The hydroiodic acid It is an aqueous solution of hydrogen iodide that is characterized by its high acidity. A definition closer to the chemical and IUPAC terminology is that it is a hydracid, whose chemical formula is HI.
However, to differentiate it from gaseous hydrogen iodide molecules, HI (g) is denoted 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..
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 H-I bond; while in HI (ac), they are actually an ion pair I- and H3OR+ interacting electrostatically (top 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 ions I- in aqueous medium.
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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 ions- and H3OR+. This dissociation can be represented by the following chemical equation:
HI (g) + HtwoO (l) => I-(ac) + H3OR+(ac)
What would be equivalent if it were written as:
HI (g) + HtwoO (l) => HI (ac)
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 ions I- and H3OR+ surrounded by water molecules hydrating them; the more concentrated the hydroiodic acid, the fewer the number of unprotonated water molecules.
Commercially, in fact, the HI concentration 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 H3OR+ with white spheres and a red one, for the oxygen atom. The cation H3OR+ features trigonal pyramid molecular geometry (viewed from a higher plane in the image).
Colorless liquid; but, it can exhibit yellowish and brown tones if it is in direct contact with oxygen. This is because the ions I- end up oxidizing to molecular iodine, Itwo. If there is a lot Itwo, it is more than likely that the triiodide anion is formed, I3-, which stains the solution brown.
127.91 g / mol.
Acre.
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) whose relative stability may be due to its commercialization over other solutions..
The 57% HI azeotrope boils at 127ºC at a pressure of 1.03 bar (GO TO ATM).
-1.78.
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 H-I bond is very weak, and it breaks easily during ionization in water. Furthermore, hydrogen bonds I- - HOHtwo+ are weak, so there is nothing to interfere with the H3OR+ react with other compounds; that is, the H3OR+ has been "free", like the I- that does not attract with too much force to his counterion.
HI is a powerful reducing agent, the main reaction product of which is Itwo.
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 [I-] [H3OR+]. This is its only name, as it is not a pure compound but a solution.
HI is an excellent source of ions I- for inorganic and organic syntheses, and is also a powerful reducing agent. For example, its 57% aqueous solution is used for the synthesis of alkyl iodides (such as CH3CHtwoI) from primary alcohols. Likewise, an OH group can be substituted for an I in a structure..
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..
Likewise, it has been used to reduce the functional groups of graphene sheets, in such a way that they can be functionalized for electronic devices..
The process begins (1) with the organo-iridium complex [Ir (CO)twoItwo]-, square plane geometry. This compound "receives" methyl iodide, CH3I, product of acidification of CH3OH with 57% HI. 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 group -CH3 like -I they bind to the metal center of iridium (2), forming an octahedral complex with a facet composed of three I ligands. One of the iodes ends up being replaced by a molecule of carbon monoxide, CO; and now (3), the octahedral complex has a facet composed of three CO ligands.
Then a rearrangement occurs: the -CH group3 it "loosens" from Ir and binds to adjacent CO (4) to form an acetyl group, -COCH3. This group is released from the iridium complex to bind to iodide ions and give CH3COI, acetyl iodide. Here the iridium catalyst is recovered, ready to participate in another catalytic cycle.
Finally, the CH3IOC undergoes a replacement of the I- per one molecule of HtwoO, whose mechanism ends up releasing HI and acetic acid.
It can be seen that a substitution of the OH group for I occurs first, followed by a second substitution for an H.
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