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.

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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.