Entropic character

Hydrophobic interactions are of entropic origin. That is to say, their formation is driven by the gain in the entropy of the system, especially involving the local structuring of the water molecules in the vicinity of the non-polar groups (Jenks, 1969 Cantor and Schimmel, 1980 Dickinson and McClements, 1995 McClements, 2005). A consequence of this entropic character is that the interactions become stronger with increasing temperature up to 60 °C. 

Hydrophobic effects are thus of practical interest. If we accept the goal of a simple, physical, molecularly valid explanation, then hydrophobic effects have also proved conceptually subtle. The reason is that hydrophobic phenomena are not tied directly to a simple dominating interaction as is the case for hydrophilic hydration of Na+, as an example. Instead hydrophobic effects are built up more collectively. In concert with this indirectness, hydrophobic effects are viewed as entropic interactions and exhibit counterintuitive temperature dependencies. An example is the cold denaturation of globular proteins. Though it is believed that hydrophobic effects stabilize compact protein structures and proteins denature when heated sufficiently, it now appears common for protein structures to unfold upon appropriate cooling. This entropic character of hydrophobic effects makes them more fascinating and more difficult. 

Our discussion of hydrophobic effects has focused on the entropic character of these phenomena. The developments above have attempted to calculate free energies and left the evaluation of entropies to a subsequent temperature differentiation. The interesting question has surfaced can hydrophobic entropies be calculated directly The current research on this question has produced interesting results and perspectives. Even though more research is called for, discussion of these issues is worthwhile here. 

The elemental potential, U, is a free energy function whose specific functional form and relative energic to entropic character are determined by the particular elements under consideration. The exact differential of... 

Moreover, because of their large size, high unsaturation content and 7r-excessive character these molecules are naturally associated, respectively, with adverse entropic factors, pronounced affinity to intramolecular pericyclization and high air sensitivity, so that their successful construction necessitated the development and use of rather unorthodox synthetic procedures. Similarly, the study of reactivity has been restricted, for the most part, to (i) internally induced reorganization and (ii) exposure to such mild reagent chemistry as was deemed necessary to induce structurally informative alterations while maintaining undesirable decomposition to a minimum. 

In recent years, direct, time-resolved methods have been extensively employed to obtain absolute kinetic data for a wide variety of alkyl radical reactions in the liquid phase, and there is presently a considerable body of data available for alkene addition reactions of a wide variety of radical types [104]. For example, rates of alkene addition reactions of the nucleophilic ferf-butyl radical (with its high-lying SOMO) have been found to correlate with alkene electron affinities (EAs), which provide a measure of the alkene s LUMO energies [105,106]. The data indicate that the reactivity of such nucleophilic radicals is best understood as deriving from a dominant SOMO-LUMO interaction, leading to charge transfer interactions which stabilize the early transition state and lower both the enthalpic and entropic barriers to reaction, with consequent rate increase. A similar recent study of the methyl radical indicated that it also had nucleophilic character, but its nucleophilic behavior is weaker than that expressed by other alkyl radicals [107]. 

In the system with three CF2 groups, i.e. 22, the radical takes on perfluoroalkyl character and the impact on cyclization rate is magnified still further. The dominant factor which has been credited for giving rise to the high reactivities of perfluoro-n-alkyl radicals in their additions to alkenes, particularly to electron-rich alkenes, is their high electrophilicities. That is, charge transfer interactions, e.g. [(CF3CF2CF2) s (alkene)l5+]] stabilize an early transition state and lower both the enthalpic and entropic barriers to reaction. 

The reorganization of the solute about reacting species is not limited to catalytic complexes, however. Studies of nucleophilicity of reactant species in ILs [262, 263] found that in some cases the nucleophilicity of solute species depends strongly on the character of the cation, though in others the association appears to be relatively weak [264]. In some cases, desolvation of solvent ions is limited by unfavorable entropic effects rather than enthalpic ones [262], This likely relates to the highly structured nature of the solvent, which can run counter to intuitive relationships between the entropy of free and bound states. Harper and Kobrak [14] reviewed this literature in detail, and we will not discuss it further here. 

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