Perspectives on Structure and Mechanism in Organic Chemistry

Preface xi Chapter 1 Fundamental Models of Organic Chemistry 1 1.1 Atoms and Molecules 1 Basic Concepts 1 Molecular Dimensions 5 1.2 Heats of Formation and Reaction 8 Experimental Determination of Heats of Formation 8 Bond Increment Calculation of Heats of Formation 10 Group Increment Calculation of Heats of Formation 11 Heats of Formation and the Concept of Protobranching 13 Homolytic and Heterolytic Bond Dissociation Energies 15 1.3 Bonding Models 18 Electronegativity and Bond Polarity 20 Complementary Theoretical Models of Bonding 23 Pictorial Representations of Bonding Concepts 27 sp3 Hybridization 28 Are There sp3 Hybrid Orbitals in Methane? 30 Hybridization and Molecular Geometry 34 Variable Hybridization 35 1.4 Complementary Models for the Double Bond 41 The σ,π Description of Ethene 41 The Bent Bond Description of Ethene 42 Predictions of Physical Properties with the Two Models 42 1.5 The Role of Complementary Models in Organic Chemistry 46 Problems 47 Chapter 2 Introduction to Computational Chemistry 53 2.1 Hückel Molecular Orbital Theory 53 Correlation of Physical Properties with Results of HMO Calculations 63 Other Parameters Generated Through HMO Theory 67 Properties of Odd Alternant Hydrocarbons 69 The Frost Circle 74 2.2 Aromaticity 75 Benzene 77 Other Aromatic Systems 81 Polycyclic Conjugated Systems 85 Larger Annulenes 90 Dewar Resonance Energy and Absolute Hardness 93 2.3 Contemporary Computational Methods 95 Extended Hückel Theory 95 Semiempirical Methods 96 Ab Initio Theory 97 2.4 Localized Molecular Orbitals 100 Perturbational Molecular Orbital Theory 104 Atoms in Molecules 108 2.5 Density Functional Theory 112 2.6 Another Look at Valence Bond Theory 114 Resonance Structures and Resonance Energies 114 Interpreting Computational Results 117 Problems 119 Chapter 3 Stereochemistry 127 3.1 Representations of Three-Dimensional Structures 127 3.2 Stereoisomerism 130 Isomerism 130 Symmetric, Asymmetric, Dissymmetric, and Nondissymmetric Molecules 133 Fischer Projections 146 Additional Stereochemical Designations 149 3.3 Physical Manifestations of Chirality 159 Optical Activity 159 Configuration and Optical Activity 161 Other Physical Properties of Stereoisomers 166 3.4 Stereotopicity 167 Stereochemical Relationships of Substituents 167 Chirotopicity and Stereogenicity 171 Problems 172 Chapter 4 Molecular Geometry and Steric Energy 183 4.1 Designation of Molecular Conformation 183 4.2 Conformational Analysis 187 Torsional Strain 187 van der Waals Strain 191 Angle Strain and Baeyer Strain Theory 193 Application of Conformational Analysis to Cycloalkanes 194 Conformational Analysis of Substituted Cyclohexanes 198 4.3 Molecular Mechanics 204 4.4 Anomeric Effect 221 4.5 Strain and Molecular Stability 225 Problems 237 Chapter 5 Reactive Intermediates 243 5.1 Reaction Coordinate Diagrams 243 5.2 Radicals 244 Early Evidence for the Existence of Radicals 244 Detection and Characterization of Radicals 246 Structure and Bonding of Radicals 251 Thermochemical Data for Radicals 253 Generation of Radicals 255 Radical Chain Reactions 256 5.3 Carbenes 263 Structure and Geometry of Carbenes 263 Generation of Carbenes 267 Reactions of Carbenes 268 5.4 Carbocations 272 Carbonium Ions and Carbenium Ions 272 Structure and Geometry of Carbocations 274 The 2-Norbornyl Cation 281 Carbocation Rearrangements 283 Radical Cations 285 5.5 Carbanions 290 Generation of Carbanions 294 Stability of Carbanions 296 Reactions of Carbanions 296 5.6 Choosing Models of Reactive Intermediates 298 Problems 299 Chapter 6 Determining Reaction Mechanisms 305 6.1 Reaction Mechanisms 305 6.2 Methods to Determine Reaction Mechanisms 306 Identification of Reaction Products 306 Determination of Intermediates 306 Crossover Experiments 311 Isotopic Labeling 313 Stereochemical Studies 314 Solvent Effects 315 Computational Studies 317 6.3 Applications of Kinetics in Studying Reaction Mechanisms 319 6.4 Arrhenius Theory and Transition State Theory 326 6.5 Reaction Barriers and Potential Energy Surfaces 337 6.6 Kinetic Isotope Effects 348 Primary Kinetic Isotope Effects 349 Secondary Kinetic Isotope Effects 354 Tunneling and Isotope Effects 359 Solvent Isotope Effects 362 6.7 Substituent Effects 363 6.8 Linear Free Energy Relationships 368 Problems 383 Chapter 7 Acid and Base Catalysis of Organic Reactions 393 7.1 Acidity and Basicity of Organic Compounds 393 Acid–Base Measurements in Solution 393 Acid–Base Reactions in the Gas Phase 402 Comparison of Gas Phase and Solution Acidities 408 Acidity Functions 410 7.2 Acid and Base Catalysis of Chemical Reactions 413 Specific Acid Catalysis 413 General Acid Catalysis 414 Brønsted Catalysis Law 417 7.3 Acid and Base Catalysis of Reactions of Carbonyl Compounds and Carboxylic Acid Derivatives 418 Addition to the Carbonyl Group 418 Enolization of Carbonyl Compounds 