Hydrogen iodide

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Hydrogen iodide
Hydrogen-iodide-2D-dimensions.svg
Hydrogen-iodide-3D-vdW.svg
Names
IUPAC name
Hydrogen iodide
Systematic IUPAC name
Iodane
Other names
Hydroiodic acid (aqueous solution)
Iodine hydride
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.030.087 OOjs UI icon edit-ltr-progressive.svg
EC Number
  • 233-109-9
KEGG
PubChem CID
RTECS number
  • MW3760000
UNII
UN number 1787 2197
  • InChI=1S/HI/h1H Yes check.svgY
    Key: XMBWDFGMSWQBCA-UHFFFAOYSA-N Yes check.svgY
  • I
Properties
HI
Molar mass 127.912 g·mol−1
AppearanceColorless gas
Odor acrid
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)
approximately 245 g/100 ml
Acidity (pKa)−10 (in water, estimate); [1] −9.5 (±1.0) [2]

2.8 (in acetonitrile) [3]

Conjugate acid Iodonium
Conjugate base Iodide
1.466 (16 °C) [4]
Structure
Terminus
0.38 D
Thermochemistry [4]
29.2 J·mol−1·K−1
Std molar
entropy (S⦵298)
206.6 J·mol−1·K−1
26.5 kJ·mol−1
1.7 kJ·mol−1
Enthalpy of fusion (ΔfH⦵fus)
2.87 kJ·mol−1
17.36 kJ·mol−1
Hazards
Occupational safety and health (OHS/OSH):
Main hazards
Toxic, corrosive, harmful and irritant
GHS labelling:
GHS-pictogram-acid.svg GHS-pictogram-skull.svg
Danger
H302, H314
P260, P264, P280, P301+P330+P331, P303+P361+P353, P304+P340, P305+P351+P338, P310, P321, P363, P405, P501
NFPA 704 (fire diamond)
NFPA 704.svgHealth 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasFlammability 0: Will not burn. E.g. waterInstability 1: Normally stable, but can become unstable at elevated temperatures and pressures. E.g. calciumSpecial hazard COR: Corrosive; strong acid or base. E.g. sulfuric acid, potassium hydroxide
3
0
1
COR
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
345 mg/kg (rat, orally) [5]
Safety data sheet (SDS) hydrogen iodide
Related compounds
Other anions
Hydrogen fluoride
Hydrogen chloride
Hydrogen bromide
Hydrogen astatide
Supplementary data page
Hydrogen iodide (data page)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
X mark.svgN  verify  (what is  Yes check.svgYX mark.svgN ?)

Hydrogen iodide (HI) is a diatomic molecule and hydrogen halide. 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 the 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

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 most concentrated solution having only four water molecules per molecule of HI. [6]

Hydroiodic acid

Hydroiodic acid is not pure hydrogen iodide, 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 radius 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

Synthesis

The industrial preparation of HI involves the reaction of I2 with hydrazine, which also yields nitrogen gas: [7]

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): [8]

H2S + I2 → 2 HI + S

Additionally, HI can be prepared by simply combining H2 and I2: This is a reversible reaction (using conditions 250°C)

H2 + I2 → 2 HI

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 : [9]

H2 + I2 + (578 nm radiation) → H2 + 2I → I···H···H···I → 2HI

In the laboratory, another method involves hydrolysis of PI3, the iodine analog 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

Solutions of hydrogen iodide are easily oxidized by air:

4 HI + O2 → 2 H2O + 2 I2
HI + I2 ⇌ HI3 [10]

HI3 is dark brown in color, which makes aged solutions of HI often appear dark brown.

Like HBr and HCl, HI adds to alkenes: [11]

HI + H2C=CH2 → H3CCH2I

HI is also used in organic chemistry to convert primary alcohols into alkyl iodides. [12] This reaction is an SN2 substitution, in which the iodide ion replaces the "activated" hydroxyl group (water):

HIsubstitution.gif

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 [12] and inert ether into more reactive species. In this example diethyl ether is split into ethanol and iodoethane:

HI ether cleavage.png

The reaction is regioselective, as iodide tends to attack the less sterically hindered ether carbon. If an excess of HI is used, the alcohol formed in this reaction will be converted to a 2nd equivalent of alkyl iodide, as in the conversion of primary alcohols into alkyl iodides.

