The potential outcome of a reaction is usually influenced by two factors: the relative stability of the products (i.e. thermodynamic factors) and the rate of product formation (i.e. kinetic factors).
Choosing the mechanism of control for a chemical reaction is important as it not only determines the composition of the product in a reaction mixture, but also determines the reaction pathway that is taken. This leads to different products due to the selectivity or stereoselectivity of the reaction. This choice of mechanism can influence your choice of reaction conditions such as concentration, temperature, pressure, and solvent.
For example, the free energy in a reversible reaction is important in both the forward and backward reaction. We can examine this using an example of a reactant A in a reversible reaction with product B.
When considering the thermodynamics of a reaction, we can look at the ΔG value for both the forward reaction and backward reaction. For example, if the forward reaction has a ΔG value of less than 0 it is spontaneous. The reactants (A) are at higher free energy than the products (B). The reverse reaction will have the same magnitude of ΔG, but it will be greater than 0 and positive; thus, the reactants B start at lower free energy than the product A. This is the thermodynamic consideration of the reactants and products in this reversible reaction. Therefore, the forward reaction is more thermodynamically favorable.
Whether the reaction is spontaneous (ΔG < 0) or non-spontaneous (ΔG >0), the kinetic barrier is the activation energy. The activation energy is the energy required to form the transition state (intermediate) that then turns into the product. All reactions usually need some energy input as the activation energy forms an unstable, high energy intermediate. A reaction with smaller activation energy will be more kinetically favorable.
In a reaction where there are two products (e.g. C-> A + B), one of the products could be more thermodynamically stable. Meanwhile, the other product could have lower activation energy and be the kinetic product. For example, product A forms faster than product B as its activation energy is the lower of the two, but product B is more stable. In this example, A is the kinetic product and is favored under kinetic control while B is the thermodynamic product and is favored under thermodynamic control.
The conditions of the reaction, such as temperature, pressure, or solvent, affect which reaction pathway may be favored: product formation under kinetic control or stability under thermodynamic control. 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. The prevalence of thermodynamic or kinetic control determines the final composition of the product when these competing reaction pathways lead to different products.
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Key Points
• Kinetic control favors the product in a reaction with has the lowest activation energy and is formed faster
• Thermodynamic control favors the more energetically stable product
Key Terms
Gibbs free energy change (G): Thermodynamic property defined in terms of system enthalpy and entropy; all spontaneous processes involve a decrease in G
Stereoselectivity: The property of a chemical reaction in which a single reactant forms an unequal mixture of stereoisomers during a non-stereospecific creation of a new stereocenter or during a non-stereospecific transformation of a pre-existing on.
Reaction conditions: The environmental conditions, such as temperature, pressure, catalysts & solvent, under which a reaction progresses optimally.
Free energy: A thermodynamic quantity equivalent to the capacity of a system to do work.
Reversible reaction: A reversible reaction is a reaction where the reactants and products react together to give the reactants back.
Forward reaction: A reaction in which products are produced from reactants and it goes from left to right in a reversible reaction.
Reverse reaction: A reaction in which reactants are produced from products and it goes from right to left in a reversible reaction.
Kinetic barrier: The magnitude of the potential barrier (sometimes called the energy barrier) separating minima of the potential energy surface pertaining to the initial and final thermodynamic state.
Activation energy: The minimum quantity of energy which the reacting species must possess in order to undergo a specified reaction.
Transition state: A particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate.
Kinetic product: The product that is formed faster in a chemical reaction.
Kinetic control: A reaction in which the product ratio is determined by the rate at which the products are formed.
Thermodynamic product: The more stable product formed in a chemical reaction.
Thermodynamic control: A reaction in which the product ratio is determined by the relative stability of the products.