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Organic Chemistry
Table of Contents
Unit 14 Overview: Conjugated Systems and UV Spectroscopy
14.1 Stability of Conjugated Dienes: Molecular Orbital Theory
14.2 Electrophilic Additions to Conjugated Dienes: Allylic Carbocations
14.3 Kinetic versus Thermodynamic Control of Reactions
14.4 The Diels–Alder Cycloaddition Reaction
14.5 Characteristics of the Diels–Alder Reaction
14.6 Diene Polymers: Natural and Synthetic Rubbers
14.7 Ultraviolet Spectroscopy
14.8 Interpreting Ultraviolet Spectra: The Effect of Conjugation
14.9 Conjugation, Color, and the Chemistry of Vision

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14.2 Electrophilic Additions to Conjugated Dienes: Allylic Carbocations

Citation:

Conjugated dienes are fascinating molecules with two double bonds separated by a single bond. This unique structure allows for electrophilic additions at both 1,2 and 1,4 positions, creating a mix of products through resonance-stabilized allylic carbocations.

The major product in these reactions depends on carbocation stability and steric factors. Conjugated dienes are more reactive than simple alkenes due to their extended π system. Kinetic and thermodynamic control can influence product distribution, adding complexity to these reactions.

Electrophilic Additions to Conjugated Dienes

Formation of addition products

  • Conjugated dienes contain two double bonds separated by a single bond allowing for electrophilic addition at the 1,2 and 1,4 positions
    • 1,2 addition occurs when the electrophile adds to the carbon adjacent to the double bond forming a carbocation intermediate (3-bromo-1-butene)
    • 1,4 addition occurs when the electrophile adds to the carbon atom two positions away from the initial double bond forming an allylic carbocation intermediate (1-bromo-2-butene)
  • Reaction proceeds through a resonance-stabilized allylic carbocation intermediate where the positive charge is delocalized over three carbon atoms increasing stability
  • Nucleophile can attack either end of the allylic carbocation leading to a mixture of 1,2 and 1,4 addition products with the ratio depending on the stability of the resulting carbocations and steric factors
  • The regioselectivity of the reaction is influenced by the relative stability of the carbocation intermediates and reaction conditions

Prediction of major products

  • Major product is determined by the relative stability of the carbocation intermediates formed during the reaction with more stable carbocations being favored
  • Carbocation stability follows the order: tertiary > secondary > primary
    • Tertiary carbocations are the most stable due to hyperconjugation and inductive effects
  • Steric factors also influence product distribution as bulky substituents can hinder the approach of the nucleophile favoring less sterically hindered products
  • Addition of HBr to 1,3-butadiene:
    1. The 1,2 addition product (3-bromo-1-butene) is favored because it forms a more stable secondary carbocation intermediate
    2. The 1,4 addition product (1-bromo-2-butene) is minor because it forms a less stable primary carbocation intermediate

Reactivity of dienes vs alkenes

  • Conjugated dienes are more reactive than simple alkenes in electrophilic addition reactions because the extended $\pi$ system allows for greater delocalization of the positive charge in the carbocation intermediate increasing stability and lowering the activation energy
  • Conjugated dienes can form resonance-stabilized allylic carbocations which are more stable than the carbocations formed from simple alkenes leading to faster reaction rates and milder reaction conditions
  • Conjugated dienes can give a mixture of 1,2 and 1,4 addition products while simple alkenes only form 1,2 addition products with the product distribution in conjugated dienes depending on the stability of the resulting carbocations and steric factors
  • Simple alkenes typically react with electrophiles to form the more stable carbocation intermediate following Markovnikov's rule but in conjugated dienes the extended conjugation can override Markovnikov's rule leading to a mixture of products

Kinetic vs Thermodynamic Control

  • The product distribution in electrophilic additions to conjugated dienes can be influenced by kinetic or thermodynamic control
  • Kinetic control favors the formation of products through the lowest energy transition state, often resulting in the faster-forming product
  • Thermodynamic control leads to the most stable product, which may differ from the kinetically favored product
  • Hammond's postulate helps predict the structure of the transition state in these reactions, relating it to the stability of nearby intermediates
  • Understanding of frontier molecular orbitals can provide insights into the reactivity and selectivity of conjugated dienes in electrophilic addition reactions

Key Terms to Review (34)