HI is subject to the same Markovnikov and anti-Markovnikov guidelines as HCl and HBr.

Although harsh by modern standards, HI was commonly employed as a reducing agent early on in the history of organic chemistry. Chemists in the 19th century attempted to prepare cyclohexane by HI reduction of benzene at high temperatures, but instead isolated the rearranged product, methylcyclopentane (see the article on cyclohexane ). As first reported by Kiliani, [13] hydroiodic acid reduction of sugars and other polyols results in the reductive cleavage of several or even all hydroxy groups, although often with poor yield and/or reproducibility. [14] In the case of benzyl alcohols and alcohols with α-carbonyl groups, reduction by HI can provide synthetically useful yields of the corresponding hydrocarbon product (ROH + 2HI → RH + H2O + I2). [11] This process can be made catalytic in HI using red phosphorus to reduce the formed I2. [15]

See also

Related Research Articles

<span class="mw-page-title-main">Acid</span> Chemical compound giving a proton or accepting an electron pair

An acid is a molecule or ion capable of either donating a proton (i.e. hydrogen ion, H+), known as a Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis acid.

<span class="mw-page-title-main">Acid–base reaction</span> Chemical reaction between an acid and a base

In chemistry, an acid–base reaction is a chemical reaction that occurs between an acid and a base. It can be used to determine pH via titration. Several theoretical frameworks provide alternative conceptions of the reaction mechanisms and their application in solving related problems; these are called the acid–base theories, for example, Brønsted–Lowry acid–base theory.

<span class="mw-page-title-main">Ether</span> Organic compounds made of alkyl/aryl groups bound to oxygen (R–O–R)

In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two organyl groups. They have the general formula R−O−R′, where R and R′ represent organyl groups. Ethers can again be classified into two varieties: if the organyl groups are the same on both sides of the oxygen atom, then it is a simple or symmetrical ether, whereas if they are different, the ethers are called mixed or unsymmetrical ethers. A typical example of the first group is the solvent and anaesthetic diethyl ether, commonly referred to simply as "ether". Ethers are common in organic chemistry and even more prevalent in biochemistry, as they are common linkages in carbohydrates and lignin.

<span class="mw-page-title-main">Halogen</span> Group of chemical elements

The halogens are a group in the periodic table consisting of six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and the radioactive elements astatine (At) and tennessine (Ts), though some authors would exclude tennessine as its chemistry is unknown and is theoretically expected to be more like that of gallium. In the modern IUPAC nomenclature, this group is known as group 17.

<span class="mw-page-title-main">Iodine</span> Chemical element, symbol I and atomic number 53

Iodine is a chemical element; it has symbol I and atomic number 53. The heaviest of the stable halogens, it exists at standard conditions as a semi-lustrous, non-metallic solid that melts to form a deep violet liquid at 114 °C (237 °F), and boils to a violet gas at 184 °C (363 °F). The element was discovered by the French chemist Bernard Courtois in 1811 and was named two years later by Joseph Louis Gay-Lussac, after the Ancient Greek Ιώδης 'violet-coloured'.

In chemistry, hydronium (hydroxonium in traditional British English) is the common name for the cation [H3O]+, also written as H3O+, the type of oxonium ion produced by protonation of water. It is often viewed as the positive ion present when an Arrhenius acid is dissolved in water, as Arrhenius acid molecules in solution give up a proton (a positive hydrogen ion, H+) to the surrounding water molecules (H2O). In fact, acids must be surrounded by more than a single water molecule in order to ionize, yielding aqueous H+ and conjugate base. Three main structures for the aqueous proton have garnered experimental support: the Eigen cation, which is a tetrahydrate, H3O+(H2O)3, the Zundel cation, which is a symmetric dihydrate, H+(H2O)2, and the Stoyanov cation, an expanded Zundel cation, which is a hexahydrate: H+(H2O)2(H2O)4. Spectroscopic evidence from well-defined IR spectra overwhelmingly supports the Stoyanov cation as the predominant form. For this reason, it has been suggested that wherever possible, the symbol H+(aq) should be used instead of the hydronium ion.

