Halide ions, concentration

In order to strengthen evidence in favour of the proposition that concerted inplane 5n2 displacement reactions can occur at vinylic carbon the kinetics of reactions of some /3-alkyl-substituted vinyliodonium salts (17) with chloride ion have been studied. Substitution and elimination reactions with formation of (21) and (22), respectively, compete following initial formation of a chloro-A, -iodane reaction intermediate (18). Both (17) and (18) undergo bimolecular substitution by chloride ion while (18) also undergoes a unimolecular (intramolecular) jS-elimination of iodoben-zene and HCl. The [21]/[22] ratios for reactions of (18a-b) increase with halide ion concentration, and there is no evidence for formation of the -isomer of (Z)-alkene (21) iodonium ion (17d) forms only the products of elimination, (22d) and (23). 

The anodic oxidation of iodide, bromide and chloride ions at illuminated ZnO electrodes, which occurs in competition with the anodic decomposition of the electrode itself, was studied as functions of halide ion concentration, illumination intensity and solution pH in order to investigate factors which affect the degree of competition. The reactivity of halide ions, obtained under fixed conditions, was in the order of I >Br >Cl, reflecting the importance of the redox potential in determining the reactivity. 

Here the rate is linear in developer concentration but independent of developer potential, halide ion concentration and rate of scavenging of oxidized developer. Such cases have not been seen experimentally. 

A practical problem frequently encountered by the analytical chemist is that of maintaining as nearly as possible a constant ionic environment while varying the concentration of one component of a mixture of electrolytes. For example, in evaluating the successive formation constants of halide complexes of a metal ion, it is necessary to vary the halide ion concentration over wide limits. The practice generally... 

If halide ion is present during hydrolysis of benzenediazonium ion or i -nitro-benzenediazonium ion, there is obtained not only the phenol, but also the aryl halide the higher the halide ion concentration, the greater the proportion of aryl halide obtained. The presence of halide ion has no effect on the rate of decomposition of benzenediazonium ion, but speeds up decomposition of the p-nitrobenzenediazonium ion. 

Table I. Influence of Alcohol and Halide-Ion Concentration on Alpha Selectivity in Glycosidations...

The most common method of determining the halide ion concentration of aqueous solutions is titration v /ith a standard solution of silver nitrate. The reaction product is solid silver halide. A titration curve for this method usually consists of a plot of pAg versus the volume of silver nitrate added. To construct titration curves, three type of calculations are required, each of which corresponds to a distinct stage in the reaction (1) preequivalence, (2) equivalence, and (3) postequivalence. Example 13-10 demonstrates how pAg is determined for each of these stages. 

From Example 20-8 we know the [Ag+] that must be exceeded to initiate precipitation of each of three silver halides, Agl, AgBr, and AgCl. We use each of these values of [Ag ] with the appropriate expression, in turn, to find the concentration of each haUde ion that remains in solution (unprecipitated). We express these halide ion concentrations as percent unprecipitated. Then we subtract each from exactly 100% to find the percentage of each halide that precipitates. 

On the other hand, when the free halide ion concentration increases, the anionic main cycle of Scheme 25 gets slower because the oxidative addition gets sloweri (see Sect B.i.d) and because the concentration of the reactive ArPdX(5)(PPh3)2 decreases due to a shift of the equilibrium toward ArPdXjlPPhjlj". However, this does not affect the branching between the main and side mechanisms. 

Whether and how readily passivation occurs will depend on both the metal and the solution environment. Even when passivation is observed, however, there is a further phenomenon which must sometimes be considered. This is pitting, a process which causes an increase in current in the passive region and which in its severest form causes complete breakdown of the passive film. It is commonest in media containing halide ion. The term derives from characteristic circular holes which appear in the passive film but their mechanism of formation is not well understood. Generally pitting becomes worse with increasing halide ion concentration and as the potential is made more positive (see Fig. 9.7) and it is thought likely that the pits... 

With alkyne substrates, haloperoxidase-catalyzed reactions yield a-haloketones (Scheme 2.230) [1783]. As with aikenes, the product distribution in the reaction with alkynes is dependent on the halide ion concentration. Both homogeneous and... 

There are of course many other kinetic studies of electrophilic aromatic substitution, particularly of the halogenation of activated aromatic species such as phenols and anilines. These are frequently complicated by the effect of pH and halide ion concentration upon equilibria between different halogenating species and between different forms of the aromatic substance. The role of bases in removing a proton has been rarely investigated directly, and much of the mechanistic information comes from a study of isotope effects, which will be considered in Chapter 12. 

Fig. 8.16. The ratio of the contributions to the Cl , Br" and I" NMR line widths of the high and low affinity binding sites of human serum albumin as a function of halide ion concentration. For the construction of the curves, see Ref. [252] from where the figure is taken...

The log Xi and log K2 for the complexes are 3.37 and 3.78. We take the halide ion concentration to be S, assuming it to remain unbound under these conditions of very low [Ag ]. Thus, in general for the three halides, we obtain from the expression... 

These electrodes are mainly anion responsive and several varieties are available for measuring the halide ion concentrations. The ion responsive material for such halide response electrodes is usually a silver halide. 

Equilibria among phenylhalocarbenes, halide ions, and phenyldihalomethide carban-ions have been spectroscopically characterized and computationally investigated for the chlorine and bromine halogens. Equilibrium constants, forward and reverse rate constants, and associated thermodynamic parameters have been reported. A simple and effective umpolung of normally electrophilic carbenes can be made available as desired by adjusting the halide ion concentration. 

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