Phosphorus triiodide

Chembox new
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
Phosphorus(III) iodide
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
Dipole =
Section7 = Chembox Hazards
ExternalMSDS =
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 phosphorus also forms a lower iodide, P₂I₄, but the existence of PI₅ is doubtful at room temperature.

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).

Reactions

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. [J. N. Denis, A. Krief, "Journal of the Chemical Society, Chemical Communications", 544-5 (1980).] Meanwhile heating a 1-iodobutane solution of PI₃ with red phosphorus causes reduction to P₂I₄.

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:

:P₄ + 6I₂ → 4PI₃.

Alternatively, PCl₃ may be converted to PI₃ by the action of hydrogen iodide or certain metal iodides.

Uses

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 phosphorus with iodine in the presence of the alcohol, e.g. the conversion of methanol to give iodomethane. [OrgSynth | author = King, C. S.; Hartman, W. W. | title = Methyl Iodide | collvol = 2 | collvolpages = 399 | year = 1943 | prep =CV2P0399]

:PI₃ + 3ROH → 3RI + HP(O)(OH)₂

These alkyl iodides are useful compounds for nucleophilic substitution reactions, and for the preparation of Grignard reagents.

References