 | |
 | |
| Names | |
|---|---|
| IUPAC name Hydrogen iodide | |
| Systematic IUPAC name Iodane | |
| Other names Hydroiodic acid (aqueous solution) Iodine hydride | |
| Identifiers | |
3D model (JSmol) | |
| ChEBI | |
| ChemSpider |
|
| ECHA InfoCard | 100.030.087 |
| EC Number |
|
| KEGG | |
| RTECS number |
|
| UNII | |
| UN number | 1787 2197 |
| |
| Properties | |
| HI | |
| Molar mass | 127.912 g·mol−1 |
| Appearance | Colorless gas |
| Odor | acrid |
| Density | 2.85 g/mL (−47 °C) |
| Melting point | −50.80 °C (−59.44 °F; 222.35 K) |
| Boiling point | −35.36 °C (−31.65 °F; 237.79 K) |
| approximately 245 g/100 ml | |
| Acidity (pKa) | −10 (in water, estimate);[1] −9.5 (±1.0)[2] 2.8 (in acetonitrile)[3] |
| Conjugate acid | Iodonium |
| Conjugate base | Iodide |
Refractive index (nD) | 1.466 (16 °C)[4] |
| Structure | |
| Terminus | |
| 0.38D | |
| Thermochemistry[4] | |
| 29.2 J·mol−1·K−1 | |
Std molar entropy(S⦵298) | 206.6 J·mol−1·K−1 |
Std enthalpy of formation(ΔfH⦵298) | 26.5 kJ·mol−1 |
Gibbs free energy(ΔfG⦵) | 1.7 kJ·mol−1 |
Enthalpy of fusion(ΔfH⦵fus) | 2.87 kJ·mol−1 |
Enthalpy of vaporization(ΔfHvap) | 17.36 kJ·mol−1 |
| Hazards | |
| Occupational safety and health (OHS/OSH): | |
Main hazards | Toxic, corrosive, harmful and irritant |
| GHS labelling: | |
  | |
| Danger | |
| H302,H314 | |
| P260,P264,P280,P301+P330+P331,P303+P361+P353,P304+P340,P305+P351+P338,P310,P321,P363,P405,P501 | |
| NFPA 704 (fire diamond) | |
| Flash point | Non-flammable |
| Lethal dose or concentration (LD, LC): | |
LD50 (median dose) | 345 mg/kg (rat, orally)[5] |
| Safety data sheet (SDS) | hydrogen iodide |
| Related compounds | |
Otheranions | Hydrogen fluoride Hydrogen chloride Hydrogen bromide Hydrogen astatide |
| Supplementary data page | |
| Hydrogen iodide (data page) | |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Hydrogen iodide (HI) is adiatomic molecule andhydrogen halide.Aqueous solutions of HI are known ashydroiodic acid or hydriodic acid, astrong acid. Hydrogen iodide and hydroiodic acid are, however, different in that the former is agas understandard conditions, whereas the latter is an aqueous solution of the gas. They are interconvertible. HI is used inorganic andinorganic synthesis as one of the primary sources ofiodine and as areducing agent.
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, the most concentrated solution having only four water molecules per molecule of HI.[6]
Hydroiodic acid is an aqueous solution of hydrogen iodide. Commercial "concentrated" hydroiodic acid usually contains 48–57% HI by mass. The solution forms anazeotrope boiling at 127°C with 57% HI, 43% water. The high acidity is caused by the dispersal of the ionic charge over the anion. Theiodide ion radius is much larger than the other commonhalides, which results in the negative charge being dispersed over a large volume. This weaker H+···I− interaction in HI facilitatesdissociation of the proton from the anion and is the reason HI is thestrongest acid of the hydrohalides.
The industrial preparation of HI involves the reaction of I2 withhydrazine, which also yieldsnitrogen gas:[7]
When the synthesis is performed in water, the HI can be purified bydistillation.
Anhydrous HI can be prepared by reaction of iodine withtetrahydronaphthalene:[8]
HI can also be distilled from a solution ofNaI or other alkali iodide that is treated with the dehydration reagentphosphorus pentoxide (which givesphosphoric acid).[9] Concentratedsulfuric acid is unsuited for acidifying iodides, as it oxidizes the iodide to elemental iodine.
An historical route to HI involves oxidation ofhydrogen sulfide with aqueous iodine:[10]
Additionally, HI can be prepared by simply combining H2 and I2:[9]
This method is usually employed to generate high-purity samples. 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 thedissociation 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 theH−H bond:[11]
In the laboratory, yet another method involveshydrolysis ofPI3, the iodineanalog ofPBr3. In this method, I2 reacts withphosphorus to createphosphorus triiodide, which then reacts with water to form HI andphosphorous acid:
Solutions of hydrogen iodide are easily oxidized by air:
HI3 is brown in color, which makes aged solutions of HI often appear dark.
Like HBr and HCl, HI adds toalkenes,[13] in a reaction that is subject to the sameMarkovnikov and anti-Markovnikov guidelines as HCl and HBr.
HI is also used in organic chemistry to convertprimary alcohols intoalkyl iodides.[14] This reaction is anSN2 substitution, in which the iodide ion replaces the "activated" hydroxyl group (water):
HI is sometimes preferred over other hydrogen halides.
HI (orHBr) can also be used to cleaveethers. Commonly, it is applied to the cleavage of aryl-alkyl ethers to give phenols and the alkyl iodide.[14] In the following idealized equationdiethyl ether is split two equivalents ofethyl iodide:
The reaction isregioselective, as iodide tends to attack the lesssterically hindered ether carbon.
HI was commonly employed as a reducing agent early on in the history of organic chemistry. Chemists in the 19th century attempted to prepare cyclohexane by HI reduction of benzene at high temperatures, but instead isolated the rearranged product, methylcyclopentane (see the article oncyclohexane). As first reported by Kiliani,[15] hydroiodic acid reduction of sugars and other polyols results in the reductive cleavage of several or even all hydroxy groups, although often with poor yield and/or reproducibility.[16] In the case of benzyl alcohols and alcohols with α-carbonyl groups, reduction by HI can provide synthetically useful yields of the corresponding hydrocarbon product (ROH + 2HI → RH + H2O + I2).[13] This process can be made catalytic in HI using red phosphorus to reduce the formed I2.[17]
Commercial processes for obtaining iodine all focus on iodide-richbrines. The purification begins by converting iodide to hydroiodic acid, which is then oxidized to iodine. The iodine is then separated by evaporation or adsorption.[18]
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