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Hydrogen Iodide

 
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Hydrogen iodide
 
 
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Hydrogen iodide (HI) is a diatomic molecule. Aqueous solutions of HI are known as iohydroic acid or hydriodic acid, a strong acid
Strong acid

A strong acid is an acid that ionizes completely in an aqueous solution , or in other terms, with a acid dissociation constant < -1.74. This generally means that in aqueous solution at standard temperature and pressure, the concentration of hydronium ions is equal to the concentration of strong acid introduced to the solution....
. 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
Organic synthesis

Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic_chemistry molecules can often contain a higher level of complexity compared to purely Inorganic_chemistry compounds, so the synthesis of organic compounds has developed into one of the most im...
 and inorganic synthesis
Inorganic chemistry

Inorganic chemistry is the branch of chemistry concerned with the properties and behavior of inorganic compounds. This field covers all chemical compounds except the myriad organic compounds , which are the subjects of organic chemistry....
 as one of the primary sources of iodine
Iodine

Iodine , is a chemical element that has the symbol I and atomic number 53. Naturally-occurring iodine is a single isotope with 74 neutrons....
 and as a reducing agent
Reducing agent

A reducing agent is the element or compound in a redox reaction that reduces another Chemical species. In doing so, it becomes oxidized, and is therefore the electron donor in the redox....
.

s a colorless gas that reacts with oxygen to give water and iodine.






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Hydrogen iodide (HI) is a diatomic molecule. Aqueous solutions of HI are known as iohydroic acid or hydriodic acid, a strong acid
Strong acid

A strong acid is an acid that ionizes completely in an aqueous solution , or in other terms, with a acid dissociation constant < -1.74. This generally means that in aqueous solution at standard temperature and pressure, the concentration of hydronium ions is equal to the concentration of strong acid introduced to the solution....
. 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
Organic synthesis

Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic_chemistry molecules can often contain a higher level of complexity compared to purely Inorganic_chemistry compounds, so the synthesis of organic compounds has developed into one of the most im...
 and inorganic synthesis
Inorganic chemistry

Inorganic chemistry is the branch of chemistry concerned with the properties and behavior of inorganic compounds. This field covers all chemical compounds except the myriad organic compounds , which are the subjects of organic chemistry....
 as one of the primary sources of iodine
Iodine

Iodine , is a chemical element that has the symbol I and atomic number 53. Naturally-occurring iodine is a single isotope with 74 neutrons....
 and as a reducing agent
Reducing agent

A reducing agent is the element or compound in a redox reaction that reduces another Chemical species. In doing so, it becomes oxidized, and is therefore the electron donor in the redox....
.

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.

Hydroiodic acid

Once again, although chemically related, hydroiodic acid is not pure HI but a mixture containing it. Commercial "concentrated" hydroiodic acid usually contains 90% - 98% HI by mass. The solution forms an azeotrope
Azeotrope

An azeotrope is a mixture of two or more liquids in such a ratio that its composition cannot be changed by simple distillation. This occurs because, when an azeotrope is boiled, the resulting vapor has the same ratio of constituents as the original mixture....
 boiling at 127 °C with 57% HI, 43% water. Hydroiodic acid is one of the strongest of all the common halide
Halide

A halide is a binary compound, of which one part is a halogen atom and the other part is an chemical element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound....
 acids due to the high stability of its corresponding conjugate base. The iodide
Iodide

An iodide ion is an iodine with a −1 electric charge. Compounds with iodine in formal oxidation state −1 are called iodides. This can include ionic compounds such as caesium iodide or covalent compounds such as phosphorus triiodide....
 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
Proton

The proton is a subatomic particle with an electric charge of +1 elementary charge. It is found in the nucleus of each atom but is also stable by itself and has a second identity as the hydrogen ion, H+....
 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
Acid dissociation constant

An acid dissociation constant, Ka, is a quantitative measure of the strong acid in solution. It is the equilibrium constant for a chemical reaction known as Dissociation in the context of acid-base reactions....
˜ 1010

HBr(g) + H2O(l) ? H3O(aq)+ + Br- (aq) Ka
Acid dissociation constant

An acid dissociation constant, Ka, is a quantitative measure of the strong acid in solution. It is the equilibrium constant for a chemical reaction known as Dissociation in the context of acid-base reactions....
˜ 109

HCl(g) + H2O(l) ? H3O(aq)+ + Cl- (aq) Ka
Acid dissociation constant

An acid dissociation constant, Ka, is a quantitative measure of the strong acid in solution. It is the equilibrium constant for a chemical reaction known as Dissociation in the context of acid-base reactions....
˜ 108

Preparation

The industrial preparation of HI involves the reaction of I2 with hydrazine
Hydrazine

Hydrazine is a chemical compound with the chemical formula N2H4. It is a colourless liquid with an ammonia-like odor and is derived from the same industrial chemistry processes that manufacture ammonia....
, which also yields nitrogen
Nitrogen

Nitrogen is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674?. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere....
 gas. 2 I2 + N2H4 ? 4 HI + N2 When performed in water, the HI must be distilled
Distillation

Distillation is a method of separation process mixtures based on differences in their Volatility in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....
.

