Dichlorine monoxide

Not to be confused with chlorine monoxide.

Dichlorine monoxide

Names
Other names Oxygen dichloride Dichlorine oxide Chlorine(I) oxide Hypochlorous oxide Hypochlorous anhydride
Identifiers
CAS Number	7791-21-1 Y
ChEBI	CHEBI:30198 Y
ChemSpider	23048 Y
Jmol 3D model	Interactive image
PubChem	24646
InChI InChI=1S/Cl2O/c1-3-2 Y Key: RCJVRSBWZCNNQT-UHFFFAOYSA-N Y InChI=1/Cl2O/c1-3-2 Key: RCJVRSBWZCNNQT-UHFFFAOYAA
SMILES ClOCl
Properties
Chemical formula	Cl2O
Molar mass	86.9054 g/mol
Melting point	−120.6 °C (−185.1 °F; 152.6 K)
Boiling point	2.0 °C (35.6 °F; 275.1 K)
Solubility in water	very soluble, hydrolyses 143 g Cl2O per 100 g water
Solubility in other solvents	soluble in CCl4
Structure
Dipole moment	0.78 ± 0.08 D
Thermochemistry
Std molar entropy (So298)	265.9 J K−1 mol−1
Std enthalpy of formation (ΔfHo298)	+80.3 kJ mol−1
Vapor pressure	{{{value}}}
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).
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Infobox references

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

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.

Preparation

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

2 Cl₂ + 2 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.

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 it must therefore be continuously removed to prevent thermal decomposition.

2 Cl₂ + Na₂CO₃ → Cl₂O + CO₂ + 2 NaCl

Dichlorine monoxide can also be formed by the reaction of calcium hypochlorite with carbon dioxide

Ca(ClO)₂ + CO₂ → CaCO₃ + Cl₂O

Structure

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; resulting in C_2V molecular symmetry. The bond angle is slightly larger than normal, likely due to steric repulsion between the bulky chlorine atoms.

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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.

Reactions

Dichlorine monoxide is highly soluble in water, 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, but the equilibrium constant ultimately favours the formation hypochlorous acid.

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, as well as in the chlorination of drinking water.

With inorganic compounds

Dichlorine monoxide reacts with metal halides, with the loss of Cl₂, to form unusual oxyhalides.

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.

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. For activated aromatics such as phenols and aryl-ethers it primarily reacts to give ring halogenated products. It has been suggested that dichlorine monoxide may be the active species in the reactions of HOCl with olefins and aromatic compounds.

Photochemistry

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.

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. which is an exceedingly high minimum explosive limit. There are conflicting reports of it exploding on exposure to strong light. Heating above 120°C, or a rapid rate of heating at lower temperatures also apparently lead to explosions. Liquid dichlorine monoxide has been reported to be shock-sensitive.