Name: Cycloaddition Reactions: MO Requirements for Photochemical Activation
Uploaded: 2022-10-11T10:04:29-04:00
Description: Some cycloaddition reactions are activated by heat, while others are initiated by light. For example, a [2 + 2] cycloaddition between two ethylene molecules occurs only in the presence of light.

Chapter 16: Dienes, Conjugated Pi Systems, and Pericyclic Reactions Back To Chapter

16.17: Cycloaddition Reactions: MO Requirements for Photochemical Activation

TABLE OF
CONTENTS

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Chapter 1: Covalent Bonding and Structure

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1.1: What is Organic Chemistry?
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1.2: Electronic Structure of Atoms
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1.3: Electron Configurations
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1.4: Chemical Bonds
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1.5: Polar Covalent Bonds
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1.6: Lewis Structures and Formal Charges
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1.7: VSEPR Theory
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1.8: Molecular Geometry and Dipole Moments
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1.9: Resonance and Hybrid Structures
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1.10: Valence Bond Theory and Hybridized Orbitals
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1.11: MO Theory and Covalent Bonding
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1.12: Intermolecular Forces and Physical Properties
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1.13: Solubility
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1.14: Introduction to Functional Groups
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1.15: Overview of Advanced Functional Groups

Chapter 2: Thermodynamics and Chemical Kinetics

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2.1: Chemical Reactions
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2.2: Enthalpy and Heat of Reaction
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2.3: Energetics of Solution Formation
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2.4: Entropy and Solvation
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2.5: Gibbs Free Energy and Thermodynamic Favorability
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2.6: Chemical and Solubility Equilibria
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2.7: Rate Law and Reaction Order
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2.8: Effect of Temperature Change on Reaction Rate
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2.9: Multi-Step Reactions
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2.10: Bond Dissociation Energy and Activation Energy
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2.11: Energy Diagrams, Transition States, and Intermediates
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2.12: Predicting Reaction Outcomes

Chapter 3: Alkanes and Cycloalkanes

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3.1: Structure of Alkanes
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3.2: Constitutional Isomers of Alkanes
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3.3: Nomenclature of Alkanes
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3.4: Physical Properties of Alkanes
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3.5: Newman Projections
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3.6: Conformations of Ethane and Propane
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3.7: Conformations of Butane
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3.8: Cycloalkanes
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3.9: Conformations of Cycloalkanes
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3.10: Conformations of Cyclohexane
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3.11: Chair Conformation of Cyclohexane
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3.12: Stability of Substituted Cyclohexanes
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3.13: Disubstituted Cyclohexanes: cis-trans Isomerism
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3.14: Combustion Energy: A Measure of Stability in Alkanes and Cycloalkanes

Chapter 4: Stereoisomerism

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4.1: Chirality
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4.2: Isomerism
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4.3: Stereoisomers
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4.4: Naming Enantiomers
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4.5: Properties of Enantiomers and Optical Activity
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4.6: Molecules with Multiple Chiral Centers
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4.7: Fischer Projections
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4.8: Racemic Mixtures and the Resolution of Enantiomers
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4.9: Stereoisomerism of Cyclic Compounds
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4.10: Chirality at Nitrogen, Phosphorus, and Sulfur
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4.11: Prochirality
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4.12: Chirality in Nature

Chapter 5: Acids and Bases

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5.1: Brønsted-Lowry Acids and Bases
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5.2: Lewis Acids and Bases
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5.3: Acid and Bases: Ka, pKa, and Relative Strengths
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5.4: Position of Equilibrium in Acid-Base Reactions
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5.5: Molecular Structure and Acidity
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5.6: Leveling Effect and Non-Aqueous Acid-Base Solutions
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5.7: Solvating Effects

