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.
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.
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.
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 of 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.
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.
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.
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