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Organic Chemistry

11.5 Characteristics of the SN1 Reaction

Organic Chemistry11.5 Characteristics of the SN1 Reaction

Table of contents
  1. Dedication and Preface
  2. 1 Structure and Bonding
    1. Why This Chapter?
    2. 1.1 Atomic Structure: The Nucleus
    3. 1.2 Atomic Structure: Orbitals
    4. 1.3 Atomic Structure: Electron Configurations
    5. 1.4 Development of Chemical Bonding Theory
    6. 1.5 Describing Chemical Bonds: Valence Bond Theory
    7. 1.6 sp3 Hybrid Orbitals and the Structure of Methane
    8. 1.7 sp3 Hybrid Orbitals and the Structure of Ethane
    9. 1.8 sp2 Hybrid Orbitals and the Structure of Ethylene
    10. 1.9 sp Hybrid Orbitals and the Structure of Acetylene
    11. 1.10 Hybridization of Nitrogen, Oxygen, Phosphorus, and Sulfur
    12. 1.11 Describing Chemical Bonds: Molecular Orbital Theory
    13. 1.12 Drawing Chemical Structures
    14. Chemistry Matters—Organic Foods: Risk versus Benefit
    15. Key Terms
    16. Summary
    17. Additional Problems
  3. 2 Polar Covalent Bonds; Acids and Bases
    1. Why This Chapter?
    2. 2.1 Polar Covalent Bonds and Electronegativity
    3. 2.2 Polar Covalent Bonds and Dipole Moments
    4. 2.3 Formal Charges
    5. 2.4 Resonance
    6. 2.5 Rules for Resonance Forms
    7. 2.6 Drawing Resonance Forms
    8. 2.7 Acids and Bases: The Brønsted–Lowry Definition
    9. 2.8 Acid and Base Strength
    10. 2.9 Predicting Acid–Base Reactions from pKa Values
    11. 2.10 Organic Acids and Organic Bases
    12. 2.11 Acids and Bases: The Lewis Definition
    13. 2.12 Noncovalent Interactions between Molecules
    14. Chemistry Matters—Alkaloids: From Cocaine to Dental Anesthetics
    15. Key Terms
    16. Summary
    17. Additional Problems
  4. 3 Organic Compounds: Alkanes and Their Stereochemistry
    1. Why This Chapter?
    2. 3.1 Functional Groups
    3. 3.2 Alkanes and Alkane Isomers
    4. 3.3 Alkyl Groups
    5. 3.4 Naming Alkanes
    6. 3.5 Properties of Alkanes
    7. 3.6 Conformations of Ethane
    8. 3.7 Conformations of Other Alkanes
    9. Chemistry Matters—Gasoline
    10. Key Terms
    11. Summary
    12. Additional Problems
  5. 4 Organic Compounds: Cycloalkanes and Their Stereochemistry
    1. Why This Chapter?
    2. 4.1 Naming Cycloalkanes
    3. 4.2 Cis–Trans Isomerism in Cycloalkanes
    4. 4.3 Stability of Cycloalkanes: Ring Strain
    5. 4.4 Conformations of Cycloalkanes
