Hypochlorite

Acidification of hypochlorites generates hypochlorous acid, which exists in an equilibrium with chlorine. A lowered pH (i.e. towards acid) drives the following reaction to the right, liberating chlorine gas, which can be dangerous:

:2 {{chem|H|+}} + {{chem|ClO|-}} + {{chem|Cl|-}} {{eqm}} {{chem|Cl|2}} + {{chem|H|2|O}}

Hypochlorites are generally unstable and many compounds exist only in solution. Lithium hypochlorite LiOCl, calcium hypochlorite Ca(OCl)₂ and barium hypochlorite Ba(ClO)₂ have been isolated as pure anhydrous compounds. All are solids. A few more can be produced as aqueous solutions. In general the greater the dilution the greater their stability. It is not possible to determine trends for the alkaline earth metal salts, as many of them cannot be formed. Beryllium hypochlorite is unheard of. Pure magnesium hypochlorite cannot be prepared; however, solid Mg(OH)OCl is known. Calcium hypochlorite is produced on an industrial scale and has good stability. Strontium hypochlorite, Sr(OCl)₂, is not well characterised and its stability has not yet been determined.{{cn|date=September 2023}}

Upon heating, hypochlorite degrades to a mixture of chloride, oxygen, and chlorates:

:2 {{chem|ClO|-}} → 2 {{chem|Cl|-}} + {{chem|O|2}}

:3 {{chem|ClO|-}} → 2 {{chem|Cl|-}} + {{chem|ClO|3|-}}

This reaction is exothermic and in the case of concentrated hypochlorites, such as LiOCl and Ca(OCl)₂, can lead to dangerous thermal runaway and is potentially explosive.{{cite journal|last=Clancey|first=V.J.|title=Fire hazards of calcium hypochlorite|journal=Journal of Hazardous Materials|year=1975|volume=1|issue=1|pages=83–94|doi=10.1016/0304-3894(75)85015-1}}

The alkali metal hypochlorites decrease in stability down the group. Anhydrous lithium hypochlorite is stable at room temperature; however, sodium hypochlorite is explosive as an anhydrous solid.{{cite book | first = Peter | last = Urben | name-list-style = vanc | date = 2006 | title = Bretherick's Handbook of Reactive Chemical Hazards | edition = 7th | volume = 1 | page = 1433 | isbn = 978-0-08-052340-8 }} The pentahydrate NaOCl·5H₂O is unstable above 0 °C,{{cite book|last=Brauer|first=G.|title=Handbook of Preparative Inorganic Chemistry; Vol. 1|year=1963|publisher=Academic Press|page=309|edition=2nd}} although the more dilute solutions encountered as household bleach are more stable. Potassium hypochlorite (KOCl) is known only in solution.{{cite book|last=Aylett|first=founded by A.F. Holleman; continued by Egon Wiberg; translated by Mary Eagleson, William Brewer; revised by Bernhard J.|title=Inorganic chemistry|year=2001|publisher=Academic Press, W. de Gruyter.|location=San Diego, Calif. : Berlin|isbn=978-0123526519|page=444|edition=1st English ed., [edited] by Nils Wiberg.}}

Lanthanide hypochlorites are also unstable; however, they have been reported as being more stable in their anhydrous forms than in the presence of water.{{cite journal|last=Vickery|first=R. C.|title=Some reactions of cerium and other rare earths with chlorine and hypochlorite|journal=Journal of the Society of Chemical Industry|date=1 April 1950|volume=69|issue=4|pages=122–125|doi=10.1002/jctb.5000690411}} Hypochlorite has been used to oxidise cerium from its +3 to +4 oxidation state.{{cite book|last=V. R. Sastri |display-authors=etal |title=Modern Aspects of Rare Earths and their Complexes.|year=2003|publisher=Elsevier|location=Burlington|isbn=978-0080536682|page=38|edition=1st}}

Hypochlorous acid itself is not stable in isolation as it decomposes to form chlorine. Its decomposition also results in some form of oxygen.

Hypochlorites react with ammonia first giving monochloramine ({{chem|NH|2|Cl}}), then dichloramine ({{chem|NHCl|2}}), and finally nitrogen trichloride ({{chem|NCl|3}}).

