Consider the following reaction:

The products are formed in the following ratios:

43%	1%	56%

The reasons for the ratios will be explained in the section on electrophilic substitution, however, what is important to note here is that the underlying reason is that the more abundant isomers simply form faster than the less abundant.  This is known as kinetic control.  However, kinetic control is not the only control of a reaction.  If the conditions are correct, it is possible to have thermodynamic control.

If a reaction is under thermodynamic control, it does not matter how fast the products are formed, only their relative stabilities matter; the less stable, the less abundant the product.

For kinetic control to occur, we normally need low temperatures, and for thermodynamic control, high temperatures.  Kinetic control also normally uses harsh reagents, and thermodynamic control less reactive ones, so that the reaction mixture may equilibrate. The following energy profile may prove useful:

From the diagram above, we can see that the kinetic product forms first because it's transition state has a lower activation energy, whereas the thermodynamic product is more stable, but its transition state requires more activation energy.  However, it is possible that the product that forms fastest is also the most stable, in which case it is both the kinetic and thermodynamic product.

An example of a reaction whose product can be change by altering the conditions to change from kinetic to thermodynamic control is the sulphonation of napthalene.

Thermodynamic	Kinetic
160ºC	80ºC