Phosphorus triiodide

Phosphorus triiodide
Names



IUPAC names Phosphorus triiodide Phosphorus(III) iodide
Other names Triiodophosphine
Identifiers
CAS Number	13455-01-1^Y
3D model (JSmol)	Interactive image
ChemSpider	75323^Y
ECHA InfoCard	100.033.302
EC Number	236-647-2
PubChem CID	83485
UNII	82QUK78Y7Y^Y
CompTox Dashboard (EPA)	DTXSID1065475
InChI InChI=1S/I3P/c1-4(2)3^Y Key: PZHNNJXWQYFUTD-UHFFFAOYSA-N^Y InChI=1/I3P/c1-4(2)3 Key: PZHNNJXWQYFUTD-UHFFFAOYAR
SMILES IP(I)I
Properties
Chemical formula	PI₃
Molar mass	411.68717 g/mol
Appearance	dark red solid
Density	4.18 g/cm³
Melting point	61.2 °C (142.2 °F; 334.3 K)
Boiling point	200 °C (392 °F; 473 K) (decomposes)
Solubility in water	Decomposes
Structure
Molecular shape	Trigonal pyramidal
Hazards
GHS labelling:^[1]
Pictograms
Signal word	Danger
Hazard statements	H314, H335
Precautionary statements	P260, P280, P301+P330+P331, P303+P361+P353, P305+P351+P338
NFPA 704 (fire diamond)	3 0 2 W
Flash point	non-flammable
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is ^YN ?) Infobox references

Chemical compound

Phosphorus triiodide (PI₃) is an inorganic compound with the formula PI₃. A red solid, it is^[2] too unstable to be stored for long periods of time; it is, nevertheless, commercially available. It is widely used in organic chemistry for converting alcohols to alkyl iodides. It is also a powerful reducing agent.

Properties

Although PI₃ is a pyramidal molecule, it has only a small molecular dipole because each P-I bond has almost no bond dipole moment. 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 various P-P-containing 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 sulfides, even at −78 °C.^[3] 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.^[4] 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:^[5]

PI₃ + 3 CH
₃OH → 3 CH
₃I + " H
₃PO
₃"

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

References

^ GHS: Sigma-Aldsrich 241555
^ 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. (1933). "Methyl Iodide". Organic Syntheses. 13: 60. doi:10.15227/orgsyn.013.0060.