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Concept
Thermodynamic versus kinetic reaction control
Summary
Thermodynamic reaction control or kinetic reaction control in a chemical reaction can decide the composition in a reaction product mixture when competing pathways lead to different products and the reaction conditions influence the selectivity or stereoselectivity. The distinction is relevant when product A forms faster than product B because the activation energy for product A is lower than that for product B, yet product B is more stable. In such a case A is the kinetic product and is favoured under kinetic control and B is the thermodynamic product and is favoured under thermodynamic control.
The conditions of the reaction, such as temperature, pressure, or solvent, affect which reaction pathway may be favored: either the kinetically controlled or the thermodynamically controlled one. Note this is only true if the activation energy of the two pathways differ, with one pathway having a lower Ea (energy of activation) than the other.
Prevalence of thermodynamic or kinetic control determines the final composition of the product when these competing reaction pathways lead to different products. The reaction conditions as mentioned above influence the selectivity of the reaction - i.e., which pathway is taken.
Asymmetric synthesis is a field in which the distinction between kinetic and thermodynamic control is especially important. Because pairs of enantiomers have, for all intents and purposes, the same Gibbs free energy, thermodynamic control will produce a racemic mixture by necessity. Thus, any catalytic reaction that provides product with nonzero enantiomeric excess is under at least partial kinetic control. (In many stoichiometric asymmetric transformations, the enantiomeric products are actually formed as a complex with the chirality source before the workup stage of the reaction, technically making the reaction a diastereoselective one. Although such reactions are still usually kinetically controlled, thermodynamic control is at least possible, in principle.)
The Diels–Alder reaction of cyclopentadiene with furan can produce two isomeric products.
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