Chapter 6: Nucleophilic Substitution and Elimination Reactions of Alkyl Halides

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6.1: Alkyl Halides
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6.2: Nucleophilic Substitution Reactions
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6.3: Nucleophiles
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6.4: Electrophiles
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6.5: Leaving Groups
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6.6: Carbocations
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6.7: SN2 Reaction: Kinetics
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6.8: SN2 Reaction: Mechanism
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6.9: SN2 Reaction: Transition State
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6.10: SN2 Reaction: Stereochemistry
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6.11: SN1 Reaction: Kinetics
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6.12: SN1 Reaction: Mechanism
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6.13: SN1 Reaction: Stereochemistry
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6.14: Predicting Products: SN1 vs. SN2
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6.15: Elimination Reactions
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6.16: E2 Reaction: Kinetics and Mechanism
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6.17: E2 Reaction: Stereochemistry and Regiochemistry
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6.18: E1 Reaction: Kinetics and Mechanism
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6.19: E1 Reaction: Stereochemistry and Regiochemistry
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6.20: Predicting Products: Substitution vs. Elimination

Chapter 7: Alkene Structure and Reactivity

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7.1: Structure and Bonding of Alkenes
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7.2: Nomenclature of Alkenes
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7.3: Degree of Unsaturation
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7.4: Isomerism in Alkenes
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7.5: Relative Stabilities of Alkenes
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7.6: Introduction to Electrophilic Addition Reactions of Alkenes
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7.7: Regioselectivity of Electrophilic Additions to Alkenes: Markovnikov's Rule

Chapter 8: Reactions of Alkenes

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8.1: Regioselectivity of Electrophilic Additions-Peroxide Effect
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8.2: Free-Radical Chain Reaction and Polymerization of Alkenes
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8.3: Halogenation of Alkenes
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8.4: Formation of Halohydrin from Alkenes
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8.5: Acid-Catalyzed Hydration of Alkenes
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8.6: Regioselectivity and Stereochemistry of Acid-Catalyzed Hydration
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8.7: Oxymercuration-Reduction of Alkenes
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8.8: Hydroboration-Oxidation of Alkenes
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8.9: Regioselectivity and Stereochemistry of Hydroboration
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8.10: Oxidation of Alkenes: Syn Dihydroxylation with Osmium Tetraoxide
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8.11: Oxidation of Alkenes: Syn Dihydroxylation with Potassium Permanganate
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8.12: Oxidation of Alkenes: Anti Dihydroxylation with Peroxy Acids
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8.13: Oxidative Cleavage of Alkenes: Ozonolysis
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8.14: Reduction of Alkenes: Catalytic Hydrogenation
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8.15: Reduction of Alkenes: Asymmetric Catalytic Hydrogenation

Chapter 9: Alkynes

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9.1: Structure and Physical Properties of Alkynes
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9.2: Nomenclature of Alkynes
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9.3: Acidity of 1-Alkynes
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9.4: Preparation of Alkynes: Alkylation Reaction
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9.5: Preparation of Alkynes: Dehydrohalogenation
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9.6: Electrophilic Addition to Alkynes: Halogenation
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9.7: Electrophilic Addition to Alkynes: Hydrohalogenation
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9.8: Alkynes to Aldehydes and Ketones: Acid-Catalyzed Hydration
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9.9: Alkynes to Aldehydes and Ketones: Hydroboration-Oxidation
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9.10: Alkynes to Carboxylic Acids: Oxidative Cleavage
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9.11: Reduction of Alkynes to cis-Alkenes: Catalytic Hydrogenation
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9.12: Reduction of Alkynes to trans-Alkenes: Sodium in Liquid Ammonia

Chapter 10: Alcohols and Phenols

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10.1: Structure and Nomenclature of Alcohols and Phenols
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10.2: Physical Properties of Alcohols and Phenols
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10.3: Acidity and Basicity of Alcohols and Phenols
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10.4: Preparation of Alcohols via Addition Reactions
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10.5: Preparation of Alcohols via Substitution Reactions
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10.6: Acid-Catalyzed Dehydration of Alcohols to Alkenes
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10.7: Alcohols from Carbonyl Compounds: Reduction
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10.8: Alcohols from Carbonyl Compounds: Grignard Reaction
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10.9: Protection of Alcohols
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10.10: Preparation of Diols and Pinacol Rearrangement
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10.11: Conversion of Alcohols to Alkyl Halides
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10.12: Oxidation of Alcohols

Chapter 11: Ethers, Epoxides, Sulfides

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11.1: Structure and Nomenclature of Ethers
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11.2: Physical Properties of Ethers
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11.3: Ethers from Alcohols: Alcohol Dehydration and Williamson Ether Synthesis
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11.4: Ethers from Alkenes: Alcohol Addition and Alkoxymercuration-Demercuration
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11.5: Ethers to Alkyl Halides: Acidic Cleavage
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11.6: Autoxidation of Ethers to Peroxides and Hydroperoxides
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11.7: Crown Ethers
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11.8: Structure and Nomenclature of Epoxides
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11.9: Preparation of Epoxides
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11.10: Sharpless Epoxidation
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11.11: Acid-Catalyzed Ring-Opening of Epoxides
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11.12: Base-Catalyzed Ring-Opening of Epoxides
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11.13: Structure and Nomenclature of Thiols and Sulfides
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11.14: Preparation and Reactions of Thiols
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11.15: Preparation and Reactions of Sulfides

Chapter 12: Aldehydes and Ketones

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12.1: Structures of Aldehydes and Ketones
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12.3: IUPAC Nomenclature of Aldehydes
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12.4: IUPAC Nomenclature of Ketones
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12.5: Common Names of Aldehydes and Ketones
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12.6: IR and UV–Vis Spectroscopy of Aldehydes and Ketones
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12.7: NMR Spectroscopy and Mass Spectrometry of Aldehydes and Ketones
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12.8: Preparation of Aldehydes and Ketones from Alcohols, Alkenes, and Alkynes
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12.9: Preparation of Aldehydes and Ketones from Nitriles and Carboxylic Acids
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12.10: Preparation of Aldehydes and Ketones from Carboxylic Acid Derivatives
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12.13: Nucleophilic Addition to the Carbonyl Group: General Mechanism
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12.14: Aldehydes and Ketones with Water: Hydrate Formation
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12.15: Aldehydes and Ketones with Alcohols: Hemiacetal Formation
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12.18: Protecting Groups for Aldehydes and Ketones: Introduction
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12.19: Acetals and Thioacetals as Protecting Groups for Aldehydes and Ketones
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12.20: Aldehydes and Ketones with HCN: Cyanohydrin Formation Overview
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12.21: Aldehydes and Ketones with HCN: Cyanohydrin Formation Mechanism
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12.22: Aldehydes and Ketones to Alkenes: Wittig Reaction Overview
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12.23: Aldehydes and Ketones to Alkenes: Wittig Reaction Mechanism
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12.24: Aldehydes and Ketones with Amines: Imine and Enamine Formation Overview
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12.25: Aldehydes and Ketones with Amines: Imine Formation Mechanism
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12.26: Aldehydes and Ketones with Amines: Enamine Formation Mechanism
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12.29: Aldehydes and Ketones to Alkanes: Wolff–Kishner Reduction
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12.30: Oxidations of Aldehydes and Ketones to Carboxylic Acids
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12.31: Reactions of Aldehydes and Ketones: Baeyer–Villiger Oxidation
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12.33: Keto–Enol Tautomerism: Mechanism

