14.3 Kinetic versus Thermodynamic Control of Reactions
3 min read•may 7, 2024
Temperature plays a crucial role in determining reaction outcomes. At lower temps, favors products that form fastest, while higher temps allow , favoring more stable products. This balance shapes the products we see in organic reactions.
Understanding kinetic vs is key to predicting and manipulating reaction outcomes. By tweaking conditions like temperature, we can steer reactions towards desired products, whether they're the fastest-forming or most stable options.
Kinetic versus Thermodynamic Control of Reactions
Temperature effects on diene addition products
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- Temperature influences relative rates of competing reaction pathways
- Lower temperatures favor kinetic product due to lower barrier and lower energy (e.g. in )
- Higher temperatures favor thermodynamic product which is more stable with lower overall Gibbs free energy () and system has enough energy to overcome higher activation energy barrier (e.g. in conjugated dienes)
- Conjugated dienes undergo 1,2-addition or 1,4-addition
- 1,2-addition typically kinetic product forming less stable intermediate (e.g. 1,2-addition of HBr to 1,3-butadiene)
- 1,4-addition typically thermodynamic product forming more stable alkene product with extended conjugation (e.g. 1,4-addition of HBr to 1,3-butadiene)
Kinetic vs thermodynamic control
- favors product that forms fastest determined by relative activation energies () of competing reaction pathways with lowest favored and kinetic products often less stable than thermodynamic products (e.g. of tertiary alkyl halides)
- Thermodynamic control favors most stable product determined by relative Gibbs free energies () of products with lowest favored and thermodynamic products often more stable than kinetic products (e.g. of primary alkyl halides)
- Interconversion between kinetic and thermodynamic products possible if activation energy barrier for interconversion is low allowing kinetic product to convert to thermodynamic product over time through equilibration (e.g. cis-trans isomerization of alkenes)
Product ratios in diene reactions
- Low temperature conditions favor kinetic control
- 1,2-addition product (kinetic product) formed in higher proportion
- Ratio of 1,2-addition to 1,4-addition products higher (e.g. 80:20 ratio of 1,2 to 1,4 addition of HBr to 1,3-butadiene at 0 ℃)
- High temperature conditions favor thermodynamic control
- 1,4-addition product (thermodynamic product) formed in higher proportion
- Ratio of 1,4-addition to 1,2-addition products higher (e.g. 20:80 ratio of 1,2 to 1,4 addition of HBr to 1,3-butadiene at 40 ℃)
- applies when interconversion between kinetic and thermodynamic products is fast relative to formation of products
- Product ratio determined by difference in energies leading to each product
- Relative stability of products does not affect product ratio under Curtin-Hammett conditions (e.g. addition of HCl to 3,3-dimethyl-1-butene)
Reaction Energy Profile Analysis
- visually represents energy changes during a reaction
- Shows relative energies of reactants, products, and transition states
- Helps identify the rate-determining step, which is typically the step with the highest activation energy
- relates transition state structure to nearby stable species
- For exothermic reactions, transition state resembles reactants
- For endothermic reactions, transition state resembles products
Key Terms to Review (21)
1,2-Addition: 1,2-Addition is a type of organic reaction where an electrophile or nucleophile adds to the first and second carbon atoms of a conjugated diene, resulting in the formation of a new carbon-carbon bond. This term is particularly relevant in the context of electrophilic additions to conjugated dienes, kinetic versus thermodynamic control of reactions, and conjugate nucleophilic additions to α,β-unsaturated aldehydes and ketones.
1,4-Addition: 1,4-Addition is a type of electrophilic addition reaction that occurs on conjugated dienes, where the electrophile adds to the 1- and 4-positions of the diene, forming a new product with an allylic carbocation intermediate. This reaction is important in the context of understanding electrophilic additions to conjugated systems, kinetic versus thermodynamic control of reactions, and conjugate nucleophilic additions to α,β-unsaturated carbonyl compounds.
Activation Energy: Activation energy is the minimum amount of energy required to initiate a chemical reaction. It represents the energy barrier that reactants must overcome in order to form products. This concept is central to understanding the mechanisms and kinetics of organic reactions.
Activation energy, ΔG‡: Activation energy (ΔG‡) is the minimum amount of energy required to initiate a chemical reaction, specifically the energy needed to reach the transition state from the reactants. It's a crucial factor in determining the rate at which a reaction will occur in organic chemistry.
