How Molecular Structure Controls the Mechanism

Both thermodynamic and kinetic factors are involved in the competition between concerted and stepwise mechanisms. The passage from the stepwise to the concerted situation is expected to arise when the ion-radical cleavage becomes faster and faster. Under these conditions, the rate-determining step of the stepwise process tends to become the initial electron transfer. Then, thermodynamics will favor one or the other mechanism according to 

TABLE 3.2. Molecular Factors Governing the Dichotomy Between Concerted and Stepwise Mechanisms 

Examples of the Prevailing Role of the Bond Dissociation Energy (Db)  

ELECTRON TRANSFER, BOND BREAKING, AND BOND FORMATION 

FIGURE 3.10. Reductive cleavage of aliphatic polychloroalakanes in a nonprotic medium. Dp is the interaction energy between clustered fragments. 

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How molecular structure controls the mechanism 133 Transition between mechanisms upon changing the driving force 134 Does concerted mechanism mean that the intermediate does not exist 140 Reaction coordinates in concerted and stepwise reactions 140... 

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