Chloride transfer equilibria

Halide abstraction reactions are very common and usually fast processes. These reactions have also proved extremely useful for two specific applications in the field of physical organic chemistry. First, for obtaining thermochemical stability data of carboca-tions through the measurement of gas-phase chloride transfer equilibria (equation 1). 

When comparisons are possible, a AAG(ci) ladder for the chloride-transfer equilibria (29) of benzylic halides can be superimposed upon the corresponding AAG(cc)h ladder for proton transfer (26). Thus a wide set of relative gas-phase stabilities of carbocations can be built up based on the same scale. 

Chloride transfer equilibria of triarylmethyl- [34] and diarylmethyl cations [35] have been determined by H NMR spectroscopy they have been combined to give a chloride affinity scale (Scheme 5). 

Scheme 5 H NMR spectroscopic determination of chloride transfer equilibria (-70° C, CD2C12). (From Ref. 35.)...

Let us first ignore ion-pairing phenomena. With this assumption, the chloride transfer equilibria (13) correspond to the chloride transfer equilibria between two carbocations which were described in Scheme 5 and thus provide a comparison of the chloride affinities of metal halides and of carbocations. One would expect the right side of this equilibrium to be favored if MCI, is the stronger Lewis acid and the left side when R+ is the stronger Lewis acid. 

Reaction 1 is the synthesis reaction and Reactions 2, 3, and 4 are the chloride-transfer equilibria that are involved in the formation of triethyl-ammonium dichlorocuprate(I). Because of the sensitivity of the liquid to oxidation, all operations are carried out in Schlenk ware. The synthesis is effected simply by mixing stoichiometric quantities of the two solids which react endothermically to form the liquid in about two minutes. 

The series of a-CFs-benzyl cations [40C ] derived from the chloride-transfer equilibrium (29) gives an excellent linear correlation for the full range of substituents down to 3,5-F2 (Fig. 28) (Mishima et al., 1990a, 1997). 

Electrodes and Galvanic Cells. The Silver-Silver Chloride Electrode. The Hydrogen Electrode. Half-cells Containing an Amalgam, Electrode. Two Cells Placed Back to Back. Cells Containing Equimolal Solutions. The Alkali Chlorides as Solutes. HC1 in Methanol or Ethanol Containing a Trace of Water. The Alkali Chlorides in Methanol-Water Mixtures. The Heal of Solution of HC1. Proton Transfer Equilibrium from Measurements of E.M.F. 

We consider a silver electrode covered with a silver chloride film in chloride solution. As shown in Fig. 4—21, the electron level of the silver-silver chloride electrode in ion transfer equilibrium is expressed by the real potential a.(A. A a w) of electrons in the silver part of the electrode as shown in Eqn. 4-27 ... 

Fig. 4-21. Electron energy levels of an ionic electrode of silver-silver chloride in ion transfer equilibrium cfia ) = Fermi level of electron in silver part of electrode snvAfCici-) = equivalent Fermi level to transfer equilibriiun of silver ions and chloride ions in silver-silver chloride electrode.

The silver-silver chloride electrode is an example of a metal electrode that participates as a member of a redox couple. The silver-silver chloride electrode consists of a silver wire or rod coated with AgCl(s) that is immersed in a chloride solution of constant activity this sets the half-cell potential. The Ag/AgCl electrode is itself considered a potentiometric electrode, as its phase boundary potential is governed by an oxidation-reduction electron transfer equilibrium reaction that occurs at the surface of the silver ... 

How can we create such a membrane for a wider range of analytes The most successful approach is to use ion-selective liquid membranes (2, 3). The liquid membranes are hydrophobic and immiscible with water, and most commonly made of plasticized poly(vinyl chloride). The selectivity is achieved by doping the membranes with a hydrophobic ion (ionic site) and a hydrophobic ligand (ionophore or carrier) that selectively and reversibly forms complexes with the analyte (Figure 7.1). Whereas the technique has been well established experimentally since the 1960s, it is only recently that the response mechanisms are fully understood. In this chapter, principles of liquid membrane ISEs will be introduced using simple concepts of ion-transfer equilibrium at water/liquid membrane interfaces. Non-equilibrium effects on the selectivity and detection limits will also be discussed. This information will enable practitioners of ISEs to better optimize experimental conditions and also to interpret data. Additionally, examples of ISEs based on commercially available ionophores are listed. More comprehensive lists of ionophore-based ISEs developed so far are available in recent lUPAC reports (4-6). 

In addition to the metal ion, the halide ion has also got an influence on the kinetic of ATRP by affecting the atom transfer equilibrium. The use of copper bromide instead of copper chloride leads to more rapidly decreasing polydispersities (/ -toluenesulfonyl chloride/copper chloride ( -TsCl/CuCl) conversion = 25%, Mn = 8500, M /Mn = 2 -TsCl/CuBr for the same conversion, Mn = 7800, M /Mn= 1.18 [294,295]). This can be assigned to the better efficiency of bromine in the deactivation step [307,308]. 

