por What is hydroiodic acid?
He hydroiodic acid It is an aqueous solution of hydrogen iodide that is characterized by its high acidity. A definition closer to chemical and IUPAC terminology is that it is a hydroacid, whose chemical formula is HI.
However, to differentiate it from the gaseous molecules of hydrogen iodide, HI(g), it is denoted as HI(aq). For this reason, in chemical equations it is important to identify the medium or physical phase in which the reactants and products are found. Even so, it is often common to confuse between hydrogen iodide and hydroiodic acid.
If the molecules compromised in their identity are observed, noticeable differences between HI(g) and HI(ac) will be found. In HI(g) there is an HI bond, while in HI(ac), they are actually a pair of ions I– and H3O+ interacting electrostatically.
On the other hand, HI(aq) is a source of HI(g), since the former is prepared by dissolving the latter in water. Because of this, unless it is in a chemical equation, HI can also be used to refer to hydroiodic acid. HI is a strong reducing agent and an excellent source of I– ions in an aqueous medium.
Hydroiodic acid structure
Hydroiodic acid consists of a solution of HI in water. Being in the water, the HI molecules completely dissociate (strong electrolyte), originating the ions I– and H3O+. This dissociation can be represented by the following chemical equation:
HI(g) + H2O(l) => I–(aq) + H3O+(aq)
Which would be equivalent if written as:
HI(g) + H2O(l) => HI(aq)
However, HI(aq) 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(aq) consists of the I– and H3O+ ions surrounded by water molecules hydrating them. The more concentrated the hydroiodic acid, the fewer the number of unprotonated water molecules.
Commercially, in fact, the concentration of HI is from 48 to 57% in water. More concentrated would be equivalent to having too fuming (and even more dangerous) acid.
Hydroiodic acid properties
Physical description: himColorless liquid, but may exhibit yellowish and brown hues if in direct contact with oxygen. This is because the I– ions they end up being oxidized to molecular iodine, I2. If there is much I2it is more than likely that the triiodide anion, I, is formed3-which stains the solution brown.
Molecular mass: 127.91 g/mol.
Smell: tobelieve.
Density: himThe density is 1.70 g/mL for the 57% HI solution, since the densities vary depending on the different HI concentrations. At this concentration an azeotrope is formed (it is distilled as a single substance and not as a mixture) whose relative stability may be due to its commercialization above other solutions.
Boiling point:eThe 57% HI azeotrope boils at 127°C at a pressure of 1.03 bar (GO TO ATM).
pKa: -1.78.
Acidity:eIt is an extremely strong acid, so strong that it is corrosive to all metals and fabrics, even rubber. This is because the HI bond is very weak and is easily broken during ionization in water. Also, hydrogen bonding I– — HOH2+ they are weak, so there is nothing to interfere for the H3EITHER+ react with other compounds, that is, H3EITHER+ is «free».
Reducing agent:el HI is a powerful reducing agent, whose main reaction product is I2.
Hydroiodic acid nomenclature
The nomenclature of 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 [I–][H3O+]. This is its only name, as it is not a pure compound, but a solution.
Hydroiodic Acid Uses
Source of iodine in organic and inorganic synthesis
HI is an excellent source of I– ions for inorganic and organic synthesis, and is also a powerful reducing agent.
Its 57% aqueous solution is used for the synthesis of alkyl iodides (such as CH3CH2I) from primary alcohols. You can also substitute an I for an OH group in a structure.
Reducing agent
Hydroiodic acid has been used to reduce 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
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 CH3OH molecule to become the acid CH3COOH.
Steps
The process begins (1) with the organo-iridium complex [Ir(CO)2I2]–, square plane geometry. This compound “receives” the methyl iodide, CH3I, the product of the acidification of CH3OH with 57% HI. Water is also produced in this reaction, and thanks to it, acetic acid is obtained at the end, while allowing HI to be recovered in the last step.
In this step, both the -CH3 and -I groups bind to the iridium metal center (2), forming an octahedral complex with a facet composed of three I ligands. One of the iodines 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 -CH3 group “releases” from Ir and joins the adjacent CO (4) to form an acetyl group, -COCH3. This group is released from the iridium complex to bind to iodide ions to give CH3COI, acetyl iodide. Here the iridium catalyst is recovered, ready to participate in another catalytic cycle.
Finally, CH3COI undergoes a substitution of I– by a molecule of H₂O, whose mechanism ends up releasing HI and acetic acid.
illicit synthesis
Hydroiodic acid has been used for the synthesis of psychotropic substances, taking advantage of its high reductive power. For example, you can reduce ephedrine (an asthma medication) in the presence of red phosphorus, to methamphetamine (top image). It can be seen that first a substitution of the OH group by I occurs, followed by a second substitution by an H.
References
Hydroiodic acid. Retrieved from en.wikipedia.org.
The Uses of Hydrodic Acid. Retrieved from science.com.
Hydrodic acid. Recovered from alpha.com.
Hydrodic acid. Retrieved from pubchem.ncbi.nlm.nih.gov.
Illustrated Glossary of Organic Chemistry: Hydroiodic acid. Retrieved from chem.ucla.edu.