6.10 Describing a Reaction: Intermediates
3 min read•may 7, 2024
are key players in organic chemistry, forming between the starting material and final product in multistep reactions. They're often reactive species like or , crucial for understanding reaction mechanisms and predicting outcomes.
visually represent the energetics of reactions, showing the relative potential energies of reactants, products, intermediates, and . These diagrams help identify rate-determining steps and overall activation energies, linking and in reaction analysis.
Reaction Intermediates and Energy Diagrams
Reaction intermediates in organic chemistry
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- Reaction intermediates form after the starting material and before the final product in a multistep reaction
- Formed from the starting material and then react further to eventually yield the final product (alkene reacting with bromine to form a intermediate)
- Not isolated because they react quickly to form the next intermediate or product
- In a multistep reaction, there can be one or more intermediates involved
- Number of intermediates always one less than the number of steps in the reaction (two-step reaction has one intermediate)
- Intermediates often reactive species such as carbocations, carbanions, or
- Can also be neutral molecules formed as a result of the first step and then undergo further reaction ( formed from addition of water to a )
- Understanding intermediates is crucial for elucidating the
Energy diagrams for two-step reactions
- Energy diagrams show the relative potential energies of the reactants, products, intermediates, and transition states in a reaction
- X-axis represents the or progress of the reaction
- Y-axis represents the potential energy of the species involved
- In a two-step reaction, there will be two transition states and one intermediate
- First transition state occurs between the reactants and the intermediate
- Second transition state occurs between the intermediate and the products
- Intermediate appears as a valley or local minimum on the energy diagram
- Has lower potential energy than the transition states but higher potential energy than the reactants and products (carbocation intermediate in reaction)
- Transition states appear as peaks or local maxima on the energy diagram
- Represent the highest energy species in the reaction and the point at which bonds are breaking and forming (transition state for formation of carbocation from leaving group departure)
- Overall () for the two-step reaction is the difference in potential energy between the reactants and the highest energy transition state
- Overall change in () for the reaction is the difference in potential energy between the reactants and the products
- Energy diagrams can be viewed as a two-dimensional representation of the
Activation energy in multistep reactions
- Overall activation energy () for a multistep reaction determined by the highest energy transition state
- Represents the minimum energy required for the reaction to proceed from reactants to products
- Step with the highest activation energy is the rate-determining step because it limits the overall rate of the reaction (formation of carbocation in SN1)
- Activation energy for each individual step can be determined from the energy diagram
- Difference in potential energy between the intermediate (or reactant for the first step) and the transition state for that step
- Overall change in free energy () for a multistep reaction is the sum of the free energy changes for each individual step
- , where n is the number of steps
- Sign and magnitude of for each step determine whether that step is thermodynamically favorable () or unfavorable ()
- Overall determines whether the entire reaction is thermodynamically favorable or unfavorable (hydrolysis of an ester is thermodynamically favorable with a negative )
Kinetics and Thermodynamics in Reaction Analysis
- Kinetics focuses on the rate of chemical reactions and the factors that influence it
- Closely related to the activation energy and transition states in the
- Thermodynamics deals with the energy changes and spontaneity of chemical reactions
- Determines the overall feasibility and direction of a reaction
- relates the structure of a transition state to the structures of nearby intermediates or products
- Useful for predicting the outcome of reactions based on their energy profiles
Key Terms to Review (21)
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.
Bromonium ion: A bromonium ion is a reactive intermediate formed during the halogenation of alkenes when a bromine molecule reacts with an alkene to form a cyclic structure where the bromine atom is covalently bonded to two carbon atoms. This ion is positively charged and highly electrophilic, making it susceptible to nucleophilic attack.
Bromonium Ion: The bromonium ion is a cyclic, three-membered ring intermediate formed during the addition of hydrobromic acid (HBr) or bromine (Br2) to alkenes. It serves as a key intermediate in various organic reactions involving the electrophilic addition of bromine to alkenes.
Carbanions: A carbanion is a negatively charged carbon atom with three bonds and a lone pair of electrons. These reactive intermediates play a crucial role in various organic chemistry reactions and concepts, including formal charges, reaction intermediates, alkyne acidity, and Grignard reagents.
Carbocations: Carbocations are positively charged carbon-centered species that serve as key intermediates in many organic reactions. They are formed when a carbon atom loses a bonded electron, resulting in a deficiency of electrons and a positive charge on the carbon.
Energy Diagrams: Energy diagrams are graphical representations that illustrate the changes in energy that occur during a chemical reaction. They provide a visual depiction of the energy profile of a reaction, including the relative energies of reactants, products, and any intermediate species involved.
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.
Halohydrin: A halohydrin is a compound containing both a halogen atom (such as chlorine, bromine, or iodine) and a hydroxyl group (-OH) on adjacent carbon atoms. These compounds are formed through the addition of a hydrogen halide (HX, where X is a halogen) to an alkene, resulting in the incorporation of both the halogen and the hydroxyl group.
Hammond's Postulate: Hammond's postulate is a fundamental concept in organic chemistry that describes the relationship between the structure and reactivity of reaction intermediates. It provides a framework for understanding the stability and reactivity of various intermediates that can form during the course of a chemical reaction.
Kinetics: Kinetics is the study of the rates and mechanisms of chemical reactions. It examines how quickly reactions occur and the factors that influence the speed of a reaction, such as temperature, pressure, and the presence of catalysts.
Potential Energy Surface: The potential energy surface is a graphical representation that depicts the potential energy of a chemical system as a function of the positions of its atoms or molecules. It provides a visual framework for understanding the energetics and dynamics of chemical reactions.
Radicals: Radicals are highly reactive species that contain one or more unpaired electrons. They play a crucial role in various chemical reactions, including those described in the topics of 'Describing a Reaction: Intermediates' and 'Uses of 13C NMR Spectroscopy'.
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 Intermediates: Reaction intermediates are unstable, short-lived species that are formed as part of a chemical reaction mechanism. They are important in understanding the step-by-step process of how a reaction occurs and the factors that influence its rate and outcome.
Reaction mechanism: A reaction mechanism is a step-by-step sequence of elementary reactions by which overall chemical change occurs. It outlines the specific way in which reactants convert to products, including the formation and breaking of bonds.
Reaction Mechanism: A reaction mechanism is the step-by-step sequence of elementary reactions by which overall chemical change occurs. It describes the detailed pathway that a reaction follows, including the formation and rearrangement of chemical bonds, the generation of intermediates, and the movement of electrons. Understanding reaction mechanisms is crucial for predicting the products of a reaction, explaining reactivity trends, and designing new synthetic pathways.
SN1: SN1, or Nucleophilic Substitution Reaction, is a type of organic reaction mechanism in which a nucleophile attacks a neutral, trigonal planar carbocation intermediate to displace a leaving group, resulting in the substitution of one functional group for another. This mechanism is characterized by a stepwise process involving the formation of a carbocation intermediate.
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 States: Transition states are short-lived, high-energy molecular configurations that represent the point of maximum energy along the reaction coordinate during a chemical transformation. They are critical in understanding the kinetics and mechanisms of organic reactions.