Hydrogen iodide

Hydrogen iodide
Systematic name

Hydrogen iodide[1] (substitutive)
Iodidohydrogen[1] (additive)
Other names

Iodane[1]
Identifiers
CAS number 10034-85-2 Y
PubChem 24841
ChemSpider 23224 Y
UNII 694C0EFT9Q Y
EC number 233-109-9
KEGG C05590 Y
MeSH hydroiodic+acid
ChEBI CHEBI:43451 Y
ChEMBL CHEMBL1233550 N
RTECS number MW3760000
Gmelin Reference 814
Jmol-3D images Image 1
Properties
Molecular formula HI
Molar mass 127.91 g mol−1
Exact mass 127.912293452 g mol-1
Appearance Colorless gas
Density 2.85 g cm-3 (at −47 °C)
Melting point

-51 °C, 222 K, -60 °F
Boiling point

-34 °C, 239 K, -29 °F
Acidity (pKa) ≈ –9 (approximate, in water, see leveling effect),[2]

2.8 (in acetonitrile)[3]
Basicity (pKb) 23.5
Refractive index (nD) 1.466
Dipole moment 0.38 D
Thermochemistry
Std enthalpy of
formation ΔfHo298		 26.40-26.60 kJ mol-1
Standard molar
entropy So298		 206.59 J K-1 mol-1
Specific heat capacity, C 228.3 mJ K-1 g-1
Hazards
MSDS External MSDS
EU Index 053-002-00-9
EU classification C
R-phrases R35
S-phrases (S1/2), S9, S26, S36/37/39, S45
NFPA 704
0
3
0
COR
Related compounds
Related compounds Hydrogen astatide

Hydrogen bromide
Hydrogen chloride
Hydrogen fluoride
Indium hydride
Rubidium hydride
Stibine
Supplementary data page
Structure and
properties		 n, εr, etc.
Thermodynamic
data		 Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
 N (verify) (what is: Y/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 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.

Contents

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, 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]).[4]

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.[5]

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[6] (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:[4]

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 in two steps.[4] The first step involves the reaction of iodine with phosphorous:

3 I2 + 2 P → 2 PI3

The second step involves water reacting with phosphorous tri-iodide

2 PI3 + 6 H2O → 6 HI + 2 H3PO3

Key reactions and applications

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.[8] 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[8] 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.

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 operators 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.[9]

Because of its listed status and closely monitored sales, clandestine operators now use red phosphorus and iodine to generate hydroiodic acid in situ.[10][11]

Use in salt industry

Hydroiodic acid can be used to synthesize sodium iodide or potassium iodide for increasing iodine content of salt.

2K(s) + 2HI(aq) -> 2KI(aq) + H2 (g)

References

  1. ^ a b c "hydrogen iodide (CHEBI:43451)". Chemical Entities of Biological Interest (ChEBI). UK: European Bioinformatics Institute. IUPAC Names. http://www.ebi.ac.uk/chebi/searchId.do?chebiId=CHEBI:43451. 
  2. ^ Perrin, D.D., Ionisation Constants of Inorganic Acids and Bases in Aqueous Solution; 2nd Ed., Pergamon Pres: Oxford, 1982.
  3. ^ 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
  4. ^ 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. http://books.google.com/?id=vEwj1WZKThEC&pg=PA432. 
  5. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Oxford: Butterworth-Heinemann. pp. 809–815. ISBN 0080379419. .
  6. ^ Georg Brauer, Handbook of Preparative Inorganic Chemistry, vol. 1, 2nd ed., 1963, p286.
  7. ^ a b Breton, G. W., P. J. Kropp, P. J.; Harvey, R. G. (2004). "Hydrogen Iodide". In L. Paquette. Encyclopedia of Reagents for Organic Synthesis. New York: Wiley & Sons. doi:10.1002/047084289X.rh039. 
  8. ^ a b Bruice, Paula Yurkanis. Organic Chemistry 4th ed. Prentice Hall: Upper Saddle River, N.J, pp. 438–439, 452 (2003).
  9. ^ Skinner, Harry F. (1990). "Methamphetamine Synthesis via HI/Red Phosphorous Reduction of Ephedrine". Forensic Science International 48 (2): 123–134. doi:10.1016/0379-0738(90)90104-7. 
  10. ^ 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. 
  11. ^ Skinner HF (1995). Microgram 28: 349. 

External links

HI He
LiI BeI2 BI3 CI4 NI3 I2O4, I2O5, I4O9 IF, IF3, IF5, IF7 Ne
NaI MgI2 AlI3 SiI4 PI3, P2I4 S ICl, ICl3 Ar
KI CaI2 Sc TiI4 VI3 Cr MnI2 Fe CoI2 NiI2 CuI ZnI2 Ga2I6 GeI2, GeI4 AsI3 Se IBr Kr
RbI SrI2 Y ZrI4 Nb Mo Tc Ru Rh Pd AgI CdI2 InI3 SnI4, SnI2 SbI3 TeI4 I Xe
CsI BaI2   Hf Ta W Re Os Ir Pt AuI Hg2I2, HgI2 TlI PbI2 BiI3 Po At Rn
Fr Ra   Rf Db Sg Bh Hs Mt Ds Rg Cn Uut Uuq Uup Uuh Uus Uuo
↓
La Ce Pr Nd Pm SmI2 Eu Gd TbI3 Dy Ho Er Tm Yb Lu
Ac ThI4 Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr