Rational thermodynamic selectivity

If the standard and reference states for the exchanger phase are defined differently, whilst maintaining the conventional states for the solution phase, a thermodynamic selectivity scale can be set up for various ions where the value of the exchange constant indicates the degree of selectivity, as first demonstrated by Bonner, Argensinger, Hogfeldt, and others. 

In this treatment the components of the exchanger phase are the mixed swollen resinates or compounds and whilst the 

The abstract thermodynamic treatment outlined above resembles Kielland s approach to ion exchange equilibria on aluminosilicates, but unlike the latter case no simplifying assumptions are made concerning the relationship between the concentrations of the resin phase components and their activity coefficients. 

Graphical integration methods are used to evaluate equations 5.26 and 5.27 which thus establish a scale for selectivity in ion exchangers. A typical selectivity series for some common cations on a sulfonic acid resin exchanging against Li is shown in Table 5.2. 

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Retention of Rohrschneider-McReynolds standards of selected chiral alcohols and ketones was measured to determine the thermodynamic selectivity parameters of stationary phases containing (- -)-61 (M = Pr, Eu, Dy, Er, Yb, n = 3, R = Mef) dissolved in poly(dimethylsiloxane) . Separation of selected racemic alcohols and ketones was achieved and the determined values of thermodynamic enantioselectivity were correlated with the molecular structure of the solutes studied. The decrease of the ionic radius of lanthanides induces greater increase of complexation efficiency for the alcohols than for the ketone coordination complexes. The selectivity of the studied stationary phases follows a common trend which is rationalized in terms of opposing electronic and steric effects of the Lewis acid-base interactions between the selected alcohols, ketones and lanthanide chelates. The retention of over fifty solutes on five stationary phases containing 61 (M = Pr, Eu, Dy, Er, Yb, n = 3, R = Mef) dissolved in polydimethylsiloxane were later measured ". The initial motivation for this work was to explore the utility of a solvation parameter model proposed and developed by Abraham and coworkers for complexing stationary phases containing metal coordination centers. Linear solvation... 

Rational reactor selection and design requires information on thermodynamics, chemical kinetics, heat and mass transport, and reactor hydrodynamics. In practice, a quantitative analysis is based on reactor models and engineering correlations. In this chapter we limit ourselves to a qualitative discussion, emphasizing principles rather than quantitative calculations. 

However, correlations have been suggested which allow prediction or rationalization of regioselectivity with a modest degree of confidence. With some significant exceptions as discussed below, the difference between the kinetic and thermodynamic selectivity has not been determined or is small. 

At the end of the discussion of rational thermodynamics we stress that in this theory we in fact study mathematical models (in this sense this theory is a part of mathematics) and only after their application in a real situation and with real material we can decide about the limits of their practical validity. Although practical application is out of scope of the theory developed here, it motivates the types of material models studied in this book and offered as various constitutive equations to be selected for particular application. Such applications motivate some concepts or procedures in the theory and also exclude some unusual properties of these models because the real materials are much more complicated to avoid, e.g., instabilities (manifested, e.g., by phase changes), we exclude zero values of some transport coeffl-eients or heat capacities. Such and similar regularity properties we add to constitutive equations and the resulting models we then denote as regular (see (3.232), (3.234), Rems, in Chap. 1, 2, 6, 8, and 9). 

Measurements of the dissolution behavior of polymorphic forms of relatively insoluble drugs are a convenient way of measuring thermodynamic parameters which, in turn, provide a rational approach to selection of the more energetic polymorphic forms of these drugs for absorption. Large differences in free energy... 

Most energetic contributions are, as we have discussed, difficult to predict and large experimental efforts have for that reason been devoted to derive systematic trends in the energetics of classes of materials. In this chapter we will try to convey an overview of periodic trends in the thermodynamic properties of inorganic compounds and we will also present selected examples illustrating some of the more usual rationalization schemes. Finally, trends in enthalpy of mixing are treated. Also here we aim to look at trends and rationalization schemes. The chapter is by no means exhaustive - only selected classes of compounds and selected rationalization schemes are discussed. 

