Electron Flow in Organic Chemistry A Decision-Based Guide to Organic Mechanisms

PAUL H. SCUDDER, PhD, is Chair of the Division of Natural Sciences and Professor of Chemistry at New College of Florida. Dr. Scudder has been teaching organic chemistry and advanced organic chemistry at the undergraduate level for thirty-four years. His current research focuses on physical organic chemistry, specifically reaction mechanisms and theory.

1 BONDING AND ELECTRON DISTRIBUTION       1

1.1     The Decision-Based Approach To Organic Chemistry     2             

1.2     Ionic And Covalent Bonding      6

1.3     Lewis Structures And Resonance Forms     8

1.4     Curved-Arrow Notation     11

1.5     Nomenclature And Abbreviations     16

1.6     An Orbital View Of Bonding (Supplemental)     18

1.7     The Shapes Of Molecules     21

1.8     Molecular Repulsions, Attractions, And Hydrogen Bonding      25

1.9     Conjugation, Vinylogy, Aromaticity     27

1.10   Summary     30

2        THE PROCESS OF BOND FORMATION         34

2.1     Energetics Control Knowledge     35

2.2     Orbital Overlap In Covalent Bond Formation     35

2.3     Orbital Interaction Diagrams     38

2.4     Polarizability And Hard And Soft Acid?Base Theory     41

2.5     Thermodynamics, Position Of Equilibrium      43

2.6     Kinetics, Rate Of Reaction     47

2.7     Solvent Stabilization Of Ions     53

2.8     Enzymatic Catalysis - Lessons From Biochemistry      55

2.9     Summary     57

3        PROTON TRANSFER AND THE PRINCIPLES OF STABILITY  61     

3.1     Introduction To Proton Transfer     62

3.2     Ranking Of Acids And Bases, The pKa Chart     63

3.3     Structural Factors That Influence Acid Strength     66

3.4     Structural Factors That Influence Base Strength     70

3.5     Carbon Acids & Ranking Of Electron-Withdrawing Groups     71

3.6     Calculation Of Keq For Proton Transfer     76

3.7     Proton Transfer Mechanisms     77

3.8     Common Errors     81

3.9     Proton Transfer Product Predictions     82

3.10   Summary     83

4        IMPORTANT REACTION ARCHETYPES       88     

4.1     Introduction To Reaction Archetypes     89

4.2     Nucleophilic Substitution At A Tetrahedral Center     89

4.3     Elimination Reactions Create Pi Bonds     110

4.4     Addition Reactions To Polarized Multiple Bonds     124

4.5     Nucleophilic Substitution At A Trigonal Planar Center     133        

4.6     Electrophilic Substitution At A Trigonal Planar Center     140

4.7     Rearrangements To An Electrophilic Carbon     144

4.8     Reaction Archetype Summary     146

5        CLASSIFICATION OF ELECTRON SOURCES         151   

5.1     Generalized Ranking Of Electron Sources     151

5.2     Nonbonding Electrons     152 

5.3     Electron-Rich Sigma Bonds     154

5.4     Electron-Rich Pi Bonds     155

5.5     Simple Pi Bonds     156

5.6     Aromatic Rings     159

5.7     Summary Of Generic Electron Sources     160

6        CLASSIFICATION OF ELECTRON SINKS      166

6.1     Generalized Ranking Of Electron Sinks     166

6.2     Electron-Deficient Species     167

6.3     Weak Single Bonds     168

6.4     Polarized Multiple Bonds Without Leaving Groups     170

6.5     Polarized Multiple Bonds With Leaving Groups     172

6.6     Summary Of Generic Electron Sinks     173

7        THE ELECTRON FLOW PATHWAYS    179   

7.1     The Dozen Most Common Pathways     180

7.2     Six Minor Pathways     191

7.3     Common Path Combinations     197

7.4     Variations On A Theme     201

7.5     Twelve Major Paths Summary And Crosschecks     208

