Hydrogen iodide
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| Names | |||
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| IUPAC name
Hydrogen iodide
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| Identifiers | |||
3D model (JSmol)
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| ChEBI | |||
| ChEMBL | |||
| ChemSpider | |||
| ECHA InfoCard | 100.030.087 | ||
| EC Number |
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| 814 | |||
| KEGG | |||
| MeSH | Hydroiodic+acid | ||
PubChem CID
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| RTECS number |
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| UNII | |||
CompTox Dashboard (EPA)
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| Properties | |||
| HI | |||
| Molar mass | 127.912 g·mol−1 | ||
| Appearance | Colorless gas | ||
| Density | 2.85 g cm-3 (at −47 °C) | ||
| Melting point | −51 °C (−60 °F; 222 K) | ||
| Boiling point | −34 °C (−29 °F; 239 K) | ||
| Acidity (pKa) | ≈ –9.5 (in water),[1] 2.8 (in acetonitrile)[2] | ||
| Basicity (pKb) | 23.5 | ||
Refractive index (nD)
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1.466 | ||
| Structure | |||
| 0.38 D | |||
| Thermochemistry | |||
Heat capacity (C)
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0.2283 J/(g·K) | ||
Std enthalpy of
formation (ΔfH⦵298) |
0.0016199 kJ mol-1 | ||
| Hazards | |||
| NFPA 704 (fire diamond) | |||
| Supplementary data page | |||
| Hydrogen iodide (data page) | |||
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Hydrogen iodide (HI) is a diatomic molecule. Aqueous solutions of HI are known as iohydroic acid or hydroiodic acid, a strong acid. Gas and aqueous solution are interconvertible. HI is used in organic and 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 gas[citation needed], the final solution having only four water molecules per molecule of HI.
Hydroiodic acid
Hydroiodic acid is a solution of pure HI in water. Commercial hydroiodic acid usually contains 57% HI by mass. The solution forms an azeotrope boiling at 127 °C with 57% HI, 43% water. Hydroiodic acid is one of the strongest of all common acids due to the high stability of its corresponding conjugate base. The iodide ion is the largest of all common halides which results in the negative charge being dispersed over a larger space. By contrast, a chloride ion is significantly smaller, meaning its negative charge is more concentrated, leading to a stronger interaction between the proton and the chloride ion. This weaker H+---I– interaction in HI facilitates dissociation of the proton from the anion, and is the reason HI is the strongest acid of the hydrohalides(except for hydroastatic acid [theoretically]).[3]
- HI(g) + H2O(l) → H3O(aq)+ + I–(aq) Ka ≈ 1010
- HBr(g) + H2O(l) → H3O(aq)+ + Br–(aq) Ka ≈ 109
- HCl(g) + H2O(l) → H3O(aq)+ + Cl–(aq) Ka ≈ 108
Preparation
The industrial preparation of HI involves the reaction of I2 with hydrazine, which also yields nitrogen gas.[4]
- 2 I2 + N2H4 → 4 HI + N2
When performed in water, the HI must be distilled.
HI can also be distilled from a solution of NaI or other alkali iodide in concentrated hypophosphorous acid[5] (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).
- H2S +I2 → 2 HI + S
Additionally HI can be prepared by simply combining H2 and I2. This method is usually employed to generate high purity samples.
- H2 + I2 → 2 HI
For many years, this reaction was considered to involve a simple bimolecular reaction between molecules of H2 and I2. However, when a mixture of the gases is irradiated with the wavelength of light equal to the dissociation energy of I2, about 578 nm, the rate increases significantly. This supports a mechanism whereby I2 first dissociates into 2 iodine atoms, which each attach themselves to a side of an H2 molecule and break the H—H bond:[3]
- H2 + I2 + 578 nm radiation → H2 + 2 I → I – - – H – - – H – - – I → 2 HI
In the laboratory, another method involves hydrolysis of PI3, the iodine equivalent of PBr3. In this method, I2 reacts with phosphorus to create phosphorus triiodide, which then reacts with water to form HI and phosphorous acid.[3]
- 3 I2 + 2 P + 6 H2O → 2 PI3 + 6 H2O → 6 HI + 2 H3PO3
Key reactions and applications
- HI will undergo oxidation if left open to air according to the following pathway:[3]
- 4 HI + O2 → 2H2O + 2 I2
- HI + I2 → HI3
HI3 is dark brown in color, which makes aged solutions of HI often appear dark brown.
