Basics for the Reactivity-Selectivity Approach

A central point to every mechanistic study of any chemical reaction is the nature of the transition state. There are not yet any experimental techniques that can directly observe transition state structures, although there are some recent experimental studies that, in the near future, could pave the way for the development of these techniques [36-37]. The chemical reaction properties that are associated with the transition state must be relied upon. Through them, the structural and energy profiles of the transition state structures can be generated. 

The Gt Gp, and Gr, are the free energies of the three stationary points on the reaction potential energy surface 5 is an operator which indicates the difference introduced as caused by some perturbation (e.g. a substituent or medium change) a is a factor that determines the position of the transition state in regard to reactants and products. It is varies from 0 to 1 and is closer to 1 for an endothermic reaction or zero for exothermic reactions. Equation (1) can be rearranged into the form 

In light of this observation, selectivity and reactivity on the basis of the energy changes along reaction coordinate for two or more competitive reactions can now be defined. If the reaction of reactant A with two competitive reagents B and C is being followed experimentally, the rate of the reaction for A + B and the rate for reaction A+ C with rate constants kA and ke respectively, can be measured. The Selectivity is defined as  

Since the difference is in the free energy of activation, AAGt, for two concurrent reactions is AGtAB - AG aC- And, since there is a linear relationship with kA/kB, the selectivity is proportional to AAGt. This is a very simplified approach to selectivity explanation and it must be noted that many assumptions must be fulfilled for its validity. The fundamental assumption for this conclusion is that the reaction under consideration obeys a rate-equilibrium relationship. For example, the principle cannot be applied for reactions that are diffusion controlled. It is also doubtful that this principle can be applied for reactions that involve very reactive species such as carbenes, radicals, and carbonium ions [1]. 

Experimentally, it is not always possible to obtain absolute rate constants, an example being the case in which a reaction is occurring very rapidly. In this case, experimentalists use relative rate constants for determining reactivity. In multistep reactions, the rate constant usually represents the rate for the slowest 

---