Phosphorus triiodide

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.

Additional recommended knowledge

What is the Correct Way to Check Repeatability in Balances?

Recognize and detect the effects of electrostatic charges on your balance

Daily Sensitivity Test

Physical 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).

Chemical properties

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 + 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.^[3] Often the PI₃ is made in situ by the reaction of red phosphorus with iodine in the presence of the alcohol.

PI₃ + 3ROH → 3RI + HP(O)(OH)₂

The alcohol is frequently used as the solvent. A primary alcohol such as 1-butanol gives a 1-iodobutane in 90% yield.

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, Journal of the Chemical Society, Chemical Communications, 544-5 (1980).
3. ^ B. S. Furnell et al., Vogel's Textbook of Practical Organic Chemistry, 5th edition, Longman/Wiley, New York, 1989.