Entropy influencing factors

A number of groups have criticized the ideas of Dauben and Noyce, especially the concept of PDC. Kamernitzsky and Akhrem, " in a thorough survey of the stereochemistry of addition reactions to carbonyl groups, accepted the existence of SAC but not of PDC. They point out that the reactions involve low energies of activation (10-13 kcal/mole) and suggest that differences in stereochemistry involve differences in entropies of activation. The effect favoring the equatorial alcohols is attributed to an electrostatic or polar factor (see also ref. 189) which may be determined by a difference in the electrostatic fields on the upper and lower sides of the carbonyl double bond, connected, for example, with the uncompensated dipole moments of the C—H bonds. The way this polar effect is supposed to influence the attack of the hydride is not made clear. 

Several factors influence the amount of entropy that a system has in a particular state. In general—... 

It is a common assumption that the influence of steric factors will be manifested mainly as a higher activation energy. In fact, there is good evidence37 to show that steric factors are mainly reflected in a less favorable entropy of activation or Arrhenius frequency factor. This is due to the degrees of freedom... 

The neutral situation, in which the entropy of the transition state does not notably differ from that of the reactant(s) in the ground state yields the standard pre-exponential factor hv = ek T/h which is of the order of 10 s . The influence of the entropy term gives rise to a range of possible pre-exponential factor from about 10 ... 

The factors influencing the values of molar enthalpies and molar entropies of hydration of ions... 

An extensive study has been performed concerning the basicity of aminoazines using the SCF MO LCAO method.165 Moreover, attention has been paid to the influence of various factors influencing the basicity of azines196 and to the relationship between the protonation and solvation entropy.167... 

All chemical reactions are influenced by two forces the tendency to achieve the most stable bonding state (for which enthalpy, H, is a useful expression) and the tendency to achieve the highest degree of randomness, expressed as entropy, S. The net driving force in a reaction is AG, the free-energy change, which represents the net effect of these two factors AG = AH- TAS. 

The mechanisms by which liquid crystals influence reaction rates need further study. Presumably, one important factor is the effect of ordering on entropy changes of various steps of the reaction. Bacon (44) has considered this factor in connection with the ability of thermotropic liquid crystals to increase the polymerization rate of phenylacetylene. 

For reactions between ions of unlike sign, solvation effects give an increase in entropy on activation. Consideration of internal structure gives a decrease in A S and the two effects could balance, or partially balance, each other. The electrostatic modifications would, however predict an increase in A S, indicating p factors greater than unity. Since the other two effects are in opposite directions to each other, the electrostatic modifications must dominate, with this being the major factor influencing the A factors. 

The impressive selectivity for e.g. fluoride over chloride drops from a factor of fifty in dichloromethane to zero in DMSO, and is also small in acetonitrile. Calorimetric measurements show, that the complexation is dominated by enthalpy, but influenced also by entropy disadvantage which is larger for Cl" than for F (it should be noted that several published data differ significantly, also as function of addition modes). Obviously, the prediction of structures and stabilities, but in particular of selectivities in different environment is still a demanding challenge. 

The classical (or semiclassical) equation for the rate constant of e.t. in the Marcus-Hush theory is fundamentally an Arrhenius-Eyring transition state equation, which leads to two quite different temperature effects. The preexponential factor implies only the usual square-root dependence related to the activation entropy so that the major temperature effect resides in the exponential term. The quadratic relationship of the activation energy and the reaction free energy then leads to the prediction that the influence of the temperature on the rate constant should go through a minimum when AG is zero, and then should increase as AG° becomes either more negative, or more positive (Fig. 12). In a quantitative formulation, the derivative dk/dT is expected to follow a bell-shaped function [83]. 

FIGURE 6.1 Factors that influence the nature of electrostatic cation adsorption at oxide surfaces, and thus, PL membrane structure, (a) Percentage of surface sites occupied by TMA+ ions compared to Na+ ions. This number is another way of representing AGexcTMA+/Na+. (b) Standard state entropy of cation adsorption. (Modified and reprinted from Sahai, N., J. Colloid Interface Sci., 252, 309, copyright 2002, and Sahai, N., Geochim. Cosmochim. Acta, 64, 3629, copyright 2000. With permission from Elsevier Science.)... 

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