Entropy of Activation and Structure

Entropy of Activation and Structure From the inception of transition state theory, entropies of activation have been discussed from the twin aspects of molecular structure and reaction mechanism. Even though there is considerable overlap between these two aspects we shall utilize a formal separation, reserving much of the discussion of mechanism for the next section. In this section our primary concern shall be the effect that structural change in a non-reacting part of a molecule has upon the entropy and enthalpy of activation for that molecule. The nature of interactions (polar, steric, and resonance) between the substituent group and the reaction center clearly relates to the problem of reaction mechanism, the solution of which involves, in the final analysis, a detailed description of the disposition of the atoms in the transition state and the interactions among them. 

Several of the reference texts and review articles on kinetics give discussions of entropies of activation in terms of structure and mechanism, and it is sufficient here only to refer briefly to a few of them. Values of AS (or its equivalent, frequency factors) for unimolecular gas reactions have been considered in some detail (Benson, 1960b Frost and Pearson, 1961c Gowenlock, 1960). Summaries of thermodynamic data for solution reactions may be found in several sources (Frost and Pearson, 1961d Moelwyn-Hughes, 1947 Pearson, 1952). 

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The interrelation of entropy of activation and structure of the activated complex may be illuminated by considering the reverse, association reaction. Taking the case of ethane for example, the entropy of activation for the unimolecular decomposition into two methyls is very large in the order of 12 eu . The association reaction of two methyls on the other hand is known to occur at nearly every collision which means that their relative orientation has no influence on their recombina-... 

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