<span class="mw-page-title-main">Ammonium</span> Chemical compound

The ammonium cation is a positively charged polyatomic ion with the chemical formula NH+4 or [NH4]+. It is formed by the protonation of ammonia. Ammonium is also a general name for positively charged (protonated) substituted amines and quaternary ammonium cations, where one or more hydrogen atoms are replaced by organic or other groups.

<span class="mw-page-title-main">Haloalkane</span> Group of chemical compounds derived from alkanes containing one or more halogens

The haloalkanes are alkanes containing one or more halogen substituents. They are a subset of the general class of halocarbons, although the distinction is not often made. Haloalkanes are widely used commercially. They are used as flame retardants, fire extinguishants, refrigerants, propellants, solvents, and pharmaceuticals. Subsequent to the widespread use in commerce, many halocarbons have also been shown to be serious pollutants and toxins. For example, the chlorofluorocarbons have been shown to lead to ozone depletion. Methyl bromide is a controversial fumigant. Only haloalkanes that contain chlorine, bromine, and iodine are a threat to the ozone layer, but fluorinated volatile haloalkanes in theory may have activity as greenhouse gases. Methyl iodide, a naturally occurring substance, however, does not have ozone-depleting properties and the United States Environmental Protection Agency has designated the compound a non-ozone layer depleter. For more information, see Halomethane. Haloalkane or alkyl halides are the compounds which have the general formula "RX" where R is an alkyl or substituted alkyl group and X is a halogen.

Demethylation is the chemical process resulting in the removal of a methyl group (CH3) from a molecule. A common way of demethylation is the replacement of a methyl group by a hydrogen atom, resulting in a net loss of one carbon and two hydrogen atoms.

The Winkler test is used to determine the concentration of dissolved oxygen in water samples. Dissolved oxygen (D.O.) is widely used in water quality studies and routine operation of water reclamation facilities to analyze its level of oxygen saturation.

<span class="mw-page-title-main">Nitrous acid</span> Chemical compound

Nitrous acid is a weak and monoprotic acid known only in solution, in the gas phase, and in the form of nitrite salts. It was discovered by Carl Wilhelm Scheele, who called it "phlogisticated acid of niter". Nitrous acid is used to make diazonium salts from amines. The resulting diazonium salts are reagents in azo coupling reactions to give azo dyes.

<span class="mw-page-title-main">Hydroiodic acid</span> Aqueous solution of hydrogen iodide

Hydroiodic acid is a colorless and aqueous solution of hydrogen iodide (HI). It is a strong acid, which is ionized completely in an aqueous solution. Concentrated solutions of hydroiodic acid are usually 48% to 57% HI.

<span class="mw-page-title-main">Phosphorus triiodide</span> Chemical compound

Phosphorus triiodide (PI3) is an inorganic compound with the formula PI3. A red solid, it is too unstable to be stored for long periods of time; it is, nevertheless, commercially available. It is widely used in organic chemistry for converting alcohols to alkyl iodides. It is also a powerful reducing agent.

<span class="mw-page-title-main">Iodic acid</span> Chemical compound (HIO3)

Iodic acid is a white water-soluble solid with the chemical formula HIO3. Its robustness contrasts with the instability of chloric acid and bromic acid. Iodic acid features iodine in the oxidation state +5 and is one of the most stable oxo-acids of the halogens. When heated, samples dehydrate to give iodine pentoxide. On further heating, the iodine pentoxide further decomposes, giving a mix of iodine, oxygen and lower oxides of iodine.

Iodometry, known as iodometric titration, is a method of volumetric chemical analysis, a redox titration where the appearance or disappearance of elementary iodine indicates the end point.

Iodine compounds are compounds containing the element iodine. Iodine can form compounds using multiple oxidation states. Iodine is quite reactive, but it is much less reactive than the other halogens. For example, while chlorine gas will halogenate carbon monoxide, nitric oxide, and sulfur dioxide, iodine will not do so. Furthermore, iodination of metals tends to result in lower oxidation states than chlorination or bromination; for example, rhenium metal reacts with chlorine to form rhenium hexachloride, but with bromine it forms only rhenium pentabromide and iodine can achieve only rhenium tetraiodide. By the same token, however, since iodine has the lowest ionisation energy among the halogens and is the most easily oxidised of them, it has a more significant cationic chemistry and its higher oxidation states are rather more stable than those of bromine and chlorine, for example in iodine heptafluoride.

