Phosphorus triiodide 1920538 209122634 2008-04-30T00:12:42Z Benjah-bmm27 126395 link to [[diphosphorus tetraiodide]] {{Chembox new | Name = Phosphorus triiodide | ImageFile = Phosphorus-triiodide-2D-dimensions.png <!-- | ImageSize = 150px --> | ImageName = Phosphorus triiodide | ImageFile1 = Phosphorus-triiodide-3D-vdW.png <!-- | ImageSize1 = 150px --> | ImageName1 = Space-filling model of the phosphorus triiodide molecule | IUPACName = Phosphorus triiodide<br />Phosphorus(III) iodide | OtherNames = Triiodophosphine | Section1 = {{Chembox Identifiers | CASNo = 13455-01-1 }} | Section2 = {{Chembox Properties | Formula = PI<sub>3</sub> | MolarMass = 411.58 g/mol | Appearance = dark red solid | Density = 4.18 g/cm<sup>3</sup>, solid | Solubility = Decomposes | MeltingPt = 61.2 °C | BoilingPt = Decomposes }} | Section3 = {{Chembox Structure | MolShape = Trigonal pyramidal | Dipole = }} | Section7 = {{Chembox Hazards | ExternalMSDS = | EUClass = not listed | FlashPt = non-flammable }} }} '''Phosphorus triiodide''' (PI<sub>3</sub>) is an unstable red solid which reacts violently with water. It is a common misconception<ref>L. G. Wade, Jr., ''Organic Chemistry'', 6th ed., p. 477, Pearson/Prentice Hall, Upper Saddle River, New Jersey, USA, 2005.</ref> that PI<sub>3</sub> is too unstable to be stored; it is, in fact, commercially available. It is widely used in organic chemistry for converting [[alcohol]]s to [[alkyl iodide]]s. It is also a powerful reducing agent. Note that [[phosphorus]] also forms a lower [[iodide]], [[diphosphorus tetraiodide|P<sub>2</sub>I<sub>4</sub>]], but the existence of PI<sub>5</sub> is doubtful at room temperature. ==Physical properties== PI<sub>3</sub> has essentially zero [[dipole moment]] in [[carbon disulfide]] solution, because the P-I bond has almost no [[bond dipole moment|dipole]]. The P-I bond is also weak; PI<sub>3</sub> is much less stable than [[Phosphorus tribromide|PBr<sub>3</sub>]] and [[Phosphorus trichloride|PCl<sub>3</sub>]], with a [[standard enthalpy change of formation|standard enthalpy of formation]] for PI<sub>3</sub> of only -46 [[kilojoule|kJ]]/ [[mole (unit)|mol]] (solid). The phosphorus atom has an [[Nuclear magnetic resonance|NMR]] chemical shift of 178 ppm (downfield of [[phosphoric acid|H<sub>3</sub>PO<sub>4</sub>]]). ==Chemical properties== 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 P-P compounds. [[Alcohol]]s likewise form [[alkyl iodide]]s, this providing the main use for PI<sub>3</sub>. PI<sub>3</sub> is also a powerful [[Redox|reducing agent]] and deoxygenating agent. It reduces [[sulfoxide]]s to [[thioether]]s, even at -78 °C.<ref>J. N. Denis, A. Krief, ''Journal of the Chemical Society, Chemical Communications'', 544-5 (1980).</ref> 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>. ==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]]: :[[Phosphorus|P<sub>4</sub>]] + 6[[Iodine|I<sub>2</sub>]] → 4PI<sub>3</sub>. Alternatively, PCl<sub>3</sub> may be converted to PI<sub>3</sub> by the action of [[hydrogen iodide]] or certain metal [[iodide]]s. ==Uses== Phosphorus triiodide is commonly used in the laboratory for the conversion of primary or secondary [[alcohol]]s to [[alkyl iodide]]s.<ref>B. S. Furnell et al., ''Vogel's Textbook of Practical Organic Chemistry'', 5th edition, Longman/Wiley, New York, 1989.</ref> Often the PI<sub>3</sub> is made ''in situ'' by the reaction of red [[phosphorus]] with [[iodine]] in the presence of the alcohol. :PI<sub>3</sub> + 3[[Alcohol|ROH]] → 3[[Alkyl iodide|RI]] + [[Phosphorous acid|HP(O)(OH)<sub>2</sub>]] The alcohol is frequently used as the [[solvent]]. A primary alcohol such as [[1-butanol]] gives a 1-iodobutane in 90% [[chemical yield|yield]]. These alkyl iodides are useful compounds for [[nucleophilic substitution]] reactions, and for the preparation of [[Grignard reaction|Grignard reagents]]. ==References== <references/> <!-- Dead note "1": N. N. Greenwood, A. Earnshaw, ''Chemistry of the Elements'', 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997. --> <!-- Dead note "2": ''Handbook of Chemistry and Physics'', 71st edition, CRC Press, Ann Arbor, Michigan, 1990. --> <!-- Dead note "3": J. March, ''Advanced Organic Chemistry'', 4th ed., p. 723, Wiley, New York, 1992. --> <!-- Dead note "4": ''The Merck Index'', 7th edition, Merck & Co, Rahway, New Jersey, USA, 1960. --> <!-- Dead note "5": A. D. F. Toy, ''The Chemistry of Phosphorus'', Pergamon Press, Oxford, UK, 1973. --> [[Category:Phosphorus compounds]] [[Category:Iodides]] [[Category:Nonmetal halides]]