- Phosphorus triiodide
Name = Phosphorus triiodide
ImageFile = Phosphorus-triiodide-2D-dimensions.png
ImageName = Phosphorus triiodide
ImageFile1 = Phosphorus-triiodide-3D-vdW.png
ImageName1 = Space-filling model of the phosphorus triiodide molecule
IUPACName = Phosphorus triiodide
OtherNames = Triiodophosphine
Section1 = Chembox Identifiers
CASNo = 13455-01-1
Section2 = Chembox Properties
Formula = PI3
MolarMass = 411.58 g/mol
Appearance = dark red solid
Density = 4.18 g/cm3, solid
Solubility = Decomposes
MeltingPt = 61.2 °C
BoilingPt = Decomposes
Section3 = Chembox Structure
MolShape = Trigonal pyramidal
Section7 = Chembox Hazards
EUClass = not listed
FlashPt = non-flammable
Phosphorus triiodide (PI3) is an unstable red solid which reacts violently with water. It is a common misconception [L. G. Wade, Jr., "Organic Chemistry", 6th ed., p. 477, Pearson/Prentice Hall, Upper Saddle River, New Jersey, USA, 2005.] 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 phosphorusalso forms a lower iodide, P2I4, but the existence of PI5 is doubtful at room temperature.
PI3 has essentially zero
dipole momentin carbon disulfidesolution, 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 phosphineand 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. [J. N. Denis, A. Krief, "Journal of the Chemical Society, Chemical Communications", 544-5 (1980).] Meanwhile heating a 1-iodobutanesolution of PI3 with red phosphoruscauses reduction to P2I4.
The usual method or preparation is by the union of the elements, often by addition of
iodineto a solution of white phosphorusin carbon disulfide:
Alternatively, PCl3 may be converted to PI3 by the action of
hydrogen iodideor certain metal iodides.
Phosphorus triiodide is commonly used in the laboratory for the conversion of primary or secondary
alcohols to alkyl iodides. [B. S. Furnell et al., "Vogel's Textbook of Practical Organic Chemistry", 5th edition, Longman/Wiley, New York, 1989.] 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 phosphoruswith iodinein the presence of the alcohol, e.g. the conversion of methanolto give iodomethane. [OrgSynth | author = King, C. S.; Hartman, W. W. | title = Methyl Iodide | collvol = 2 | collvolpages = 399 | year = 1943 | prep =CV2P0399]
These alkyl iodides are useful compounds for
nucleophilic substitutionreactions, and for the preparation of Grignard reagents.
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