: {{chem|NH|3}} + {{chem|ClO|-}} → {{chem|HO|-}} + {{chem|NH|2}}Cl

: {{chem|NH|2}}Cl + {{chem|ClO|-}} → {{chem|HO|-}} + {{chem|NHCl|2}}

: {{chem|NHCl|2}} + {{chem|ClO|-}} → {{chem|HO|-}} + {{chem|NCl|3}}

Hypochlorite salts are formed by the reaction between chlorine and alkali and alkaline earth metal hydroxides. The reaction is performed at close to room temperature to suppress the formation of chlorates. This process is widely used for the industrial production of sodium hypochlorite (NaClO) and calcium hypochlorite (Ca(ClO)₂).

:Cl₂ + 2 NaOH → NaCl + NaClO + H₂O

:2 Cl₂ + 2 Ca(OH)₂ → CaCl₂ + Ca(ClO)₂ + 2 H₂O

Large amounts of sodium hypochlorite are also produced electrochemically via an un-separated chloralkali process. In this process brine is electrolyzed to form {{chem|Cl|2}} which dissociates in water to form hypochlorite. This reaction must be conducted in non-acidic conditions to prevent release of chlorine:

:2 {{chem|Cl|-}} → {{chem|Cl|2}} + 2 e⁻

: {{chem|Cl|2}} + {{chem|H|2|O}} {{eqm}} {{chem|H|Cl|O}} + {{chem|Cl|-}} + {{chem|H|+}}

Some hypochlorites may also be obtained by a salt metathesis reaction between calcium hypochlorite and various metal sulfates. This reaction is performed in water and relies on the formation of insoluble calcium sulfate, which will precipitate out of solution, driving the reaction to completion.

: Ca(ClO)₂ + MSO₄ → M(ClO)₂ + CaSO₄

image:TBuOCl.svg is a rare example of a stable organic hypochlorite.{{cite book|doi=10.1002/047084289X.rb388.pub2|chapter=t-Butyl Hypochlorite|title=Encyclopedia of Reagents for Organic Synthesis|year=2006|last1=Simpkins|first1=Nigel S.|last2=Cha|first2=Jin K.|isbn=0471936235}}]]

Hypochlorite esters are in general formed from the corresponding alcohols, by treatment with any of a number of reagents (e.g. chlorine, hypochlorous acid, dichlorine monoxide and various acidified hypochlorite salts).

Chloroperoxidases are enzymes that catalyze the chlorination of organic compounds. This enzyme combines the inorganic substrates chloride and hydrogen peroxide to produce the equivalent of Cl⁺, which replaces a proton in hydrocarbon substrate:

:R-H + Cl⁻ + H₂O₂ + H⁺ → R-Cl + 2 H₂O

The source of Cl⁺ is hypochlorous acid (HOCl).{{cite journal|last1=Hofrichter|first1=M.|last2=Ullrich|first2=R.|last3=Pecyna|first3=Marek J.|first4=Christiane |last4= Liers|first5=Taina |last5=Lundell|journal=Appl Microbiol Biotechnol|year=2010|volume=87|issue=3|doi=10.1007/s00253-010-2633-0|pmid=20495915| pages= 871–897 | title=New and classic families of secreted fungal heme peroxidases|s2cid=24417282}} Many organochlorine compounds are biosynthesized in this way.

In response to infection, the human immune system generates minute quantities of hypochlorite within special white blood cells, called neutrophil granulocytes.{{cite journal|doi=10.1007/s00726-012-1361-4|pmid=22810731|pmc=3894431|title=Taurine and inflammatory diseases|journal=Amino Acids|volume=46|issue=1|pages=7–20|year=2014|last1=Marcinkiewicz|first1=Janusz|last2=Kontny|first2=Ewa}} These granulocytes engulf viruses and bacteria in an intracellular vacuole called the phagosome, where they are digested.

