Free enthalpic effect

The mechanism of free-radical addition follows the pattern discussed in Chapter 14 (pp. 894-895). The method of principal component analysis has been used to analyze polar and enthalpic effect in radical addition reactions. A radical is generated by... 

From the viewpoint of molecular interactions, the number of fundamentally distinct chromatographic stationary phases is very limited.17 One mechanism for adsorption to the stationary phase is solvophobic, or mobilestationary phase transfer free energy effects, in which the adsorption of an analyte to the stationary phase liberates bound solvent. There is often an accompanying enthalpic component to such binding through dispersion interactions. Another mechanism for adsorption is that of specific interactions,... 

Once we have determined the entropy and enthalpy of polymerization, we can calculate the free energy of the process at a variety of temperatures. The only time this is problematic is when we are working near the temperatures of transition as there are additional entropic and enthalpic effects due to crystallization. From the free energy of polymerization, we can predict the equilibrium constant of the reaction and then use this and Le Chatelier s principle to design our polymerization vessels to maximize the percent yield of our process. 

Because the enthalpic effect was considered the sole driving force in free-radical processes, it was long erroneously believed that high reactivity had always to be associated with low selectivity. This is one of the reasons why radical chemistry was thought to be useless for selective synthesis of complex molecules, until Barton, in 1960 [1], showed the synthetic potentiality of free-radical reactions. 

Minisci, F., Fontana, F., Vismara, E. Polar and enthalpic effects in free-radical reactions. Free-radical diazo coupling and reactivity of carbohydrate radicals. Gazz. Chim. Ital. 1993, 123, 9-18. 

Several conclusions emerge from this analysis. The entropic contribution constitutes the major part in the change in free energy and amounts, in "normal" Hammett relationships, to approximately 80 per cent, the rest coming from the enthalpic effect. 

From the discussion so far it is clear that the action of a template has a pronounced entropic rather than an enthalpic effect. The reason for this is that the free motion... 

F. Minisci, C. Punta, F. Recupero, F. Fontana, G. F. Pedulli, A new, highly selective synthesis of aromatic aldehydes by aerobic free-radical oxidation of benzylpic alcohols, catalysed by N-hydroxyphtalimide under mild conditions. Polar and enthalpic effects, Chem. Commum. (2002) 68. 

The family of methods that make direct use of experimentally determined protein structures has their fundamental basis in direct physical simulation. Figure 2.4 cartoons the process of protein and ligand in a solvated and unbound state (left) moving into a bound state (right). While the cartoon may appear simple, the specific physical interactions each have multiple enthalpic and entropic effects, some of which are indirect The association of a hydrophobic Ugand surface with a complementary hydrophobic protein surface involves a direct enthalpic effect from the Van der Waals interactions. But the indirect effects include those from solvent molecules that had occupied the protein site in a semiordered state, which are released as a consequence of ligand occupancy near the protein. These solvent molecules may achieve greater entropy when free within solvent, and they may make more enthalpically favorable... 

The Flory-Huggins theory can be improved to incorporate new terms. For example, the Prigogine-Flory-Patterson theory introduces an entropic contribution due to free volume effects in order to obtain more realistic results [70]. This contribution does not modify the solubility criterion as stated above. When the Flory interaction parameter (/) is considered to have both an enthalpic component and... 

Free radical chemistry continues to be of importance in the field and a survey has been published on polar and enthalpic effects in free radical reactions. A further short review dealt with free radicals, carbenes and nitrenes at the anomeric centre of carbohydrates. A useful survey has appeared of the application of carbohydrate based chiral auxilliaries in stereoselective syntheses which covers a range of reactions, for example cycloaddition processes, reductions and Strecker reactions. ... 

X is a temperature-dependent dimensionless parameter that is specific to the solvent/polymer pair, where x At Tis a measure of the free energy penalty paid whenever a solvent molecule is in contact with a polymer segment, h being the Boltzmann constant. Usually x = a+ b T, where the contributions are from nonideal entropic and enthalpic effects respectively. As x increases, the solvent and polymer segments dishke each other more, resulting eventually in phase separation. 

AG is always negative and the decrease in free energy can be due to enthalpic effects (change in AH) or entropic interactions (change in AS). 

The most direct effect of defects on tire properties of a material usually derive from altered ionic conductivity and diffusion properties. So-called superionic conductors materials which have an ionic conductivity comparable to that of molten salts. This h conductivity is due to the presence of defects, which can be introduced thermally or the presence of impurities. Diffusion affects important processes such as corrosion z catalysis. The specific heat capacity is also affected near the melting temperature the h capacity of a defective material is higher than for the equivalent ideal crystal. This refle the fact that the creation of defects is enthalpically unfavourable but is more than comp sated for by the increase in entropy, so leading to an overall decrease in the free energy... 

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