ELECTRON FLOWIN ORGANIC CHEMISTRY

Paul H . Scudder

1

BONDING AND ELECTRON DISTRIBUTION

1

2 THE PROCESS OF BOND FORMATION

3 4

3 STABILITY AND REACTIVITY OF INTERMEDIATES

5 6

4 CLASSIFICATION OF ELECTRON SOURCES

7 7

5

CLASSIFICATION OF ELECTRON SINKS .

8 9

6 THE ELECTRON FLOW PATHWAYS

9 9

7 INTERACTION OF ELECTRON SOURCES AND SINKS 13 6

8

DECISIONS, DECISIONS

18 1

9 CHOOSING THE MOST PROBABLE PATH

19 4

10 ONE ELECTRON PROCESSES

24 1

APPENDIX

25 6

HINTS TO PROBLEMS FROM CHAPTERS 7, 8, AND 9 27 9

EXERCISE ANSWERS

28 1

INDEX

353

1

BONDING AND ELECTRON DISTRIBUTION

1

1 .1 Introduction 21 .2 Lewis Structures and Resonance Forms 31 .3 Curved-Arrow Notation 71.4 Nomenclature and Abbreviations 10

1 .5 Hybridization and Bonding 121 .6 Orbital Interaction Diagrams (Supplemental) 1 91 .7 Conjugation 221 .8 Molecular Orbital Theory for Acyclic Systems (Supplemental) 24

1 .9 Aromaticity, Cyclic Systems 28

2 THE PROCESS OF BOND FORMATION

3 4

2.1 Thermodynamics, Position of Equilibrium 3 5

2.2 Kinetics, Rate of Reaction 3 82.3 The Basics of Barriers 4 32.4 Orbital Overlap in Bond Formation 442.5 Microscopic Reversibility, Stereoelectronics 4 5

2.6 Steric Limitations 472.7 HOMO-LUMO Interactions 4 72.8 Hard and Soft Acid-Base Theory 4 92.9 Perturbation of the HOMO and LUMO (Supplemental) 5 2

3

STABILITY AND REACTIVITY OF INTERMEDIATES

5 6

3.1 Ion Stability, Solvation, and Media Effects 5 63.2 Ranking of Acids and Bases, the pKa Chart 59

3.3 Calculation of Keq for Proton Transfer 6 1

3.4 Carbocation Stability and Trends 6 33 .5 Ranking of Electron-Donor Groups 673.6 Carbanion Stability and Trends 683.7 Ranking of Electron-Withdrawing Groups 7 03.8 Ranking of Leaving Groups 7 13 .9 The ApKa Rule 72

4

CLASSIFICATION OF ELECTRON SOURCES

7 7

4 .1 Generalized Ranking of Electron Sources 7 74 .2 Nonbonding Electrons 784 .3 Electron-Rich Sigma Bonds 8 14 .4 Electron-Rich Pi Bonds 8 24.5 Simple Pi Bonds 844.6 Aromatic Rings 854.7 Summary of Generic Electron Sources 86

5

CLASSIFICATION OF ELECTRON SINKS

8 9

5.1 Generalized Ranking of Electron Sinks 895.2 Electron-Deficient Species 905.3 Weak Single Bonds 9 15.4 Polarized Multiple Bonds Without Leaving Groups 925 .5 Polarized Multiple Bonds With Leaving Groups 945.6 Summary of Generic Electron Sinks 96

6 THE ELECTRON FLOW PATHWAYS

9 9

6.1 The Dozen Most Common Pathways 1006.2 Common Path Combinations 11 26.3 Eight Minor Pathways 11 56.4 Variations

1246.5 Common Errors 1266.6 Major Path Summary 132

7

INTERACTION OF ELECTRON SOURCES AND SINKS

13 6

7.1 Source and Sink Matrix 1377.2 Detailed Matrix-Cell Discussions 1387.3 Electrophilic Additions 1577.4 Rearrangements with Loss of a Leaving Group 1667.5 Nu-L Reactions 16 77.6 Fragmentations 1707.7 Concerted Six-Electron Pericyclic Reactions 17 17 .8 Miscellaneous Reactions 1747.9 Product Matrix Summary 177

8

DECISIONS, DECISIONS

18 1

8 .1 Decision Point Recognition

18 18 .2 Multiple Additions 18 28.3 Ionization of Leaving Groups 18 38.4 Ambident Nucleophiles 18 38 .5 Substitution vs . Elimination

1868 .6 Ambident Electrophiles 1898.7 Intermolecular vs . Intramolecular 1908.8 Kinetic vs. Thermodynamic Enolate Formation 19 1

9 CHOOSING THE MOST PROBABLE PATH

19 4

9 .1 Problem-Solving in General 19 59 .2 General Mechanistic Cross-Checks 1999.3 The Path-Selection Process 2009.4 Reaction Mechanism Strategies 2029.5 Worked Mechanism Examples 2039.6 Worked Product Prediction Examples 21 99.7 Methods for Testing Mechanisms 233

10 ONE-ELECTRON PROCESSES

24 1

10 .1 Radical Structure and Stability 24 110 .2 Charged Radicals 24410.3 Radical Path Initiation 24510 .4 Paths for Radicals Reacting with Neutrals 24610.5 Other Radical Paths 24710.6 Radical Path Combinations 24910.7 Dissolving Metal Reductions 25 110 .8 Electron Transfer Catalysis 25210 .9 One-Electron Path Summary 253

APPENDIX (A COLLECTION OF IMPORTANT TOOLS)

25 6

General Bibliography 25 6Abbreviations Used in This Text 25 7Functional Group Glossary 25 8Composite pKa Chart 26 1

Bond Strength Table 26 4The Twelve Generic Electron Sources 265The Eighteen Generic Electron Sinks 26 5Flow Charts for the Classification of Electron Sources and Sinks 266Pathway Summary 267Substitution vs. Elimination Decision Matrices 27 1Addition/elimination Energy Surfaces 27 2Trends Guide 273Thermodynamics and Kinetics 276Generation of Alternate Paths, Reaction Cubes 276

HINTS TO PROBLEMS FROM CHAPTERS 7, 8, AND 9

27 9

EXERCISE ANSWERS

28 1

INDEX

353