Conversion of Amides into Carboxylic Acids Hydrolysis Amides undergo hydrolysis to yield carboxylic acids plus ammonia or an amine on heating in either aqueous acid or aqueous base. The conditions required for amide hydrolysis are more severe than those required for the hydrolysis of add chlorides or esters but the mechanisms are similar. Acidic hydrolysis reaction occurs by nucleophilic addition of water to the protonated amide, followed by transfer of a proton from oxygen to nitrogen to make the nitrogen a better leaving group and subsequent elimination. The steps are reversible, with the equilibrium shifted toward product by protonation of NH3 in the final step. 

The cyclic diazastannylene 1 has been found to be very suitable for this type of reaction 1S5) (cf. also Sect. 4.1). In Eqs. (43) and (44) the chlorine atoms of the Lewis acids are transferred to the divalent tin atom resulting in the formation of 57 and 76 and tin(II) chloride, the latter being insoluble in benzene. In (45) the solubility of the produced compounds is again important because SnS precipitates from the solution thus, the equilibrium is shifted to the right (in Eqs. (43)-(45) R denotes tert-butyl). 

The most dramatic rate retardations of proton transfers have been observed when the acidic or basic site is contained within a molecular cavity. The first kinetic and equilibrium studies of the protonation of such a basic site were made with large ring bicyclic diamines [72] (Simmons and Park, 1968 Park and Simmons, 1968a). It was also observed (Park and Simmons, 1968b) that chloride ion could be trapped inside the diprotonated amines. The binding of metal ions and small molecules by macrocyclic compounds is now a well-known phenomenon (Pedersen, 1967, 1978 Lehn, 1978). In the first studies of proton encapsulation, equilibrium and kinetic measurements were made with several macrobicyclic diamines [72] using an nmr technique. 

Solvent extraction, sometimes called liquid-liquid extraction, involves the selective transfer of a substance from one liquid phase to another. Usually, an aqueous solution of the sample is extracted with an immiscible organic solvent. For example, if an aqueous solution of iodine and sodium chloride is shaken with carbon tetrachloride, and the liquids allowed to separate, most of the iodine will be transferred to the carbon tetrachloride layer, whilst the sodium chloride will remain in the aqueous layer. The extraction of a solute in this manner is governed by the Nernstpartition or distribution law which states that at equilibrium, a given solute will always be distributed between two essentially immiscible liquids in the same proportions. Thus, for solute A distributing between an aqueous and an organic solvent,... 

Figure 1. Solute transfer across an idealised eukaryote epithelium. The solute must move from the bulk solution (e.g. the external environment, or a body fluid) into an unstirred layer comprising water/mucus secretions, prior to binding to membrane-spanning carrier proteins (and the glycocalyx) which enable solute import. Solutes may then move across the cell by diffusion, or via specific cytosolic carriers, prior to export from the cell. Thus the overall process involves 1. Adsorption 2. Import 3. Solute transfer 4. Export. Some electrolytes may move between the cells (paracellular) by diffusion. The driving force for transport is often an energy-requiring pump (primary transport) located on the basolateral or serosal membrane (blood side), such as an ATPase. Outward electrochemical gradients for other solutes (X+) may drive import of the required solute (M+, metal ion) at the mucosal membrane by an antiporter (AP). Alternatively, the movement of X+ down its electrochemical gradient could enable M+ transport in the same direction across the membrane on a symporter (SP). A, diffusive anion such as chloride. Kl-6 refers to the equilibrium constants for each step in the metal transfer process, Kn indicates that there may be more than one intracellular compartment involved in storage. See the text for details...

Another possibility is that the reactivity of the active end would be influenced by complexing, e.g., with an added transfer agent. One example of this is the polymerization of styrene by stannic chloride in the presence of thiophene (T) [134] which can be interpreted on the supposition that there is equilibrium complex formation between the growing end and thiophene ... 

Sometimes a metal electrode may be directly responsible to the concentration of an anion which either gives rise to a complex or a precipitate with the respective cations of the metal. Therefore, they are termed as second-order electrodes as they respond to an ion not directly involved in the electron transfer process. The silver-silver chloride electrode, as already described in Section 16.3.1.1.3, is a typical example of a second-order electrode. In this particular instance, the coated Ag wire when dipped in a solution, sufficient AgCl dissolves to saturate the layer of solution just in contact with the respective electrode surface. Thus, the Ag+ ion concentration in the said layer of solution may be determined by the status of the solubility product (Kvfa equilibrium ... 

From Eqns. 4-27 and 4-28, the equilibrium electrode potential, , is obtained for the transfer of silver ions and chloride ions at the silver-silver chloride electrode as shown in Eqn. 4-29 ... 

A hydride transfer from the methylene group to the copper ion followed by the oxidation of the latter by the oxidizing agent is also unlikely, in view of the fact that the deamination reaction is not affected by the addition of nucleophiles like chloride ions, which would be expected to interfere with a pre-equilibrium involving hydride ions. 

---