Enthalpic effect

Table 1 shows the kinetic data available for the (TMSjsSiH, which was chosen because the majority of radical reactions using silanes in organic synthesis deal with this particular silane (see Sections III and IV). Furthermore, the monohydride terminal surface of H-Si(lll) resembles (TMSjsSiH and shows similar reactivity for the organic modification of silicon surfaces (see Section V). Rate constants for the reaction of primary, secondary, and tertiary alkyl radicals with (TMSIsSiH are very similar in the range of temperatures that are useful for chemical transformations in the liquid phase. This is due to compensation of entropic and enthalpic effects through this series of alkyl radicals. Phenyl and fluorinated alkyl radicals show rate constants two to three orders of magnitude... 

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... 

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. 

How do entropic and enthalpic effects determine the miscibility of polymers in blends Which term dominates the formation of a miscible blend ... 

In an ideal SEC experiment, enthalpic effects are not operating (AH = 0) and, thus, AEac equals 1. Accordingly, the distribution coefficient of ideal SEC is only a function of AS, and Kd is given by Equation 17.7 ... 

In conclusion one can say that SEC is a very powerful method for polymer characterization, especially in combination with other composition sensitive or absolute calibration methods. A big advantage is also that the sample amount is fairly small, typically 10 mg. For more complex polymers, such as polyelectrolytes, enthalpic effects often become dominant and also for rather high molecular weight polymers chromatographic methods such as field-flow fraction (FFF) techniques might be more suitable. For fast routine measurements linear columns are often used. 

The above definition of the enthalpic effects of conjugation is not unique. A second definition decouples the two double bonds by an alternative hydrogenation process (equation 19). 

The exchange of BTM-n cations is mainly governed by enthalpic effects in contrast to alkylammonium exchange, the behaviour of which is primarily determined by entropy changes (All being essentially zero). This suggests that electrostatic interactions, which are enhanced due to better delocalization, are also important. dgp spacings of 1.45 nm confirm a flat orientation of these ions. 

Let us suppose that the acetic acid content of the final aqueous solution is 5%, corresponding to a ratio of approximately 1 mol of CH3COOH to 60 mol of H2O. As the yield of reaction 2.1 will be near 100% (recall that reaction 2.2 is rather exothermic, implying a very high equilibrium constant see section 2.9), the same value will be used for the molar ratio (H2 O) / n (C 2115OII), despite the increased total amount of substance of water in the reaction products. In the present case, the difference of 1 mol of water between the product and the reactant mixtures has a negligible enthalpic effect. The enthalpies associated with the solution of ethanol and acetic acid in 60 mol of water are derived from literature data [17] as Asin//(1) = -10.0 0.1 kJ mol-1 and Asin//(3) = —1.0 0.1 kJ mol-1. This calculation will be detailed in section 2.5. 

The efiect is thus not related to geometrical constraints induced on complexes anchored in mesoporous charmels (sometimes also called as confinement efiect, even if this definition is not properly correct), neither to shape-selectivity effects as possible in zeolites, since the size of mesoporous charmels is much larger than those of micro-porous materials. Instead, an effective modification on the characteristics of the fluids is observed due to the electrostatic field generated by the charmel walls. This is an enthalpic effect versus an entropic effect as observed when the modification is instead related to limitations in the translation modes of molecules. Recently, it was also demonstrated that wall curvature influence the molecular orientation of the... 

Table III. Values of the Enthalpic Effect of Hydrophobic Hydration Hb(H20), the Number of Water Molecules per Alkyl Group N/4 Correlated with It, and the Mean Deviations of the Calculated from...

According to Grumberg [116], it appears likely that Wvjsc may be a close approximation to the heat of mixing. Table 1.7 shows that the Wvisc values are proportional to the X values. The large negative values of WviSc for p - xylene and mesitylene in a polar solvent can be taken as experimental evidence of an effective enthalpic effect in these binary systems relative to toluene - 2 - propanol system, which influence the value of the preferential adsorption coefficient X. 

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. 

The perfluoro alkylation of protonated N-heteroaromatic bases (Scheme 14.5a) gives high conversion, because of the high enthalpic effect, which can be ascribed to the stronger C—C bond formed when Rr rather than R radicals add to the substrate [26]. However, Rf-being an electrophilic radical, low selectivity is observed. 

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