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.
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.
Markovnikov’s rule: Markovnikov's rule predicts the outcome of the electrophilic addition of hydrogen halides to alkenes, stating that the hydrogen atom will attach to the carbon with more hydrogen atoms, and the halide will attach to the more substituted carbon. This rule helps in determining the major product of addition reactions in organic chemistry.
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.
Resonance: Resonance is a fundamental concept in organic chemistry that describes the ability of certain molecules to exist in multiple equivalent structures or resonance forms. This phenomenon arises from the delocalization of electrons within the molecule, leading to the stabilization of the overall structure and the distribution of electron density across multiple atoms.
Nucleophilic Attack: Nucleophilic attack is a fundamental chemical reaction in which a nucleophile, an electron-rich species, attacks an electrophilic (electron-deficient) center, forming a new covalent bond. This process is central to understanding many organic reactions, including polar reactions, addition reactions, and substitution reactions.
Hyperconjugation: Hyperconjugation is a type of conjugation in organic chemistry where the sigma bonds of alkyl groups (such as methyl or ethyl) interact with adjacent pi bonds, leading to increased stability of the molecule. This stabilizing effect is particularly important in understanding the stability of carbocations and the orientation of electrophilic additions.
Protonation: Protonation is the process of adding a proton (H+) to a molecule or atom, resulting in the formation of a positively charged species. This fundamental chemical reaction is central to various organic chemistry topics, as it can significantly influence the reactivity and stability of molecules.
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.
Regioselectivity: Regioselectivity refers to the preference of a chemical reaction to occur at a specific site or region of a molecule, leading to the formation of one regioisomeric product over another. This concept is particularly important in the context of electrophilic addition reactions of alkenes, electrophilic aromatic substitution, and other organic transformations.
HBr: HBr, or hydrobromic acid, is a strong acid composed of hydrogen (H) and bromine (Br). It is an important reagent in organic chemistry, commonly used in various reactions and processes, including the addition of HBr to alkenes, the preparation of alkyl halides from alcohols, and electrophilic additions to conjugated dienes.
Markovnikov: Markovnikov is a principle that describes the preferred regiochemistry of electrophilic addition reactions to unsymmetrical alkenes. It states that in the addition of an unsymmetrical reagent, such as a hydrogen halide, to an alkene, the electrophilic portion of the reagent will add to the carbon of the alkene that can best stabilize the resulting carbocation intermediate.
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.
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.
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.
1,3-Butadiene: 1,3-Butadiene is a simple conjugated diene, composed of four carbon atoms with two carbon-carbon double bonds separated by a single carbon-carbon bond. This structural feature gives 1,3-butadiene unique chemical properties and reactivity that are important in various organic chemistry topics.
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.
Carbocation Stability: Carbocations are positively charged carbon atoms that are formed as intermediates in many organic reactions. The stability of a carbocation is a crucial factor in determining the mechanism and outcome of these reactions. Carbocation stability is a key concept that connects various topics in organic chemistry, including electrophilic additions, the SN1 reaction, and the reactivity of conjugated dienes.
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.
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.
3-bromo-1-butene: 3-bromo-1-butene is an organic compound with a bromine atom attached to the third carbon of a four-carbon alkene chain. This term is relevant in the context of understanding the stability of the allyl radical and the electrophilic additions to conjugated dienes, which involve the formation of allylic carbocations.
1-bromo-2-butene: 1-bromo-2-butene is an organic compound with the formula CH3CH=CHCH2Br. It is a halogenated alkene that is important in the context of understanding the stability of the allyl radical and the electrophilic additions to conjugated dienes, specifically the formation of allylic carbocations.
Allylic Position: The allylic position refers to the carbon atom adjacent to a carbon-carbon double bond. This term is particularly relevant in the context of electrophilic additions to conjugated dienes, where the allylic position plays a crucial role in the formation of allylic carbocations.
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.
Allylic Carbocations: An allylic carbocation is a positively charged carbon atom that is adjacent to a carbon-carbon double bond. These reactive intermediates are formed during electrophilic addition reactions to conjugated dienes and play a crucial role in determining the products of these transformations.
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.
Diene: A diene is a hydrocarbon compound that contains two carbon-carbon double bonds. Dienes are important in the context of various organic chemistry topics, including electrophilic additions to conjugated dienes, the Diels-Alder reaction, cycloaddition reactions, and pericyclic reactions.
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.
π-complex: A π-complex, also known as a pi-complex, is a type of molecular interaction that occurs between an electron-rich aromatic system, such as a benzene ring, and an electrophilic species. This interaction involves the delocalized π-electrons of the aromatic system and the electrophile, forming a stabilized intermediate.
Vinylic Position: The vinylic position refers to the carbon-carbon double bond in a conjugated diene system. It is a key structural feature that influences the reactivity and products of electrophilic additions to these types of compounds.
σ-complex: A σ-complex, also known as a sigma complex, is an intermediate species formed during electrophilic addition reactions to conjugated dienes. It is characterized by the formation of a three-membered ring structure that stabilizes the allylic carbocation, a key step in the mechanism of these reactions.
Kinetic Product: The kinetic product refers to the product that is formed in a chemical reaction as a result of the fastest or most favorable pathway, rather than the thermodynamically most stable product. It is an important concept in understanding the outcome of electrophilic additions to conjugated dienes and the formation of allylic carbocations.
Thermodynamic Product: The thermodynamic product refers to the most stable and energetically favorable product formed in a chemical reaction. It is the product that is favored by thermodynamics, or the overall decrease in free energy of the system, rather than the kinetics of the reaction.
Frontier Molecular Orbitals: Frontier molecular orbitals refer to the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in a molecule. These orbitals play a crucial role in understanding and predicting the reactivity and behavior of molecules, particularly in the context of electrophilic additions to conjugated dienes, the Diels-Alder cycloaddition reaction, electrocyclic reactions, and photochemical electrocyclic reactions.