid, gas |
| Spectral data | UV, IR, NMR, MS |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references |
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Phosphorus triiodide (PI3) is an unstable red solid which reacts violently with water. It is a common misconception[1] that PI3 is too unstable to be stored; it is, in fact, commercially available. It is widely used in organic chemistry for converting alcohols to alkyl iodides. It is also a powerful reducing agent. Note that phosphorus also forms a lower iodide, P2I4, but the existence of PI5 is doubtful at room temperature.
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[edit] Physical properties
PI3 has essentially zero dipole moment in carbon disulfide solution, because the P-I bond has almost no dipole. The P-I bond is also weak; PI3 is much less stable than PBr3 and PCl3, with a standard enthalpy of formation for PI3 of only -46 kJ/ mol (solid). The phosphorus atom has an NMR chemical shift of 178 ppm (downfield of H3PO4).
[edit] Chemical properties
Phosphorus triiodide reacts vigorously with water, producing phosphorous acid (H3PO3) and hydroiodic acid (HI), along with smaller amounts of phosphine and P-P compounds. Alcohols likewise form alkyl iodides, this providing the main use for PI3.
PI3 is also a powerful reducing agent and deoxygenating agent. It reduces sulfoxides to thioethers, even at -78 °C.[2] Meanwhile heating a 1-iodobutane solution of PI3 with red phosphorus causes reduction to P2I4.
[edit] Preparation
The usual method or preparation is by the union of the elements, often by addition of iodine to a solution of white phosphorus in carbon disulfide:
Alternatively, PCl3 may be converted to PI3 by the action of hydrogen iodide or certain metal iodides.
[edit] Uses
Phosphorus triiodide is commonly used in the laboratory for the conversion of primary or secondary alcohols to alkyl iodides.[3] Often the PI3 is made in situ by the reaction of red phosphorus with iodine in the presence of the alcohol.
- PI3 + 3ROH → 3RI + HP(O)(OH)2
The alcohol is frequently used as the solvent. A primary alcohol such as 1-butanol gives a 1-iodobutane in 90% yield.
These alkyl iodides are useful compounds for nucleophilic substitution reactions, and for the preparation of Grignard reagents.
[edit] References
- ^ L. G. Wade, Jr., Organic Chemistry, 6th ed., p. 477, Pearson/Prentice Hall, Upper Saddle River, New Jersey, USA, 2005.
- ^ J. N. Denis, A. Krief, Journal of the Chemical Society, Chemical Communications, 544-5 (1980).
- ^ B. S. Furnell et al., Vogel's Textbook of Practical Organic Chemistry, 5th edition, Longman/Wiley, New York, 1989.
