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Oxides of chlorine

Chlorine forms a series of oxides ( [link] ) in which the chlorine has the formal oxidation states +1, +4, +6, and +7. The physical properties of the oxides are summarized in [link] . While, the oxides of chlorine are not very stable (in fact several are shock sensitive and are prone to explode) the conjugate oxyacids are stable.

Physical properties of the oxides of chlorine.
Compound Mp (°C) Bp (°C)
Cl 2 O -116 4
ClO 2 -5.9 10
Cl 2 O 4 -117 44.5
Cl 2 O 6 3.5 unstable
Cl 2 O 7 -91.5 82

Dichlorine monoxide (Cl 2 O, [link] a) is a yellowish-red gas that is prepared by the reaction of chlorine with mercury oxide, [link] , or with a solution of chlorine in CCl 4 .

The structure of (a) Cl 2 O, (b) ClO 2 , (c) Cl 2 O 4 , and (d) Cl 2 O 7 .

When heated or subject to a spark, Cl 2 O explodes to Cl 2 and O 2 . Dichlorine monoxide reacts with water to form an orange-yellow solution of hypochlorous acid, [link] .

Chlorine dioxide (ClO 2 ) is a yellowish gas at room temperature and is commonly used in industry as an oxidizing agent. The best synthesis of ClO 2 involves the reduction of potassium chlorate (KClO 3 ) by oxalic acid at 90 °C, since the CO 2 formed acts as a diluent for the highly explosive ClO 2 . On an industrial scale ClO 2 is made by the exothermic reaction of sodium chlorate with SO 2 in sulfuric acid, [link] . The photolysis of ClO 2 yields a dark brown solid with the formula Cl 2 O 3 ; however, its facile explosive decomposition precludes study.

The structure of ClO 2 ( [link] b) is equivalent to SO 2 with one extra electron, resulting in a paramagnetic unpaired electron species. Unusually, despite the unpaired electron configuration, ClO 2 shows no tendency to dimerize. This is unlike the analogous NO 2 molecule.

Dichlorine tetraoxide (Cl 2 O 4 ) is commonly called chlorine perchlorate as a consequence of its structure ( [link] c). Dichlorine hexaoxide (Cl 2 O 6 ) is an unstable red oil that has the ionic structure in the solid state: [ClO 2 ] + [ClO 4 ] - .

Dichlorine heptoxide (Cl 2 O 7 ) is a relatively stable oil, that is prepared by the dehydration of perchloric acid at -10 °C, [link] , followed by vacuum distillation. The structure of Cl 2 O 7 ( [link] d) has been determined by gas phase electron diffraction.

The reaction of Cl 2 O 7 with alcohols and amines yields alkyl perchlorates (ROClO 3 ) and amine perchlorates (R 2 NClO 3 ), respectively.

Fluorides of chlorine

Given the isolobal relationship between the halogens it is not surprising that the mixed dihalogens can be prepared, e.g., ClF, ICl, and BrCl. Chlorine fluoride is a highly reactive gas (Bp = -100.1 °C) that is a powerful fluorinating agent, and is prepared by the oxidation of chlorine by chlorine trifluoride, [link] .

The higher electronegativity of fluorine as compared to chlorine ( [link] ), and the ability of chlorine to form more than one bond, means that higher fluorides of chlorine are also known, i.e., ClF 3 and ClF 5 . Chlorine trifluoride (CF 3 , Bp = 11.75 °C) is a useful fluorinating agent, that is prepared by the high temperature reaction of elemental chlorine and fluorine, is a useful fluorinating age. The gaseous pentafluoride (ClF 5 , Bp = -31.1 °C) is prepared by the reaction of potassium chloride with fluorine, [link] .

The structure of ClF 3 is T-shaped with two lone pairs on chlorine ( [link] a), while that of ClF 5 is square pyramidal with a single lone pair on chlorine ( [link] b).

The structures of (a) ClF 3 and (b) ClF 5 .

In general the halogen fluorides are very reactive; explosive reactions occur with organic compounds. They are all powerful fluorinating agents when diluted with nitrogen, and the order of reactivity follows:

Like most halogen fluorides, ClF, ClF 3 and ClF 5 all react with strong bases (e.g., alkali metal fluorides) to form anions, [link] and [link] , and strong acids (e.g., AsF 5 and SbF 5 ) to form cations, [link] , [link] , and [link] .

Questions & Answers

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Source:  OpenStax, Chemistry of the main group elements. OpenStax CNX. Aug 20, 2010 Download for free at http://cnx.org/content/col11124/1.25
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