Polyhalogen ions

The iodine is made soluble by combining with the aqueous iodide ion  

I It is a Lewis acid-Lewis base reaction the iodine atoms in I2 each have eight outer electrons. But in one of them this jumps to ten when 1 acts as a Lewis base, donating a non-bonding pair to I2, which acts as the Lewis acid. 

Many other polyhalide anions can be formed by reactions of the type shown in Equation 6.61. If CsF reacts with ClF3(g) at 100 °C, CsClF4(s) is formed a similar reaction with BrFs gives CsBrF. In these reactions, CIF3 and BrF5 act as Lewis acids, accepting a non-bonding pair from the F ion in CsF  

In the case of CsBrF6, it appears that the BrFg ion is regular octahedral. This confounds the predictions of VSEPR theory, which indicates seven repulsion axes. However, it is worth remembering that the ion is not free, but in a crystal, where its shape could be influenced by interaction with surrounding ions. 

So far we have considered reactions in which a halogen or interhalogen expands its valence shell by accepting a halide non-bonding pair. But reactions are also possible in which the interhalogen valence shell contracts through loss of a halide ligand. 

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The electrogenerative mode has also proved successful in direct and indirect electrocatalytic brominations and fluorinations under mild conditions. Vaporized bromine in nitrogen was used at the cathode to brominate alkenes at the anode, in a process similar to the chlorination above (47). Since bromine can form polyhalogen ions in solution. Bra , Br, , which could react nonelectrochemically, the electrolyte was flowed over the catalyst to remove such ions. As with direct chlorination, bromohydrins and dibromoalkanes were formed at platinum anodes. 

Polyelectronic atom, 20-43 Polyhalide ions, 839-843 Polyhalogen cations, 848 Polymers, and phosphazene polymers, 773-775 Polynuclear carbonyl complexes, 633-639... 

Compared with monohaloalkanes, polyhalogen compounds have quite different reactivities and behavior toward nucleophiles and bases. Thus di-chloromethane reacts with hydroxide ion by an SN2 mechanism much less readily than methyl chloride. The chloromethanol formed then undergoes a rapid E2 elimination to give methanal (formaldehyde), a substance that exists in water largely as dihydroxymethane ... 

The m.s. data, particularly now that recent techniques give a best-fit elemental composition of molecular ions and fragment ions which facilitates speculation of fission processes, provides in association with the above techniques, further information of structural value. Polyhalogenated compounds are particularly easily handled by this technique. 

Polyhalogenated aromatic hydrocarbons (e.g. hex-achlorobenzene (HCB, CeCle) and polychorobiphenyls (PCBs)) are rapidly degraded by superoxide ion in DMF to bicarbonate and halide ions. Because halogen-bearing intermediates are not detected, the initial nucleophilic attack is the rate-determining step. The rates of reaction exhibit a direct correlation with the electrophilicity of the substrate (reduction potential) (e.g. CeCle, E° = -1.48 V vs. SCE ki/[S] = 1 X lO M- s and 1,2,4-C6H3C13, E° = -2.16 V ki/[S] = 2x 10-2 M- s ). 

Molecular ions can be particularly complex for polyhalogenated compounds. Taking the natural abundance of Cl into account (see Problem 12.23), calculate the masses of the molecular ions of the following formulas. What are the relative percentages of each ion ... 

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