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Chapter 16

Dienes, Conjugated Pi Systems, and Pericyclic Reactions

Structure of Conjugated Dienes
Structure of Conjugated Dienes
Introduction Conjugated dienes are compounds characterized by the presence of alternating double and single bonds. In a conjugated system like ...
Stability of Conjugated Dienes
Stability of Conjugated Dienes
Introduction A comparison of the enthalpies of hydrogenation of dienes reveals that conjugated dienes release less heat on hydrogenation, rendering them ...
π Molecular Orbitals of 1,3-Butadiene
π Molecular Orbitals of 1,3-Butadiene
Conjugated dienes have lower heats of hydrogenation than cumulated and isolated dienes, making them more stable. The enhanced stabilization of conjugated ...
π Molecular Orbitals of the Allyl Cation and Anion
π Molecular Orbitals of the Allyl Cation and Anion
An allyl group is a three-carbon conjugated system where the sp³-hybridized allylic carbon is bonded to a CH=CH2 group via a single bond. Allyl ...
π Molecular Orbitals of the Allyl Radical
π Molecular Orbitals of the Allyl Radical
Allyl radicals are three-carbon conjugated systems. They are readily formed as intermediates in halogenation reactions of alkenes involving the addition ...
Electrophilic 1,2- and 1,4-Addition of HX to 1,3-Butadiene
Electrophilic 1,2- and 1,4-Addition of HX to 1,3-Butadiene
The electrophilic addition of hydrogen halides such as HBr to alkenes and nonconjugated dienes gives a single product as per Markovnikov’s rule. ...
Electrophilic 1,2- and 1,4-Addition of X<sub>2</sub> to 1,3-Butadiene
Electrophilic 1,2- and 1,4-Addition of X2 to 1,3-Butadiene
Electrophilic addition of halogens to alkenes proceeds via a cyclic halonium ion to form a 1,2-dihalide or a vicinal dihalide. Conjugated dienes react ...
Electrophilic Addition of HX to 1,3-Butadiene: Thermodynamic vs Kinetic Control
Electrophilic Addition of HX to 1,3-Butadiene: Thermodynamic vs Kinetic Control
The addition of a hydrogen halide to 1,3-butadiene gives a mixture of 1,2- and 1,4-adducts. Since more substituted alkenes are more stable, the 1,4-adduct ...
UV&ndash;Vis Spectroscopy of Conjugated Systems
UV–Vis Spectroscopy of Conjugated Systems
Organic compounds with conjugated double bonds show strong absorption features in the UV–visible region of the electromagnetic spectrum attributed ...
UV&ndash;Vis Spectroscopy: Woodward&ndash;Fieser Rules
UV–Vis Spectroscopy: Woodward–Fieser Rules
UV–Visible absorption spectra of conjugated dienes arise from the lowest energy π → π* transitions. The ...
Pericyclic Reactions: Introduction
Pericyclic Reactions: Introduction
Pericyclic reactions are organic reactions that occur via a concerted mechanism without generating any intermediates. The reactions proceed through the ...
Thermal and Photochemical Electrocyclic Reactions: Overview
Thermal and Photochemical Electrocyclic Reactions: Overview
Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two ...
Thermal Electrocyclic Reactions: Stereochemistry
Thermal Electrocyclic Reactions: Stereochemistry
The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction ...
Photochemical Electrocyclic Reactions: Stereochemistry
Photochemical Electrocyclic Reactions: Stereochemistry
The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. ...
Cycloaddition Reactions: Overview
Cycloaddition Reactions: Overview
Cycloadditions are one of the most valuable and effective synthesis routes to form cyclic compounds. These are concerted pericyclic reactions between two ...
Cycloaddition Reactions: MO Requirements for Thermal Activation
Cycloaddition Reactions: MO Requirements for Thermal Activation
Thermal cycloadditions are reactions where the source of activation energy needed to initiate the reaction is provided in the form of heat. A typical ...
Cycloaddition Reactions: MO Requirements for Photochemical Activation
Cycloaddition Reactions: MO Requirements for Photochemical Activation
Some cycloaddition reactions are activated by heat, while others are initiated by light. For example, a [2 + 2] cycloaddition between two ethylene ...
[4+2] Cycloaddition of Conjugated Dienes: Diels&ndash;Alder Reaction
[4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction
The Diels–Alder reaction is an example of a thermal pericyclic reaction between a conjugated diene and an alkene or alkyne, commonly referred to as ...
Diels&ndash;Alder vs Retro-Diels&ndash;Alder Reaction: Thermodynamic Factors
Diels–Alder vs Retro-Diels–Alder Reaction: Thermodynamic Factors
The Diels–Alder reaction is thermally reversible, meaning that the reaction reverts to the starting diene and dienophile under suitable ...
Diels&ndash;Alder Reaction Forming Cyclic Products: Stereochemistry
Diels–Alder Reaction Forming Cyclic Products: Stereochemistry
The Diels–Alder reaction is one of the robust methods for synthesizing unsaturated six-membered rings. The reaction involves a concerted cyclic ...
Diels&ndash;Alder Reaction Forming Bridged Bicyclic Products: Stereochemistry
Diels–Alder Reaction Forming Bridged Bicyclic Products: Stereochemistry
Diels–Alder reactions between cyclic dienes locked in an s-cis configuration and dienophiles yield bridged bicyclic products. Dienophiles with one ...
Diels&ndash;Alder Reaction: Characteristics of Dienes
Diels–Alder Reaction: Characteristics of Dienes
The Diels–Alder reaction brings together a diene and a dienophile to form a six-membered ring. Both components have unique characteristics that ...
Diels&ndash;Alder Reaction: Characteristics of Dienophiles
Diels–Alder Reaction: Characteristics of Dienophiles
In a Diels–Alder reaction, the diene is usually an electron-rich system and acts as a nucleophile, whereas the dienophile is electron-deficient and ...
Thermal Sigmatropic Reactions: Overview
Thermal Sigmatropic Reactions: Overview
Sigmatropic rearrangements are a class of pericyclic reactions in which a σ bond migrates from one part of a π system to another. These are ...
[3,3] Sigmatropic Rearrangement of 1,5-Dienes: Cope Rearrangement
[3,3] Sigmatropic Rearrangement of 1,5-Dienes: Cope Rearrangement
The Cope rearrangement is classified as a [3,3] sigmatropic shift in 1,5-dienes, leading to a more stable, isomeric 1,5-diene. The reaction involves a ...
[3,3] Sigmatropic Rearrangement of Allyl Vinyl Ethers: Claisen Rearrangement
[3,3] Sigmatropic Rearrangement of Allyl Vinyl Ethers: Claisen Rearrangement
The Claisen rearrangement is a [3,3] sigmatropic rearrangement of allyl vinyl ethers to unsaturated carbonyl compounds. The rearrangement is a concerted ...
Woodward&ndash;Hoffmann Selection Rules and Microscopic Reversibility
Woodward–Hoffmann Selection Rules and Microscopic Reversibility
Electrocyclic reactions, cycloadditions, and sigmatropic rearrangements are concerted pericyclic reactions that proceed via a cyclic transition state. ...
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