<span class="mw-page-title-main">Magnesium iodide</span> Chemical compound

Magnesium iodide is an inorganic compound with the chemical formula MgI2. It forms various hydrates MgI2·xH2O. Magnesium iodide is a salt of magnesium and hydrogen iodide. These salts are typical ionic halides, being highly soluble in water.

Iron(II) iodide is an inorganic compound with the chemical formula FeI2. It is used as a catalyst in organic reactions.

Samarium(III) iodide is an inorganic compound, a salt of samarium and hydroiodic acid with the chemical formula SmI
3.

Europium(III) iodide is an inorganic compound containing europium and iodine with the chemical formula EuI3.

References

  1. ↑ Bell, R.P. The Proton in Chemistry. 2nd ed., Cornell University Press, Ithaca, NY, 1973.
  2. ↑ Trummal, A.; Lipping, L.; Kaljurand, I.; Koppel, I. A.; Leito, I. "Acidity of Strong Acids in Water and Dimethyl Sulfoxide" J. Phys. Chem. A. 2016, 120, 3663-3669. doi:10.1021/acs.jpca.6b02253
  3. ↑ Raamat, E.; Kaupmees, K.; Ovsjannikov, G.; Trummal, A.; Kütt, A.; Saame, J.; Koppel, I.; Kaljurand, I.; Lipping, L.; Rodima, T.; Pihl, V.; Koppel, I. A.; Leito, I. "Acidities of strong neutral Brønsted acids in different media." J. Phys. Org. Chem. 2013, 26, 162-170. doi:10.1002/poc.2946
  4. 1 2 CRC handbook of chemistry and physics: a ready-reference book of chemical and physical data. William M. Haynes, David R. Lide, Thomas J. Bruno (2016-2017, 97th ed.). Boca Raton, Florida. 2016. ISBN   978-1-4987-5428-6. OCLC   930681942.{{cite book}}: CS1 maint: location missing publisher (link) CS1 maint: others (link)
  5. ↑ Hydrogen iodide: toxicity
  6. ↑ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN   0-12-352651-5.
  7. ↑ Greenwood, N. N. and A. Earnshaw. The Chemistry of the Elements. 2nd ed. Oxford: Butterworth-Heineman. p 809–815. 1997.
  8. ↑ Joseph Louis Gay-Lussac (1815), "A Memoir on Iodine", Annals of Philosophy , 5: 101
  9. ↑ Holleman, A. F. Wiberg, E. Inorganic Chemistry. San Diego: Academic Press. p. 371, 432–433. 2001.
  10. ↑ Magnuson, J.A.; Wolfenden, J.H. (December 1, 1956). "Hydrogen Tri-iodide". Journal of Physical Chemistry . 60 (12): 1665–1666. doi:10.1021/j150546a021 . Retrieved May 26, 2022.
  11. 1 2 Breton, G. W., P. J. Kropp, P. J.; Harvey, R. G. "Hydrogen Iodide" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. doi : 10.1002/047084289X.
  12. 1 2 Bruice, Paula Yurkanis. Organic Chemistry 4th ed. Prentice Hall: Upper Saddle River, N. J, 2003 p. 438–439, 452.
  13. ↑ Kiliani, Heinrich (1886-01-01). "Ueber die Constitution der Dextrosecarbonsäure". Berichte der Deutschen Chemischen Gesellschaft. 19 (1): 1128–1130. doi:10.1002/cber.188601901251. ISSN   1099-0682.
  14. ↑ Perlin, A. S.; Purves, C. B. (1953-03-01). "Kiliani's Reduction of Glucose and Fructose Cyanohydrins to the Corresponding Heptanoic Acids and Lactones". Canadian Journal of Chemistry. 31 (3): 227–236. doi: 10.1139/v53-033 . ISSN   0008-4042.
  15. ↑ Dobmeier, Michael; Herrmann, Josef M; Lenoir, Dieter; König, Burkhard (2012-03-02). "Reduction of benzylic alcohols and α-hydroxycarbonyl compounds by hydriodic acid in a biphasic reaction medium". Beilstein Journal of Organic Chemistry. 8 (1): 330–336. doi:10.3762/bjoc.8.36. PMC   3302097 . PMID   22423302.
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