Dichlorine monoxide

Last updated
Dichlorine monoxide
Other names
Oxygen dichloride
Dichlorine oxide
Chlorine (I) oxide
Hypochlorous oxide
Hypochlorous anhydride
3D model (JSmol)
ECHA InfoCard 100.029.312 OOjs UI icon edit-ltr-progressive.svg
PubChem CID
  • InChI=1S/Cl2O/c1-3-2 Yes check.svgY
  • InChI=1/Cl2O/c1-3-2
  • ClOCl
Molar mass 86.9054 g/mol
Appearancebrownish-yellow gas
Melting point −120.6 °C (−185.1 °F; 152.6 K)
Boiling point 2.0 °C (35.6 °F; 275.1 K)
very soluble, hydrolyses 143 g Cl2O per 100 g water
Solubility in other solventssoluble in CCl4
0.78 ± 0.08 D
Std molar
265.9 J K−1 mol−1
+80.3 kJ mol−1
GHS labelling:
GHS-pictogram-acid.svg GHS-pictogram-pollu.svg
H290, H314, H400, H411
P234, P260, P264, P273, P280, P301+P330+P331, P303+P361+P353, P304+P340, P305+P351+P338, P310, P311, P321, P363, P390, P391, P405, P406, P501
NFPA 704 (fire diamond)
Safety data sheet (SDS) [1]
Related compounds
Other cations
Nitrous oxide, dibromine monoxide, water
Related compounds
Oxygen difluoride, chlorine dioxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Yes check.svgY  verify  (what is  Yes check.svgYX mark.svgN ?)

Dichlorine monoxide is an inorganic compound with the molecular formula Cl2O. It was first synthesised in 1834 by Antoine Jérôme Balard, [2] who along with Gay-Lussac also determined its composition. In older literature it is often referred to as chlorine monoxide, [3] which can be a source of confusion as that name now refers to the ClO radical.


At room temperature it exists as a brownish-yellow gas which is soluble in both water and organic solvents. Chemically, it is a member of the chlorine oxide family of compounds, as well as being the anhydride of hypochlorous acid. It is a strong oxidiser and chlorinating agent.


The earliest method of synthesis was to treat mercury(II) oxide with chlorine gas. [3] However, this method is expensive, as well as highly dangerous due to the risk of mercury poisoning.

2 Cl2 + HgO → HgCl2 + Cl2O

A safer and more convenient method of production is the reaction of chlorine gas with hydrated sodium carbonate at 20–30 °C. [3]

2 Cl2 + 2 Na2CO3 + H2O → Cl2O + 2 NaHCO3 + 2 NaCl
2 Cl2 + 2 NaHCO3 → Cl2O + 2 CO2 + 2 NaCl + H2O

This reaction can be performed in the absence of water but requires heating to 150–250 °C; as dichlorine monoxide is unstable at these temperatures [4] it must therefore be continuously removed to prevent thermal decomposition.

2 Cl2 + Na2CO3 → Cl2O + CO2 + 2 NaCl


The structure of dichlorine monoxide is similar to that of water and hypochlorous acid, with the molecule adopting a bent molecular geometry (due to the lone pairs on the oxygen atom) and resulting in C2V molecular symmetry. The bond angle is slightly larger than normal, likely due to steric repulsion between the bulky chlorine atoms.

In the solid state, it crystallises in the tetrahedral space group I41/amd, making it isostructural to the high pressure form of water, ice VIII. [5]


Dichlorine monoxide is highly soluble in water, [6] where it exists in an equilibrium with HOCl. The rate of hydrolysis is slow enough to allow the extraction of Cl2O with organic solvents such as CCl4, [3] but the equilibrium constant ultimately favours the formation of hypochlorous acid. [7]

2 HOCl ⇌ Cl2O + H2O K (0 °C) = 3.55x10−3 dm3/mol

Despite this, it has been suggested that dichlorine monoxide may be the active species in the reactions of HOCl with olefins and aromatic compounds, [8] [9] as well as in the chlorination of drinking water. [10]

