Hydrogen iodide

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Hydrogen iodide
Hydrogen iodide
Hydrogen iodide
IUPAC name
Other names Hydriodic acid, Hydroiodic acid
Identifiers
CAS number 10034-85-2 Yes check.svgY
PubChem 24841
RTECS number MW3760000
Properties
Molecular formula HI
Molar mass 127.904 g/mol
Appearance Colorless gas
Density 2.85 g/mL (-47 °C)
Melting point

–50.80 °C (222.35 K)[1]
Boiling point

–34.36 °C (237.79 K)
Acidity (pKa) –10
Refractive index (nD) 1.466
Structure
Molecular shape Terminus
Dipole moment 0.38 D
Thermochemistry
Std enthalpy of
formation ΔfHo298		 0.2072 kJ/g
Specific heat capacity, C 0.2283 J/(g·K)
Hazards
MSDS External MSDS
R-phrases R20, R21, R22, R35
S-phrases S7, S9, S26, S45
NFPA 704
NFPA 704.svg
0
3
0
COR
Flash point Non-flammable.
Related compounds
Other anions "Bond Type: Polar covalent"
Hydrogen fluoride
Hydrogen chloride
Hydrogen bromide
Supplementary data page
Structure and
properties		 n, εr, etc.
Thermodynamic
data		 Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
 Yes check.svgY (what is this?)  (verify)
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 hydriodic acid, a strong acid. Hydrogen iodide and hydriodic 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.

Contents

[edit] 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 hydriodic acid. It is exceptionally soluble in water, giving hydriodic acid. One liter of water will dissolve 425 liters of HI, the final solution having only four water molecules per molecule of HI.[1]

[edit] Hydriodic acid

Once again, although chemically related, hydriodic acid is not pure HI but a mixture containing it. Commercial "concentrated" hydriodic acid usually contains 90-98% HI by mass. The solution forms an azeotrope boiling at 127 °C with 57% HI, 43% water. Hydriodic acid is one of the strongest of all the common halide acids due to the high stability of its corresponding conjugate base. The iodide ion is much larger than the other common halides which results in the negative charge being dispersed over a larger 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.Also has been known to have been created in volcanic explosions. 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.[1]

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

[edit] Preparation

The industrial preparation of HI involves the reaction of I2 with hydrazine, which also yields nitrogen gas.[2]

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 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 hydriodic 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:[1]

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.[1]

3 I2 + 2 P + 6 H2O → 2 PI3 + 6 H2O → 6 HI + 2 H3PO3

[edit] Key reactions and applications

  • HI will undergo oxidation if left open to air according to the following pathway:[1]
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[4]. 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.

HIsubstitution.gif

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[4] 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.

DEEcleavage.gif

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.[3]

[edit] Illicit use of hydriodic acid

Lab using the HI/P method

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 hydriodic acid and red phosphorus under heat, HI reacts with pseudoephedrine to form iodoephedrine, an intermediate which is reduced primarily to methamphetamine.[5].

Because of its listed status and closely monitored sales, clandestine chemists now use red phosphorus and iodine to generate hydriodic acid in situ[6][7].

[edit] Use in salt industry

Hydriodic acid can be used to synthesize Sodium iodide or Potassium iodide for increasing iodine content of salt.

[edit] References

  1. ^ a b c d e f Wiberg, Egon; Wiberg, Nils; Holleman, Arnold Frederick (2001). Inorganic chemistry. Academic Press. pp. 371,432–433. ISBN 0123526515. http://books.google.com/books?id=vEwj1WZKThEC&pg=PA432. 
  2. ^ Greenwood, N.N. and A. Earnshaw (1997). The Chemistry of the Elements (2 ed.). Oxford: Butterworth-Heineman. pp. 809–815. 
  3. ^ 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:10.1002/047084289. 
  4. ^ a b Bruice, Paula Yurkanis. Organic Chemistry 4th ed. Prentice Hall: Upper Saddle River, N.J, pp. 438-439, 452 (2003).
  5. ^ Skinner, Harry F. (1990). "Methamphetamine Synthesis via HI/Red Phosphorous Reduction of Ephedrine". Forensic Science International 48: 128–134. http://designer-drugs.com/pte/12.162.180.114/dcd/chemistry/meth.hi-rp.html. 
  6. ^ Skinner HF (1995). "Identification and quantitation of hydriodic acid manufactured from iodine, red phosphorus and water". Journal of the Clandestine Laboratory Investigation Chemists Association 5: 12. http://www.designer-drugs.com/pte/12.162.180.114/dcd/chemistry/clic.html. 
  7. ^ Skinner HF (1995). Microgram 28: 349. 

[edit] 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 Bi Po At Rn
Fr Ra   Rf Db Sg Bh Hs Mt Ds Rg Uub 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
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