Tetrahydrofuran thermodynamic constants

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... 

A potentiometric method for determination of ionization constants for weak acids and bases in mixed solvents and for determination of solubility product constants in mixed solvents is described. The method utilizes glass electrodes, is rapid and convenient, and gives results in agreement with corresponding values from the literature. After describing the experimental details of the method, we present results of its application to three types of ionization equilibria. These results include a study of the thermodynamics of ionization of acetic acid, benzoic acid, phenol, water, and silver chloride in aqueous mixtures of acetone, tetrahydrofuran, and ethanol. The solvent compositions in these studies were varied from 0 to ca. 70 mass % nonaqueous component, and measurements were made at several temperatures between 10° and 40°C. 

As a part of the continuing studies of the effect of dipolar aprotic solvent plus water mixtures on these specific solute-solvent interactions, we have examined the first and second dissociation steps of glycine and computed their associated thermodynamic quantities in 10 mass % tetrahydrofuran-water (THF-H20) solvents (dielectric constant, c = 71.8 at 298.15 K), 30 mass % THF-H20 (c — 56.6 at 298.15 K), and 50 mass % THF-H20 (c — 40.0 at 298.15 K) from 278.15 to 328.15 K. 

These are equilibria whose positions depend on the relative acidities of the proton sources. The more acidic compound provides the greater amount of organolithium and so the greater the difference in the acidity, the larger the equilibrium constant. However, kinetic acidities must be considered as well as thermodynamic (pK values) e.g., PhH (pKj 37) is not metallated by alkyllithiums [pK of alkanes > 40 in tetrahydrofuran (THF)], despite large difference in pK values. The more reactive RLi-TMED (tetra-methylethylenediamine) complexes, however, provide PhLi. The equilibrium can be driven to the product side by removing the product hydrocarbon or precipitating the RLi. 

With fluorenyl caesium (FICs) in dimethoxymethane, addition of tetrahydro-furan appears to generate triple ions, Fl2Cs and FICs2, in equilibrium with ion pairs which are only weakly dissociated into free ions. The derived thermodynamic data indicate that dimethoxymethane, although having a lower dielectric constant than tetrahydrofuran, specifically solvates Cs". ... 

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