IODINE ACID (HIO2): PROPERTIES AND USES - CHEMISTRY - 2025

The iodous acid is a chemical compound of f'ormula HIO 2. This acid, as well as its salts (known as iodites), are extremely unstable compounds that have been observed but never isolated.

It is a weak acid, which means that it does not completely dissociate. In the anion, iodine is in oxidation state III and has a structure analogous to chlorous acid or bromous acid, as illustrated in Figure 1.

Figure 1: Structure of Iodine Acid

Despite the compound being unstable, iodine acid and its iodite salts have been detected as intermediates in the conversion between iodides (I ^-) and iodates (IO ₃ ^-).

Its instability is due to a dismutation reaction (or disproportionation) to form hypoiodoid acid and iodic acid, which is analogous to chlorous and bromous acids in the following way:

2HIO _{2 ->} HIO + HIO ₃

In Naples in 1823, the scientist Luigi Sementini wrote a letter to E. Daniell, secretary of the royal institution in London, where he explained a method for obtaining iodine acid.

In the letter, he said that considering that the formation of nitrous acid was, by combining nitric acid with what he called nitrous gas (possibly N ₂ O), iodine acid could be formed in the same way by reacting iodic acid with oxide. of iodine, a compound he had discovered.

In doing so, he obtained a yellowish-amber liquid that lost its color in contact with the atmosphere (Sir David Brewster, 1902).

Later, the scientist M. Wöhler discovered that Sementini's acid is a mixture of iodine chloride and molecular iodine, since the iodine oxide used in the reaction was prepared with potassium chlorate (Brande, 1828).

Physical and chemical properties

As mentioned above, iodine acid is an unstable compound that has not been isolated, so its physical and chemical properties are theoretically obtained through calculations and computational simulations (Royal Society of Chemistry, 2015).

Iodine acid has a molecular weight of 175.91 g / mol, a density of 4.62 g / ml in the solid state, and a melting point of 110 degrees centigrade (iodous acid, 2013-2016).

It also has a solubility in water of 269 g / 100 ml at 20 degrees centigrade (being a weak acid), has a pKa of 0.75, and has a magnetic susceptibility of −48.0 · 10−6 cm3 / mol (National Center for Biotechnology Information, nd).

Since iodine acid is an unstable compound that has not been isolated, there is no risk of handling it. It has been found by theoretical calculations that iodine acid is not flammable.

Applications

Nucleophilic acylation

Iodine acid is used as a nucleophile in nucleophilic acylation reactions. The example is given with the acylation of trifluoroacetyls such as 2,2,2-trifluoroacetyl bromide, 2,2,2-trifluoroacetyl chloride, 2,2,2-trifluoroacetyl fluoride, and 2,2,2-trifluoroacetyl iodide to form the iodosyl 2,2,2 trifluoroacetate as illustrated in figure 2.1, 2.2, 2.3 and 2.4 respectively.

Figure 2: iodosyl 2,2,2 trifluoroacetate formation reactions

Iodine acid is also used as a nucleophile for the formation of iodosyl acetate by reacting it with acetyl bromide, acetyl chloride, acetyl fluoride and acetyl iodide as shown in figures 3.1, 3.2, 3.3 and 3.4 respectively (GNU Free Documentation, sf).

Figure 2: iodosyl acetate formation reactions.

Dismutation reactions

Dismutation or disproportionation reactions are a type of oxide reduction reaction, where the substance that is oxidized is the same that is reduced.

In the case of halogens, as they have oxidation states of -1, 1, 3, 5 and 7, different products of dismutation reactions can be obtained depending on the conditions used.

In the case of iodine acid, the example of how it reacts to form hypoiodine acid and iodic acid of the form was mentioned above.

2HIO _{2 ->} HIO + HIO ₃

Recent studies have analyzed the iodine acid dismutation reaction by measuring the concentrations of protons (H ⁺), iodate (IO3 ^-) and the acidic hypoiodite cation (H ₂ IO ⁺) in order to better understand the mechanism of acid dismutation. iodine (Smiljana Marković, 2015).

A solution was prepared containing the intermediate species I ³⁺. A mixture of iodine (I) and iodine (III) species was prepared by dissolving iodine (I ₂) and potassium iodate (KIO ₃), in the ratio 1: 5, in concentrated sulfuric acid (96%). In this solution a complex reaction proceeds, which can be described by the reaction:

I ₂ + 3IO ₃ ^- + 8H ^{+ -–>} 5IO ⁺ + H ₂ O

Species I ³⁺ are stable only in the presence of excess iodate added. Iodine prevents the formation of I ³⁺. The IO ⁺ ion obtained in the form of iodine sulfate (IO) ₂ SO ₄), rapidly decomposes in acidic aqueous solution and forms I ³⁺, represented as the acid HIO ₂ or the ionic species IO3 ^-. Subsequently, a spectroscopic analysis was performed to determine the value of the concentrations of the ions of interest.

This presented a procedure for the evaluation of pseudo-equilibrium concentrations of hydrogen, iodate and H ₂ OI ⁺ ions, important kinetic and catalytic species in the process of disproportionation of iodine acid, HIO ₂.

Bray – Liebhafsky reactions

A chemical clock or oscillation reaction is a complex mixture of reacting chemical compounds, in which the concentration of one or more components changes periodically, or when sudden changes in properties occur after a predictable induction time.

They are a class of reactions that serve as an example of non-equilibrium thermodynamics, resulting in the establishment of a non-linear oscillator. They are theoretically important because they show that chemical reactions do not have to be dominated by equilibrium thermodynamic behavior.

The Bray-Liebhafsky reaction is a chemical clock first described by William C. Bray in 1921 and is the first oscillation reaction in a stirred homogeneous solution.

Iodine acid is used experimentally for the study of this type of reaction when it is oxidized with hydrogen peroxide, finding a better agreement between the theoretical model and experimental observations (Ljiljana Kolar-Anić, 1992).

References

1. Brande, WT (1828). A manual of chemistry, on the basis of Professor Brande's. Boston: University of Harvard.
2. GNU Free Documentation. (sf). iodous acid. Retrieved from chemsink.com: chemsink.com
3. iodous acid. (2013-2016). Retrieved from molbase.com: molbase.com
4. Ljiljana Kolar-Anić, GS (1992). Mechanism of the Bray – Liebhafsky reaction: effect of the oxidation of iodous acid by hydrogen peroxide. Chem. Soc., Faraday Trans 1992,88, 2343-2349.
5. National Center for Biotechnology Information. (nd). PubChem Compound Database; CID = 166623. Retrieved from pubchem.com:pubchem.ncbi.nlm.nih.gov.
6. Royal Society of Chemistry. (2015). Iodous acid ChemSpider ID145806. Retrieved from ChemSpider: chemspider.com
7. Sir David Brewster, RT (1902). The London and Edinburgh Philosophical Magazine and Journal of Science. london: university of london.
8. Smiljana Marković, RK (2015). Disproportionation reaction of iodous acid, HOIO. Determination of the concentrations of the relevant ionic species H +, H2OI +, and IO3 -.