Basic Mechanistic Concepts Kinetic versus Thermodynamic Control, Hammonds Postulate, the Curtin-Hammett Principle

Basic Mechanistic Concepts Kinetic Versus Thermodynamic Control, Hammond s Postulate, the Curtin-Hammett Principle 

Product composition may be governed by the equilibrium thermodynamics of the system. When this is true, the product composition is said to be governed by thermodynamic control. Alternatively, product composition may be governed by competing rates of formation of the possible products. This is called kinetic control 

Let us consider cases 1-3 in Fig. 4.12. In case 1, AG s for formation of transition states A and B from the reactant R are much less than AG s for 

In case 3, the barrier separating A and B is very small relative to that for formation of A from R. A and B will equilibrate rapidly relative to formation of A. The product mixture will reflect thermodynamic control, with the ratio of A and B being determined by the relative stabilities of the products rather than their rates of formation. 

The idea of kinetic versus thermodynamic control can be illustrated by discussing briefly the formation of enolate anions from unsymmetrical ketones. A more complete discussion of this topic is given in Chapter 7 and in Part B, Chapter 1. Any ketone with more than one type of a proton can give rise to at least two enolates when a proton is abstracted. Many studies, particularly those of House, have shown that the ratio of the two possible enolates depends on the reaction conditions. If the base is strong and sterically bulky and there are no hydroxylic solvents present, enolate 6 is the major product. When equilibrium is established between 5 and 6 by making enolate formation reversible by using a hydroxylic solvent, however, the dominant enolate is 5. Thus, 6 is the product of kinetic control The structural reason 

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