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Hydrogen iodide
IUPAC name	Hydrogen iodide
Other names	Hydriodic acid, Hydroiodic acid
Identifiers
CAS number	10034-85-2
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)
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	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
	(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.

[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) K_a ≈ 10¹⁰

HBr_(g) + H₂O_(l) → H₃O^_(aq)+ + Br^–_(aq) K_a ≈ 10⁹

HCl_(g) + H₂O_(l) → H₃O^_(aq)+ + Cl^–_(aq) K_a ≈ 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.

Like HBr and HCl, HI add to alkenes^[3]

HI + H₂C=CH₂ → H₃CCH₂I

HI is also used in organic chemistry to convert primary alcohols into alkyl halides^[4]. This reaction is an S_N2 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 S_N1 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^[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.

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

^ ^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.
^ Greenwood, N.N. and A. Earnshaw (1997). The Chemistry of the Elements (2 ed.). Oxford: Butterworth-Heineman. pp. 809–815.
^ ^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.
^ ^a ^b Bruice, Paula Yurkanis. Organic Chemistry 4th ed. Prentice Hall: Upper Saddle River, N.J, pp. 438-439, 452 (2003).
^ 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.
^ 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.
^ Skinner HF (1995). Microgram 28: 349.

[edit] External links

International Chemical Safety Card 1326

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

[chem-0] ^ ^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.

[1] Greenwood, N.N. and A. Earnshaw (1997). The Chemistry of the Elements (2 ed.). Oxford: Butterworth-Heineman. pp. 809–815.

[Breton-2] 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.

[Bruice-3] Bruice, Paula Yurkanis. Organic Chemistry 4th ed. Prentice Hall: Upper Saddle River, N.J, pp. 438-439, 452 (2003).

[4] 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.

[5] 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.

[6] Skinner HF (1995). Microgram 28: 349.

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