    6. 4.5 Conformations of Cyclohexane
    7. 4.6 Axial and Equatorial Bonds in Cyclohexane
    8. 4.7 Conformations of Monosubstituted Cyclohexanes
    9. 4.8 Conformations of Disubstituted Cyclohexanes
    10. 4.9 Conformations of Polycyclic Molecules
    11. Chemistry Matters—Molecular Mechanics
    12. Key Terms
    13. Summary
    14. Additional Problems
  6. 5 Stereochemistry at Tetrahedral Centers
    1. Why This Chapter?
    2. 5.1 Enantiomers and the Tetrahedral Carbon
    3. 5.2 The Reason for Handedness in Molecules: Chirality
    4. 5.3 Optical Activity
    5. 5.4 Pasteur’s Discovery of Enantiomers
    6. 5.5 Sequence Rules for Specifying Configuration
    7. 5.6 Diastereomers
    8. 5.7 Meso Compounds
    9. 5.8 Racemic Mixtures and the Resolution of Enantiomers
    10. 5.9 A Review of Isomerism
    11. 5.10 Chirality at Nitrogen, Phosphorus, and Sulfur
    12. 5.11 Prochirality
    13. 5.12 Chirality in Nature and Chiral Environments
    14. Chemistry Matters—Chiral Drugs
    15. Key Terms
    16. Summary
    17. Additional Problems
  7. 6 An Overview of Organic Reactions
    1. Why This Chapter?
    2. 6.1 Kinds of Organic Reactions
    3. 6.2 How Organic Reactions Occur: Mechanisms
    4. 6.3 Polar Reactions
    5. 6.4 An Example of a Polar Reaction: Addition of HBr to Ethylene
    6. 6.5 Using Curved Arrows in Polar Reaction Mechanisms
    7. 6.6 Radical Reactions
    8. 6.7 Describing a Reaction: Equilibria, Rates, and Energy Changes
    9. 6.8 Describing a Reaction: Bond Dissociation Energies
    10. 6.9 Describing a Reaction: Energy Diagrams and Transition States
    11. 6.10 Describing a Reaction: Intermediates
    12. 6.11 A Comparison Between Biological Reactions and Laboratory Reactions
    13. Chemistry Matters—Where Do Drugs Come From?
    14. Key Terms
    15. Summary
    16. Additional Problems
  8. 7 Alkenes: Structure and Reactivity
    1. Why This Chapter?
    2. 7.1 Industrial Preparation and Use of Alkenes
    3. 7.2 Calculating the Degree of Unsaturation
    4. 7.3 Naming Alkenes
    5. 7.4 Cis–Trans Isomerism in Alkenes
    6. 7.5 Alkene Stereochemistry and the E,Z Designation
    7. 7.6 Stability of Alkenes
    8. 7.7 Electrophilic Addition Reactions of Alkenes
    9. 7.8 Orientation of Electrophilic Additions: Markovnikov’s Rule
    10. 7.9 Carbocation Structure and Stability
    11. 7.10 The Hammond Postulate
    12. 7.11 Evidence for the Mechanism of Electrophilic Additions: Carbocation Rearrangements