However, the reaction of 1,3-cycloheptadiene is less regioselective. Isoprene and E,E-2,4-hexadiene afford 1,2-/1,4-adducts in ratios of 87 13 and 83 17, respectively. The high selectivity for 1,2-addition (>95%) to 1,3-pentadiene is opposite to the corresponding oxymercuration of the same diene, which has been reported159 to give mainly 1,4-adducts. The different regiochemistry has therefore been explained by assuming that sulfomercu-ration occurs under kinetic control whereas oxymercuration occurs under thermodynamic control. 

Examination of the reactivity of acyclic (diene)Fe(CO)3 complexes indicates that this nucleophilic addition is reversible. The reaction of (C4H6)Fe(CO)3 with strong carbon nucleophiles, followed by protonation, gives olefinic products 195 and 196 (Scheme 49)187. The ratio of 195 and 196 depends upon the reaction temperature and time. Thus, for short reaction time and low temperature (0.5 h, —78 °C) the product from attack at C2 (i.e. 195) predominates while at higher temperature and longer reaction time (2 h, 0 °C) the product from attack at Cl (i.e. 196) predominates. This selectivity is rationalized by kinetically controlled attack at the more electron-poor carbon (C2) at low temperature. Nucleophilic attack is reversible and, under conditions where an equilibrium is established, the thermodynamically more stable (allyl)Fe(CO)3" is favored. The regioselectivity for nucleophilic attack on substituted (diene)Fe(CO)3 complexes has been reported187. The... 

The benefits from tuning the solvent system can be tremendous. Again, remarkable opportunities exist for the fruitful exploitation of the special properties of supercritical and near-critical fluids as solvents for chemical reactions. Solution properties may be tuned, with thermodynamic conditions or cosolvents, to modify rates, yields, and selectivities, and supercritical fluids offer greatly enhanced mass transfer for heterogeneous reactions. Also, both supercritical fluids and near-critical water can often replace environmentally undesirable solvents or catalysts, or avoid undesirable byproducts. Furthermore, rational design of solvent systems can also modify reactions to facilitate process separations (Eckert and Chandler, 1998). 

Since its discovery by Pasteur in 1853,5 classical resolution by selective crystallization of diastereo-isomers, despite wide and frequent use, remains to a large degree a method of trial and error. Various attempts to rationalize classical resolutions and predict a successful combination of race-mate and resolving agent by computational approaches so far have not been crowned with remarkable success.6 Even when the crystal structures of both diastereoisomeric salts are known, molecular modeling calculations do not provide a basis for a reliable prediction. Only recently has some progress been made in the calculation of the relative thermodynamic stability of ephedrine-cyclic phosphoric acid 4 diastereoisomers,7 a diastereoisomeric salt frequently used as a model system (vide infra). 

Use of the potential of a galvanic cell to measure the concentration of an electroactive species developed later than a number of other electrochemical methods. In part this was because a rational relation between the electrode potential and the concentration of an electroactive species required the development of thermodynamics, and in particular its application to electrochemical phenomena. The work of J. Willard Gibbs1 in the 1870s provided the foundation for the Nemst equation.2 The latter provides a quantitative relationship between potential and the ratio of concentrations for a redox couple [ox l[red ), and is the basis for potentiometry and potentiometric titrations.3 The utility of potentiometric measurements for the characterization of ionic solutions was established with the invention of the glass electrode in 1909 for a selective potentiometric response to hydronium ion concentrations.4 Another milestone in the development of potentiometric measurements was the introduction of the hydrogen electrode for the measurement of hydronium ion concentrations 5 one of many important contributions by Professor Joel Hildebrand. Subsequent development of special glass formulations has made possible electrodes that are selective to different monovalent cations.6"8 The idea is so attractive that intense effort has led to the development of electrodes that are selective for many cations and anions, as well as several gas- and bioselective electrodes.9 The use of these electrodes and the potentiometric measurement of pH continue to be among the most important applications of electrochemistry. 

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