Hydrogen iodide 1930814 226169804 2008-07-17T03:39:06Z LorenzoB 2054613 Typo {{Chembox new | Name = Hydrogen iodide | ImageFile = Hydrogen-iodide-2D-dimensions.png <!-- | ImageSize = 120px --> | ImageName = Hydrogen iodide | ImageFile1 = Hydrogen-iodide-3D-vdW.svg <!-- | ImageSize1 = 150px --> | ImageName1 = Hydrogen iodide | IUPACName = Hydrogen iodide | OtherNames = hydroiodic acid | Name = Thermodynamic Data | Section1 = {{Chembox Identifiers | CASNo = 10034-85-2 | RTECS = MW3760000 }} | Section2 = {{Chembox Properties | Formula = HI | MolarMass = 127.904 g/mol | Appearance = Colorless gas. | Density = 2.85 g/mL (-47 °C) | MeltingPt = –50.80 °C (184.55 K) | BoilingPt = –35.36 °C (237.79 K) | Solubility in water = approximately 232 g/ml | pKa = –10 }} | Section3 = {{Chembox Structure | MolShape = Terminus | Dipole = 0.38 [[Debye|D]] }} | Section7 = {{Chembox Hazards | ExternalMSDS = [http://www.camd.lsu.edu/msds/h/hydrogen_iodide.htm hydrogen iodide]<br /> [http://ptcl.chem.ox.ac.uk/MSDS/HY/hydroiodic_acid.html hydroiodic acid] | MainHazards = Toxic, corrosive. | NFPA-H = 3 | NFPA-F = | NFPA-R = 1 | NFPA-O = COR | FlashPt = Non-flammable. | RPhrases = {{R20}}, {{R21}}, {{R22}}, {{R35}} | SPhrases = {{S7}}, {{S9}}, {{S26}}, {{S45}} }} | Section8 = {{Chembox Other | OtherAnions = [[Flouride]]<br />[[Chloride]]<br />[[Bromide]]}} }} '''Hydrogen iodide''' (HI) is a [[diatomic molecule]]. Aqueous solutions of HI are known as '''hydroiodic acid''' or '''hydriodic acid''', a [[strong acid]]. Hydrogen iodide and hydroiodic acid are, however, different in that the former is a gas under standard conditions; whereas, the other is an aqueous solution of said gas. They are interconvertible. HI is used in [[organic synthesis|organic]] and [[Inorganic chemistry#Synthetic inorganic chemistry|inorganic synthesis]] as one of the primary sources of [[iodine]] and as a [[reducing agent]]. ==Properties of hydrogen iodide== HI is a colorless gas that reacts with oxygen to give water and iodine. With moist air, HI gives a mist (or fumes) of hydroiodic acid. It is exceptionally soluble in water, giving hydroiodic acid. One liter of water will dissolve 425 liters of HI, the final solution having only four water molecules per molecule of HI.<ref>Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.</ref> ===Hydroiodic acid=== Once again, although chemically related, hydroiodic acid is not pure HI but a mixture containing it. Commercial "concentrated" hydroiodic acid usually contains 48% - 57% HI by mass. The solution forms an [[azeotrope]] boiling at 127 °C with 57% HI, 43% water. Hydroiodidic acid is one of the strongest of all the common [[halide]] acids because the [[electronegativity]] of [[iodine]] is weaker than the rest of the other common halides. The high acidity is caused by the dispersal of the ionic charge over the anion. The [[iodide]] ion is much larger than the other common halides which results in the negative charge being dispersed over a large space. By contrast, a chloride ion is much smaller, meaning its negative charge is more concentrated, leading to a stronger interaction between the [[proton]] and the chloride ion. This weaker H<sup>+</sup>---I<sup>−</sup> interaction in HI facilitates [[dissociate|dissociation]] of the proton from the anion, and is the reason HI is the strongest acid of the