The Hypochlorous acid It is an inorganic compound whose chemical formula is HClO. It corresponds to the least oxidized of the oxoacids of chlorine, since it contains only one oxygen atom. From it they derive the hypochlorite anion, ClO-, and its salts, widely used as commercial water disinfectants.
HClO is the strongest oxidizing and antimicrobial agent generated when chlorine gas dissolves in water. Its antiseptic action has been known for more than a century, even before chlorine solutions were used to clean the wounds of soldiers in World War I..
Its discovery in fact dates back to the year 1834, by the French chemist Antoine Jérôme Balard, who achieved the partial oxidation of chlorine by bubbling it in an aqueous suspension of mercury oxide, HgO. Since then, it has been used as a disinfectant and an antiviral agent..
Chemically speaking, HClO is an oxidizing agent that ends up giving up its chlorine atom to other molecules; In other words, chlorinated compounds can be synthesized with it, with chloroamines being of great relevance in the development of new antibiotics..
In the 1970s, it was discovered that the body is able to naturally produce this acid through the action of the enzyme myeloperoxidase; enzyme that acts on peroxides and chloride anions during phagocytosis. Thus, from the same organism this "killer" of intruders can arise, but on a harmless scale for its own well-being.
Article index
The upper image shows the structure of HClO. Note that the formula contradicts the structure: the molecule is H-O-Cl and not H-Cl-O; however, the latter is usually preferred to be able to compare it directly with its more oxidized counterparts: HClOtwo, HClO3 and HClO4.
Acid hydrogen, H+, released by HClO is located on the OH group attached to the chlorine atom. Note also the notable length differences in the O-H and Cl-O bonds, the latter being the longest due to the lesser degree of overlap of the chlorine orbitals, more diffuse, with those of oxygen..
The HOCl molecule can hardly remain stable under normal conditions; cannot be isolated from its aqueous solutions without being disproportionate or released as chlorine gas, Cltwo.
Therefore, there are no anhydrous crystals (not even hydrates thereof) of hypochlorous acid; And to date, there is also no indication that they can be prepared by extravagant methods. If they could crystallize, the HClO molecules would interact with each other through their permanent dipoles (negative charges oriented towards oxygen).
HClO is a monoprotic acid; that is, you can only donate one H+ to the aqueous medium (which is where it is formed):
HClO (aq) + HtwoO ↔ ClO-(ac) + H3OR+(ac) (pKa = 7.53)
From this equilibrium equation it is observed that a decrease in H ions3OR+ (an increase in the basicity of the medium), favors the formation of more hypochlorite anions, ClO-. Consequently, if you want to keep a solution of ClO relatively stable- the pH needs to be basic, which is achieved with NaOH.
Its dissociation constant, pKa, calls into question that HClO is a weak acid. Therefore, when handling it concentrated, you would not have to worry so much about the H ions3OR+, but by the HClO itself (given its high reactivity and not because of its corrosivity).
It was mentioned that the chlorine atom in HClO has an oxidation number of +1. This means that it hardly requires the gain of a single electron to return to its ground state (Cl0) and be able to form the molecule Cltwo. Consequently, the HClO will be reduced to Cltwo and HtwoOr, oxidizing another species more quickly compared to the same Cltwo or ClO-:
2HClO (aq) + 2H+ + 2e- ↔ Cltwo(g) + 2HtwoO (l)
This reaction already allows us to see how stable HClO is in its aqueous solutions..
Its oxidizing power is not only measured by the formation of Cltwo, but also because of its ability to give up its chlorine atom. For example, it can react with nitrogenous species (including ammonia and nitrogenous bases), to produce chloroamines:
HClO + N-H → N-Cl + HtwoOR
Note that an N-H bond of an amino group (-NHtwo) mostly, and is replaced by one N-Cl. The same happens with the O-H bonds of hydroxyl groups:
HClO + O-H → O-Cl + HtwoOR
These reactions are crucial and explain the disinfectant and antibacterial action of HClO..