Chapter 13: Carboxylic Acids

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13.1: IUPAC Nomenclature of Carboxylic Acids
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13.4: Physical Properties of Carboxylic Acids
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13.5: Acidity of Carboxylic Acids
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13.6: Substituent Effects on Acidity of Carboxylic Acids
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13.7: IR and UV–Vis Spectroscopy of Carboxylic Acids
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13.8: NMR and Mass Spectroscopy of Carboxylic Acids
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13.9: Preparation of Carboxylic Acids: Overview
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13.10: Preparation of Carboxylic Acids: Hydrolysis of Nitriles
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13.11: Preparation of Carboxylic Acids: Carboxylation of Grignard Reagents
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13.12: Reactions of Carboxylic Acids: Introduction
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13.13: Carboxylic Acids to Esters: Acid-Catalyzed (Fischer) Esterification Overview
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13.14: Carboxylic Acids to Esters: Acid-Catalyzed (Fischer) Esterification Mechanism
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13.15: Carboxylic Acids to Methylesters: Alkylation using Diazomethane
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13.16: Carboxylic Acids to Acid Chlorides
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13.17: Carboxylic Acids to Primary Alcohols: Hydride Reduction
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13.18: Loss of Carboxy Group as CO2: Decarboxylation of β-Ketoacids
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13.19: Loss of Carboxy Group as CO2: Decarboxylation of Malonic Acid Derivatives

Chapter 14: Carboxylic Acid Derivatives

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14.1: Carboxylic Acid Derivatives: Overview
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14.2: Nomenclature of Carboxylic Acid Derivatives: Acid Halides, Esters, and Acid Anhydrides
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14.3: Nomenclature of Carboxylic Acid Derivatives: Amides and Nitriles
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14.4: Structures of Carboxylic Acid Derivatives
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14.5: Physical Properties of Carboxylic Acid Derivatives
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14.6: Acidity and Basicity of Carboxylic Acid Derivatives
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14.7: Spectroscopy of Carboxylic Acid Derivatives
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14.8: Relative Reactivity of Carboxylic Acid Derivatives
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14.9: Nucleophilic Acyl Substitution of Carboxylic Acid Derivatives
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14.10: Acid Halides to Carboxylic Acids: Hydrolysis
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14.11: Acid Halides to Esters: Alcoholysis
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14.12: Acid Halides to Amides: Aminolysis
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14.13: Acid Halides to Alcohols: LiAlH4 Reduction
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14.14: Acid Halides to Alcohols: Grignard Reaction
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14.15: Acid Halides to Ketones: Gilman Reagent
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14.16: Preparation of Acid Anhydrides
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14.17: Reactions of Acid Anhydrides
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14.18: Esters to Carboxylic Acids: Saponification
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14.19: Esters to Carboxylic Acids: Acid-Catalyzed Hydrolysis
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14.20: Esters to Alcohols: Hydride Reductions
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14.21: Esters to Alcohols: Grignard Reaction
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14.22: Preparation of Amides
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14.23: Amides to Carboxylic Acids: Hydrolysis
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14.24: Amides to Amines: LiAlH4 Reduction
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14.25: Preparation of Nitriles
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14.26: Nitriles to Carboxylic Acids: Hydrolysis
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14.27: Nitriles to Ketones: Grignard Reaction
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14.28: Nitriles to Amines: LiAlH4 Reduction