With inorganic compounds

Dichlorine monoxide reacts with metal halides, with the loss of Cl2, to form unusual oxyhalides. [11] [12] [3]

VOCl3 + Cl2O → VO2Cl + 2 Cl2
TiCl4 + Cl2O → TiOCI2 + 2 Cl2
SbCI5 + 2 CI2O → SbO2CI + 4 Cl2

Similar reactions have also been observed with certain inorganic halides. [13] [14]

AsCI3 + 2 CI2O → AsO2CI + 3 Cl2
NOCl + Cl2O → NO2Cl + Cl2

With organic compounds

Dichlorine monoxide is an effective chlorinating agent. It can be used for either the side-chain or ring chlorination of deactivated aromatic substrates. [15] For activated aromatics such as phenols and aryl-ethers it primarily reacts to give ring halogenated products. [16] It has been suggested that dichlorine monoxide may be the active species in the reactions of HOCl with olefins and aromatic compounds. [8] [9]


Dichlorine monoxide undergoes photodissociation, eventually forming O2 and Cl2. The process is primarily radical based, with flash photolysis showing radical hypochlorite (ClO·) to be a key intermediate. [17]

2 Cl2O → 2 Cl2 + O2

Explosive properties

Dichlorine monoxide is explosive, although there is a lack of modern research into this behaviour. Room temperature mixtures with oxygen could not be detonated by an electric spark until they contained at least 23.5% Cl2O [18] which is an exceedingly high minimum explosive limit. There are conflicting reports of it exploding on exposure to strong light. [19] [20] Heating above 120 °C, or a rapid rate of heating at lower temperatures also apparently lead to explosions. [3] Liquid dichlorine monoxide has been reported to be shock-sensitive. [21]

Related Research Articles

<span class="mw-page-title-main">Chlorine</span> Chemical element, symbol Cl and atomic number 17

Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine is a yellow-green gas at room temperature. It is an extremely reactive element and a strong oxidising agent: among the elements, it has the highest electron affinity and the third-highest electronegativity on the revised Pauling scale, behind only oxygen and fluorine.

<span class="mw-page-title-main">Sodium hypochlorite</span> Chemical compound (known in solution as bleach)

Sodium hypochlorite is an inorganic chemical compound with the formula NaOCl, comprising a sodium cation and a hypochlorite anion. It may also be viewed as the sodium salt of hypochlorous acid. The anhydrous compound is unstable and may decompose explosively. It can be crystallized as a pentahydrate NaOCl·5H
, a pale greenish-yellow solid which is not explosive and is stable if kept refrigerated.

<span class="mw-page-title-main">Hypochlorous acid</span> Chemical compound

Hypochlorous acid is a weak acid that forms when chlorine dissolves in water, and itself partially dissociates, forming hypochlorite, ClO. HClO and ClO are oxidizers, and the primary disinfection agents of chlorine solutions. HClO cannot be isolated from these solutions due to rapid equilibration with its precursor, chlorine.

<span class="mw-page-title-main">Hypochlorite</span> Ion

In chemistry, hypochlorite is an anion with the chemical formula ClO. It combines with a number of cations to form hypochlorite salts. Common examples include sodium hypochlorite and calcium hypochlorite. The Cl-O distance in ClO is 1.69 Å.

<span class="mw-page-title-main">Nitrogen trichloride</span> Chemical compound

Nitrogen trichloride, also known as trichloramine, is the chemical compound with the formula NCl3. This yellow, oily, pungent-smelling and explosive liquid is most commonly encountered as a byproduct of chemical reactions between ammonia-derivatives and chlorine (for example, in swimming pools). Alongside monochloramine and dichloramine, trichloramine is responsible for the distinctive 'chlorine smell' associated with swimming pools, where the compound is readily formed as a product from hypochlorous acid reacting with ammonia and other nitrogenous substances in the water, such as urea from urine.

