HYDROIODIC ACID (HI): STRUCTURE, PROPERTIES AND USES - CHEMISTRY - 2025

The hydroiodic acid is an aqueous solution of hydrogen iodide which is characterized by its high acidity. A definition closer to chemical terminology and IUPAC, is that it is a hydracid, whose chemical formula is HI.

However, to differentiate it from gaseous hydrogen iodide molecules, HI (g) is denoted as HI (aq). It is for this reason that in chemical equations it is important to identify the medium or physical phase in which the reactants and products are found. Even so, confusion between hydrogen iodide and hydroiodic acid is common.

Ions of hydroiodic acid. Source: Gabriel Bolívar.

If the molecules committed in their identity are observed, noticeable differences will be found between HI (g) and HI (ac). In HI (g), there is an HI bond; while in HI (ac), they are actually a pair of I ^- and H ₃ O ⁺ ions interacting electrostatically (upper image).

On the other hand, HI (ac) is a source of HI (g), since the first is prepared by dissolving the second in water. Because of this, unless it is in a chemical equation, HI can be used to refer to hydroiodic acid as well. HI is a strong reducing agent and an excellent source of I ^- ions in aqueous medium.

Structure of hydroiodic acid

Hydroiodic acid, as just explained, consists of a solution of HI in water. Being in water, the HI molecules completely dissociate (strong electrolyte), originating the I ^- and H ₃ O ⁺ ions. This dissociation can be represented by the following chemical equation:

HI (g) + H ₂ O (l) => I ^- (aq) + H ₃ O ⁺ (aq)

What would be equivalent if it were written as:

HI (g) + H ₂ O (l) => HI (aq)

However, HI (ac) does not reveal at all what has happened to the gaseous HI molecules; it only indicates that they are in an aqueous medium.

Therefore, the true structure of HI (ac) consists of the I ^- and H ₃ O ⁺ ions surrounded by water molecules, hydrating them; the more concentrated the hydroiodic acid, the smaller the number of unprotonated water molecules.

Commercially in fact the concentration of HI is 48 to 57% in water; more concentrated would be equivalent to having an acid that is too fuming (and even more dangerous).

In the image, it can be seen that the anion I ^- is represented by a purple sphere, and H ₃ O ⁺ with white spheres and a red sphere, for the oxygen atom. The H ₃ O ⁺ cation has trigonal pyramid molecular geometry (seen from a higher plane in the image).

Properties

Physical description

Colorless liquid; but, it can exhibit yellowish and brown tones if it is in direct contact with oxygen. This is because the I ^- ions end up oxidizing to molecular iodine, I ₂. If there is a lot of I ₂, it is more than likely that the triiodide anion, I ₃ ^{- is formed}, which turns the solution brown.

Molecular mass

127.91 g / mol.

Odor

Acre.

Density

The density is 1.70 g / mL for the 57% HI solution; since, the densities vary depending on the different concentrations of HI. At this concentration an azeotrope is formed (it is distilled as a single substance and not as a mixture) to whose relative stability it may be due its commercialization over other solutions.

Boiling point

The 57% HI azeotrope boils at 127 ° C at a pressure of 1.03 bar (GO TO ATM).

pKa

-1.78.

Acidity

It is an extremely strong acid, so much so that it is corrosive to all metals and fabrics; even for rubbers.

This is because the HI bond is very weak, and it breaks easily during ionization in water. Furthermore, hydrogen bonds I ^- - HOH ₂ ⁺ are weak, so there is nothing to interfere with H ₃ O ⁺ reacting with other compounds; that is to say, the H ₃ O ⁺ has become “free”, like the I ^- which does not attract its counterion with too much force.

Reducing agent

HI is a powerful reducing agent, the main reaction product of which is I ₂.

Nomenclature

The nomenclature for hydroiodic acid derives from the fact that iodine "works" with a single oxidation state: -1. And also, the same name indicates that it has water within its structural formula. This is its only name, as it is not a pure compound but a solution.

Applications

Source of iodine in organic and inorganic syntheses

HI is an excellent source of I ions ^- for inorganic and organic synthesis, and is also a powerful reducing agent. For example, its 57% aqueous solution is used for the synthesis of alkyl iodides (such as CH ₃ CH ₂ I) from primary alcohols. Likewise, an OH group can be substituted for an I.

Reducing agent

Hydroiodic acid has been used to reduce, for example, carbohydrates. If glucose dissolved in this acid is heated, it will lose all its OH groups, obtaining the hydrocarbon n-hexane as a product.

It has also been used to reduce the functional groups of graphene sheets, so that they can be functionalized for electronic devices.

Cativa Process

Catalytic cycle diagram for the Cativa process. Source: Ben Mills. HI is also used for the industrial production of acetic acid using the Cativa process. This consists of a catalytic cycle in which the carbonylation of methanol occurs; that is, a carbonyl group, C = O, is introduced to the CH ₃ OH molecule to transform it into the acid CH ₃ COOH.

Steps

The process begins (1) with the organo-iridium complex ^-, flat square geometry. This compound "receives" the methyl iodide, CH ₃ I, product of the acidification of CH ₃ OH with HI at 57%. Water is also produced in this reaction, and thanks to it, acetic acid is finally obtained, while allowing the HI to be recovered in the last step.

In this step, both the –CH ₃ and –I group join the iridium metal center (2), forming an octahedral complex with a facet made up of three I ligands. One of the iodes ends up being replaced by a carbon monoxide molecule, CO; and now (3), the octahedral complex has a facet composed of three CO ligands.

Then, a rearrangement occurs: the –CH ₃ group "lets go" from Ir and binds to the adjacent CO (4) to form an acetyl group, -COCH ₃. This group is released from the iridium complex to bind to iodide ions and give CH ₃ COI, acetyl iodide. Here the iridium catalyst is recovered, ready to participate in another catalytic cycle.

Finally, CH ₃ COI undergoes a substitution of I ^- by a molecule of H ₂ O, whose mechanism ends up releasing HI and acetic acid.

Illicit syntheses

Reduction reaction of ephedrine with hydroiodic acid and red phosphorus to methamphetamine. Source: Methamphetamine_from_ephedrine_with_HI_ru.svg: Ring0 derivative work: materialscientist (talk). Hydroiodic acid has been used for synthesis of psychotropic substances taking advantage of its high reductive power. For example, you can reduce ephedrine (a medicine for treating asthma) in the presence of red phosphorus, to methamphetamine (top image).

It can be seen that a substitution of the OH group by I occurs first, followed by a second substitution by an H.

References

1. Wikipedia. (2019). Hydroiodic acid. Recovered from: en.wikipedia.org
2. Andrews, Natalie. (April 24, 2017). The Uses of Hydriodic Acid. Sciencing. Recovered from: sciencing.com
3. Alfa Aesar, Thermo Fisher Scientific. (2019). Hydriodic acid. Recovered from: alfa.com
4. National Center for Biotechnology Information. (2019). Hydriodic acid. PubChem Database., CID = 24841. Recovered from: pubchem.ncbi.nlm.nih.gov
5. Steven A. Hardinger. (2017). Illustrated Glossary of Organic Chemistry: Hydroiodic acid. Recovered from: chem.ucla.edu
6. Reusch William. (May 5, 2013). Carbohydrates. Recovered from: 2.chemistry.msu.edu
7. In Kyu Moon, Junghyun Lee, Rodney S. Ruoff & Hyoyoung Lee. (2010). Reduced graphene oxide by chemical graphitization. DOI: 10.1038 / ncomms1067.