How Does Hypochlorous Acid React and Why Is It Important?
Hypochlorous Acid is described as a weak acid having the chemical formula HOCl. It is also referred to as either Chlorine hydroxide or hypochloric acid or even as Hydrogen hypochlorite. In 1834, it was discovered by a French chemist named "Antoine Jerome Balard." It is also termed as an oxyacid of chlorine.
It also contains the monovalent chlorine that functions either as an oxidizing agent or a reducing agent. It functions as a human metabolite and is an unstable acid. It belongs to the reactive oxygen species family, and it is the conjugate acid of a hypochlorite.
Properties of Hypochlorous Acid – HOCl
Hypochlorous Acid Structure – HOCl
(Image to be added soon)
Reactions
Reaction with the Protein Amino Groups
Hypochlorous acid readily reacts with the amino acids that contain amino group side-chains, with chlorine from HClO, with hydrogen displacement, resulting in an organic chloramine. The chlorinated amino acids decompose rapidly, but the protein chloramines are long-lived and retain a few oxidative capacities. A chemist named "Thomas et al." concluded from their results that several organic chloramines decayed by the internal rearrangement. The same fewer available NH2 groups promoted attack on the peptide bond, resulting in the protein's cleavage.
Davies and McKenna also found that 10 mM or greater HClO is required to fragment Vivo's proteins. With these consistent results, later, it was proposed that the chloramine encounters a molecular rearrangement, releasing ammonia and HCl to produce an aldehyde. Then, the aldehyde group can react further with the other amino group to produce a Schiff’s base, causing protein aggregation cross-linking.
Reaction with the Lipids
The hypochlorous acid reacts with the unsaturated bonds in lipids, but not on the saturated bonds. Moreover, the ClO− ion does not participate in this particular reaction. This reaction takes place by hydrolysis with the addition of chlorine to any of the carbons and one hydroxyl to the other.
Then, chlorohydrin is the resulting compound. The polar chlorine disrupts the lipid bilayers and could increase its permeability. When chlorohydrin formation occurs in lipid bilayers of red blood cells, the increased permeability occurs. Disruption could take place if necessary chlorohydrin is produced. Also, the addition of preformed chlorohydrin to the red blood cells will affect permeability. Cholesterol chlorohydrin has also been observed, but it does not affect permeability, and it is also believed that Cl2 is wholly responsible for this reaction.
Hypochlorites
Hypochlorites are the hypochlorous acid salts, and the commercially essential hypochlorites are sodium hypochlorite and calcium hypochlorite.
Production of Hypochlorites using Electrolysis
Solutions of the hypochlorites are formed by electrolysis of an aqueous solution of sodium chloride. The resulting solution's composition depends on the anode's pH. In acid conditions, the formed solution will have a high concentrated hypochlorous acid. However, it also contains dissolved gaseous chlorine, which is corrosive at a neutral pH; the solution will be nearly 25% hypochlorite and 75% hypochlorous acid. A few of the chlorine gas formed will dissolve, forming the hypochlorite ions. Also, hypochlorites can be produced by the chlorine gas disproportionation in alkaline solutions.
Formation of Hypochlorous Acid
When the acids are added to the hypochlorous acid's aqueous salts (like sodium hypochlorite in the commercial bleach solution), the resultant reaction is directed to the left and thus forms chlorine gas. Hence, the stable hypochlorite bleaches formation happened by dissolving chlorine gas into the basic water solutions, like sodium hydroxide.
The acid can also be prepared by dissolving the dichlorine monoxide with water; at standard aqueous conditions, where, currently, anhydrous hypochlorous acid is not possible to prepare because of the readily reversible equilibrium between it to its anhydride:
2 HOCl ⇌ Cl2O + H2O K (at 0 °C) = 3.55 10−3 dm3 mol−1
Either the presence of light or the transition metal oxides of nickel, copper, cobalt accelerates the exothermic decomposition into oxygen and hydrochloric acid:
Uses of Hypochlorous Acid
Let us know some of the important uses of hypochlorous acid as listed below.
HClO converts alkenes to chlorohydrins in organic synthesis.
Hypochlorous acid is generated in biology in activated neutrophils by the chloride ion’s myeloperoxidase mediated peroxidation and contributes to the bacteria's destruction.
In water distribution and food services, a specialized equipment to generate HClO weak solutions from salt and water is used at times to form adequate quantities of safe disinfectant to treat water supplies and food preparation surfaces.
Hypochlorous acid is an active sanitizer in water treatment in hypochlorite-based products, like in a swimming pool.
Similarly, in yachts and ships, the marine sanitation devices use electricity in converting seawater into hypochlorous acid to disinfect the macerated faecal waste prior to discharging into the sea.