Allyl Carbocation: An allyl carbocation is a resonance-stabilized positive charge that is delocalized across three carbon atoms, typically formed during the course of organic reactions. This term is particularly relevant in the context of understanding kinetic versus thermodynamic control of reactions.
Cis-trans Isomerism: Cis-trans isomerism is a type of stereoisomerism in organic chemistry where two molecules have the same molecular formula and connectivity, but differ in the spatial arrangement of their atoms. This isomerism arises when carbon-carbon double bonds restrict rotation, leading to distinct orientations of substituents on either side of the double bond.
Conjugated Dienes: Conjugated dienes are organic compounds with two carbon-carbon double bonds that are separated by a single carbon-carbon bond. This arrangement of alternating double and single bonds creates a system of delocalized pi electrons, which gives conjugated dienes unique stability and reactivity properties.
Curtin-Hammett Principle: The Curtin-Hammett principle describes the relationship between the kinetic and thermodynamic control of a reaction, where the observed product distribution is determined by the relative rates of the competing pathways rather than their relative stabilities.
Hammond postulate: The Hammond postulate suggests that the transition state of a chemical reaction resembles the structure and energy of the nearest stable species, whether reactants or products. It is particularly useful in understanding the reactivity of alkenes in organic chemistry by predicting the outcome of reactions and their mechanisms.
Hammond Postulate: The Hammond Postulate is a principle in organic chemistry that describes the relationship between the structure of the transition state in a chemical reaction and the relative stability of the reactants and products. It states that if two transition states have similar energies, the one leading to the more stable product will be favored.
Kinetic control: Kinetic control in organic chemistry refers to reaction conditions under which the product distribution is determined by the rate at which products are formed, favoring the formation of products that are formed fastest. These conditions often lead to products that are not necessarily the most stable but are reached more quickly due to lower activation energies.
Kinetic Control: Kinetic control refers to the principle that the initial product formed in a reaction is determined by the reaction pathway that has the lowest activation energy, regardless of the thermodynamic stability of the final products. It describes how the kinetics of a reaction, rather than just the thermodynamics, can dictate the outcome of a transformation.
Reaction Coordinate Diagram: A reaction coordinate diagram is a graphical representation that depicts the energy changes that occur during the course of a chemical reaction. It provides a visual tool to understand the energetics and kinetics of a reaction, which are crucial in the study of organic chemistry mechanisms, equilibria, and reaction control.
SN1 reaction: An SN1 reaction is a two-step nucleophilic substitution process in organic chemistry where the bond between the carbon and leaving group breaks before the nucleophile adds to the carbocation intermediate. It typically occurs with tertiary alkyl halides or molecules that can stabilize a positive charge well.
SN1 Reaction: The SN1 reaction, or Substitution Nucleophilic Unimolecular reaction, is a type of nucleophilic substitution mechanism in organic chemistry where a nucleophile replaces a leaving group in a two-step process involving the formation of a carbocation intermediate. This reaction is characterized by its unique step-wise mechanism and is influenced by factors such as the stability of the carbocation intermediate and the nature of the nucleophile and leaving group.
SN2 reaction: In organic chemistry, an SN2 reaction is a type of nucleophilic substitution where a nucleophile strongly attacks an electrophilic center in one step, leading to the simultaneous displacement of a leaving group. This reaction mechanism is characterized by its bimolecular nature, involving two reacting species in the rate-determining step.
SN2 Reaction: The SN2 reaction, or bimolecular nucleophilic substitution, is a type of organic reaction where a nucleophile attacks the backside of a carbon atom bearing a leaving group, resulting in the displacement of the leaving group and the inversion of stereochemistry at the carbon center.
Thermodynamic control: In organic chemistry, thermodynamic control describes conditions under which the products of a reaction are determined by the relative stability of the products rather than the rates at which they are formed. This often results in the formation of the most stable product over time, even if it is not the most rapidly produced.
Thermodynamic Control: Thermodynamic control refers to the principle that the most stable and thermodynamically favored product will be the predominant product of a reaction, regardless of the kinetic pathway. It is a concept that governs the outcome of various organic chemistry reactions, including those related to energy diagrams, elimination reactions, electrophilic additions to conjugated dienes, and the dehydration of aldol products.
Transition state: In organic chemistry, the transition state is a high-energy, temporary condition where reactants are transformed into products during a chemical reaction. It represents the point of maximum energy on the energy diagram before the formation of products.
Transition State: The transition state is a key concept in organic chemistry that describes the highest-energy intermediate along the reaction pathway. It represents the point where the reactants are being converted into products, with the system at its most unstable and energetically unfavorable configuration.