Chapter 10: Conjugation in Alkadienes and Allylic Systems Ch 10 contents

Kinetic and Thermodynamic Control

The potential outcome of a reaction is usually influenced by two factors:

  1. the relative stability of the products  (i.e. thermodynamic factors)
  2. the rate of product formation  (i.e. kinetic factors)
The following simple reaction coordinate diagram provides a basis for the key issues about kinetic and thermodynamic control:
 
Consider the case where a starting material, SM, can react to give two different products, P1 and P2 via different pathways (represented by green and blue  lines). 

Reaction 1 via pathway 1 (green)  generates P1.
This will be the faster reaction since it has a more stable transition state, TS1, and therefore a lower activation barrier. So P1 is the kinetic product. 

Reaction 2 via pathway 2 (blue) generates P2.
P2 is the more stable product since it is at lower energy than P1. So P2 is the thermodynamic product.
 

 

 Reaction coordinate diagram showing kinetic and thermodynamic products
Now consider what happens as we alter the reaction temperature and therefore the average energy of the molecules changes.

1.  At low temperature, the reaction preferentially proceeds along the green path to P1 and stops since they lack sufficient energy to reverse to SM,  i.e. it is irreversible, so the product ratio of the reaction is dictated by the rates of formation of P1 and P2, k1: k2.

2. At some slightly higher temperature, reaction 1 will become reversible while reaction 2 remains irreversible. So although P1 may form initially, over time it will revert to SM and react to give the more stable P2.

3. At high temperature, both reaction 1 and 2 are reversible and the product ratio of the reaction is dictated by the equilibrium constants for P1 and P2, K1 : K2.

Summary


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