Chapter 15: α-Carbon Chemistry: Enols, Enolates, and Enamines

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15.1: Reactivity of Enols
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15.2: Reactivity of Enolate Ions
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15.3: Types of Enols and Enolates
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15.4: Enolate Mechanism Conventions
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15.5: Regioselective Formation of Enolates
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15.6: Stereochemical Effects of Enolization
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15.7: Acid-Catalyzed α-Halogenation of Aldehydes and Ketones
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15.8: Base-Promoted α-Halogenation of Aldehydes and Ketones
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15.9: Multiple Halogenation of Methyl Ketones: Haloform Reaction
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15.10: α-Halogenation of Carboxylic Acid Derivatives: Overview
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15.11: α-Bromination of Carboxylic Acids: Hell–Volhard–Zelinski Reaction
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15.12: Reactions of α-Halocarbonyl Compounds: Nucleophilic Substitution
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15.13: Nitrosation of Enols
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15.14: C–C Bond Formation: Aldol Condensation Overview
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15.15: Base-Catalyzed Aldol Addition Reaction
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15.16: Acid-Catalyzed Aldol Addition Reaction
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15.17: Dehydration of Aldols to Enals: Base-Catalyzed Aldol Condensation
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15.18: Dehydration of Aldols to Enones: Acid-Catalyzed Aldol Condensation
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15.19: Intramolecular Aldol Reaction
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15.20: C–C Bond Cleavage: Retro-Aldol Reaction
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15.21: Crossed Aldol Reactions: Overview
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15.22: Crossed Aldol Reaction Using Weak Bases
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15.23: Ketones with Nonenolizable Aromatic Aldehydes: Claisen–Schmidt Condensation
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15.24: Crossed Aldol Reaction Using Strong Bases: Directed Aldol Reaction
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15.26: Aldol Condensation with β-Diesters: Knoevenagel Condensation
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15.27: Esters to β-Ketoesters: Claisen Condensation Overview
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15.28: Esters to β-Ketoesters: Claisen Condensation Mechanism
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15.29: Aldol Condensation vs Claisen Condensation
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15.30: Intramolecular Claisen Condensation of Dicarboxylic Esters: Dieckmann Cyclization
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15.31: β-Dicarbonyl Compounds via Crossed Claisen Condensations
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15.32: α-Alkylation of Ketones via Enolate Ions
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15.33: Factors Affecting α-Alkylation of Ketones: Choice of Base
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15.34: Alkylation of β-Ketoester Enolates: Acetoacetic Ester Synthesis
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15.35: Alkylation of β-Diester Enolates: Malonic Ester Synthesis
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15.36: Conjugate Addition to α,β-Unsaturated Carbonyl Compounds
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15.37: Conjugate Addition (1,4-Addition) vs Direct Addition (1,2-Addition)
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15.38: Conjugate Addition of Enolates: Michael Addition
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15.39: Cyclohexenones via Michael Addition and Aldol Condensation: The Robinson Annulation
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15.40: Synthesis of α-Substituted Carbonyl Compounds: The Stork Enamine Reaction
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15.42: Nonenolizable Aldehydes to Acids and Alcohols: The Cannizzaro Reaction

Chapter 16: Dienes, Conjugated Pi Systems, and Pericyclic Reactions

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16.1: Structure of Conjugated Dienes
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16.2: Stability of Conjugated Dienes
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16.3: π Molecular Orbitals of 1,3-Butadiene
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16.4: π Molecular Orbitals of the Allyl Cation and Anion
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16.5: π Molecular Orbitals of the Allyl Radical
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16.6: Electrophilic 1,2- and 1,4-Addition of HX to 1,3-Butadiene
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16.7: Electrophilic 1,2- and 1,4-Addition of X2 to 1,3-Butadiene
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16.8: Electrophilic Addition of HX to 1,3-Butadiene: Thermodynamic vs Kinetic Control
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16.9: UV–Vis Spectroscopy of Conjugated Systems
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16.10: UV–Vis Spectroscopy: Woodward–Fieser Rules
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16.11: Pericyclic Reactions: Introduction
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16.12: Thermal and Photochemical Electrocyclic Reactions: Overview
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16.13: Thermal Electrocyclic Reactions: Stereochemistry
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16.14: Photochemical Electrocyclic Reactions: Stereochemistry
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16.15: Cycloaddition Reactions: Overview
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16.16: Cycloaddition Reactions: MO Requirements for Thermal Activation
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16.17: Cycloaddition Reactions: MO Requirements for Photochemical Activation
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16.18: [4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction
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16.19: Diels–Alder vs Retro-Diels–Alder Reaction: Thermodynamic Factors
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16.20: Diels–Alder Reaction Forming Cyclic Products: Stereochemistry
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16.21: Diels–Alder Reaction Forming Bridged Bicyclic Products: Stereochemistry
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16.22: Diels–Alder Reaction: Characteristics of Dienes
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16.23: Diels–Alder Reaction: Characteristics of Dienophiles
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16.24: Thermal Sigmatropic Reactions: Overview
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16.25: [3,3] Sigmatropic Rearrangement of 1,5-Dienes: Cope Rearrangement
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16.26: [3,3] Sigmatropic Rearrangement of Allyl Vinyl Ethers: Claisen Rearrangement
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16.27: Woodward–Hoffmann Selection Rules and Microscopic Reversibility

