7.10 The Hammond Postulate
3 min read•may 7, 2024
The helps predict structures in chemical reactions. It's key for understanding how alkenes react, especially in . The stability of forming carbocations plays a big role in these reactions.
Alkene reactivity in electrophilic addition depends on stability. More stable carbocations form faster, leading to quicker reactions for more substituted alkenes. This concept is crucial for predicting and explaining reaction rates and outcomes.
The Hammond Postulate and Electrophilic Addition to Alkenes
Hammond postulate and transition states
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- States structure of resembles nearest stable species on
- Transition state closer in energy to reactants will more closely resemble reactants in structure ()
- Transition state closer in energy to products will more closely resemble products in structure ()
- Relative energies of reactants, transition state, and products determine transition state structure
- : transition state closer in energy to reactants, resembles reactants more closely
- : transition state closer in energy to products, resembles products more closely
- The is crucial in understanding and predicting the nature of transition states
Application to alkene protonation
- transition state involves partial carbocation formation and partial positive charge on proton
- Forming carbocation stability influences transition state structure and stability
- More stable carbocation (tertiary > secondary > primary) results in transition state resembling carbocation product
- More advanced formation
- More fully developed positive charge on carbocation
- Less stable carbocation (primary < secondary < tertiary) results in transition state resembling alkene reactant
- Less advanced C-H bond formation
- Less developed positive charge on carbocation
- More stable carbocation (tertiary > secondary > primary) results in transition state resembling carbocation product
- Hammond postulate predicts transition state for more stable carbocation formation is lower in energy and more product-like compared to less stable carbocation formation
- The for the reaction is influenced by the stability of the forming carbocation
Carbocation stability in reaction rates
- rate to alkene depends on stability of forming
- More stable carbocations (tertiary > secondary > primary) form faster than less stable carbocations
- Hammond postulate explains reaction rate trend
- More stable carbocation formation transition state is lower in energy and more product-like
- Lower energy transition state is easier to reach, resulting in faster reaction rate ()
- Less stable carbocation formation transition state is higher in energy and more reactant-like
- Higher energy transition state is more difficult to reach, resulting in slower reaction rate ()
- More stable carbocation formation transition state is lower in energy and more product-like
- Alkenes forming more stable carbocations (more substituted alkenes) react faster in electrophilic addition reactions than alkenes forming less stable carbocations (less substituted alkenes)
- Reaction rate: tertiary alkenes > secondary alkenes > primary alkenes
- The rate-determining step in these reactions often involves the formation of the carbocation intermediate
Energetics and Thermodynamics
- Potential energy diagrams illustrate the energy changes throughout the reaction progress
- changes during the reaction influence the overall and reaction spontaneity
- The relationship between kinetics and thermodynamics in these reactions can be visualized using potential energy diagrams
Key Terms to Review (27)
2-methylpropene: 2-methylpropene, also known as isobutylene, is a branched-chain alkene with the molecular formula C₄H₈. It is an important organic compound that is relevant in the context of several topics in organic chemistry, including the addition of HBr to ethylene, the stability of alkenes, Markovnikov's rule, and the Hammond postulate.
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.
Alkene Protonation: Alkene protonation is a fundamental reaction in organic chemistry where a hydrogen ion (proton) is added to an alkene, resulting in the formation of a carbocation intermediate. This process is particularly relevant in the context of the Hammond Postulate, which describes the relationship between the structure of reaction intermediates and the activation energy barriers of chemical reactions.
C-H Bond: The C-H bond is a covalent bond formed between a carbon atom and a hydrogen atom. This bond is fundamental to organic chemistry and is present in a wide range of organic compounds, from simple alkanes to complex biomolecules. The C-H bond is crucial in understanding the structure, reactivity, and stability of organic molecules.
Carbocation: A carbocation is a positively charged carbon atom that is part of an organic molecule. These reactive intermediates play a crucial role in various organic reactions, including electrophilic additions, nucleophilic substitutions, and elimination reactions.
Carbocation Intermediate: A carbocation intermediate is a positively charged carbon atom that acts as a reactive species in various organic chemistry reactions. These intermediates are formed during the course of a reaction and play a crucial role in determining the outcome and mechanism of the transformation.