Part of the digestion mechanism involves an enzyme-mediated respiratory burst, which produces reactive oxygen-derived compounds, including superoxide (which is produced by NADPH oxidase). Superoxide decays to oxygen and hydrogen peroxide, which is used in a myeloperoxidase-catalysed reaction to convert chloride to hypochlorite.{{cite journal|author1=Harrison, J. E. |author2=J. Schultz|year = 1976|title = Studies on the chlorinating activity of myeloperoxidase|journal = Journal of Biological Chemistry|volume = 251|issue = 5|pages = 1371–1374|doi=10.1016/S0021-9258(17)33749-3|pmid = 176150|doi-access = free}}{{cite journal|author = Thomas, E. L.|year = 1979|title = Myeloperoxidase, hydrogen peroxide, chloride antimicrobial system: Nitrogen-chlorine derivatives of bacterial components in bactericidal action against Escherichia coli|journal = Infect. Immun.|volume = 23|issue = 2|pages = 522–531|pmid = 217834|pmc = 414195|doi = 10.1128/IAI.23.2.522-531.1979}}{{cite journal|last1=Albrich|first1=JM|last2=McCarthy|first2=CA|last3=Hurst|first3=JK|title=Biological reactivity of hypochlorous acid: implications for microbicidal mechanisms of leukocyte myeloperoxidase.|journal=Proceedings of the National Academy of Sciences of the United States of America|date=January 1981|volume=78|issue=1|pages=210–4|pmid=6264434|doi=10.1073/pnas.78.1.210|pmc=319021|bibcode=1981PNAS...78..210A|doi-access=free}}

Low concentrations of hypochlorite were also found to interact with a microbe's heat shock proteins, stimulating their role as intra-cellular chaperone and causing the bacteria to form into clumps that will eventually die off.{{cite journal

| last = Jakob

| first = U.

|author2=J. Winter |author3=M. Ilbert |author4=P.C.F. Graf |author5=D. Özcelik

| title = Bleach Activates A Redox-Regulated Chaperone by Oxidative Protein Unfolding

| journal = Cell

| volume = 135

| issue = 4

| pages = 691–701

| publisher = Elsevier

| date = 14 November 2008

| url= | doi =10.1016/j.cell.2008.09.024

| pmid = 19013278

| pmc = 2606091 }} The same study found that low (micromolar) hypochlorite levels induce E. coli and Vibrio cholerae to activate a protective mechanism, although its implications were not clear.

In some cases, the base acidity of hypochlorite compromises a bacterium's lipid membrane.{{citation needed|date=June 2018}}

Hypochlorites, especially of sodium ("liquid bleach", "Javel water") and calcium ("bleaching powder") are widely used, industrially and domestically, to whiten clothes, lighten hair color and remove stains. They were the first commercial bleaching products, developed soon after that property was discovered in 1785 by French chemist Claude Berthollet.

Hypochlorites are widely used as broad spectrum disinfectants and deodorizers. That application started soon after French chemist Labarraque discovered those properties, around 1820 (still before Pasteur formulated his germ theory of disease).

Hypochlorite is the strongest oxidizing agent of the chlorine oxyanions. This can be seen by comparing the standard half cell potentials across the series; the data also shows that the chlorine oxyanions are stronger oxidizers in acidic conditions.{{Cotton&Wilkinson5th|page=564}}

Hypochlorite is a sufficiently strong oxidiser to convert Mn(III) to Mn(V) during the Jacobsen epoxidation reaction and to convert {{chem|Ce|3+}} to {{chem|Ce|4+}}.

This oxidising power is what makes them effective bleaching agents and disinfectants.

In organic chemistry, hypochlorites can be used to oxidise primary alcohols to carboxylic acids.{{cite book|last3=Warren|first1=Jonathan|last1=Clayden|authorlink1=Jonathan Clayden|first2=Nick|last2=Greeves|first3=Stuart|title=Organic Chemistry|publisher=Oxford University Press|location=Oxford|isbn=978-0-19-927029-3|page=195|edition=2nd|year=2012}}

Hypochlorite salts can also serve as chlorinating agents. For example, they convert phenols to chlorophenols. Calcium hypochlorite converts piperidine to N-chloropiperidine.

Chlorine can be the nucleus of oxyanions with oxidation states of −1, +1, +3, +5, or +7. Chlorine can also assume oxidation state +4 as seen in the neutral compound chlorine dioxide ClO₂.

• Chlorine oxide

{{reflist}}

{{Hypochlorites}}