- HI + H2C=CH2 → H3CCH2I
HI is also used in organic chemistry to convert primary alcohols into alkyl halides.[7] This reaction is an SN2 substitution, in which the iodide ion replaces the "activated" hydroxyl group (water). HI is preferred over other hydrogen halides in polar protic solvents 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. The large iodide anion is less solvated and more reactive in polar protic solvents and thus causes the reaction to proceed faster because of stronger partial bonds in the transition state. This reaction also occurs for secondary and tertiary alcohols, but substitution occurs via the SN1 pathway.
HI (or HBr) can also be used to cleave ethers into alkyl iodides and alcohols, in a reaction similar to the substitution of alcohols. This type of cleavage is siginficant because it can be used to convert a chemically stable[7] 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 sterically hindered ether carbon.
Hydroiodic acid is subject to the same 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.[6]
Illicit use of hydroiodic acid
Hydriodic acid is currently listed as a 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 pseudoephedrine (recovered from decongestant pills) with hydroiodic acid and red phosphorus under heat, HI reacts with pseudoephedrine to form iodoephedrine, an intermediate which is reduced primarily to methamphetamine.[8]
Because of its listed status and closely monitored sales, clandestine chemists now use red phosphorus and iodine to generate hydroiodic acid in situ.[9][10]
Use in salt industry
Hydroiodic acid can be used to synthesize sodium iodide or potassium iodide for increasing iodine content of salt.
References
- ^ Perrin, D. D. Dissociation constants of inorganic acids and bases in aqueous solution. Butterworths, London, 1969.
- ^ Kütt, A.; Rodima, T.; Saame, J.; Raamat, E.; Mäemets, V.; Kaljurand, I.; Koppel, I. A.; Garlyauskayte, R. Yu.; Yagupolskii, Y. L.; Yagupolskii, L. M.; Bernhardt, E.; Willner, H.; Leito, I. Equilibrium Acidities of Superacids. J. Org. Chem. 2011, 76, 391–395. DOI: 10.1021/jo101409p
- ^ a b c d Wiberg, Egon; Wiberg, Nils; Holleman, Arnold Frederick (2001). Inorganic chemistry. Academic Press. pp. 371, 432–433. ISBN 0-12-352651-5.
{{cite book}}: CS1 maint: multiple names: authors list (link) - ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. pp. 809–815. ISBN 978-0-08-037941-8..
- ^ Georg Brauer, Handbook of Preparative Inorganic Chemistry, vol. 1, 2nd ed., 1963, p286.
- ^ a b Breton, G. W., P. J. Kropp, P. J.; Harvey, R. G. (2004). "Hydrogen Iodide". In L. Paquette (ed.). Encyclopedia of Reagents for Organic Synthesis. New York: Wiley & Sons. doi:10.1002/047084289X.rh039.
{{cite book}}: CS1 maint: multiple names: authors list (link) - ^ a b Bruice, Paula Yurkanis. Organic Chemistry 4th ed. Prentice Hall: Upper Saddle River, N.J, pp. 438–439, 452 (2003).
- ^ Skinner, Harry F. (1990). "Methamphetamine Synthesis via HI/Red Phosphorous Reduction of Ephedrine". Forensic Science International. 48: 123–134. doi:10.1016/0379-0738(90)90104-7.
- ^ Skinner HF (1995). "Identification and quantitation of hydroiodic acid manufactured from iodine, red phosphorus and water". Journal of the Clandestine Laboratory Investigation Chemists Association. 5: 12.[unreliable source?]
- ^ Skinner HF (1995). Microgram. 28: 349.
{{cite journal}}: Missing or empty|title=(help)[unreliable source?]
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