Phosphorus triiodide (PI<sub>3</sub>) is an inorganic compound with the formula PI<sub>3</sub>. A red solid, it is too unstable to be stored for long periods of time; it is, nevertheless, commercially available. It is widely used in organic chemistry for converting alcohols to alkyl iodides and also serves as a powerful reducing agent.
Although PI<sub>3</sub> 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; PI<sub>3</sub> is much less stable than PBr<sub>3</sub> and PCl<sub>3</sub>, with a standard enthalpy of formation for PI<sub>3</sub> of only âÂÂ46 kJ/ mol (solid). The phosphorus atom has an NMR chemical shift of 178 ppm (downfield of H<sub>3</sub>PO<sub>4</sub>).
Phosphorus triiodide reacts vigorously with water, producing phosphorous acid (H<sub>3</sub>PO<sub>3</sub>) 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<sub>3</sub>.
PI<sub>3</sub> is also a powerful reducing agent and deoxygenating agent. It reduces sulfoxides to sulfides, even at âÂÂ78 ðC. Meanwhile, heating a 1-iodobutane solution of PI<sub>3</sub> with red phosphorus causes reduction to P<sub>2</sub>I<sub>4</sub>.
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:
Alternatively, PCl<sub>3</sub> may be converted to PI<sub>3</sub> by the action of hydrogen iodide or certain metal iodides.
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<sub>3</sub> 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:
These alkyl iodides are useful compounds for nucleophilic substitution reactions, and for the preparation of Grignard reagents.