HI can also be distilled from a solution of NaI
Sodium iodide

Sodium iodide is a white, crystalline salt with chemical formula SodiumIodine used in radiation detection, treatment of iodine deficiency, and as a reactant in the Finkelstein reaction....
 or other alkali iodide in concentrated phosphoric acid
Phosphoric acid

Phosphoric acid, also known as orthophosphoric acid or phosphoric acid, is a mineral acid having the chemical formula Hydrogen3PhosphorusOxygen4....
 (note that sulfuric acid
Sulfuric acid

Sulfuric acid, hydrogen2sulfuroxygen4, is a strong mineral acid. It is soluble in water at all concentrations. Sulfuric acid has many applications, and is one of the top products of the chemical industry....
 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
Hydrogen sulfide

Hydrogen sulfide is the chemical compound with the chemical formula Hydrogen2Sulfur. This colorless, toxic and flammable gas is partially responsible for the foul odor of egg and flatulence....
 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:

H2 + I2 + 578 nm radiation ? H2 + 2 I ? I - - - H - - - H - - - I ? 2 HI

In the laboratory, another method involves hydrolysis
Hydrolysis

Hydrolysis is a chemical reaction during which one or more water are split into hydrogen and hydroxide ions which may go on to participate in further reactions....
 of PI3
Phosphorus triiodide

Phosphorus triiodide is an unstable red solid which reacts violently with water. It is a common misconception that PI3 is too unstable to be stored; it is, in fact, commercially available....
, the iodine equivalent of PBr3
Phosphorus tribromide

Phosphorus tribromide is a colourless liquid with the formula PhosphorusBromine3. It fumes in air due to hydrolysis and has a penetrating odour....
. In this method, I2 reacts with phosphorus
Phosphorus

Phosphorus is the chemical element that has the symbol P and atomic number 15. The name comes from the and . A Valency nonmetal of the nitrogen group, phosphorus is commonly found in inorganic phosphate minerals....
 to create phosphorus triiodide
Phosphorus triiodide

Phosphorus triiodide is an unstable red solid which reacts violently with water. It is a common misconception that PI3 is too unstable to be stored; it is, in fact, commercially available....
, which then reacts with water to form HI and phosphorous acid
Phosphorous acid

Phosphorous acid is the Compound described by the chemical formula H3PO3. It can be formulated as HP2 and therefore contains phosphorus in oxidation state +3....
. 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.
  • Like HBr and HCl, HI add to alkenes
HI + H2C=CH2 ? H3CCH2I

HI is also used in organic chemistry to convert primary alcohols into alkyl halides. 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 and inert ether into more reactive species. In this example diethyl ether
Diethyl ether

Diethyl ether, also known as ether and ethoxyethane, is a clear, colorless, and highly flammable liquid with a low boiling point and a characteristic odor....
 is cleaved into ethanol and iodoethane. The reaction is regioselective, as iodide tends to attack the less sterically hindered
Steric effects

Steric effects arise from the fact that each atom within a molecule occupies a certain amount of space. If atoms are brought too close together, there is an associated cost in energy due to overlapping electron clouds , and this may affect the molecule's preferred shape and chemical reaction....
 ether carbon.

HI is subject to the same Markovnikov
Markovnikov's rule

In organic chemistry, Markovnikov's rule or Markownikoff's rule is an observation based on Zaitsev's rule. It was formulated by the Russian chemist Vladimir Vasilevich Markovnikov in 1870 ....
 and anti-Markovnikov guidelines as HCl and HBr.
  • HI reduces certain a-substituted ketones and alcohols replacing the a substituent with a hydrogen atom.


Illicit Use of Hydroiodic Acid

Hydroiodic acid is currently listed as a Federal DEA List I Chemical
DEA list of chemicals

The United States Drug Enforcement Administration maintains lists regarding not only the classification of illicit drugs . It also maintains List I of chemicals and List II of chemicals, which contain chemicals which are used to manufacture the controlled substances/illicit drugs....
. Owing to its usefulness as a reducing agent, reduction with HI and red phosphorus has become the most popular method to produce methamphetamine
Methamphetamine

is a stimulant and sympathomimetics psychoactive drug. It is a member of the family of phenylethylamines. The levorotary levomethamphetamine is an over-the-counter drug and used in Vicks Inhalers for nasal decongestion and does not possess the Central nervous system activity of dextro or racemic methamphetamine....
 in the United States. Clandestine chemists react pseudoephedrine (recovered from decongestant pills) with hydroiodic acid and red phosphorus under heat, HI reacts with psuedoephedrine to form iodoephedrine, an intermediate which is reduced primarily to methamphetamine. Due to its listed status and closely monitored sales, clandestine chemists now use red phosphorus and iodine to generate hydroiodic acid in situ.

Footnotes


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