PI<sub>3</sub> - Phosphorus Triiodide

What is Phosphorus Triiodide?

Phosphorus Triiodide is a red solid unstable chemical compound with the formula PI3. Phosphorus triiodide a hexagonal red crystalline material is prepared by the direct action of iodine on white phosphorus in carbon disulfide. The molecule of phosphorus triiodide is pyramidal in shape with very low polarity of the phosphorus iodine bond. This is consistent with the observed zero dipole moment of the compound in carbon disulfide solution.

Other names – Triiodophosphine, Phosphorus(III) iodide


Phosphorus Triiodide


4.18 g/cm³

Molecular Weight/ Molar Mass

411.6872 g/mol

Boiling Point

200 °C

Melting Point

61.2 °C

Chemical Formula


Phosphorus Triiodide Structure – PI3

Phosphorus Triiodide Structure

Physical Properties of Phosphorus Triiodide – PI3




Dark red solid

Covalently-Bonded Unit


Heavy Atom Count





Soluble in water

Chemical Properties of Phosphorus Triiodide – PI3

  • Phosphorus triiodide reacts violently with water forms phosphorus acid and hydrogen iodide. The chemical equation is given below.
  • PI3 + 3H2O → H3PO3 + 3HI

  • Phosphorus triiodide reacts with methyl iodide and mercury to form a substance close to a diiodide complex of trimethylphosphine mercury.
  • PI3 + 3CH3I + Hg → (CH3)3PHgI2

Uses of Phosphorus Triiodide – PI3

  • Used as a reagent for replacing hydroxyl groups by chlorine.
  • Used as a classic reagent for the conversion of aliphatic alcohols into iodides.
  • Used in the preparation of phosphoric acid by combining with water.
  • Used in the preparation of iodopyrazines instead of bromide yields 15-20% in refluxing 1,1,2-trichloroethane.

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