Stille reaction chloride ions

Although widely used in the past and still used in special cases, the industrial sulfation with chlorosulfonic acid presents several problems which have caused the decline of this technique in favor of the more advantageous sulfation method with sulfur trioxide. These problems consist of evolution of the highly corrosive hydrogen chloride, heat transfer characteristics of the reaction, and the comparatively high level of chloride ion in the sulfated product compared with alcohol and alcohol ether sulfates obtained with sulfur trioxide. 

It is sometimes said that this electrode is reversible with respect to the anion. This claim must be examined in more detail. An electrode potential that depends on anion activity still constitutes no evidence that the anions are direct reactants. Two reaction mechanisms are possible at this electrode, a direct transfer of chloride ions across the interface in accordance with Eq. (3.34) or the combination of the electrode reaction... 

In aqueous acetic acid, the disproportionation of the platinum still occurs quite rapidly, and it can be suppressed further by adding mineral acid. Hydrochloric acid is often used, but this has a disadvantage in that the exchange rate is inversely proportional to the chloride ion concentration. Perchloric acid has been found to be more satisfactory (55). The platinum(II) catalyst most used is sodium or potassium tetrachloropla-tinate(II). An aromatic compound added to the reaction mixture also inhibits disproportionation of the platinum(II) complex—benzene, pyrene, and other aromatics have been used. A comparative study of the effect of various aromatics on the H—D exchange in alkanes has been carried out (55). Even under optimum conditions, the disproportionation [Eq. (4)] still takes place, and the catalytic platinum(II) is slowly removed from the reaction mixture. To get useful rates of exchange in alkanes, temperatures of 100° to 120°C have to be used, and the disproportionation rate increases with temperature. 

The exact catalytic cycle is still under debate [89]. Studies of model compounds provide insight into the mechanism, but these model reactions differ from the actual catalytic cycle, and so may follow a different mechanistic pathway [90-92]. Kinetic studies show that the conversion of ethene follows the rate law in Eq. (3.1). This supports a pre-equilibrium that involves the dissociation of two chloride ions and one proton, thus explaining the sensitivity of the reaction to the presence of chloride ions. 

Notice that these reactions take place with allylic chlorides. We should not expect an alkyl chloride to be particularly good at Sn2 reactions as chloride ion is only a moderate leaving group and we should normally prefer alkyl bromides or iodides. AUylic chlorides are more reactive because of the alkene. Even if the reaction occurs by a simple Sn2 mechanism without rearrangement, the alkene is still making the molecule more electrophilic. 

In the second step, acetate ion (pICaH about 5) and HCI (pXa 7) set up an equilibrium with acetic acid and chloride ion. The difference in pJC values is still great and that equilihrium too wiU be right over on one side. However, the main point is that the pK values of acetic acid and PhNH+ are about the same so there will be about the same amount of each - the equilihrium constant will be about 1. Most of the amine will still be in solution as the cation but there will be enough free amine to react. If no sodium acetate were added, the amine would all be in solution as the cation and there would be essentially no free amine to act as a nucleophile. It would be simpler but no reaction would occur. 

The availability of equipment to measure molar conductivity of solutions was turned to good use. It is interesting to note that coordination chemists still make use of physical methods heavily in their quest to assign structures - it s just that the extent and sophistication of instrumentation has grown enormously in a century. What conductivity could tell the early coordination chemist was some further information about the apparently ionic species inferred to exist through the silver ion precipitation reactions. This is best illustrated for a series of platinum(IV) complexes with various amounts of chloride ion and ammonia present (Figure 3.1). From comparison of measured molar conductivity with conductivities of known compounds, the number of ions present in each of the complexes could be determined. We now understand these results in terms of modern formulation of the complexes as octahedral platinum(IV) compounds with coordinated ammonia, where coordinated chloride ions make up any shortfall in the fixed coordination number of six. This leaves in most cases some free ionic chloride ions to balance the charge on the complex cation. 

Chloride ions behave as a poison for the equilibration reaction (see example 3 in Table 2). Most of these Cl ions are located on the support in the vicinity of the metal particles. However, it was shown by EXAFSl °l and by SIMSl that some Cl atoms were still bound to the metals after high-temperature oxidation or reduction, which can explain the inhibiting role of chlorine in equilibration. 

Since NH4 is a weaker acid than H30, the reaction will go only a little to the right, but still far enough that a solution of ammonium chloride is acidic (pH < 7). Note that in the above reactions H2O can act as either an acid or a base. Note also that Na (aq) and Cl (aq) do not affect the above equilibria and are omitted from the net ionic equations for the hydrolysis reactions such ions are termed "spectator" ions since they do not change the acid-base equilibrium, even though they are present in solution. 

These problems can be alleviated by electrolyzing, for example, a mixed solution of HCl and CuCl2 [38-40], The anode reaction is still the oxidation of chloride ion. The cathode reaction becomes the reduction of the cupric ion to cuprous ... 

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