<span class="mw-page-title-main">Sodium chlorate</span> Chemical compound

Sodium chlorate is an inorganic compound with the chemical formula NaClO3. It is a white crystalline powder that is readily soluble in water. It is hygroscopic. It decomposes above 300 °C to release oxygen and leaves sodium chloride. Several hundred million tons are produced annually, mainly for applications in bleaching pulp to produce high brightness paper.

<span class="mw-page-title-main">Dichlorine heptoxide</span> Chemical compound

Dichlorine heptoxide is the chemical compound with the formula Cl2O7. This chlorine oxide is the anhydride of perchloric acid. It is produced by the careful distillation of perchloric acid in the presence of the dehydrating agent phosphorus pentoxide:

<span class="mw-page-title-main">Gold(III) chloride</span> Chemical compound

Gold(III) chloride, traditionally called auric chloride, is a compound of gold and chlorine with the molecular formula Au2Cl6. The "III" in the name indicates that the gold has an oxidation state of +3, typical for many gold compounds. Gold(III) chloride is hygroscopic and decomposes in visible light. This compound is a dimer of AuCl3. This compound has few uses, although it catalyzes various organic reactions.

<span class="mw-page-title-main">Sulfuryl chloride</span> Chemical compound

Sulfuryl chloride is an inorganic compound with the formula SO2Cl2. At room temperature, it is a colorless liquid with a pungent odor. Sulfuryl chloride is not found in nature, as can be inferred from its rapid hydrolysis.

In chemical nomenclature, the IUPAC nomenclature of inorganic chemistry is a systematic method of naming inorganic chemical compounds, as recommended by the International Union of Pure and Applied Chemistry (IUPAC). It is published in Nomenclature of Inorganic Chemistry. Ideally, every inorganic compound should have a name from which an unambiguous formula can be determined. There is also an IUPAC nomenclature of organic chemistry.

<span class="mw-page-title-main">BCDMH</span> Chemical compound

1-Bromo-3-chloro-5,5-dimethylhydantoin is a chemical structurally related to hydantoin. It is a white crystalline compound with a slight bromine and acetone odor and is insoluble in water, but soluble in acetone.

<span class="mw-page-title-main">Vanadium oxytrichloride</span> Chemical compound

Vanadium oxytrichloride is the inorganic compound with the formula VOCl3. This yellow distillable liquid hydrolyzes readily in air. It is an oxidizing agent. It is used as a reagent in organic synthesis. Samples often appear red or orange owing to an impurity of vanadium tetrachloride.

Monochloramine, often called chloramine, is the chemical compound with the formula NH2Cl. Together with dichloramine (NHCl2) and nitrogen trichloride (NCl3), it is one of the three chloramines of ammonia. It is a colorless liquid at its melting point of −66 °C (−87 °F), but it is usually handled as a dilute aqueous solution, in which form it is sometimes used as a disinfectant. Chloramine is too unstable to have its boiling point measured.

<span class="mw-page-title-main">Chlorine perchlorate</span> Chemical compound

Chlorine perchlorate is a chemical compound with the formula Cl2O4. This chlorine oxide is an asymmetric oxide, with one chlorine atom in +1 oxidation state and the other +7, with proper formula ClOClO3. It is produced by the photodimerization of chlorine dioxide (ClO2) at room temperature by 436 nm ultraviolet light:

The Pinnick oxidation is an organic reaction by which aldehydes can be oxidized into their corresponding carboxylic acids using sodium chlorite (NaClO2) under mild acidic conditions. It was originally developed by Lindgren and Nilsson. The typical reaction conditions used today were developed by G. A. Kraus. H.W. Pinnick later demonstrated that these conditions could be applied to oxidize α,β-unsaturated aldehydes. There exist many different reactions to oxidize aldehydes, but only a few are amenable to a broad range of functional groups. The Pinnick oxidation has proven to be both tolerant of sensitive functionalities and capable of reacting with sterically hindered groups. This reaction is especially useful for oxidizing α,β-unsaturated aldehydes, and another one of its advantages is its relatively low cost.