Early Transition State: The early transition state refers to the initial stage of the transition state in a chemical reaction, where the reactants have only slightly deviated from their original structures and the reaction is just beginning to progress towards the products. This concept is closely related to the Hammond Postulate, which describes the relationship between the transition state and the reaction energetics.
Electrophilic Addition: Electrophilic addition is a type of organic reaction where an electrophile, a species that is attracted to electrons, adds to the carbon-carbon double bond of an alkene. This results in the formation of a new carbon-carbon single bond and the incorporation of the electrophile into the molecule.
Electrophilic addition reaction: An electrophilic addition reaction is a chemical process in which an electrophile reacts with a nucleophile, typically an alkene or alkyne, forming a new sigma bond by adding across the double or triple bond. This reaction is key in organic synthesis, resulting in the addition of atoms or groups to the carbon atoms involved in the multiple bond.
Endothermic Reaction: An endothermic reaction is a type of chemical reaction in which the system absorbs energy from the surroundings in the form of heat. This process requires an input of energy to proceed, as the products of the reaction have a higher energy state than the reactants.
Ethylene: Ethylene is a colorless, flammable gas with the chemical formula C₂H₄. It is the simplest alkene and is widely used in the chemical industry for the production of various organic compounds and polymers. Ethylene is a key term that connects to several important topics in organic chemistry, including the structure of alkenes, chemical bonding, and industrial applications.
Exothermic Reaction: An exothermic reaction is a chemical process that releases energy in the form of heat to the surrounding environment. This type of reaction is characterized by a decrease in the overall energy of the system, as the products of the reaction have less energy than the reactants.
Free Energy: Free energy is a measure of the useful work that can be extracted from a thermodynamic system. It represents the amount of energy available to do work while accounting for the system's entropy and the constraints imposed by the environment. This concept is crucial in understanding chemical reactions, equilibria, and the energy changes associated with various processes in chemistry and biochemistry.
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.
Late Transition State: The late transition state is a concept in organic chemistry that describes the structure of the transition state in a chemical reaction, particularly in the context of the Hammond Postulate. It refers to a transition state that is more product-like, meaning it more closely resembles the structure of the reaction's products rather than the reactants.
Potential Energy Diagram: A potential energy diagram is a graphical representation that illustrates the changes in potential energy of a system as a function of a specific reaction coordinate or structural parameter. It provides a visual depiction of the energy barriers and energy minima associated with the different conformations or states of a molecule or a reaction pathway.
Primary Carbocation: A primary carbocation is a positively charged carbon atom that has three single-bonded substituents and one hydrogen atom attached to it. These carbocations are the least stable type of carbocation due to the limited ability to delocalize the positive charge.
Reaction coordinate: A reaction coordinate is a parameter that represents progress along the pathway from reactants to products during a chemical reaction. It charts the energy changes that occur during the transformation, typically depicted on an energy diagram.
Reaction Coordinate: The reaction coordinate is a conceptual tool used to describe the progress of a chemical reaction. It represents the path that the reactants take as they transform into products, with the highest point on the path corresponding to the transition state of the reaction.
Reaction Kinetics: Reaction kinetics is the study of the rates and mechanisms of chemical reactions. It examines the factors that influence the speed and efficiency of a reaction, such as temperature, pressure, and the presence of catalysts. This concept is crucial in understanding organic reactions, as the rate and pathway of a reaction can have a significant impact on the products formed and the overall efficiency of the process.
Secondary Carbocation: A secondary carbocation is a positively charged carbon atom that has two alkyl groups attached to it. These types of carbocations are more stable than primary carbocations due to the ability of the alkyl groups to stabilize the positive charge through hyperconjugation.
Tertiary Carbocation: A tertiary carbocation is a positively charged carbon atom that has three alkyl groups attached to it, making it a highly stable intermediate in organic reactions. This term is crucial in understanding various topics related to electrophilic additions, carbocation stability, and reaction mechanisms.
Thermodynamics: Thermodynamics is the study of the relationships between heat, work, temperature, and energy. It describes the transformations of energy and the direction of these transformations, which is crucial for understanding chemical reactions and biological processes.
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.