    13. Chemistry Matters—Bioprospecting: Hunting for Natural Products
    14. Key Terms
    15. Summary
    16. Additional Problems
  9. 8 Alkenes: Reactions and Synthesis
    1. Why This Chapter?
    2. 8.1 Preparing Alkenes: A Preview of Elimination Reactions
    3. 8.2 Halogenation of Alkenes: Addition of X2
    4. 8.3 Halohydrins from Alkenes: Addition of HO-X
    5. 8.4 Hydration of Alkenes: Addition of H2O by Oxymercuration
    6. 8.5 Hydration of Alkenes: Addition of H2O by Hydroboration
    7. 8.6 Reduction of Alkenes: Hydrogenation
    8. 8.7 Oxidation of Alkenes: Epoxidation and Hydroxylation
    9. 8.8 Oxidation of Alkenes: Cleavage to Carbonyl Compounds
    10. 8.9 Addition of Carbenes to Alkenes: Cyclopropane Synthesis
    11. 8.10 Radical Additions to Alkenes: Chain-Growth Polymers
    12. 8.11 Biological Additions of Radicals to Alkenes
    13. 8.12 Reaction Stereochemistry: Addition of H2O to an Achiral Alkene
    14. 8.13 Reaction Stereochemistry: Addition of H2O to a Chiral Alkene
    15. Chemistry Matters—Terpenes: Naturally Occurring Alkenes
    16. Key Terms
    17. Summary
    18. Summary of Reactions
    19. Additional Problems
  10. 9 Alkynes: An Introduction to Organic Synthesis
    1. Why This Chapter?
    2. 9.1 Naming Alkynes
    3. 9.2 Preparation of Alkynes: Elimination Reactions of Dihalides
    4. 9.3 Reactions of Alkynes: Addition of HX and X2
    5. 9.4 Hydration of Alkynes
    6. 9.5 Reduction of Alkynes
    7. 9.6 Oxidative Cleavage of Alkynes
    8. 9.7 Alkyne Acidity: Formation of Acetylide Anions
    9. 9.8 Alkylation of Acetylide Anions
    10. 9.9 An Introduction to Organic Synthesis
    11. Chemistry Matters—The Art of Organic Synthesis
    12. Key Terms
    13. Summary
    14. Summary of Reactions
    15. Additional Problems
  11. 10 Organohalides
    1. Why This Chapter?
    2. 10.1 Names and Structures of Alkyl Halides
    3. 10.2 Preparing Alkyl Halides from Alkanes: Radical Halogenation
    4. 10.3 Preparing Alkyl Halides from Alkenes: Allylic Bromination
    5. 10.4 Stability of the Allyl Radical: Resonance Revisited
    6. 10.5 Preparing Alkyl Halides from Alcohols
    7. 10.6 Reactions of Alkyl Halides: Grignard Reagents
    8. 10.7 Organometallic Coupling Reactions
    9. 10.8 Oxidation and Reduction in Organic Chemistry
    10. Chemistry Matters—Naturally Occurring Organohalides
    11. Key Terms
    12. Summary
    13. Summary of Reactions
    14. Additional Problems
  12. 11 Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations
    1. Why This Chapter?
    2. 11.1 The Discovery of Nucleophilic Substitution Reactions
    3. 11.2 The SN2 Reaction
    4. 11.3 Characteristics of the SN2 Reaction
    5. 11.4 The SN1 Reaction
    6. 11.5 Characteristics of the SN1 Reaction
    7. 11.6 Biological Substitution Reactions
    8. 11.7 Elimination Reactions: Zaitsev’s Rule
    9. 11.8 The E2 Reaction and the Deuterium Isotope Effect
    10. 11.9 The E2 Reaction and Cyclohexane Conformation
    11. 11.10 The E1 and E1cB Reactions
    12. 11.11 Biological Elimination Reactions
    13. 11.12 A Summary of Reactivity: SN1, SN2, E1, E1cB, and E2
    14. Chemistry Matters—Green Chemistry
    15. Key Terms
    16. Summary
    17. Summary of Reactions
    18. Additional Problems
  13. 12 Structure Determination: Mass Spectrometry and Infrared Spectroscopy
    1. Why This Chapter?
    2. 12.1 Mass Spectrometry of Small Molecules: Magnetic-Sector Instruments
    3. 12.2 Interpreting Mass Spectra
    4. 12.3 Mass Spectrometry of Some Common Functional Groups
    5. 12.4 Mass Spectrometry in Biological Chemistry: Time-of-Flight (TOF) Instruments
