Halates disproportionation

Reaction (11.4) is really a disproportionation reaction of the halate(I) anion 3XO 2X -E XO. ) Reaction (11.3) is favoured by the use of dilute alkali and low temperature, since the halate(I) anions, XO are thermally unstable and readily disproportionate (i.e. reaction (11.4)). The stability of the halate(I) anion, XO , decreases from chlorine to iodine and the iodate(I) ion disproportionates very rapidly even at room temperature. 

The stability of the halate(I) ion decreases, as expected, from C10 to 10 and only the chlorate(I) ion can be considered reasonably stable even in aqueous solution. Solid sodium bromate(I). NaBrO (with five or seven molecules of water of crystallisation) can be obtained, but on standing or warming it disproportionates ... 

The modes of thermal decomposition of the halates and their complex oxidation-reduction chemistry reflect the interplay of both thermodynamic and kinetic factors. On the one hand, thermodynamically feasible reactions may be sluggish, whilst, on the other, traces of catalyst may radically alter the course of the reaction. In general, for a given cation, thermal stability decreases in the sequence iodate > chlorate > bromate, but the mode and ease of decomposition can be substantially modified. For example, alkali metal chlorates decompose by disproportionation when fused ... 

For all three halates (in the absence of disproportionation) the preferred mode of decomposition depends, again, on both thermodynamic and kinetic considerations. Oxide formation tends to be favoured by the presence of a strongly polarizing cation (e.g. magnesium, transition-metal and lanthanide halates), whereas halide formation is observed for alkali-metal, alkaline- earth and silver halates. 

The trivalent [P04] and [As04] ions react similarly. Examples of anions that give insoluble Hg(I) compounds in this way include halides, pseudohalides, halates, carboxylates and sulfate. A trace of HNO3 or HCIO4 is often added to the solution of the Hg(I) nitrate or perchlorate to prevent disproportionation induced by alkali. Table 1 lists common Hg(I) derivatives prepared in this way and includes values of the solubility products of the sparingly soluble Hg(I) compounds where these are measured. A similar reaction is used to prepare HgjCO, from a soluble bicarbonate ... 

As mentioned in Section I, disproportionation occurs most rapidly at a pH corresponding to the pKa value [Eq. (12). The velocity of formation of halate increases greatly in the order C103 < Br03 < 107 this order correlates directly with the leaving-group tendencies of the respective halide ion (see Table I). 

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... 

Introducing a halogen into a hot basic solution causes a disproportionation process. Part of the halogen is reduced to the halide anion and another part of it is oxidized to the halate anion (equation 117). 

The hypohalites (XO ), halites (X02 ), and halates (X03 ) are oxidizing agents formed by aqueous disproportionation reactions [see Group 7A(17) Family Portrait]. Potassium chlorate is the oxidizer in safety matches. You may have heated it in the lab to form small amounts of O2 ... 

With hot alkaU, however, the same reactants give different products. The hypohalite anions, the initial products of the reaction, disproportionate on warming to halate (XO ) and halide (X , X = Cl, Br, I) ... 

Noszticzius, Z. Noszticzius, E. Schelly, Z. A. 1983. On the Use of Ion-Selective Electrodes for Monitoring Oscillating Reactions. 2. Potential Response of Bromide-and Iodide-Selective Electrodes in Slow Corrosive Processes. Disproportionation of Bromous and lodous Acid. A Lotka-Volterra Model for the Halate Driven Oscillators, J. Phys. Chem. 87, 510-524. 

Aqueous solutions of chloric and bromic acids are formed by disproportionation of CI2 or Br2 in hot alkaline solution [Eq. (15) and (16)] or by treating the appropriate barium halate salt with sulfuric acid and removing the precipitated barium sulfate [similar to Eq. (41)]. Concentration of these solutions leads to decomposition of the acid. 

Sodium chlorate, NaClOy, is the most commercially important halate salt. It is prepared commercially on a large scale by electrolyzing hot brine and allowing the chlorine gas liberated at the anode to disproportionate in the basic solution that forms at the cathode. The major uses of sodium chlorate are as a bleaching agent for paper pulp, as a defoliant, and as an intermediate in the production... 

In the hypohalous-hypohalite section, we saw that the heavier three halogens can be hydrolyzed to hypohalites [Equation (18.32)] and that these in turn can disproportionate with varying rates (IO > BrO > CIO ) to the halates and halides [Equation (18.33)]. Given these reactions, it is not surprising that hot alkaline solutions of chlorine and bromine go directly to chlorate and bromate, respectively. The overall reaction is represented in Equation (18.40) ... 

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