Phosphorus triiodide

Phosphorus triiodide
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Template:Longitem	PI3
Molar mass	411.68717 g/mol
Appearance	dark red solid
Density	4.18 g/cm3
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Template:Longitem	Trigonal pyramidal
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Phosphorus triiodide (PI₃) is an inorganic compound with the formula PI₃. A red solid, it is too unstable to be stored for long periods of time; it is, nevertheless, commercially available.^[1] It is widely used in organic chemistry for converting alcohols to alkyl iodides and also serves as 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.^[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:

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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]

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

References

↑ L. G. Wade, Jr., Organic Chemistry, 6th ed., p. 477, Pearson/Prentice Hall, Upper Saddle River, New Jersey, USA, 2005.
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↑ B. S. Furnell et al., Vogel's Textbook of Practical Organic Chemistry, 5th edition, Longman/Wiley, New York, 1989.
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