Theoretical treatments of gradations in reaction rates

Energy surface treatments [53, 54] of the reaction between sodium and a monohalide are based upon the principle [55] that these reactions can be represented by the superposition of two energy surfaces. The first of these relates to the reaction process involving homopolar forces, the second to ionic forces. This can be represented as 

This method, in a simplified form, has been used for the discussion of gradations of reaction rate by making use of the result that when the sodium atom approaches the alkyl halide molecule no appreciable change in energy is caused even though the Na. . . X separation is equal to the normal separation distance of the ion pair. Consequently, a cross-section 

Warhurst [4] has also invoked a geometric effect to account for the increased rate coefficients in some dichlorocompounds. Thus the six-fold increase in the rate coefficient on passing from vinyl chloride to cis-dichloroethylene is ascribed primarily to the ability of the sodium to approach two chlorine atoms up to the ion-pair distance in the latter case. 

These concepts represent the theoretical framework for the interpretation of the majority of the published data for sodium flame reactions. Subsequent to the publication of Warhurst s review it was suggested by Smith and co-workers [63, 64] that there should be a correlation of the activation energy of sodium—alkyl chloride reactions with the net electronic charge on the halogen atom, the higher the charge 

A general correlation [65] of sodium flame reaction rates with electron absorption coefficients has been made. This correlation covers a range of rate coefficients varying by a factor of three powers of ten. It is illustrative rather than conclusive and deserves further consideration. A formal similarity could also exist between the mechanisms for dissociative electron attachment of organic halides [66, 67] and the sodium flame reactions. These mechanisms can be either a direct dissociative mechanism 

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