    6. 12.5 Spectroscopy and the Electromagnetic Spectrum
    7. 12.6 Infrared Spectroscopy
    8. 12.7 Interpreting Infrared Spectra
    9. 12.8 Infrared Spectra of Some Common Functional Groups
    10. Chemistry Matters—X-Ray Crystallography
    11. Key Terms
    12. Summary
    13. Additional Problems
  14. 13 Structure Determination: Nuclear Magnetic Resonance Spectroscopy
    1. Why This Chapter?
    2. 13.1 Nuclear Magnetic Resonance Spectroscopy
    3. 13.2 The Nature of NMR Absorptions
    4. 13.3 Chemical Shifts
    5. 13.4 Chemical Shifts in 1H NMR Spectroscopy
    6. 13.5 Integration of 1H NMR Absorptions: Proton Counting
    7. 13.6 Spin–Spin Splitting in 1H NMR Spectra
    8. 13.7 1H NMR Spectroscopy and Proton Equivalence
    9. 13.8 More Complex Spin–Spin Splitting Patterns
    10. 13.9 Uses of 1H NMR Spectroscopy
    11. 13.10 13C NMR Spectroscopy: Signal Averaging and FT–NMR
    12. 13.11 Characteristics of 13C NMR Spectroscopy
    13. 13.12 DEPT 13C NMR Spectroscopy
    14. 13.13 Uses of 13C NMR Spectroscopy
    15. Chemistry Matters—Magnetic Resonance Imaging (MRI)
    16. Key Terms
    17. Summary
    18. Additional Problems
  15. 14 Conjugated Compounds and Ultraviolet Spectroscopy
    1. Why This Chapter?
    2. 14.1 Stability of Conjugated Dienes: Molecular Orbital Theory
    3. 14.2 Electrophilic Additions to Conjugated Dienes: Allylic Carbocations
    4. 14.3 Kinetic versus Thermodynamic Control of Reactions
    5. 14.4 The Diels–Alder Cycloaddition Reaction
    6. 14.5 Characteristics of the Diels–Alder Reaction
    7. 14.6 Diene Polymers: Natural and Synthetic Rubbers
    8. 14.7 Ultraviolet Spectroscopy
    9. 14.8 Interpreting Ultraviolet Spectra: The Effect of Conjugation
    10. 14.9 Conjugation, Color, and the Chemistry of Vision
    11. Chemistry Matters—Photolithography
    12. Key Terms
    13. Summary
    14. Summary of Reactions
    15. Additional Problems
  16. 15 Benzene and Aromaticity
    1. Why This Chapter?
    2. 15.1 Naming Aromatic Compounds
    3. 15.2 Structure and Stability of Benzene
    4. 15.3 Aromaticity and the Hückel 4n + 2 Rule
    5. 15.4 Aromatic Ions
    6. 15.5 Aromatic Heterocycles: Pyridine and Pyrrole
    7. 15.6 Polycyclic Aromatic Compounds
    8. 15.7 Spectroscopy of Aromatic Compounds
    9. Chemistry Matters—Aspirin, NSAIDs, and COX-2 Inhibitors
    10. Key Terms
    11. Summary
    12. Additional Problems
  17. 16 Chemistry of Benzene: Electrophilic Aromatic Substitution
    1. Why This Chapter?
    2. 16.1 Electrophilic Aromatic Substitution Reactions: Bromination
    3. 16.2 Other Aromatic Substitutions
    4. 16.3 Alkylation and Acylation of Aromatic Rings: The Friedel–Crafts Reaction
    5. 16.4 Substituent Effects in Electrophilic Substitutions
    6. 16.5 Trisubstituted Benzenes: Additivity of Effects
    7. 16.6 Nucleophilic Aromatic Substitution
    8. 16.7 Benzyne
    9. 16.8 Oxidation of Aromatic Compounds
    10. 16.9 Reduction of Aromatic Compounds
    11. 16.10 Synthesis of Polysubstituted Benzenes
    12. Chemistry Matters—Combinatorial Chemistry
    13. Key Terms
    14. Summary
    15. Summary of Reactions
    16. Additional Problems
  18. 17 Alcohols and Phenols
    1. Why This Chapter?
    2. 17.1 Naming Alcohols and Phenols
    3. 17.2 Properties of Alcohols and Phenols