Chapter 17: Aromatic Compounds

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17.1: Aromatic Compounds: Overview
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17.2: Nomenclature of Aromatic Compounds with a Single Substituent
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17.3: Nomenclature of Aromatic Compounds with Multiple Substituents
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17.4: Structure of Benzene: Kekulé Model
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17.5: Structure of Benzene: Molecular Orbital Model
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17.7: Criteria for Aromaticity and the Hückel 4n + 2 Rule
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17.8: Hückel's Rule Diagram of π MOs: Frost Circle
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17.9: Frost Circles for Different Conjugated Systems
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17.13: Aromatic Hydrocarbon Anions: Structural Overview
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17.14: Aromatic Hydrocarbon Cations: Structural Overview
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17.15: Five-Membered Heterocyclic Aromatic Compounds: Overview
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17.19: NMR Spectroscopy of Aromatic Compounds

Chapter 18: Reactions of Aromatic Compounds

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18.4: NMR Spectroscopy of Benzene Derivatives
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18.5: Reactions at the Benzylic Position: Oxidation and Reduction
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18.7: Reactions at the Benzylic Position: Halogenation
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18.8: Electrophilic Aromatic Substitution: Overview
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18.9: Electrophilic Aromatic Substitution: Chlorination and Bromination of Benzene
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18.10: Electrophilic Aromatic Substitution: Fluorination and Iodination of Benzene
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18.11: Electrophilic Aromatic Substitution: Nitration of Benzene
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18.12: Electrophilic Aromatic Substitution: Sulfonation of Benzene
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18.13: Electrophilic Aromatic Substitution: Friedel–Crafts Alkylation of Benzene
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18.14: Electrophilic Aromatic Substitution: Friedel–Crafts Acylation of Benzene
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18.15: Limitations of Friedel–Crafts Reactions
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18.16: Directing Effect of Substituents: ortho–para-Directing Groups
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18.17: Directing Effect of Substituents: meta-Directing Groups
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18.18: ortho–para-Directing Activators: –CH3, –OH, –⁠NH2, –OCH3
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18.19: ortho–para-Directing Deactivators: Halogens
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18.20: meta-Directing Deactivators: –NO2, –CN, –CHO, –⁠CO2R, –COR, –CO2H
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18.21: Directing and Steric Effects in Disubstituted Benzene Derivatives
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18.23: Nucleophilic Aromatic Substitution: Addition–Elimination (SNAr)
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18.24: Nucleophilic Aromatic Substitution: Elimination–Addition
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18.25: Nucleophilic Aromatic Substitution of Aryldiazonium Salts: Aromatic SN1
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18.26: Reduction of Benzene to Cyclohexane: Catalytic Hydrogenation
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18.28: Benzene to 1,4-Cyclohexadiene: Birch Reduction Mechanism
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18.29: Hydrolysis of Chlorobenzene to Phenol: Dow Process
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18.30: Benzene to Phenol via Cumene: Hock Process
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18.32: Oxidation of Phenols to Quinones

