Thermodynamic versus Kinetic Control of Reactions

If the product composition of a reaction is governed by the equilibrium thermodynamics of the system, i.e. if formation of the thermodynamically more stable product is the driving-force of the reaction, the reaction is said to be thermodymically controlled.

If the product composition is governed by competing rates of formation of products, i.e. if the product with the lowest activation energy is formed, the reaction is said to be under kinetic control.

In Case I both products A and B are energetically significantly more stable than reactant R. The reverse reaction from A or B to R is energetically disfavoured and the reaction is effectively irreversible (regardless of what the ration of A to B is). Whether A or B are formed will depend on the activation energy to reach TSA or TSB. This is an example of kinetic control.

In Case II the actvation energies for the formation of A and B from R are very similar. Formation of A is only slightly more favourable than formation of B. This reaction can be under kinetic or thermodynamic control. Under carefully controlled conditions the reaction can be stopped at A (the kinetic product). Under more forcing conditions, i.e. higher temperatures, A can be converted into B, the thermodymanic product.

In Case III TSA has a much higher energy barrier than TSB. When R is converted into A it will equilibrate rapidly to give B. Temperature adjustment will not change the ratio of A to B (mostly B). This reaction is under thermodynamic control.

Examples for thermodynamic versus kinetic control are

• regioselectivity of enolate formation (Tutorial Link: Carbonyl Chemistry I)

• endo- versus exo- selectivity in the Diels-Alder Reaction (Tutorial Link: Pericyclic Chemistry)