Phosphorus triiodide

Phosphorus triiodide

Phosphorus triiodide
Space-filling model of the phosphorus triiodide molecule
Names
IUPAC names Phosphorus triiodide Phosphorus(III) iodide
Other names Triiodophosphine
Identifiers
CAS Number	13455-01-1 N
ChemSpider	75323 Y
Jmol 3D model	Interactive image
PubChem	83485
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
Vapor pressure	{{{value}}}
Supplementary data page
Structure and properties	Refractive index (n), Dielectric constant (εr), etc.
Thermodynamic data	Phase behaviour solid–liquid–gas
Spectral data	UV, IR, NMR, MS
N verify (what is YN ?)
Infobox references

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.

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. 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. 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 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.

See also

• Nitrogen triiodide
• Diphosphorus tetraiodide
• Phosphorus trifluoride
• Phosphorus trichloride
• Phosphorus tribromide
• Phosphorus pentaiodide