出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/06/25 07:56:53」(JST)
| Names | |
|---|---|
| IUPAC name
Hydrogen iodide
|
|
| Other names
hydroiodic acid
|
|
| Identifiers | |
|
CAS Registry Number
|
10034-85-2 Y |
| RTECS number | MW3760000 |
| Properties | |
|
Chemical formula
|
HI |
| Molar mass | 127.904 g/mol |
| Appearance | Colorless gas |
| 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) |
|
Solubility in water
|
approximately 245 g/100 ml |
| Acidity (pKa) | −10 (in water, estimate)[1] 2.8 (in acetonitrile)[2] |
| Structure | |
|
Molecular shape
|
Terminus |
|
Dipole moment
|
0.38 D |
| Hazards | |
| Main hazards | Toxic, corrosive, Harmful and Irritant |
| Safety data sheet | See: data page hydrogen iodide |
| R-phrases | R20, R21, R22, R35 |
| S-phrases | S7, S9, S26, S45 |
| NFPA 704 |
0
3
1
COR
|
| Flash point | Non-flammable |
| Related compounds | |
|
Other anions
|
Fluoride Chloride |
| Supplementary data page | |
|
Structure and
properties |
Refractive index (n), Dielectric constant (εr), etc. |
|
Thermodynamic
data |
Phase behaviour solid–liquid–gas |
|
Spectral data
|
UV, IR, NMR, MS |
|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|
|
| N verify (what is: Y/N?) | |
| Infobox references | |
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 and inorganic synthesis as one of the primary sources of iodine and as a reducing 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, the most concentrated solution having only four water molecules per molecule of HI.[3]
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. 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+---I− interaction in HI facilitates dissociation of the proton from the anion, and is the reason HI is the strongest acid of the hydrohalides.
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≈ 106
The industrial preparation of HI involves the reaction of I2 with hydrazine, which also yields nitrogen gas.[4]
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 phosphoric acid (note that concentrated 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).
Additionally HI can be prepared by simply combining H2 and I2. 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 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:[5]
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.
HI3 is dark brown in color, which makes aged solutions of HI often appear dark brown.
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 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 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 significant because it can be used to convert a chemically stable[7] and inert ether into more reactive species. In this example diethyl ether is split into ethanol and iodoethane. The reaction is regioselective, as iodide tends to attack the less sterically hindered ether carbon.
HI is subject to the same Markovnikov and anti-Markovnikov guidelines as HCl and HBr.
Hydroiodic 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 nasal 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] This reaction is stereospecific, producing only (d)-methamphetamine.
Due to its listed status and closely monitored sales, clandestine chemists now use red phosphorus and iodine to generate hydroiodic acid in situ.[9]
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.
|
||||||
|
全文を閲覧するには購読必要です。 To read the full text you will need to subscribe.
| 関連記事 | 「iodide」 |