General Relationships between Thermodynamic Stability and Reaction Rates

General Relationships between Thermodynamic Stability and Reaction Rates 

Note that the form of this third-order kinetic expression is identical to that in the case where the second step was rate determining and would not distinguish between the two possibilities. 

Experimental studies of this base-catalyzed condensation have revealed that it is third order, indicating that either the second or fourth step must be rate determining. Studies on the intermediate I obtained by an alternative synthesis have shown that is about four times as large as so that about 80% of I goes on to product. These reactions are faster than the overall reaction under the same conditions, so the second step must be rate controlling.  

These examples illustrate the relationship between kinetic results and the determination of the reaction mechanism. Kinetic results can exclude from consideration all mechanisms that require a rate law different from the observed one. It is often true, however, that related mechanisms give rise to identical predicted rate expressions. In that case, the mechanisms are kinetically equivalent and a choice between them is not possible solely on the basis of kinetic data. A further limitation on the information that kinetic studies provide should also be recognized. Although the data can give the composition of the activated complex for the rate-determining and preceding steps, it provides no information about the structure of the activated complex or any intermediates. Sometimes the structure can be inferred from related chemical experience, but it is never established solely by kinetic data. 

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