422 Hydrolysis of Acetals 426 Acid-Catalyzed Hydrolysis of Esters 428 Alkaline Hydrolysis of Esters 431 Hydrolysis of Amides 437 Problems 441 Chapter 8 Substitution Reactions 449 8.1 Introduction 449 8.2 Nucleophilic Aliphatic Substitution 450 8.3 The SN1 Reaction 453 Kinetics 453 Structural Effects in SN1 Reactions 454 Solvent Polarity and Nucleophilicity 455 Solvated Ions and Ion Pairs 459 Anchimeric Assistance in SN1 Reactions 464 Nonclassical Carbocations in SN1 Reactions 469 8.4 The SN2 Reaction 471 Stereochemistry 471 Solvent Effects 473 Substrate Effects 477 8.5 Quantitative Measures of Nucleophilicity 480 Brønsted Correlations 481 Hard–Soft Acid–Base Theory and Nucleophilicity 482 Edwards Equations 483 Swain-Scott Equation 484 Mayr Equations 485 The α-Effect 488 Leaving Group Effects in SN2 Reactions 489 Aliphatic Substitution and Single Electron Transfer 490 8.6 Electrophilic Aromatic Substitution 495 The SEAr Reaction 495 Quantitative Measurement of SEAr Rate Constants: Partial Rate Factors 498 Lewis Structures as Models of Reactivity in SEAr Reactions 500 8.7 Nucleophilic Aromatic and Vinylic Substitution 504 Nucleophilic Aromatic Substitution 504 Nucleophilic Vinylic Substitution 509 8.8 Substitution Involving Benzyne Intermediates 511 8.9 Radical-Nucleophilic Substitution 518 8.10 The Impermanence of Mechanistic Labels 521 Problems 521 Chapter 9 Elimination Reactions 529 9.1 Introduction 529 9.2 Dehydrohalogenation and Related 1,2-Elimination Reactions 534 Potential Energy Surfaces for 1,2-Elimination 534 Competition Between Substitution and Elimination 540 Stereochemistry of 1,2-Elimination Reactions 541 Elimination Reactions to Produce Alkynes 547 Regiochemistry of 1,2-Elimination Reactions 548 9.3 Other 1,2-Elimination Reactions 558 Dehalogenation of Vicinal Dihalides 558 Dehydration of Alcohols 561 Deamination of Amines 568 Pyrolytic Eliminations 572 Problems 578 Chapter 10 Addition Reactions 587 10.1 Introduction 587 10.2 Addition of Halogens to Alkenes 588 Electrophilic Addition of Bromine to Alkenes 588 Role of Charge-Transfer Complexes in Bromine Addition Reactions 592 Kinetics of Bromine Addition Reactions 593 Solvent Effects in Bromine Additions 596 Reversibility of Bromine Addition 598 Intermediates in the Addition of Bromine to Alkyl-Substituted Alkenes 599 Intermediates in the Addition of Bromine to Aryl-Substituted Alkenes 604 Summary of Bromine Addition 608 Addition of Other Halogens to Alkenes 609 10.3 Other Addition Reactions 618 Addition of Hydrogen Halides to Alkenes 618 Hydration of Alkenes 625 Oxymercuration 628 Hydroboration 632 Epoxidation 637 Electrophilic Addition to Alkynes and Cumulenes 639 Nucleophilic Addition to Alkenes and Alkynes 647 Nucleophilic Addition to Carbonyl Compounds 651 Problems 656 Chapter 11 Pericyclic Reactions 661 11.1 Introduction 661 11.2 Electrocyclic Transformations 665 Definitions and Selection Rules 665 MO Correlation Diagrams 670 State Correlation Diagrams 675 11.3 Sigmatropic Reactions 678 Selection Rules for Sigmatropic Reactions 679 Other Examples of Sigmatropic Reactions 687 11.4 Cycloaddition Reactions 691 Introduction 691 Ethene Dimerization 692 The Diels–Alder Reaction 694 Selection Rules for Cycloaddition Reactions 698 11.5 Other Pericyclic Reactions 705 Cheletropic Reactions 705 Double Group Transfer Reactions 707 Ene Reactions 709 11.6 A General Selection Rule for Pericyclic Reactions 711 11.7 Alternative Conceptual Models for Pericyclic Reactions 713 Frontier Molecular Orbital Theory 713 Hückel and Möbius Aromaticity of Transition Structures 719 Synchronous and Nonsynchronous Pericyclic Reactions 725 Potential Energy Surfaces and Ambimodal Reactions 729 11.8 Reaction Dynamics and Potential Energy Surfaces 729 Problems 735 Chapter 12 Organic Photochemistry 745 12.1 Energy and Electronic States 745 12.2 Photophysical Processes 747 Designation of Spectroscopic Transitions 748 Selection Rules for Radiative Transitions 754 Fluorescence and Phosphorescence 756 Energy Transfer and Electron Transfer 759 12.3 Photochemical Kinetics 763 Actinometry and Quantum Yield Determinations 763 Rate Constants for Unimolecular Processes 764 Transient Detection and Monitoring 765 Bimolecular Decay of Excited States: Stern–Volmer Kinetics 768 12.4 Physical Properties of Excited States 770 Acidity and Basicity in Excited States 770 Bond Angles and Dipole Moments of Excited-State Molecules 774 12.5 Representative Photochemical Reactions 777 Photochemical Reactions of Alkenes and Dienes 778 Photochemical Reactions of Carbonyl Compounds 790 Photochemical Reactions of α,ß-Unsaturated Carbonyl Compounds 798 Photochemical Reactions of Aromatic Compounds 800 Photosubstitution Reactions 802 σ Bond Photodissociation Reactions 803 Singlet Oxygen and Organic Photochemistry 808 12.6 Applications of Organic Photochemistry 811 Problems 822 References for Selected Problems 831 Index 837