hydrohalides. HI<sub>(g)</sub> + H<sub>2</sub>O<sub>(l)</sub> → H<sub>3</sub>O<sup><sub>(aq)</sub>+</sup> + I<sup>-</sup> <sub>(aq)</sub> [[acid dissociation constant|K<sub>a</sub>]]≈ 10<sup>10</sup> HBr<sub>(g)</sub> + H<sub>2</sub>O<sub>(l)</sub> → H<sub>3</sub>O<sup><sub>(aq)</sub>+</sup> + Br<sup>-</sup> <sub>(aq)</sub> [[acid dissociation constant|K<sub>a</sub>]]≈ 10<sup>9</sup> HCl<sub>(g)</sub> + H<sub>2</sub>O<sub>(l)</sub> → H<sub>3</sub>O<sup><sub>(aq)</sub>+</sup> + Cl<sup>-</sup> <sub>(aq)</sub> [[acid dissociation constant|K<sub>a</sub>]]≈ 10<sup>8</sup> ==Preparation== The industrial preparation of HI involves the reaction of I<sub>2</sub> with [[hydrazine]], which also yields [[nitrogen]] gas.<ref>Greenwood, N.N. and A. Earnshaw. ''The Chemistry of the Elements''. 2nd ed. Oxford: Butterworth-Heineman. p 809-815. 1997.</ref> ::2 I<sub>2</sub> + N<sub>2</sub>H<sub>4</sub> → 4 HI + N<sub>2</sub> When performed in water, the HI must be [[distillation|distilled]]. HI can also be distilled from a solution of [[sodium iodide|NaI]] or other alkali iodide in concentrated [[phosphoric acid]] (note that [[sulfuric acid]] will not work for acidifying iodides as it will oxidize the iodide to elemental iodine). Another way HI may be prepared is by bubbling [[hydrogen sulfide]] steam through an aqueous solution of Iodine, forming hydroiodic acid (which is distilled) and elemental sulfur (this is filtered). ::H<sub>2</sub>S +I<sub>2</sub> → 2 HI + S Additionally HI can be prepared by simply combining H<sub>2</sub> and I<sub>2</sub>. This method is usually employed to generate high purity samples. ::H<sub>2</sub> + I<sub>2</sub> → 2 HI For many years, this reaction was considered to involve a simple bimolecular reaction between molecules of H<sub>2</sub> and I<sub>2</sub>. However, when a mixture of the gases is irradiated with the wavelength of light equal to the [[dissociation energy]] of I<sub>2</sub>, about 578 nm, the rate increases significantly. This supports a mechanism whereby I<sub>2</sub> first dissociates into 2 iodine atoms, which each attach themselves to a side of an H<sub>2</sub> molecule and break the H -- H bond:<ref>Holleman, A.F. Wiberg, E. ''Inorganic Chemistry.'' San Diego: Academic Press. p 371, 432-433. 2001.</ref> ::H<sub>2</sub> + I<sub>2</sub> + 578 nm radiation → H<sub>2</sub> + 2 I → I - - - H - - - H - - - I → 2 HI In the laboratory, another method involves [[hydrolysis]] of [[phosphorus triiodide|PI<sub>3</sub>]], the iodine equivalent of [[phosphorus tribromide|PBr<sub>3</sub>]]. In this method, I<sub>2</sub> reacts with [[phosphorus]] to create [[phosphorus triiodide]], which then reacts with water to form HI and [[phosphorous acid]]. ::3 I<sub>2</sub> + 2 P + 6 H<sub>2</sub>O → 2 PI<sub>3</sub> + 6 H<sub>2</sub>O → 6 HI + 2 H<sub>3</sub>PO<sub>3</sub> ==Key reactions and applications== *HI will undergo oxidation if left open to air according to the following pathway:' ::4 HI + O<sub>2</sub> → 2H<sub>2</sub>O + 2 I<sub>2</sub> ::HI + I<sub>2</sub> → HI<sub>3</sub> HI<sub>3</sub> is dark brown in color, which makes aged solutions of HI often appear dark brown. *Like HBr and HCl, HI add to [[alkenes]]<ref name="Breton" /> ::HI + H<sub>2</sub>C=CH<sub>2</sub> → H<sub>3</sub>CCH<sub>2</sub>I HI is also used in organic chemistry to convert [[primary alcohols]] into [[alkyl halides]]<ref name="Bruice" />. This