Chlorine Oxides

compounds of chlorine and oxygen: Cl₂O, ClO₂, Cl₂O₆, Cl₂O₇, and Cl₂O₈.

Chlorine monoxide, Cl₂O (an ahydride of hypochlorous acid), is a yellow-brown gas, with a pungent odor similar to the odor of chlorine. It has a melting point of –121°C and a boiling point of 2°C. A vigorous oxidizing agent, it is an unstable compound, which slowly decomposes under normal conditions and may explode spontaneously in concentrated and condensed form: 2Cl₂O = 2Cl₂ + O₂. It is readily soluble in water (about 200 volumes of Cl₂O dissolve in one volume of water) and carbon tetrachloride. Upon dissolving in water, it yields hypochlorous acid, HCIO. Chlorine monoxide may be produced by the reaction of chlorine with freshly precipitated dry mercuric oxide: 2HgO + 2Cl₂ = HgCl₂ + Cl₂O. In industry, aqueous solutions of chlorine monoxide are produced by the chlorination of carbonates of alkali or alkaline-earth metals in water. Chlorine monoxide is used in the production of calcium hypochlorite.

Chlorine dioxide, ClO₂ (a mixed anhydride of chlorous acid and chloric acid), is a yellow-orange gas, with an unpleasant odor and a melting point of –59°C and a boiling point of 10°C. It is a strong oxidizing agent, especially in an acidic medium, and is explosive in concentrated form. Chlorine dioxide decomposes gradually in the light, a process accompanied by explosion above 50°C. It is readily soluble in water, sulfuric acid, acetic acid, and carbon tetrachloride. Upon reaction with alkaline solutions, it forms chlorites and chlorates. Chlorine dioxide is produced by the partial reduction of chlorates by SO₂, oxalic acid, or hydrochloric acid and, in small amounts, by the action of chlorine on sodium chlorite:

2NaClO₂ + Cl₂ = 2NaCl + 2ClO₂

It is commonly used in the form of an aqueous solution or in gaseous form in a mixture with air to bleach and sterilize wood pulp, fabrics, and flour. The reaction of ClO₂ with aqueous solutions of NaOH in the presence of a reducing agent is the basis of the industrial production of chlorites.

Chlorine trioxide, Cl₂O₆ (a mixed anhydride of perchloric acid and chloric acid), is a fuming, dark red liquid, with a melting point of 3.5°C and a boiling point of 203°C. It explodes upon contact with readily oxidizable compounds. In the crystalline state, it has the structure of chloryl perchlorate, ClO₂⁺ClO₄^–. It reacts vigorously with water. Chlorine trioxide may be produced by the oxidation of chlorine dioxide with ozone or by the reaction of chlorates with fluorine: 2KClO₃ + F₂ = 2KF + Cl₂O₆. It is always present in the products of the thermal decompostion of perchloric acid. It has no practical applications.

Chlorine heptoxide, Cl₂O₇ (an anhydride of perchloric acid), is a colorless liquid, with a melting point of –93°C and a boiling point of 83°C. It decomposes slowly upon storage, with the production of colored decomposition products that are lower oxides of chlorine. It is spontaneously explosive, especially in the presence of its decomposition products. Chlorine heptoxide dissolves in carbon tetrachloride at room temperature and reacts with water to form perchloric acid. It explodes upon contact with iodine. Chlorine heptoxide may be obtained by the dehydration of perchloric acid in the presence of phosphorus pentoxide or oleum or by the low-temperature electrolysis of concentrated perchloric acid on a platinum anode. It has no practical applications.

Cl₂O₈, an intermediate in the electrolysis of perchloric acid and perchlorate salts, has not been isolated as a separate compound.

In addition to the oxygen compounds of chlorine mentioned above, Cl₂O₄ is also mentioned in the literature. This compound has the structure of chlorine perchlorate, ClOClO₃, and may be obtained by the reaction of chlorine fluorosulfonate and CsClO₄.