Thermodynamic and Kinetic
Control of Organic reactions
By
Dr. Mehboob Peeran
The following scheme illustrates the process of kinetic
and thermodynamic controls
Conclusions:
In reactions that proceed through different pathways leading to different
products, if the major product is that derived from the pathway of
lower energy of activation then the reaction is under kinetically
control (even if the other product is more stable).
[A] / [B] = k1 / k2 that is the product proportion
is the ratio of the rate constants of the two pathways.
If the major product is the more sable product then the reaction is
under thermodynamic control.
[A] / [B] = K where K is the equilibrium constant (the equilibrium
is between the two isomeric products)
For this to be possible the reaction conditions should be suitable
for establishing equilibrium between the reactants and the kinetically
controlled products. The major product is the one, which is associated
with lower free energy (ΔG0). In kinetic controlled
reactions the major product is the one that has a lower activation
energy barrier (ΔG‡) for the reaction pathway.
Comparison |
Under
kinetic control |
Under
Thermodynamic control |
The product proportion reflects the relative rates of formation of
the processes.
The reaction can be under kinetic control before
equilibrium conditions are established. |
The product proportion reflects the relative free energies of the products.(that
is the thermodynamic stability)
The reaction is under thermodynamic control when equilibrium conditions
are established. |
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