    4. 17.3 Preparation of Alcohols: A Review
    5. 17.4 Alcohols from Carbonyl Compounds: Reduction
    6. 17.5 Alcohols from Carbonyl Compounds: Grignard Reaction
    7. 17.6 Reactions of Alcohols
    8. 17.7 Oxidation of Alcohols
    9. 17.8 Protection of Alcohols
    10. 17.9 Phenols and Their Uses
    11. 17.10 Reactions of Phenols
    12. 17.11 Spectroscopy of Alcohols and Phenols
    13. Chemistry Matters—Ethanol: Chemical, Drug, and Poison
    14. Key Terms
    15. Summary
    16. Summary of Reactions
    17. Additional Problems
  19. 18 Ethers and Epoxides; Thiols and Sulfides
    1. Why This Chapter?
    2. 18.1 Names and Properties of Ethers
    3. 18.2 Preparing Ethers
    4. 18.3 Reactions of Ethers: Acidic Cleavage
    5. 18.4 Cyclic Ethers: Epoxides
    6. 18.5 Reactions of Epoxides: Ring-Opening
    7. 18.6 Crown Ethers
    8. 18.7 Thiols and Sulfides
    9. 18.8 Spectroscopy of Ethers
    10. Chemistry Matters—Epoxy Resins and Adhesives
    11. Key Terms
    12. Summary
    13. Summary of Reactions
    14. Additional Problems
    15. Preview of Carbonyl Chemistry
  20. 19 Aldehydes and Ketones: Nucleophilic Addition Reactions
    1. Why This Chapter?
    2. 19.1 Naming Aldehydes and Ketones
    3. 19.2 Preparing Aldehydes and Ketones
    4. 19.3 Oxidation of Aldehydes and Ketones
    5. 19.4 Nucleophilic Addition Reactions of Aldehydes and Ketones
    6. 19.5 Nucleophilic Addition of H2O: Hydration
    7. 19.6 Nucleophilic Addition of HCN: Cyanohydrin Formation
    8. 19.7 Nucleophilic Addition of Hydride and Grignard Reagents: Alcohol Formation
    9. 19.8 Nucleophilic Addition of Amines: Imine and Enamine Formation
    10. 19.9 Nucleophilic Addition of Hydrazine: The Wolff–Kishner Reaction
    11. 19.10 Nucleophilic Addition of Alcohols: Acetal Formation
    12. 19.11 Nucleophilic Addition of Phosphorus Ylides: The Wittig Reaction
    13. 19.12 Biological Reductions
    14. 19.13 Conjugate Nucleophilic Addition to α,β‑Unsaturated Aldehydes and Ketones
    15. 19.14 Spectroscopy of Aldehydes and Ketones
    16. Chemistry Matters—Enantioselective Synthesis
    17. Key Terms
    18. Summary
    19. Summary of Reactions
    20. Additional Problems
  21. 20 Carboxylic Acids and Nitriles
    1. Why This Chapter?
    2. 20.1 Naming Carboxylic Acids and Nitriles
    3. 20.2 Structure and Properties of Carboxylic Acids
    4. 20.3 Biological Acids and the Henderson–Hasselbalch Equation
    5. 20.4 Substituent Effects on Acidity
    6. 20.5 Preparing Carboxylic Acids
    7. 20.6 Reactions of Carboxylic Acids: An Overview
    8. 20.7 Chemistry of Nitriles
    9. 20.8 Spectroscopy of Carboxylic Acids and Nitriles
    10. Chemistry Matters—Vitamin C
    11. Key Terms
    12. Summary
    13. Summary of Reactions
    14. Additional Problems
  22. 21 Carboxylic Acid Derivatives: Nucleophilic Acyl Substitution Reactions
    1. Why This Chapter?
    2. 21.1 Naming Carboxylic Acid Derivatives
    3. 21.2 Nucleophilic Acyl Substitution Reactions
    4. 21.3 Reactions of Carboxylic Acids
    5. 21.4 Chemistry of Acid Halides
    6. 21.5 Chemistry of Acid Anhydrides
    7. 21.6 Chemistry of Esters
    8. 21.7 Chemistry of Amides
    9. 21.8 Chemistry of Thioesters and Acyl Phosphates: Biological Carboxylic Acid Derivatives
