Bromide to iodide

The elimination of sulfur dioxide is apparently more difficult than that of either carbon monoxide or carbon dioxide When fluorosulfonyldifluoroacetyl halide (chloride, bromide, or iodide) is photolyzed, carbon monoxide is quantitatively eliminated to give halodifluoromethanesulfonyl fluonde with increasing ease from chloride to bromide to iodide [95, 100] (equation 67)... 

The passage from bromides to iodides thus illustrates the strategy to be followed to obtain information on the redox properties of radicals. It is, however,... 

The synthesis of the cation is typically performed by alkylation of an amine, phosphine or sulfide, most commonly using an alkyl halide [ ]. In most cases the reaction is carried out with chloro-, bromo- and iodoalkanes as readily available alkylating reagents, with the reaction conditions becoming more gentle changing from chloride to bromide to iodide, as can be expected for nucleophilic substitution... 

All four halides are known. In the gas phase, they consist of linear X-Zn-X molecules with intemuclear distances (in A) as a function of X as follows F, 1.81 Cl, 2.05 Br, 2.21 I, 2.38. The diflnoride has the highest melting point (872 °C), the melting points rising thereafter from chloride to bromide to iodide (275, 394, and 446 °C, respectively). ZnF2 may be prepared by the action of HF on the metal or by the action of heat on the tetraflnoroborate Zn(BF4)2. It has the typically ionic 6 3 rutile structure in which zinc is six coordinated by... 

Halogen exchange from chlorides or bromides to iodides... 

Halide exchange from the lower halides to iodine is often desirable due to the higher reactivity of iodides in nucleophilic substitutions, reductions, organometallic or radical reactions (Scheme 30). Conversion of chlorides and bromides to iodides with sodium iodide in acetone is called the Finkelstein reaction. This halide exchange is an equilibrium process, which is shifted to the iodinated products due to precipitation of the less soluble sodium bromide or chloride from acetone. Best results are obtained when the reaction mixture is free of water. 

It is obvious from the foregoing discussion that the enthalpies of mixing for charge-unsymmetrical systems do not follow the simple conformal solution theory. When the anion in a strontium halide-alkali metal halide mixture from chloride to bromide and from bromide to iodide is changed, the enthalpy of mixing is decreasing. For all systems, the enthalpy interaction parameter, k, is a linear function of 512 with the usual exception for lithium-containing systems. Two important features of the k versus 512 plot should be emphasized ... 

However, a reduction in the complexation ability of the melt-anion leads to a shift in the equilibrium (3.6.3) to the left, which finally results in a reduction in the metal-oxide s solubility. The substitution of bromide ions by iodide ions (transfer from bromide to iodide-melts) leads to the strengthening of the basic properties (which cause a decrease in the oxide solubility) and of the complexation ability (which results in the solubility increase). For molten Csl, the simultaneous action of both factors leads to an increase in the metal-oxide solubilities. However, iodide ion belongs to the group of soft bases, and this is the main reason for the solubilities of oxides in iodide-melts remaining lower than that in the molten alkali-metal chlorides. 

Schay [82] has reported on the reactions of both sodium and potassium with the hydrogen halides using the nozzle flame method. The rate coefficients increased for both metals in the order from chloride to bromide to iodide. A similar result was obtained [83] using the diffusion flame technique. The most accurate measurements [84] obtained by the diffusion flame apparatus give rate coefficients for reaction with sodium at 511°K of 4.1 X 10 (HCl) and 3.4 x 10 (DCl) cm mole sec. The mechanism of the reaction has been discussed [53] in terms of the energy surfaces involved and of the zero point energy contribution to the activation energy. 

Resemblance of bromide to iodide goitrogenic effects of bromide... 

The nature of the cation grafted and the counteranion used, have unsurprisingly a direct effect on the efficiency of the catalysis. Whereas changing the anion from chloride or bromide to iodide led to a loss of activity only for the iodide (after 2 h Cl 65% Br 66% 1 44%), decreasing the acidity by changing the cation from dimethyl pyridinium to the unsubstituted pyridinium led in comparison to a decrease in conversion from 74% to 51% (5 h). 

The tetrakis-sulphoxides (67) have been reported as a new type of PT catalyst in a solid-liquid mode, for example in 5n reactions of 1-bromo-octane. Nucleophilic substitutions of 1-bromo-octane and benzyl bromide are again the test reactions in a report on the use of the sucrose-ethylene oxide adducts (68a) as PT catalysts in both solid-liquid and liquid-liquid modes. The methacrylate ester derivative (68b) has been polymerized to a cross-linked gel that acts in a TC capacity for the same reactions. In a related approach some modified dextran anion exchangers carrying lipophilic substituents, such as the modified hydroxypropylated dextran gel shown in (69), have been synthesized and shown to catalyse displacement reactions including the alkyl bromide to iodide transformation under TC conditions. ... 

The comparative reactivity order for AnthT ys.. Anth with chlorides and bromides is contrary to what is expected on the basis of their respective reduction potentials (17). This rate ratio changes by an order of magnitude in going from chlorides to bromides to iodides. For the iodides the rates of the two reactions are comparable but this surprising similarity may be fortuitous and may not in fact mean a similarity in mechanisms. Further discussion of this point appears in a subsequent section. 

We also examined the Finkelstein reaction of octyl halide involving a conversion from bromide to iodide or from iodide to bromide (see Equation 2) [26]. The results are summarized in Table IV. The Finkelstein reaction proceeded without any catalysts, because halide salts are soluble in various solvents. But, when catalyst was combined in this system, the rate was enhanced clearly by 1.5-2 times (see Table IV). In the case involving a conversion of octyl bromide to iodide, 9c showed the maximum rate for KI and Rbl in acetone or acetonitrile, indicating that its complexation with the alkali metal ion is an important factor to accelerate this reaction. Note that 9c has the largest rate constant among the catalysts for all metal iodides examined in this reaction. 

In view of this mode of operation, it is easy to understand why those anions which form less soluble compounds with silver than does the measured ion interfere with these halide electrodes. The number of interfering ions decreases from chloride to bromide to iodide and finally to sulfide, since correspondingly fewer anions exist which can produce less soluble silver salts. Only Hg " interferes with the sulfide electrode. This is because Hg forms an equally insoluble material with sulfide which can deposit on the electrode surface. In determining sulfide ion certain precautions are necessary. Sulfide is sometimes inclined to associate with hydrogen ions ... 

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