Enthalpy change factors affecting

Now we know something of the factors that affect equilibrium. Enthalpy change, AH, is important. Basically, the side of the equilibrium with the stronger bonds will be favored by enthalpy. Also important is the temperature-dependent entropy change, AS, term. The two terms may act in concert or oppose each other. 

A theoretical study of nucleophilic substitution, from a charge density perspective, has used the concept of bond bundles , which allows the prediction of novel activating groups for arene substitutions. A survey of the factors affecting the activation parameters in nucleophilic substitution has concluded that charge development in the ttansition state is determined mainly by the internal part of the activation enthalpy change. ... 

In aqueous solution the factors which affect the enthalpy of formation of a complex ion are complicated by the presence of an effect caused by a change in the enthalpy of solvation. If we consider the reaction... 

The free energy of activation is composed of an entropy activation (AS ) and an enthalpy of activation AH ). The former is associated with the pre-exponential factor A of the Arrhenius equation and the latter with the experimental act, which defines the sensitivity of reaction rate to temperature. In some reactions, substituents affect ac, (or AH ) and AH° primarily, while A (or AS ) and A S° change only slightly. In these cases, AG and In k vary in the same way as act, and In k varies linearly with AG°. Frequently, however, In k and In K are correlated, even though both A and act vary as the substituent changes. Thil can happen if AH (or act) and AS are correlated, and similar relationships are observed between the values of AH° and AS 1 for the reactions this is called the compensation effect. The net effect is a simple LFER in which the change in k in less than if E.,cl or A changed alone. It is easy to show that this situation leads to... 

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