Chapter 19: Amines

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19.1: Amines: Introduction
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19.2: Nomenclature of Primary Amines
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19.3: Nomenclature of Secondary and Tertiary Amines
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19.4: Nomenclature of Aryl and Heterocyclic Amines
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19.5: Structure of Amines
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19.6: Physical Properties of Amines
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19.7: Basicity of Aliphatic Amines
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19.8: Basicity of Aromatic Amines
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19.9: Basicity of Heterocyclic Aromatic Amines
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19.10: NMR Spectroscopy Of Amines
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19.12: Mass Spectrometry of Amines
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19.13: Preparation of Amines: Alkylation of Ammonia and Amines
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19.14: Preparation of 1° Amines: Azide Synthesis
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19.15: Preparation of 1° Amines: Gabriel Synthesis
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19.16: Preparation of Amines: Reduction of Oximes and Nitro Compounds
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19.17: Preparation of Amines: Reduction of Amides and Nitriles
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19.18: Preparation of Amines: Reductive Amination of Aldehydes and Ketones
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19.19: Preparation of 1° Amines: Hofmann and Curtius Rearrangement Overview
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19.20: Preparation of 1° Amines: Hofmann and Curtius Rearrangement Mechanism
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19.21: Amines to Amides: Acylation of Amines
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19.22: Amines to Alkenes: Hofmann Elimination
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19.24: Amines to Alkenes: Cope Elimination
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19.25: 1° Amines to Diazonium or Aryldiazonium Salts: Diazotization with NaNO2 Overview
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19.26: 1° Amines to Diazonium or Aryldiazonium Salts: Diazotization with NaNO2 Mechanism
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19.27: 2° Amines to N-Nitrosamines: Reaction with NaNO2
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19.28: Diazonium Group Substitution with Halogens and Cyanide: Sandmeyer and Schiemann Reactions
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19.29: Diazonium Group Substitution: –OH and –H
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19.30: Aryldiazonium Salts to Azo Dyes: Diazo Coupling
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19.31: Amines to Sulfonamides: The Hinsberg Test

Chapter 20: Radical Chemistry

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20.1: Radicals: Electronic Structure and Geometry
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20.4: Electron Paramagnetic Resonance (EPR) Spectroscopy: Organic Radicals
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20.5: Radical Formation: Overview
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20.6: Radical Formation: Homolysis
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20.7: Radical Formation: Abstraction
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20.8: Radical Formation: Addition
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20.9: Radical Formation: Elimination
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20.10: Radical Reactivity: Overview
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20.11: Radical Reactivity: Steric Effects
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20.12: Radical Reactivity: Concentration Effects
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20.13: Radical Reactivity: Electrophilic Radicals
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20.14: Radical Reactivity: Nucleophilic Radicals
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20.15: Radical Reactivity: Intramolecular vs Intermolecular
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20.16: Radical Autoxidation
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20.17: Radical Oxidation of Allylic and Benzylic Alcohols
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20.18: Radical Substitution: Halogenation of Alkanes and Alkyl Substituents
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20.19: Radical Halogenation: Thermodynamics
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20.21: Radical Halogenation: Stereochemistry
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20.22: Radical Substitution: Allylic Chlorination
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20.23: Radical Substitution: Allylic Bromination
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20.24: Radical Substitution: Hydrogenolysis of Alkyl Halides with Tributyltin Hydride
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20.25: Radical Anti-Markovnikov Addition to Alkenes: Overview
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20.26: Radical Anti-Markovnikov Addition to Alkenes: Mechanism
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20.27: Radical Anti-Markovnikov Addition to Alkenes: Thermodynamics
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20.28: Vicinal Diols via Reductive Coupling of Aldehydes or Ketones: Pinacol Coupling Overview
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20.30: Alkenes via Reductive Coupling of Aldehydes or Ketones: McMurry Reaction
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20.31: α-Hydroxy Ketones via Reductive Coupling of Esters: Acyloin Condensation Overview

