Phosphorus(III) iodide (phosphorus triiodide)

Structure and properties

PI3 exists as discrete PI3 molecules in the solid state with a trigonal pyramidal geometry around phosphorus, similar to PCl3, because of the lone pair on the phosphorus atom. The P–I bonds are relatively long and polarizable compared with P–Cl or P–Br bonds, which makes PI3 more sensitive and less thermally stable. It hydrolyzes readily in the presence of water to give phosphorous acid and hydrogen iodide, releasing corrosive fumes; more detail is available at additional resource.

Preparation and reactions

PI3 is typically prepared by combining elemental phosphorus and iodine, often in an inert organic solvent or by using red phosphorus with iodine to generate the reagent in situ. The direct reaction is driven by formation of the P–I bonds, but isolated PI3 can be unstable and is often formed as needed for a particular transformation. It is a strong iodinating and reducing agent and undergoes hydrolysis, oxidation, and ligand-exchange reactions; see chemical data for context.

Uses in organic synthesis

One important laboratory use of PI3 is converting alcohols into alkyl iodides. The reagent converts primary and secondary alcohols to the corresponding iodides, generally by an SN2-type substitution that inverts configuration at a stereogenic center. Because pure PI3 is uncommon commercially, chemists frequently generate it in situ from red phosphorus and iodine when performing these conversions or employ alternative iodination methods depending on substrate sensitivity. Practical examples and protocols are discussed in synthetic notes.

Safety, handling and notable distinctions

PI3 is corrosive, releases hydrogen iodide upon contact with moisture, and can decompose on heating. Good laboratory practice requires working under inert atmosphere, using suitable gloves and eye protection, and quenching or hydrolyzing residues carefully before disposal. Unlike PCl3 or PBr3, PI3 is less commonly stored as a pure reagent because of stability and handling concerns. For safety guidance consult safety information.