Aprotic solvents thermodynamic measurements

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... 

The reactions discussed in the following sections take place in aprotic solvents, and reference to known or estimated thermodynamic basicities will relate to DM SO unless otherwise noted, since DM SO is the polar aprotic solvent in which most thermodynamic acidities have been measured [55-58]. Values of pK determined in DM SO can usually be assumed to parallel values in DMF [59, 60], MeCN, and other polar aprotic solvents whereas pK values (and relative pK values) related to water and other hydroxylic solvents can be very different. 

In order for such dissection of thermodynamic parameters to be possible in general, it is clearly essential that a sufficient body of data (8AGtr, 8AHU, 5AStr) on individual species be available. Measurements of such quantities are increasingly being made, and a recent excellent report contains an up to date summary (Cox, 1973 see also Abraham, 1973 Cox et al., 1974). Some of the currently available data referring to transfer of ionic species from water to various dipolar aprotic solvents are presented in Tables 1 and 2. 

Walden products, whereas in all dipolar aprotic solvents rj °(K+) 17X0 (Cl-). This suggests a large decrease in size of solvation shell of Cl-, relative to K+, on transfer from water. The quantitative significance of these electrochemical results with regard to calculated hydration numbers is in some doubt but they do appear to substantiate, at least in this particular case, conclusions drawn from thermodynamic measurements. 

Since all reactions discussed in this chapter are carried out in aprotic solvents, reference to known or estimated thermodynamic basicities relates to dimethyl sulfoxide (DMSO) unless otherwise noted. This is because DMSO is the polar aprotic solvent in which most thermodynamic acidities have been measured, especially by Bordwell and coworkers [7-10]. The term is used throughout, although in the literature terms such as pA a, and are often used interchangeably for values determined in... 

This book is based on the reactions of thermal electrons with molecules. The ECD, negative-ion chemical ionization (NICI) mass spectrometry, and polaro-graphic reduction in aprotic solvents methods are used to determine the kinetic and thermodynamic parameters of these reactions. The chromatograph gives a small pure sample of the molecule. The temperature dependence of the response of the ECD and NIMS is measured to determine fundamental properties. The ECD measurements are verified and extended by correlations with half-wave reduction potentials in aprotic solvents, absorption spectra of aromatic hydrocarbons and donor acceptor complexes, electronegativities, and simple molecular orbital theory. 

Now let us return to the approaches connected with the estimation of the primary medium effect for protons, log y0 n+, that are used for obtaining quantitative information on the acidity of pure protolytic or aprotic solvents relative to the standard solution of a strong acid in water. From the thermodynamics, these are known to be a measure of the Gibbs free energy of proton transfer from the standard solution in water to the one in a non-aqueous solvent (M). This parameter is connected with the energy of proton resolvation in the following way ... 

Well-behaved reversible reference electrodes are a prerequisite for good thermodynamic and kinetic measurements. There exist two excellent publications - concerned with reference electrodes in non-aqueous solvents, which should be consulted for details. Some aspects of electrode performance are discussed in Chapter 3. Many of the reference electrodes used in aqueous systems are useful in non-aqueous solvents with some notable exceptions. In many aprotic solvents the mercurous halide electrodes have been found to be unreliable due to the disproportionation reaction (which occurs to a small extent in water)... 

In this chapter, the thermodynamics and kinetics of the steps involved in proton-transfer reactions in aprotic solvents of low dielectric constant will be considered, and special attention will be paid to dissecting the overall reaction 1 into a sequence of elementary processes, each of which will be discussed and related to the measured kinetic parameters kf and kj,. 

The thermodynamic and analytical aspects of acid-base reactions in aprotic solvents are surveyed in reviews by Davis [1, 2]. The correlation of acid-base strength in water and aprotic solvents is of major importance. Early kinetic work by Bell and co-workers on the acid catalysis of (i) the ethyldiazoacetate-phenol interaction [3] (ii) the rearrangement of N-bromoacetanilide [4] and (iii) the inversion of /-menthone [5] established an order of acid strengths in aprotic media and the importance of intra-molecular hydrogen bonds e.g in picric acid). A thermodynamic method using reference acids and bases is more direct, and Bell and Bayles [6] employed the indicator acid Bromophenol Blue to obtain a basicity order for weak amine bases. Kinetic measurements on these systems have recently been made, and are considered in detail in Section 7. 

It should be born in mind, however, that the activation parameters calculated refer to the sum of several reactions, whose enthalpy and/or entropy changes may have different signs from those of the decrystalUzation proper. Specifically, the contribution to the activation parameters of the interactions that occur in the solvent system should be taken into account. Consider the energetics of association of the solvated ions with the AGU. We may employ the extra-thermodynamic quantities of transfer of single ions from aprotic to protic solvents as a model for the reaction under consideration. This use is appropriate because recent measurements (using solvatochromic indicators) have indicated that the polarity at the surface of cellulose is akin to that of aliphatic alcohols [99]. Single-ion enthalpies of transfer indicate that Li+ is more efficiently solvated by DMAc than by alcohols, hence by cellulose. That is, the equilibrium shown in Eq. 7 is endothermic ... 

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