HClO is unstable almost everywhere you look at it. For example, the hypochlorite anion is disproportionate in chlorine species with oxidation numbers of -1 and +5, more stable than +1 in HClO (H+Cl+ORtwo-):
3ClO-(aq) ↔ 2Cl-(ac) + ClO3-(ac)
This reaction would again shift the equilibrium towards the disappearance of HClO. Likewise, HClO participates directly in a parallel equilibrium with water and chlorine gas:
Cltwo(g) + HtwoO (l) ↔ HClO (aq) + H+(ac) + Cl-(ac)
That is why trying to heat a solution of HClO to concentrate it (or isolate it) leads to the production of Cltwo, which is identified as a yellow gas. Likewise, these solutions cannot be exposed to light for too long, nor to the presence of metallic oxides, as they decompose Cltwo (HClO disappears even more):
2Cltwo + 2HtwoO → 4HCl + Otwo
HCl reacts with HClO to generate more Cltwo:
HClO + HCl → Cltwo + HtwoOR
And so on until there is no more HClO.
One of the methods for preparing or synthesizing hypochlorous acid has already been implicitly explained: dissolving chlorine gas in water. Another quite similar method consists of dissolving the anhydride of this acid in water: dichloro monoxide, CltwoOR:
CltwoO (g) + HtwoO (l) ↔ 2HClO (aq)
Again there is no way to isolate pure HClO, since evaporating the water would shift the equilibrium to the formation of CltwoOr, gas that would escape from the water.
On the other hand, it has been possible to prepare more concentrated solutions of HClO (20%) using mercuric oxide, HgO. To do this, chlorine is dissolved in a volume of water just at its freezing point, in such a way that chlorinated ice is obtained. Then this same ice is stirred, and as it melts, it mixes with the HgO:
2Cltwo + HgO + 12HtwoO → 2HClO + HgCltwo + 11HtwoOR
The 20% HClO solution can finally be vacuum distilled.
A simpler and safer method of preparing hypochlorous acid solutions is to use brines as raw material instead of chlorine. Brines are rich in chloride anions, Cl-, which through an electrolysis process can be oxidized to Cltwo:
2HtwoO → Otwo + 4H+ + 4e-
2Cl- ↔ 2e- + Cltwo
These two reactions occur at the anode, where chlorine is produced which immediately dissolves to give rise to HClO; while in the cathode compartment, the water is reduced:
2HtwoO + 2e- → 2OH- + Htwo
In this way, HClO can be synthesized on a commercial to industrial scale; and these solutions obtained from brines are, in fact, the commercially available products of this acid.
HClO can be used as an oxidizing agent to oxidize alcohols to ketones, and to synthesize chloroamines, chloroamides, or chlorohydrins (starting from alkenes).
However, all its other uses can be encompassed in one word: biocide. It is a killer of fungi, bacteria, viruses, and a neutralizer of toxins released by pathogens.
The immune system of our body synthesizes its own HClO by the action of the enzyme myeloperoxidase, helping the white blood cells to eradicate the intruders that cause the infection.
Countless studies suggest various mechanisms of the action of HClO on the biological matrix. This donates its chlorine atom to the amino groups of certain proteins, and also oxidizes their SH groups present to S-S disulfide bridges, resulting in their denaturation..
Likewise, it stops DNA replication by reacting with nitrogenous bases, affects the complete oxidation of glucose, and can also deform the cell membrane. All these actions end up causing the death of germs.
That is why HClO solutions end up being used to:
-Treatment of infectious wounds and gangrene
-Disinfect water supplies
-Sterilizing agent for surgical material, or tools used in veterinary medicine, medicine and dentistry
-Disinfectant of any type of surface or object in general: bars, handrails, coffee machines, ceramics, glass tables, laboratory counters, etc..
-Synthesize chloroamines that serve as less aggressive antibiotics, but at the same time more durable, specific and stable than HClO itself
HClO solutions can be dangerous if they are highly concentrated, as they can react violently with species prone to oxidation. In addition, they tend to release gaseous chlorine when destabilized, so they must be stored under a rigorous security protocol..
HClO is so reactive towards germs that where it is watered it disappears instantly, without posing a risk later to those who touch the surfaces treated by it. The same happens inside the organism: it decomposes quickly, or is neutralized by any species in the biological environment.
When generated by the body itself, it is presumable that it can tolerate low concentrations of HClO. However, if it is highly concentrated (used for synthetic purposes and not disinfectants) it can have undesirable effects by also attacking healthy cells (of the skin, for example).
Yet No Comments