Hydroiodic acid

Hydroiodic acid

Hydrogen iodide
CAS number 10034-85-2 YesY
RTECS number MW3760000
Molecular formula HI
Molar mass 127.904 g/mol
Appearance Colorless gas
Density 2.85 g/mL (-47 °C)
Melting point

–50.80 °C (184.55 K)

Boiling point

–35.36 °C (237.79 K)

Solubility in water approximately 245 g/ml
Acidity (pKa) –10 (in water, estimate)[1]

2.8 (in acetonitrile)[2]

Molecular shape Terminus
Dipole moment 0.38 D
MSDS External MSDS
R-phrases R20, R21, R22, R35
S-phrases S7, S9, S26, S45
Main hazards Toxic, corrosive.
NFPA 704
Flash point Non-flammable.
Related compounds
Other anions Fluoride
Supplementary data page
Structure and
n, εr, etc.
Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
 N (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
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.

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.[3]

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. 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]

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 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).

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:[5]

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 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:'
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 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 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 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.

  • HI reduces certain α-substituted ketones and alcohols replacing the α substituent with a hydrogen atom.[6]

Illicit Use of Hydroiodic Acid

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 antihistamine 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 stereoselective, producing (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.

External links

  • International Chemical Safety Card 1326