In chemistry, molecular oxohalides (oxyhalides) are a group of chemical compounds in which both oxygen and halogen atoms are attached to another chemical element A in a single molecule. They have the general formula AOmXn, where X = fluorine (F), chlorine (Cl), bromine (Br), and/or iodine (I). The element A may be a main group element, a transition element or an actinide. The term oxohalide, or oxyhalide, may also refer to minerals and other crystalline substances with the same overall chemical formula, but having an ionic structure.

<span class="mw-page-title-main">Chlorine peroxide</span> Chemical compound

Chlorine peroxide is a molecular compound with formula ClOOCl. Chemically, it is a dimer of the chlorine monoxide radical (ClO·). It is important in the formation of the ozone hole. Chlorine peroxide catalytically converts ozone into oxygen when it is irradiated by ultraviolet light.

<span class="mw-page-title-main">Vanadyl perchlorate</span> Chemical compound

Vanadyl perchlorate or vanadyl triperchlorate is a golden yellow coloured liquid or crystalline compound of vanadium, oxygen and perchlorate group. The substance consists of molecules covalently bound and is quite volatile.

<span class="mw-page-title-main">Chlorine trifluoride oxide</span> Chemical compound

Chlorine oxide trifluoride or chlorine trifluoride oxide is a corrosive liquid molecular compound with formula ClOF3. It was developed secretly as a rocket fuel oxidiser.

<span class="mw-page-title-main">Acetyl hypochlorite</span> Chemical compound

Acetyl hypochlorite, also known as chlorine acetate, is a chemical compound with the formula CH3COOCl. It is a photosensitive colorless liquid that is a short lived intermediate in the Hunsdiecker reaction.