Chapter 21: Synthetic Polymers

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21.2: Characteristics and Nomenclature of Homopolymers
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21.3: Characteristics and Nomenclature of Copolymers
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21.4: Polymers: Defining Molecular Weight
30
21.5: Polymers: Molecular Weight Distribution
30
21.6: Polymer Classification: Architecture
30
21.7: Polymer Classification: Crystallinity
30
21.8: Polymer Classification: Stereospecificity
30
21.10: Radical Chain-Growth Polymerization: Overview
30
21.11: Radical Chain-Growth Polymerization: Mechanism
30
21.12: Radical Chain-Growth Polymerization: Chain Branching
30
21.13: Anionic Chain-Growth Polymerization: Overview
30
21.14: Anionic Chain-Growth Polymerization: Mechanism
30
21.16: Cationic Chain-Growth Polymerization: Mechanism
30
21.17: Ziegler–Natta Chain-Growth Polymerization: Overview
30
21.19: Step-Growth Polymerization: Overview
30
21.20: Molecular Weight of Step-Growth Polymers
30
21.22: Types of Step-Growth Polymers: Polyesters
30
21.24: Olefin Metathesis Polymerization: Overview
30
21.25: Olefin Metathesis Polymerization: Ring-Opening Metathesis Polymerization (ROMP)
30
21.26: Olefin Metathesis Polymerization: Acyclic Diene Metathesis (ADMET)

Full Table of Contents

JoVE Core
Organic Chemistry

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Cycloaddition Reactions: MO Requirements for Photochemical Activation

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16.16: Cycloaddition Reactions: MO Requirements for Thermal Activation

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16.18: [4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction

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TRANSCRIPT

Not all cycloaddition reactions take place under thermal conditions.

While the [4 + 2] cycloaddition is thermally allowed, the [2 + 2] cycloaddition requires photochemical activation.

But why?

Recall that a symmetry mismatch between the ground state HOMO and LUMO of ethylene prevents the reaction from occurring in a concerted suprafacial manner under thermal conditions.

Alternatively, photochemical excitation of one of the ethylene molecules promotes an electron from the ground state HOMO to the LUMO.

This leaves the excited state HOMO with the correct symmetry to interact with the ground state LUMO of the other ethylene molecule, resulting in a concerted suprafacial overlap of the two components.

In summary, for photochemical cycloadditions, the key HOMO–LUMO interaction is between the electronic ground state of one component and the excited state of the other. In comparison, thermal cycloadditions occur between the ground state frontier orbitals of both components.

16.17: 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 molecules occurs only in the presence of light. It is photochemically allowed but thermally forbidden.

Thermally-induced [2 + 2] cycloadditions are symmetry forbidden. This is because the ground state HOMO of one ethylene molecule and the LUMO of the other ethylene are out of phase, preventing a concerted suprafacial-suprafacial overlap.

Absorption of UV light by one of the ethylene molecules promotes an electron from the ground state HOMO to the LUMO. The excited state HOMO now has the required symmetry to interact with the LUMO of the other ethylene via a concerted symmetry-allowed pathway.

Chapter 1: Covalent Bonding and Structure

Chapter 2: Thermodynamics and Chemical Kinetics

Chapter 3: Alkanes and Cycloalkanes

Chapter 4: Stereoisomerism

Chapter 5: Acids and Bases

Chapter 6: Nucleophilic Substitution and Elimination Reactions of Alkyl Halides

Chapter 7: Alkene Structure and Reactivity

Chapter 8: Reactions of Alkenes

Chapter 9: Alkynes

Chapter 10: Alcohols and Phenols

Chapter 11: Ethers, Epoxides, Sulfides

Chapter 12: Aldehydes and Ketones

Chapter 13: Carboxylic Acids

Chapter 14: Carboxylic Acid Derivatives

Chapter 15: α-Carbon Chemistry: Enols, Enolates, and Enamines

Chapter 16: Dienes, Conjugated Pi Systems, and Pericyclic Reactions

Chapter 17: Aromatic Compounds

Chapter 18: Reactions of Aromatic Compounds

Chapter 19: Amines

Chapter 20: Radical Chemistry

Chapter 21: Synthetic Polymers

16.17: Cycloaddition Reactions: MO Requirements for Photochemical Activation

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