Phosphorus triiodide

Template:Chembox new 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.

## Contents

* 1 Physical properties
* 2 Chemical properties
* 3 Preparation
* 4 Uses
* 5 References

## Physical properties

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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

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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

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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

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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

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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.