Chloride affinities

These results, which pertain to stable-ion conditions, provide strong evidence that foe most stable structure for foe norbomyl cation is foe symmetrically bridged nonclassical ion. How much stabilization does foe a bridging provide An estimate based on molecular mechanics calculations and a foermodynamic cycle suggests a stabilization of about 6 1 kcal/mol. An experimental value based on mass-spectrometric measurements is 11 kcal/mol. Gas-phase Itydride affinity and chloride affinity data also show foe norbomyl cation to be especially stable. ... 

The high chloride affinity and selectivity afforded by cholapods such as 4.97 suggests that they may be of use in the treatment of cystic fibrosis (Section 4.1) by acting as artificial chloride transporters. Chloride transport has been measured in an artificial vesicle system in which the fluorescent dye lucigenin is incorporated into the inner aqueous pore of the vesicle in the presence of a background... 

For the chloronium ions (RC1R )+ it was found that for any given R the bond energy D R+—C1R ) decreases with increasing electronic stabilization of R+, which follows the order Me+ < Et+ < 2-Pr+ < c-pentyf, t-Bu+, 2-norbornyl+14. This is the same order as found for D(R+—Cl"), i.e. the chloride affinity of R+4. 

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). 

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. 

The relative chloride affinities of the benzhydryl cations from Scheme 5 are now graphically displayed in Scheme 7 (bottom, right). The correlation between the chloride affinities of diarylcarbenium ions and the ethan-olysis rate constants of the corresponding diarylmethyl chlorides [39] allows the chloride affinity scale of the diarylmethyl cations to extend to the less stabilized systems (Scheme 7, bottom left). In the AG° scale shown in Scheme 7, 10 kJ mol-1 corresponds to K = 374. 

If the chloride transfer from diarylmethyl chlorides to metal chlorides MCI followed Eq. (13), one might derive relative chloride affinities of MCI and Aryl2CH + from the same type of NMR experiments, which led to the relative chloride affinities of various Aryl2CH + species. In this way one would also obtain a chloride affinity scale of various metal halides. The situation is more complex, however ... 

Scheme 7 Relative chloride affinities of diarylcarbenium ions and of metal chlorides in CH2C12 (-70° C). (From Ref. 40.)...

These numbers show that, except in highly dilute solutions, carbocations are predominantly paired in CH2CI2, and chloride affinities can... 

Experiments similar to those described in Fig. 1, and titrations of Aryl2CH + MCl,T+]/MCl mixtures with NR4+C1 solutions have been performed to characterize the relative chloride affinities of other Lewis acids MCI, [40], It turned out, however, that the well-defined behavior of BCI3 could not be observed with other systems. The positions of all other metal chlorides in Scheme 7, therefore, have only qualitative character. Relatively small concentrations of SbCl5 and GaCl3 were sufficient to completely ionize (p-CICeHUhCHCl, the weakest chloride donor of this series. The values of these ionization constants could not be deter-... 

Very small concentrations of FeCl3 were sufficient to completely ionize ToI2CHCI, indicating a high chloride affinity of FeCl3. More accurate comparisons with less stabilized carbenium ions failed because of the low solubility of FeCI3. 

Although the chloride affinities of most metal chlorides thus determined are only approximate, comparison with an analogous scale deter-... 

According to Eq. (17), AGd° is negative if AG 0 < AG. A value of -50 kJ mol-1 was estimated for AGa° (-70° C) above, so that carbocation salts can only add to olefins to form covalent products if AG > -50 kJ mol-1. As AG depends both on the structure of R+ and on the Lewis acidity of MCI, we can conclude that the thermodynamic driving force for Case d increases with decreasing stabilization of R+ and decreasing chloride-ion affinity of MC1 . Because the magnitude of AG can be estimated from the relative chloride affinities of carbocations and Lewis acidic metal halides in Scheme 7 (Section II.G), one can derive which carbocationic salts might add to alkenes with formation of covalent products. 

A receptor for solvent-separated ion pairs is receptor 10, a combination of a dibenzo-18-crown-6 and abridging 1,3-phenyldicarboxamide. In the presence of 1 mol equivalent of metal cation, chloride affinities are enhanced in the following order K+ (ninefold enhancement), Na+ (eightfold enhancement), and Cs" " (no enhancement). An X-ray crystal structure shows that the receptor binds sodium chloride as a solvent-shared ion pair (Figure 8). 

Second, the same steric shielding effect in 20 is also responsible for a lack of observed cooperativity in ion pair binding. No enhancement in chloride affinity occurred upon potassium complexation to the adjacent crown ether (K = 300 M with or without potassium cations present). Hence, the axle induced an insulating effect on the through-space electrostatic interaction between the bound potassium and chloride ions, resulting in deviation away from the synergistic behavior of macrocycle 17. 

Zhu, j. Cole, R. B. Ranking of gas-phase acidities and chloride affinities of monosaccharides and linkage specificity in collision-induced decompositions of Negative Ion Electrospray-Generated chloride adducts of oligosaccharides. J. Am. Soc. Mass Spectrom. 2001, 12, 1193-1204. 

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