FAQs on Hypochlorous Acid: Structure, Properties, and Uses
1. What is hypochlorous acid (HClO)?
Hypochlorous acid, represented by the chemical formula HClO or HOCl, is a weak acid that is formed when chlorine gas dissolves in water. It is a powerful oxidising agent and is primarily known for its disinfectant properties, making it the active ingredient in many bleaches and cleaning solutions. Despite containing chlorine, it is chemically distinct from hydrochloric acid (HCl).
2. How is the structure of hypochlorous acid organised?
The structure of hypochlorous acid (HOCl) consists of a central oxygen atom singly bonded to both a hydrogen atom and a chlorine atom (H-O-Cl). Due to the presence of two lone pairs of electrons on the oxygen atom, the molecule has a bent or angular geometry. This arrangement of atoms and lone pairs makes the molecule polar.
3. What are the key physical and chemical properties of hypochlorous acid?
Hypochlorous acid exhibits several distinct properties:
- Physical State: It exists as a colourless aqueous solution.
- Odour: It has a strong, pungent odour similar to chlorine.
- Acidity: It is a weak acid, meaning it only partially ionises in water.
- Oxidising Agent: It is a strong oxidising agent, which is the basis for its bleaching and disinfecting actions.
- Stability: The acid is highly unstable and readily decomposes into hydrochloric acid and oxygen, especially in the presence of light or heat.
4. What are the main applications of hypochlorous acid in daily life and industry?
Hypochlorous acid is used in various fields due to its effective antimicrobial properties. Its main applications include:
- Disinfection: It is widely used to disinfect swimming pools, drinking water, and surfaces.
- Wound Care: In dilute, stabilised forms, it is approved for use as an antiseptic to clean wounds and treat skin infections.
- Food Safety: It is used to sanitise food preparation surfaces and wash fresh produce to eliminate pathogens.
- Cosmetics: It is used in some skin cleansers and baby products for its gentle, non-irritating cleaning action.
5. What are hypochlorites and how do they relate to hypochlorous acid?
Hypochlorites are the salts derived from hypochlorous acid. They are formed when the acidic hydrogen of HClO is replaced by a metal ion, such as sodium (Na⁺) or calcium (Ca²⁺). The most common examples are sodium hypochlorite (NaClO), the main component of household bleach, and calcium hypochlorite (Ca(ClO)₂), used for disinfecting swimming pools. These salts are more stable than the acid itself and release hypochlorous acid when dissolved in water.
6. Why is hypochlorous acid (HClO) considered a weak acid?
Hypochlorous acid is classified as a weak acid because it does not completely ionise in water. The equilibrium for its dissociation (HClO ⇌ H⁺ + ClO⁻) lies significantly to the left, meaning only a small fraction of the acid molecules release their hydrogen ions (protons). The high electronegativity of the oxygen atom holds the hydrogen atom relatively tightly, preventing full dissociation, which is characteristic of strong acids like HCl.
7. How does the structure of hypochlorous acid (HClO) differ from chlorous acid (HClO₂)?
The primary difference lies in the number of oxygen atoms bonded to the central chlorine atom. In hypochlorous acid (HClO), the chlorine atom is bonded to one oxygen atom. In chlorous acid (HClO₂), the chlorine atom is bonded to two oxygen atoms (HO-Cl=O). This additional oxygen atom in HClO₂ makes it a stronger acid than HClO.
8. How does hypochlorous acid act as a disinfectant?
Hypochlorous acid acts as a disinfectant through its powerful oxidising ability. Because it is a small, uncharged molecule, it can easily penetrate the cell walls of bacteria, viruses, and fungi. Once inside, it oxidises essential components like proteins and enzymes, disrupting their structure and function. This leads to the rapid inactivation and death of the microorganism.
9. Is it safe to handle hypochlorous acid?
While dilute, commercially prepared hypochlorous acid solutions are safe for skin and surface contact, the pure or concentrated acid is unstable and corrosive. It decomposes easily, releasing chlorine gas, which is toxic. Therefore, it is typically generated on-site for industrial use or sold in stabilised, low-concentration formulations for consumer products. Standard laboratory safety precautions, such as using gloves and eye protection, are necessary when handling it.
10. How does hypochlorous acid react with amino acids?
Hypochlorous acid reacts rapidly with the amino groups (-NH₂) in amino acids. The chlorine atom from HClO displaces a hydrogen atom on the amino group, forming a compound known as an organic chloramine (R-NHCl). This reaction is significant because it is one of the ways HClO inactivates proteins and enzymes in pathogens, contributing to its antimicrobial effect.
