Advanced Organic Chemistry

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1 >... / ' :. <.. Advanced Organic Chemistry FOURTH EDITION Part A: Structure and Mechanisms FRANCIS A. CAREY and RICHARD J. SUNDBERG University of Virginia Charlottesville, Virginia Kluwer Academic /Plenum Publishers New York, Boston, Dordrecht, London, Moscow

2 Contents of Part A Chapter 1. Chemical Bonding and Structure Valence Bond Description of Chemical Bonding Orbital Hybridization Resonance Bond Energy, Polarity, and Polarizability \ Bond Energies Electronegativity and Polarity Polarizability Hardness and Softness Molecular Orbital Theory and Methods Hiickel Molecular Orbital Theory Qualitative Application of Molecular Orbital Theory Application of Molecular Orbital Theory to Reactivity Interactions between a and n Systems Hyperconjugation Other Quantitative Descriptions of Molecular Structure Atoms in Molecules Electron Density Functionals Modern Valence Bond Approaches 64 General References 65 Problems 65 Chapter 2. Principles of Stereochemistry Enantiomeric Relationships Diastereomeric Relationships Stereochemistry of Reactions Prochiral Relationships 105 vii

3 viii General References 114 Problems 114 CONTENTS OF PART A Chapter 3. Conformational, Steric, and Stereoelectronic Effects Strain and Molecular Mechanics Conformations of Acyclic Molecules Conformations of Cyclohexane Derivatives Carbocyclic Rings Other Than Six-Membered The Effect of Heteroatoms on Conformational Equilibria The Anomeric Effect Conformational Effects on Reactivity Angle Strain and Its Effect on Reactivity Relationships between Ring Size and Rate of Cyclization Torsional and Stereoelectronic Effects on Reactivity 171 General References 177 Problems 177 Chapter 4. Study and Description of Organic Reaction Mechanismns Thermodynamic Data Kinetic Data Substituent Effects and Linear Free-Energy Relationships Basic Mechanistic Concepts: Kinetic versus Thermodynamic Control, Hammond's Postulate, the Curtin-Hammett Principle Kinetic versus Thermodynamic Control Hammond's Postulate The Curtin-Hammett Principle Isotope Effects Isotopes in Labeling Experiments Characterization of Reaction Intermediates Catalysis by Bransted Acids'and Bases Lewis Acid Catalysis Solvent Effects Substituent Effects in the Gas Phase Stereochemistry Conclusion 248 General References 248 Problems 250 Chapter 5. Nucleophilic Substitution The Limiting Cases Substitution by the Ionization (S^l) Mechanism The Limiting Cases Substitution by the Direct Displacement (S N 2) Mechanism Detailed Mechanistic Description and Borderline Mechanisms 269

4 5.4. Carbocations Nucleophilicity and Solvent Effects Leaving-Group Effects Steric and Strain Effects on Substitution and Ionization Rates : Effects of Conjugation on Reactivity Stereochemistry of Nucleophilic Substitution Neighboring-Group Participation Mechanism of Rearrangements of Carbocations The Norbornyl Cation and Other Nonclassical Carbocations 327 General References 334 Problems 335 IX CONTENTS OF PART A Chapter 6. Polar Addition and Elimination Reactions Addition of Hydrogen Halides to Alkenes Acid-Catalyzed Hydration and Related Addition Reactions Addition of Halogens Electrophilic Additions Involving Metal Ions Additions to Alkynes and Allenes The E2, El, and Elcb Mechanisms Regiochemistry of Elimination Reactions Stereochemistry of E2 Elimination Reactions Dehydration of Alcohols Eliminations Not Involving C-H Bonds 393 General References 398 Problems 398 Chapter 7. Carbanions and Other Nucleophilic Carbon Species Acidity of Hydrocarbons Carbanions Stabilized by Functional Groups Enols and Enamines Carbanions as Nucleophiles in S N 2 Reactions ' 432 General References 439 Problems 440 Chapter 8. Reactions of Carbonyl Compounds Hydration and Addition of Alcohols to Aldehydes and Ketones Addition-Elimination Reactions of Ketones and Aldehydes Addition of Carbon Nucleophiles to Carbonyl Groups Reactivity of Carbonyl Compounds toward Addition Ester Hydrolysis Aminolysis of Esters Amide Hydrolysis Acylation of Nucleophilic Oxygen and Nitrogen Groups 484

5 X 8.9. Intramolecular Catalysis 488 General References 495 CONTENTS OF PART A,,.-, Problems 496 Chapter 9. Aromaticity The Concept of Aromaticity The Annulenes Aromaticity in Charged Rings Homoaromaticity Fused-Ring Systems Heterocyclic Rings 540 General References 543 Problems 543 Chapter 10. Aromatic Substitution Electrophilic Aromatic Substitution Reactions Structure-Reactivity Relationships Reactivity of Polycyclic and Heteroaromatic Compounds Specific Substitution Mechanisms Nitration Halogenation Protonation and Hydrogen Exchange Friedel-Crafts Alkylation and Related Reactions Friedel-Crafts Acylation and Related Reactions Coupling with Diazonium Compounds Substitution of Groups Other Than Hydrogen Nucleophilic Aromatic Substitution by the Addition-Elimination Mechanism Nucleophilic Aromatic Substitution by the Elimination-Addition Mechanism 593 General References 597 Problems 597 Chapter 11. Concerted Pericyclic Reactions Electrocyclic Reactions Sigmatropic Rearrangements Cycloaddition Reactions 636 General References 651 Problems 651 Chapter 12. Free-Radical Reactions Generation and Characterization of Free Radicals 663

6 Background 663 x Stable and Persistent Free Radicals 664,,- _. _... r-r. J- i T ^ J- x sr*, CONTENTS OF PART t Direct Detection or Radical Intermediates Sources of Free Radicals Structural and Stereochemical Properties of Radical Intermediates Charged Radical Species Characteristics of Reaction Mechanisms Involving Radical Intermediates Kinetic Characteristics of Chain Reactions Structure-Reactivity Relationships Free-Radical Substitution Reactions Halogenation Oxidation Free-Radical Addition Reactions Addition of Hydrogen Halides Addition of Halomethanes Addition of Other Carbon Radicals Addition of Thiols and Thiocarboxylic Acids Halogen, Sulfur, and Selenium Group Transfer Reactions Intramolecular Free-Radical Reactions Rearrangement and Fragmentation Reactions of Free Radicals Rearrangement Reactions Fragmentation Electron-Transfer Reactions Involving Transition-Metal Ions SRNI Substitution Processes 727 General References 733 Problems 734 Chapter 13. Photochemistry General Principles Orbital Symmetry Considerations Related to Photochemical Reactions Photochemistry of Carbonyl Compounds, Photochemistry of Alkenes and Dienes Photochemistry of Aromatic Compounds 779 General References 781 Problems 781 References to Problems 791 Index 807

Benzene Benzene is best represented as a resonance hybrid:

Benzene Benzene is best represented as a resonance hybrid: Electrophilic Aromatic Substitution (EAS) is a substitution reaction usually involving the benzene ring; more specifically it is a reaction in which the hydrogen atom of an aromatic ring is replaced as