Nitration and aromatic reactivity A. The theoretical

It is the purpose of this chapter to provide a resume of theoretical concepts which are used in discussing aromatic reactivity. Extended discussion is imnecessary, for many of the concepts are the common currency of organic chemistry, and have been frequently expounded. -  

Most of the reactions with which organic chemists are concerned involve poly-atomic molecules, and occur in solution at temperatures not far removed from the ambient. There is not at present the faintest possibility of chemical theory predicting the absolute rates of such processes. 

There are available from experiment, for such reactions, measurements of rates and the familiar Arrhenius parameters and, much more rarely, the temperature coefficients of the latter. The theories which we use, to relate structure to the ability to take part in reactions, provide static models of reactants or transition states which quite neglect thermal energy. Enthalpies of activation at zero temperature would evidently be the quantities in terms of which to discuss these descriptions, but they are unknown and we must enquire which of the experimentally available quantities is most appropriately used for this purpose. 

In transition state theory, the rate constant, k, is given by the following 

Admitting the impossibility of calculating absolute rates, we can concern ourselves with the effect of a structural modification to a particular reactant which we take as a point of reference if the rate constant for the reaction involving the modified compound is k, and that for the 

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