Extrathermodynamic applications

Theoretically, the problem has been attacked by various approaches and on different levels. Simple derivations are connected with the theory of extrathermodynamic relationships and consider a single and simple mechanism of interaction to be a sufficient condition (2, 120). Alternative simple derivations depend on a plurality of mechanisms (4, 121, 122) or a complex mechanism of so called cooperative processes (113), or a particular form of temperature dependence (123). Fundamental studies in the framework of statistical mechanics have been done by Riietschi (96), Ritchie and Sager (124), and Thorn (125). Theories of more limited range of application have been advanced for heterogeneous catalysis (4, 5, 46-48, 122) and for solution enthalpies and entropies (126). However, most theories are concerned with reactions in the condensed phase (6, 127) and assume the controlling factors to be solvent effects (13, 21, 56, 109, 116, 128-130), hydrogen bonding (131), steric (13, 116, 132) and electrostatic (37, 133) effects, and the tunnel effect (4,... 

Tihe two methods of structure-activity correlation which have received the most application in the past decade are the Hansch multiple parameter method, or the so-called extrathermodynamic approach, and the Free-Wilson, or additive model. The basic differences and similarities of these methods are discussed in this presentation. 

Kubinyi H. The Free-Wilson method and its relationship to the extrathermodynamic approach. In Ramsden CA, ed. Quantitative Drug DesignHansch C, Sammes PG, Taylor JB, eds. Comprehensive Medicinal Chemistry. The Rational Design, Mechanistic Study and Therapeutic Application of Chemical Compounds 1990 Vol. 4. Oxford Pergamon Press, 1990 589-643. 

It is shown that rate expressions in terms of activities are appropriate for the liquid-phase tertiary alkyl ethyl ether synthesis system. The rate expressions are based upon the application of the thermodynamic transition-state theory to the elementary steps within the LHHW formalism. Extrathermodynamic correlations that relate the kinetics to the reaction thermodynamics can also be rationalized within this framework and are experimentally observed for this family of tertiary ethers. 

A more quantitative prediction of activity coefficients can be done for the simplest cases [18]. However, for most electrolytes, beyond salt concentrations of 0.1 M, predictions are a tedious task and often still impossible, although numerous attempts have been made over the past decades [19-21]. This is true all the more when more than one salt is involved, as it is usually the case for practical applications. Ternary salt systems or even multicomponent systems with several salts, other solutes, and solvents are still out of the scope of present theory, at least, when true predictions without adjusted parameters are required. Only data fittings are possible with plausible models and with a certain number of adjustable parameters that do not always have a real physical sense [1, 5, 22-27]. It is also very difficult to calculate the activity coefficients of an electrolyte in the presence of other electrolytes and solutes. Even the definition is difficult, because electrolyte usually dissociate, so that extrathermodynamical ion activity coefficients must be defined. The problem is even more complex when salts are only partially dissociated or when complex equilibriums of gases, solutes, and salts are involved, for example, in the case of CO2 with acids and bases [28, 29]. 

In spite of extensive theoretical and experimental studies on solvent and solute effects based on the operators defined above, there are no general theoretical models available as yet that can predict these effects with any certainty. From a practical point of view, therefore, one must look into methods other than those based on the application of formal thermodynamics. Thus, we turn to the extrathermodynamic approach. 

The significance of the above-described findings consists in that they make it possible to determine to what extent the extrathermodynamic relationships found to be useful for structure-property correlations in solution chemistry are applicable to the description of intrinsic characteristics of reactions which are not connected with the solvent effects. 

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