    10. 21.9 Polyamides and Polyesters: Step-Growth Polymers
    11. 21.10 Spectroscopy of Carboxylic Acid Derivatives
    12. Chemistry Matters—β-Lactam Antibiotics
    13. Key Terms
    14. Summary
    15. Summary of Reactions
    16. Additional Problems
  23. 22 Carbonyl Alpha-Substitution Reactions
    1. Why This Chapter?
    2. 22.1 Keto–Enol Tautomerism
    3. 22.2 Reactivity of Enols: α-Substitution Reactions
    4. 22.3 Alpha Halogenation of Aldehydes and Ketones
    5. 22.4 Alpha Bromination of Carboxylic Acids
    6. 22.5 Acidity of Alpha Hydrogen Atoms: Enolate Ion Formation
    7. 22.6 Reactivity of Enolate Ions
    8. 22.7 Alkylation of Enolate Ions
    9. Chemistry Matters—Barbiturates
    10. Key Terms
    11. Summary
    12. Summary of Reactions
    13. Additional Problems
  24. 23 Carbonyl Condensation Reactions
    1. Why This Chapter?
    2. 23.1 Carbonyl Condensations: The Aldol Reaction
    3. 23.2 Carbonyl Condensations versus Alpha Substitutions
    4. 23.3 Dehydration of Aldol Products: Synthesis of Enones
    5. 23.4 Using Aldol Reactions in Synthesis
    6. 23.5 Mixed Aldol Reactions
    7. 23.6 Intramolecular Aldol Reactions
    8. 23.7 The Claisen Condensation Reaction
    9. 23.8 Mixed Claisen Condensations
    10. 23.9 Intramolecular Claisen Condensations: The Dieckmann Cyclization
    11. 23.10 Conjugate Carbonyl Additions: The Michael Reaction
    12. 23.11 Carbonyl Condensations with Enamines: The Stork Enamine Reaction
    13. 23.12 The Robinson Annulation Reaction
    14. 23.13 Some Biological Carbonyl Condensation Reactions
    15. Chemistry Matters—A Prologue to Metabolism
    16. Key Terms
    17. Summary
    18. Summary of Reactions
    19. Additional Problems
  25. 24 Amines and Heterocycles
    1. Why This Chapter?
    2. 24.1 Naming Amines
    3. 24.2 Structure and Properties of Amines
    4. 24.3 Basicity of Amines
    5. 24.4 Basicity of Arylamines
    6. 24.5 Biological Amines and the Henderson–Hasselbalch Equation
    7. 24.6 Synthesis of Amines
    8. 24.7 Reactions of Amines
    9. 24.8 Reactions of Arylamines
    10. 24.9 Heterocyclic Amines
    11. 24.10 Spectroscopy of Amines
    12. Chemistry Matters—Green Chemistry II: Ionic Liquids
    13. Key Terms
    14. Summary
    15. Summary of Reactions
    16. Additional Problems
  26. 25 Biomolecules: Carbohydrates
    1. Why This Chapter?
    2. 25.1 Classification of Carbohydrates
    3. 25.2 Representing Carbohydrate Stereochemistry: Fischer Projections
    4. 25.3 D,L Sugars
    5. 25.4 Configurations of the Aldoses
    6. 25.5 Cyclic Structures of Monosaccharides: Anomers
    7. 25.6 Reactions of Monosaccharides
    8. 25.7 The Eight Essential Monosaccharides
    9. 25.8 Disaccharides
    10. 25.9 Polysaccharides and Their Synthesis
    11. 25.10 Some Other Important Carbohydrates
    12. Chemistry Matters—Sweetness
    13. Key Terms
    14. Summary
    15. Summary of Reactions
    16. Additional Problems
  27. 26 Biomolecules: Amino Acids, Peptides, and Proteins
    1. Why This Chapter?
    2. 26.1 Structures of Amino Acids
    3. 26.2 Amino Acids and the Henderson–Hasselbalch Equation: Isoelectric Points
    4. 26.3 Synthesis of Amino Acids
    5. 26.4 Peptides and Proteins
    6. 26.5 Amino Acid Analysis of Peptides
    7. 26.6 Peptide Sequencing: The Edman Degradation
    8. 26.7 Peptide Synthesis
