Phosphorus triiodide

Phosphorus triiodide
IUPAC name	Phosphorus triiodide Phosphorus(III) iodide
Other names	Triiodophosphine
Identifiers
CAS number	13455-01-1
PubChem	83485
Properties
Molecular formula	PI3
Molar mass	411.68717 g/mol
Appearance	dark red solid
Density	4.18 g/cm3
Melting point	61.2 °C
Boiling point	200 °C (decomp)
Solubility in water	Decomposes
Structure
Molecular shape	Trigonal pyramidal
Hazards
MSDS	External MSDS
EU classification	not listed
NFPA 704	0 3 2
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 references

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.

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.^[2] 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 + 6 I₂ → 4 PI₃

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.^[3] 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:^[4]

PI₃ + 3 CH₃OH → 3 CH₃I + H₃PO₃

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

References

1. ^ L. G. Wade, Jr., Organic Chemistry, 6th ed., p. 477, Pearson/Prentice Hall, Upper Saddle River, New Jersey, USA, 2005.
2. ^ 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. 
3. ^ B. S. Furnell et al., Vogel's Textbook of Practical Organic Chemistry, 5th edition, Longman/Wiley, New York, 1989.
4. ^ King, C. S.; Hartman, W. W. (1943), "Methyl Iodide", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=CV2P0399 ; Coll. Vol. 2: 399