HYPOCHLOROUS ACID (HCLO): STRUCTURE, PROPERTIES, USES, SYNTHESIS - CHEMISTRY - 2025

The hypochlorous acid is an inorganic compound with the chemical formula 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 that is 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 the First World War.

Hypochlorous acid molecule represented by a ball-and-stick model. Source: Ben Mills and Jynto

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; that is to say, with it chlorinated compounds can be synthesized, being chloroamines 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 emerge, but on a harmless scale for its own well-being.

Structure

The upper image shows the structure of HClO. Note that the formula contradicts the structure: the molecule is HO-Cl and not H-Cl-O; however, the latter is usually preferred in order to be able to compare it directly with its more oxidized counterparts: HClO ₂, HClO ₃ and HClO ₄.

Chemical structure of hypochlorous acid.

The acidic hydrogen, H ⁺, released by HClO is located in the OH group attached to the chlorine atom. Note also the notable differences in length in the OH 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 barely remain stable under normal conditions; It cannot be isolated from its aqueous solutions without being disproportionate or released as chlorine gas, Cl ₂.

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).

Properties

Acidity

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) + H ₂ O ↔ ClO ^- (aq) + H ₃ O ⁺ (aq) (pKa = 7.53)

From this equilibrium equation it is observed that a decrease in H ₃ O ⁺ ions (an increase in the basicity of the medium) favors the formation of more hypochlorite anions, ClO ^-. Consequently, if a solution of ClO ^- is to be kept relatively stable, the pH must be basic, which is achieved with NaOH.

Its dissociation constant, pKa, makes it doubtful that HClO is a weak acid. Therefore, when handling it concentrated, you should not worry so much about the H ₃ O ⁺ ions, but about the HClO itself (given its high reactivity and not because of its corrosivity).

Oxidizing agent

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 (Cl ⁰) and to be able to form the Cl ₂ molecule. Consequently, HClO will be reduced to Cl ₂ and H ₂ O, oxidizing another species more rapidly compared to the same Cl ₂ or ClO ^-:

2HClO (aq) + 2H ⁺ + 2e ^- ↔ Cl ₂ (g) + 2H ₂ O (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 Cl ₂, but also by 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 + NH → N-Cl + H ₂ O

Note that an NH bond is broken, of an amino group (-NH ₂) for the most part, and is replaced by an N-Cl. The same happens with the OH bonds of hydroxyl groups:

HClO + OH → O-Cl + H ₂ O

These reactions are crucial and explain the disinfecting and antibacterial action of HClO.

Stability

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 ⁺ O ^2-):

3ClO ^- (aq) ↔ 2Cl ^- (aq) + ClO ₃ ^- (aq)

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:

Cl ₂ (g) + H ₂ O (l) ↔ HClO (aq) + H ⁺ (aq) + Cl ^- (aq)

That is why trying to heat a solution of HClO to concentrate it (or isolate it) leads to the production of Cl ₂, which is identified as a yellow gas. Likewise, these solutions cannot be exposed to light for too long, nor to the presence of metal oxides, as they decompose Cl ₂ (HClO disappears even more):

2Cl ₂ + 2H ₂ O → 4HCl + O ₂

HCl reacts with HClO to generate more Cl ₂:

HClO + HCl → Cl ₂ + H ₂ O

And so on until there is no more HClO.

Synthesis

Water and chlorine

One of the methods for preparing or synthesizing hypochlorous acid has already been implicitly explained: by dissolving chlorine gas in water. Another quite similar method consists in dissolving the anhydride of this acid in water: dichloro monoxide, Cl ₂ O:

Cl ₂ O (g) + H ₂ O (l) ↔ 2HClO (aq)

Again there is no way to isolate pure HClO, since evaporating the water would shift the equilibrium to the formation of Cl ₂ O, a 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:

2Cl ₂ + HgO + 12H ₂ O → 2HClO + HgCl ₂ + 11H ₂ O

The 20% HClO solution can finally be distilled under vacuum.

Electrolysis

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 Cl ₂:

2H ₂ O → O ₂ + 4H ⁺ + 4e ^-

2Cl ^- ↔ 2e ^- + Cl ₂

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:

2H ₂ O + 2e ^- → 2OH ^- + H ₂

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.

Applications

General features

HClO can be used as an oxidizing agent to oxidize alcohols to ketones, and to synthesize chloroamines, chloroamides, or chlorohydrins (starting from the 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.

Our body's immune system 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 its SH groups present to SS disulfide bridges, resulting in their denaturation.

Likewise, it stops DNA replication by reacting with nitrogenous bases, it affects the complete oxidation of glucose, and it can also deform the cell membrane. All these actions end up causing the germs to die.

Disinfection and cleaning

That is why HClO solutions end up being used to:

-Treatment of infectious and gangrenous wounds

-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

Risks

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).

References

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