reaction is an S<sub>N</sub>2 substitution, in which the iodide ion replaces the "activated" hydroxyl group (water). HI is perfered over other hydrogen halides because the iodide ion is a much better nucleophile than bromide or chloride, so the reaction can take place at a reasonable rate without much heating. This reaction also occurs for secondary and tertiary alcohols, but substitution occurs via the S<sub>N</sub>1 pathway. [[Image:HIsubstitution.gif|600px]] HI (or HBr) can also be used to cleave [[ethers]] into alkyl iodides and alcohols, in a reaction simmilar to the substitution of alcohols. This type of cleavage is siginficant because it can be used to convert a chemically stable<ref name="Bruice">Bruice, Paula Yurkanis. ''Organic Chemistry'' 4th ed. Prentice Hall: Upper Saddle River, N.J, 2003 p438-439, 452</ref> and inert ether into more reactive species. In this example [[diethyl ether]] is cleaved into ethanol and iodoethane. The reaction is regioselective, as iodide tends to attack the less [[steric effects|sterically hindered]] ether carbon. [[Image:DEEcleavage.gif|550px]] HI is subject to the same [[Markovnikov's rule|Markovnikov]] and anti-Markovnikov guidelines as HCl and HBr. *HI reduces certain α-substituted ketones and alcohols replacing the α substituent with a hydrogen atom.<ref name="Breton">Breton, G. W., P. J. Kropp, P. J.; Harvey, R. G. “Hydrogen Iodide” in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.</ref> ==Illicit Use of Hydroiodic Acid== Hydroiodic acid is currently listed as a [[DEA list of chemicals|Federal DEA List I Chemical]]. Owing to its usefulness as a reducing agent, reduction with HI and red phosphorus has become the most popular method to produce methamphetamine in the United States. Clandestine chemists react psuedoephedrine (recovered from antihistamine pills) with hydroiodic acid and red phosphorus under heat, HI reacts with psuedoephedrine to form iodoephedrine, an intermediate which is reduced primarily to methamphetamine<ref>Skinner, Harry F. "Methamphetamine Synthesis via HI/Red Phosphorous Reduction of Ephedrine". Forensic Science International, 48 128-134 (1990)</ref>. This reaction is stereoselective, producing (d)-methamphetamine. [[Image:methlabs.jpg|thumb|Lab using the HI/P method]] Due to its listed status and closely monitored sales, clandestine chemists now use red phosphorus and iodine to generate hydroiodic acid ''in situ''<ref>Skinner HF. "Identification and quantitation of hydriodic acid manufactured from iodine, red phosphorus and water" . Journal of the Clandestine Laboratory Investigation Chemists Association 1995;5(4):12; Microgram 1995;28(11):349</ref>. ==References== <references/> See also: Nishikata, E., T.; Ishii, and T. Ohta. “Viscosities of Aqueous Hydrochloric Acid Solutions, and Densities and Viscosities of Aqueous Hydroiodic Acid Solutions”. J. Chem. Eng. Data. ''26''. 254-256. 1981. ==External links== * [http://www.ilo.org/public/english/protection/safework/cis/products/icsc/dtasht/_icsc13/icsc1326.htm International Chemical Safety Card 1326] [[Category:Hydrogen compounds]] [[Category:Iodides]] [[Category:Acids]] [[Category:Nonmetal halides]] [[Category:DEA List I chemicals]] [[cs:Jodovodík]] [[de:Iodwasserstoff]] [[fr:Iodure d'hydrogène]] [[nl:Waterstofjodide]] [[pt:Iodeto de hidrogénio]] [[ru:Иодоводород]] [[sv:Vätejodid]] [[zh:碘化氢]]