Hydrogen iodide is a diatomic molecule. Aqueous solutions of HI are known as
iohydroic acid or
hydroiodic acid, a
strong acidA strong acid is an acid that ionizes completely in an aqueous solution by losing one proton, according to the equationFor sulfuric acid which is diprotic, the "strong acid" designation refers only to dissociation of the first protonMore precisely, the acid must be stronger in aqueous solution than...
. Gas and aqueous solution are interconvertible. HI is used in
organicOrganic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...
and inorganic synthesis as one of the primary sources of
iodineIodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
and as a
reducing agentA reducing agent is the element or compound in a reduction-oxidation reaction that donates an electron to another species; however, since the reducer loses an electron we say it is "oxidized"...
.
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
azeotropeAn 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 common acids due to the high stability of its corresponding conjugate base. The
iodideAn iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. This page is for the iodide ion and its salts. For information on organoiodides, see organohalides. In everyday life, iodide is most commonly encountered as a component of iodized salt,...
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
protonThe proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
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]).
- HI(g) + H2O(l) → H3O(aq)+ + I–(aq) Ka
An acid dissociation constant, Ka, is a quantitative measure of the strength of an 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
An acid dissociation constant, Ka, is a quantitative measure of the strength of an 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
An acid dissociation constant, Ka, is a quantitative measure of the strength of an 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 I
2 with
hydrazineHydrazine is an inorganic compound with the formula N2H4. It is a colourless flammable liquid with an ammonia-like odor. Hydrazine is highly toxic and dangerously unstable unless handled in solution. Approximately 260,000 tons are manufactured annually...
, which also yields
nitrogenNitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
gas.
- 2 I2 + N2H4 → 4 HI + N2
When performed in water, the HI must be
distilledDistillation is a method of separating mixtures based on differences in volatilities of components 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
NaISodium iodide is a white, crystalline salt with chemical formula NaI used in radiation detection, treatment of iodine deficiency, and as a reactant in the Finkelstein reaction.-Uses:Sodium iodide is commonly used to treat and prevent iodine deficiency....
or other alkali iodide in concentrated
hypophosphorous acidHypophosphorous acid is a phosphorus oxoacid and a powerful reducing agent with molecular formula H3PO2. Inorganic chemists refer to the free acid by this name , or the acceptable name of phosphinic acid. It is a colorless low-melting compound, which is soluble in water, dioxane, and alcohols...
(note that
sulfuric acidSulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
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 sulfideHydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
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 H
2 and I
2. 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 H
2 and I
2. However, when a mixture of the gases is irradiated with the wavelength of light equal to the dissociation energy of I
2, about 578 nm, the rate increases significantly. This supports a mechanism whereby I
2 first dissociates into 2 iodine atoms, which each attach themselves to a side of an H
2 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
hydrolysisHydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
of
PI3Phosphorus 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. It is widely used in organic chemistry for converting alcohols to alkyl iodides. It is also a powerful...
, the iodine equivalent of
PBr3Phosphorus tribromide is a colourless liquid with the formula PBr3. It fumes in air due to hydrolysis and has a penetrating odour. It is widely used in the laboratory for the conversion of alcohols to alkyl bromides.-Preparation:...
. In this method, I
2 reacts with
phosphorusPhosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
to create
phosphorus triiodidePhosphorus 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. It is widely used in organic chemistry for converting alcohols to alkyl iodides. It is also a powerful...
, which then reacts with water to form HI and
phosphorous acidPhosphorous acid is the compound described by the formula H3PO3. This acid is diprotic , not triprotic as might be suggested by this formula. Phosphorous acid is as an intermediate in the preparation of other phosphorus compounds.-Nomenclature and tautomerism:H3PO3 is more clearly described with...
.
- 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
HI
3 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 S
N2 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 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 and inert ether into more reactive species. In this example
diethyl etherDiethyl ether, also known as ethyl ether, simply ether, or ethoxyethane, is an organic compound in the ether class with the formula . It is a colorless, highly volatile flammable liquid with a characteristic odor...
is cleaved into ethanol and iodoethane. The reaction is regioselective, as iodide tends to attack the less
sterically hinderedSteric 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 reactivity.-Steric...
ether carbon.
Hydroiodic acid is subject to the same
MarkovnikovIn 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 α-substituted ketones and alcohols replacing the α substituent with a hydrogen atom.
Illicit use of hydroiodic acid
Hydriodic acid is currently listed as a
Federal DEA List I ChemicalThe 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 psychostimulant of the phenethylamine and amphetamine class of psychoactive drugs...
in the United States. Clandestine chemists 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.
Because of its listed status and closely monitored sales, clandestine chemists now use red phosphorus and iodine to generate hydroiodic acid
in situ.
Use in salt industry
Hydroiodic acid can be used to synthesize
sodium iodideSodium iodide is a white, crystalline salt with chemical formula NaI used in radiation detection, treatment of iodine deficiency, and as a reactant in the Finkelstein reaction.-Uses:Sodium iodide is commonly used to treat and prevent iodine deficiency....
or
potassium iodidePotassium iodide is an inorganic compound with the chemical formula KI. This white salt is the most commercially significant iodide compound, with approximately 37,000 tons produced in 1985. It is less hygroscopic than sodium iodide, making it easier to work with...
for increasing
iodineIodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
content of salt.
- 2K(s) + 2HI(aq) -> 2KI(aq) + H2 (g)
External links