|Jmol-3D images||Image 1|
|Molar mass||411.68717 g/mol|
|Appearance||dark red solid|
200 °C (decomp)
|Solubility in water||Decomposes|
|Molecular shape||Trigonal pyramidal|
|EU classification||not listed|
|Supplementary data page|
|n, εr, etc.|
Solid, liquid, gas
|Spectral data||UV, IR, NMR, MS|
| (what is: / ?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Phosphorus triiodide (PI3) is an unstable red solid which reacts violently with water. 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 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.
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).
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. Meanwhile heating a 1-iodobutane solution of PI3 with red phosphorus causes reduction to P2I4.
- P4 + 6 I2 → 4 PI3
Phosphorus triiodide is commonly used in the laboratory for the conversion of primary or secondary alcohols to alkyl iodides. The alcohol is frequently used as the solvent, on top of being the reactant. Often the PI3 is made in situ by the reaction of red phosphorus with iodine in the presence of the alcohol; for example, the conversion of methanol to give iodomethane:
- PI3 + 3 CH3OH → 3 CH3I + H3PO3
See also 
- Nitrogen triiodide
- Diphosphorus tetraiodide
- Phosphorus trifluoride
- Phosphorus trichloride
- Phosphorus tribromide
- 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 (1980). "Phosphorus tri-iodide (PI3), a powerful deoxygenating agent". J. Chem. Soc., Chem. Commun. (12): 544–5. doi:10.1039/C39800000544.
- B. S. Furnell et al., Vogel's Textbook of Practical Organic Chemistry, 5th edition, Longman/Wiley, New York, 1989.
- King, C. S.; Hartman, W. W. (1943), "Methyl Iodide", Org. Synth.; Coll. Vol. 2: 399