Phosphorus triiodide
# Phosphorus triiodide

Phosphorus triiodide $(PI_3)$ is an inorganic compound with the formula $PI_3.$ A red solid, it is too unstable to be stored; it is, nevertheless, 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, $P_2I_4,$ but the existence of $PI_5$ is doubtful at room temperature.

## Properties

$PI_3$ has a low dipole moment in carbon disulfide solution, because the P-I bond has almost no dipole. The P-I bond is also weak; $PI_3$ is much less stable than $PBr_3$ and $PCl_3,$ with a standard enthalpy of formation for $PI_3$ of only −46 kJ/ mol (solid). The phosphorus atom has an NMR chemical shift of 178 ppm (downfield of $H_3PO_4).$

## Reactions

Phosphorus triiodide reacts vigorously with water, producing phosphorous acid $(H_3PO_3)$ 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 $PI_3.$

$PI_3$ is also a powerful reducing agent and deoxygenating agent. It reduces sulfoxides to sulfides, even at −78 °C. Meanwhile, heating a 1-iodobutane solution of $PI_3$ with red phosphorus causes reduction to $P_2I_4.$

## 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:

    $P_4$ + 6 $I_2$ → 4 $PI_3$

Alternatively, $PCl_3$ may be converted to $PI_3$ 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 $PI_3$ 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:

    $PI_3$ + 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.

