Phosphorus triiodide

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Phosphorus triiodide
Phosphorus triiodide
Space-filling model of the phosphorus triiodide molecule
IUPAC names
Phosphorus triiodide
Phosphorus(III) iodide
Other names
3D model (JSmol)
ECHA InfoCard 100.033.302
Molar mass 411.68717 g/mol
Appearance dark red solid
Density 4.18 g/cm3
Melting point 61.2 °C (142.2 °F; 334.3 K)
Boiling point 200 °C (392 °F; 473 K) (decomposes)
Trigonal pyramidal
Safety data sheet See: data page
not listed
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gas Reactivity code 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g., phosphorus Special hazards (white): no codeNFPA 704 four-colored diamond
Flash point non-flammable
Supplementary data page
Refractive index (n),
Dielectric constantr), etc.
Phase behaviour
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YesYN ?)
Infobox references

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.


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).


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.


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 + 6 I2 → 4 PI3

Alternatively, PCl3 may be converted to PI3 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.[3] The alcohol is frequently used as the solvent, on top of being the reactant. Often the PI3 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:[4]

PI3 + 3 CH
→ 3 CH
+ H

These alkyl iodides are useful compounds for nucleophilic substitution reactions, and for the preparation of Grignard reagents.

See also[edit]


  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 (1980). "Phosphorus tri-iodide (PI3), a powerful deoxygenating agent". J. Chem. Soc., Chem. Commun. (12): 544–5. doi:10.1039/C39800000544. 
  3. ^ B. S. Furnell et al., Vogel's Textbook of Practical Organic Chemistry, 5th edition, Longman/Wiley, New York, 1989.
  4. ^ King, C. S.; Hartman, W. W. (1943). "Methyl Iodide". Organic Syntheses.  ; Collective Volume, 2, p. 399