Anion solvating ability

Though the computed free energy profile for the 8, 2 reaction in water is unimodal, it seemed likely that there should be a solvent that could yield a nonconcerted profile, that is, one with ion-molecule or solvent-separated intermediates. A solvent with diminished anion solvating ability should reduce the energy increase upon desolvation of the nucleophile. This could at least reintroduce the ion-molecule complexes as intermediates. Though hydrocar-... 

In Chapter 20, F rca iu derives a novel measure of solvent nu-cleophilicities. First, protonation equilibria of benzene derivatives are used to measure acidity of a set of acids. Then, by measurement of the effect of weakly basic solvents (such as trifluoroacetic acid) on these equilibria, basicities of the solvents are calculated. Interestingly, these solvent basicities compare quite well with the solvent nucleophilicities calculated by Peterson (discussed previously). F rca iu also considers the anion-solvating ability of hydrogen-bonding solvents and concludes, in agreement with Chapter 17 and in disagreement with Chapters 18 and 19, that solvent electrophilicity must be considered as a separate variable. 

The anion solvation ability of dimethylformamide (DMF) and propylene carbonate (PC), is similar to that of DMSO, e.g. chloride ion is stronger solvated by water than by DMF. The stability constants of copper(II) chloro complexes were found therefore to be many orders of magnitude higher in N-N-dimethylformamide than in water [El 80]. 

TABLE 8.10 Rate Constants, log(k/s" ), for PTC Reactions A, B, C, D, and E Described in the Text in Solvents Arranged According to their Relative Pemittivities, vrith their Anion Solvating Abilities, the Acceptor Number AN, Also Shovm... 

In the discussion of the relative acidity of carboxylic acids in Chapter 1, the thermodynamic acidity, expressed as the acid dissociation constant, was taken as the measure of acidity. It is straightforward to determine dissociation constants of such adds in aqueous solution by measurement of the titration curve with a pH-sensitive electrode (pH meter). Determination of the acidity of carbon acids is more difficult. Because most are very weak acids, very strong bases are required to cause deprotonation. Water and alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for generation of hydrocarbon anions. Any strong base will deprotonate the solvent rather than the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, tetrahydrofuran (THF), and dimethoxyethane (DME) are used, but for equilibrium measurements solvents that promote dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such as DMSO and cyclohexylamine are used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitate dissociation... 

In mixed solvent systems the difference in the solvating abilities of solvent molecules S and S2 causes a selective solvation of cations and anions [119,120],... 

This classification is not rigid. Protic solvents are particularly good anion solvators due to their hydrogen-bonding ability (Symons and Thomas, 1981). The observation... 

A solvent, in addition to permitting the ionic charges to separate and the electrolyte solution to conduct an electrical current, also solvates the discrete ions, firstly by ion-dipole or ion-induced dipole interactions and secondly by more direct interactions, such as hydrogen bonding to anions or electron pair donation to cations. The latter interactions, thus, depend on the Lewis acidity and basicity, respectively, of the solvents (Table 4.3). The redox properties of the ions at an electrode therefore depend on their being solvated, and the solvent effects on electrode potentials or polarographic half wave potentials, or similar quantities in voltammetry are manifested through the different solvation abilities of the solvents. 

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