8        INTERACTION OF ELECTRON SOURCES AND SINKS   213

8.1     Source And Sink Correlation Matrix     214

8.2     H-A Sinks Reacting With Common Sources     214

8.3     Y?L Sinks Reacting With Common Sources     218

8.4     sp3 C?L Sinks Reacting With Common Sources     222

8.5     C=Y Sinks Reacting With Common Sources     227

8.6     R?C?Y Sinks Reacting With Common Sources     233

8.7     C=C?Ewg Sinks Reacting With Common Sources     235

8.8     L?C=Y Sinks Reacting With Common Sources     237

8.9     Miscellaneous Reactions     240

8.10   Metal Ions As Electron Sinks     242

8.11   Rearrangements To An Electrophilic Center     243

8.12   Nu-L Reactions     244

8.13   Product Matrix Summary     248

9        DECISIONS, DECISIONS      251   

9.1     Decision Point Recognition     252

9.2     Multiple Additions     252

9.3     Regiochemistry & Stereochemistry Of Enolate Formation     254

9.4     Ambident Nucleophiles     255

9.5     Substitution Vs. Elimination     258

9.6     Ambident Electrophiles     262

9.7     Intermolecular Vs. Intramolecular     263

9.8     To Migrate Or Not To An Electrophilic Center     264

9.8     Summary     266

10      CHOOSING THE MOST PROBABLE PATH    269            

10.1   Problem-Solving In General      270

10.2   General Mechanistic Cross-Checks     274

10.3   The Path-Selection Process     276

10.4   Reaction Mechanism Strategies     278

10.5   Worked Mechanism Examples     279

10.6   Product Prediction Strategies     297

10.7   Worked Product Prediction Examples     297

10.8   Methods For Testing Mechanisms     313

10.9   Lessons from Biochemical Mechanisms     319

10.10          Summary     321

11      ONE-ELECTRON PROCESSES      326   

11.1   Radical Structure And Stability     326

11.2   Radical Path Initiation     329

11.3   Major Paths For Radicals Reacting With Neutrals     330

11.4   Unimolecular Radical Paths     332

11.5   Termination Radical Paths     333

11.6   Radical Path Combinations     333

11.7   Approaches To Radical Mechanisms     336

11.8   Single Electron Transfer, S.E.T., And Charged Radicals     338

11.9   Dissolving Metal Reductions     339

11.10 Electron Transfer Initiated Processes     340

11.11 One-Electron Path Summary     340

12      QUALITATIVE M.O. THEORY & PERICYCLIC REACTIONS    343   

12.1   Review Of Orbitals As Standing Waves     344

12.2   Molecular Orbital Theory For Linear Pi Systems     344

12.3   Molecular Orbital Theory For Cyclic Conjugated PI Systems     348

12.4   Perturbation Of The HOMO And LUMO     351

12.5   Delocalization Of Sigma Electrons (Supplemental)     352

12.6   Concerted Pericyclic Cycloaddition Reactions     353

12.7   Concerted Pericyclic Electrocyclic Reactions     357

12.8   Concerted Pericyclic Sigmatropic Rearrangements     359

12.9   Pericyclic Reactions Summary     361       

APPENDIX (A COLLECTION OF IMPORTANT TOOLS)   364            

General Bibliography     364

Abbreviations Used in This Text     365

Functional Group Glossary     366

Composite pKa  Chart     369

Bond Strength Table     372

Generic Classification Guide     373

Flow Charts for the Classification of Electron Sources and Sinks     375

Pathway Summary     375

Trends Guide     380

Major Routes Summary     384

Major Decisions Guide     388

Thermodynamics and Kinetics     390

Generation of Alternate Paths, Reaction Cubes     390

Organic Structure Elucidation Strategies     393

Notes on Nomenclature     399

HINTS TO PROBLEMS FROM CHAPTERS 8, 9, AND 10  404   

INDEX       407

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