Halate ions

The oxidizing power of the halate ions in aqueous solution, as measured by their standard reduction potentials (p. 854), decreases in the sequence bromate > chlorate > iodate but the rates of reaction follow the sequence iodate > bromate > chlorate. In addition, both the thermodynamic oxidizing power and the rate of reaction depend markedly on the hydrogen-ion concentration of the solution, being substantially greater in acid than in alkaline conditions (p, 855). 

As with FCIO2 and FIO2, hydrolysis regenerates the halate ion, the reaction with FBr02 being of explosive violence. Hydrolysis in basic solution at 0° can be represented as... 

Halate ions are trigonal pyramidal, with Csv symmetry, as shown below ... 

The chlorate, bromate, and iodate ions are stable in solution, and many salts derived from these anions are familiar compounds. Although, as we have seen, the halate ions result from basic hydrolysis of the elemental halogens in warm solution, such a hydrolysis is generally not the method of choice for preparation of pure halates, since five sixths of the halogen... 

The situation, however, is complicated by the tendency of the hypohalite ions to disproportionate further in basic solution to produce the halate ions ... 

Finally, we must consider the possibility of production of perhalate ions by disproportionation of the halate ions. Since the acids H0X02 and HOXO3 are all strong, these equilibria are independent of pH. The reaction... 

All the halic acids are strong acids and arc powerful oxidizing agents the mechanisms of reduction, e.g., by I- are very complicated.2 11 The halate ions, XO3, are all pyramidal, as is to be expected from the presence of an octet, with one unshared pair, in the halogen valence shell. 

Another example of the limitations of the data in Figure 17.14 is the inference that O2 should oxidize I and Br at pH 0. Further, the fact that CI2 rather than O2 is evolved when hydrochloric acid is electrolysed is a consequence of the high overpotential for O2 evolution at most surfaces (see worked example 17.3). Despite some limitations. Figure 17.14 does provide some useful information for example, the more powerful oxidizing properties of periodate and perbromate than of perchlorate when these species are being reduced to halate ions, and the more weakly oxidizing powers of iodate and iodine than of the other halates or halogens respectively. 

Oxidation with Hypohalites in Alkaline Solutions. In alkaline solution the halogens exist as hypohalous acid and hypohalite ions. The oxidation is likely to be more drastic than for the free halogens. Thus, whereas free iodine will not act as an oxidant, hypoiodite is a powerful oxidizing agent. Hypobromite and hypochlorite particularly are likely to produce oxidation of primary and secondary alcoholic groups and cause cleavage of carbon-carbon bonds. As noted above, the processes are complicated by the tendency of hypohalite to be converted to halate ions. 

Studies of the oxidations of oxovanadium(iv) by halate ions have been reported. In the reaction with bromate ... 

The structures of the intermediates may involve bonding between the vanadium atom and one of the oxygens of the halate ion, with subsequent fission of this halogen-oxygen bond. 

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