  1. "CHLORINE MONOXIDE". CAMEO Chemicals. National Oceanic and Atmospheric Administration. Retrieved 12 May 2015.
  2. Balard, A.J. (1834). "Recherches sur la nature des combinaisons décolorantes du chlore" [Investigations into the nature of bleaching compounds of chlorine]. Annales de Chimie et de Physique. 2nd series (in French). 57: 225–304.
  3. 1 2 3 4 5 6 Renard, J. J.; Bolker, H. I. (1 August 1976). "The chemistry of chlorine monoxide (dichlorine monoxide)". Chemical Reviews. 76 (4): 487–508. doi:10.1021/cr60302a004.
  4. Hinshelwood, Cyril Norman; Prichard, Charles Ross (1923). "CCCXIII.—A homogeneous gas reaction. The thermal decomposition of chlorine monoxide. Part I". Journal of the Chemical Society, Transactions. 123: 2730–2738. doi:10.1039/CT9232302730.
  5. Minkwitz, R.; Bröchler, R.; Borrmann, H. (1 January 1998). "Tieftemperatur-Kristallstruktur von Dichlormonoxid, Cl2O". Zeitschrift für Kristallographie. 213 (4): 237–239. Bibcode:1998ZK....213..237M. doi:10.1524/zkri.1998.213.4.237.
  6. Davis, D. S. (1942). "Nomograph for the Solubility of Chlorine Monoxide in Water". Industrial & Engineering Chemistry. 34 (5): 624. doi:10.1021/ie50389a021.
  7. Aylett, founded by A.F. Holleman ; continued by Egon Wiberg ; translated by Mary Eagleson, William Brewer ; revised by Bernhard J. (2001). Inorganic chemistry (1st English ed., [edited] by Nils Wiberg. ed.). San Diego, Calif. : Berlin: Academic Press, W. de Gruyter. p. 442. ISBN   9780123526519.
  8. 1 2 Swain, C. Gardner; Crist, DeLanson R. (1 May 1972). "Mechanisms of chlorination by hypochlorous acid. The last of chlorinium ion, Cl+". Journal of the American Chemical Society. 94 (9): 3195–3200. doi:10.1021/ja00764a050.
  9. 1 2 Sivey, John D.; McCullough, Corey E.; Roberts, A. Lynn (1 May 2010). "Chlorine Monoxide (Cl2O) and Molecular Chlorine (Cl2) as Active Chlorinating Agents in Reaction of Dimethenamid with Aqueous Free Chlorine". Environmental Science & Technology. 44 (9): 3357–3362. Bibcode:2010EnST...44.3357S. doi:10.1021/es9038903. PMID   20302364.
  10. Powell, Steven C. (1 May 2010). "The active species in drinking water chlorination: the case for Cl2O". Environmental Science & Technology. 44 (9): 3203. Bibcode:2010EnST...44.3203P. doi:10.1021/es100800t. PMID   20302368.
  11. Oppermann, H. (1967). "Untersuchungen an Vanadinoxidchloriden und Vanadinchloriden. I. Gleichgewichte mit VOCl3, VO2Cl und VOCl2". Zeitschrift für anorganische und allgemeine Chemie. 351 (3–4): 113–126. doi:10.1002/zaac.19673510302.
  12. Dehnicke, Kurt (1961). "Titan(IV)-Oxidchlorid TiOCl2". Zeitschrift für anorganische und allgemeine Chemie. 309 (5–6): 266–275. doi:10.1002/zaac.19613090505.
  13. Dehnicke, Kurt (1 December 1964). "Über die Oxidchloride PO2Cl, AsO2Cl und SbO2Cl". Chemische Berichte. 97 (12): 3358–3362. doi:10.1002/cber.19640971215.
  14. Martin, H. (1 January 1966). "Kinetic Relationships between Reactions in the Gas Phase and in Solution". Angewandte Chemie International Edition in English. 5 (1): 78–84. doi:10.1002/anie.196600781.
  15. Marsh, F. D.; Farnham, W. B.; Sam, D. J.; Smart, B. E. (1 August 1982). "Dichlorine monoxide: a powerful and selective chlorinating reagent". Journal of the American Chemical Society. 104 (17): 4680–4682. doi:10.1021/ja00381a032.
  16. Sivey, John D.; Roberts, A. Lynn (21 February 2012). "Assessing the Reactivity of Free Chlorine Constituents Cl2, Cl2O, and HOCl Toward Aromatic Ethers". Environmental Science & Technology. 46 (4): 2141–2147. Bibcode:2012EnST...46.2141S. doi:10.1021/es203094z. PMID   22211432.
  17. Basco, N.; Dogra, S. K. (22 June 1971). "Reactions of Halogen Oxides Studied by Flash Photolysis. II. The Flash Photolysis of Chlorine Monoxide and of the ClO Free Radical". Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences. 323 (1554): 401–415. Bibcode:1971RSPSA.323..401B. doi:10.1098/rspa.1971.0112. S2CID   98084403.
  18. Cady, George H.; Brown, Robert E. (September 1945). "Minimum Explosive Concentration of Chlorine Monoxide Diluted with Oxygen". Journal of the American Chemical Society. 67 (9): 1614–1615. doi:10.1021/ja01225a501.
  19. Iredale, T.; Edwards, T. G. (April 1937). "Photoreaction of Chlorine Monoxide and Hydrogen". Journal of the American Chemical Society. 59 (4): 761. doi:10.1021/ja01283a504.
  20. Wallace, Janet I.; Goodeve, C. F. (1 January 1931). "The heats of dissociation of chlorine monoxide and chlorine dioxide". Transactions of the Faraday Society. 27: 648. doi:10.1039/TF9312700648.
  21. Pilipovich, Donald; Lindahl, C. B.; Schack, Carl J.; Wilson, R. D.; Christe, Karl O. (1972). "Chlorine trifluoride oxide. I. Preparation and properties". Inorganic Chemistry. 11 (9): 2189–2192. doi:10.1021/ic50115a040. ISSN   0020-1669.