    9. 26.8 Automated Peptide Synthesis: The Merrifield Solid-Phase Method
    10. 26.9 Protein Structure
    11. 26.10 Enzymes and Coenzymes
    12. 26.11 How Do Enzymes Work? Citrate Synthase
    13. Chemistry Matters—The Protein Data Bank
    14. Key Terms
    15. Summary
    16. Summary of Reactions
    17. Additional Problems
  28. 27 Biomolecules: Lipids
    1. Why This Chapter?
    2. 27.1 Waxes, Fats, and Oils
    3. 27.2 Soap
    4. 27.3 Phospholipids
    5. 27.4 Prostaglandins and Other Eicosanoids
    6. 27.5 Terpenoids
    7. 27.6 Steroids
    8. 27.7 Biosynthesis of Steroids
    9. Chemistry Matters—Saturated Fats, Cholesterol, and Heart Disease
    10. Key Terms
    11. Summary
    12. Additional Problems
  29. 28 Biomolecules: Nucleic Acids
    1. Why This Chapter?
    2. 28.1 Nucleotides and Nucleic Acids
    3. 28.2 Base Pairing in DNA
    4. 28.3 Replication of DNA
    5. 28.4 Transcription of DNA
    6. 28.5 Translation of RNA: Protein Biosynthesis
    7. 28.6 DNA Sequencing
    8. 28.7 DNA Synthesis
    9. 28.8 The Polymerase Chain Reaction
    10. Chemistry Matters—DNA Fingerprinting
    11. Key Terms
    12. Summary
    13. Additional Problems
  30. 29 The Organic Chemistry of Metabolic Pathways
    1. Why This Chapter?
    2. 29.1 An Overview of Metabolism and Biochemical Energy
    3. 29.2 Catabolism of Triacylglycerols: The Fate of Glycerol
    4. 29.3 Catabolism of Triacylglycerols: β-Oxidation
    5. 29.4 Biosynthesis of Fatty Acids
    6. 29.5 Catabolism of Carbohydrates: Glycolysis
    7. 29.6 Conversion of Pyruvate to Acetyl CoA
    8. 29.7 The Citric Acid Cycle
    9. 29.8 Carbohydrate Biosynthesis: Gluconeogenesis
    10. 29.9 Catabolism of Proteins: Deamination
    11. 29.10 Some Conclusions about Biological Chemistry
    12. Chemistry Matters—Statin Drugs
    13. Key Terms
    14. Summary
    15. Additional Problems
  31. 30 Orbitals and Organic Chemistry: Pericyclic Reactions
    1. Why This Chapter?
    2. 30.1 Molecular Orbitals of Conjugated Pi Systems
    3. 30.2 Electrocyclic Reactions
    4. 30.3 Stereochemistry of Thermal Electrocyclic Reactions
    5. 30.4 Photochemical Electrocyclic Reactions
    6. 30.5 Cycloaddition Reactions
    7. 30.6 Stereochemistry of Cycloadditions
    8. 30.7 Sigmatropic Rearrangements
    9. 30.8 Some Examples of Sigmatropic Rearrangements
    10. 30.9 A Summary of Rules for Pericyclic Reactions
    11. Chemistry Matters—Vitamin D, the Sunshine Vitamin
    12. Key Terms
    13. Summary
    14. Additional Problems
  32. 31 Synthetic Polymers
    1. Why This Chapter?
    2. 31.1 Chain-Growth Polymers
    3. 31.2 Stereochemistry of Polymerization: Ziegler–Natta Catalysts
    4. 31.3 Copolymers
    5. 31.4 Step-Growth Polymers
    6. 31.5 Olefin Metathesis Polymerization
    7. 31.6 Intramolecular Olefin Metathesis
    8. 31.7 Polymer Structure and Physical Properties
    9. Chemistry Matters—Degradable Polymers
    10. Key Terms
    11. Summary
    12. Additional Problems
  33. A | Nomenclature of Polyfunctional Organic Compounds
  34. B | Acidity Constants for Some Organic Compounds
  35. C | Glossary
  36. D | Periodic Table
  37. Answer Key
    1. Chapter 1
    2. Chapter 2
    3. Chapter 3
    4. Chapter 4
    5. Chapter 5
    6. Chapter 6
    7. Chapter 7
    8. Chapter 8
    9. Chapter 9
    10. Chapter 10
    11. Chapter 11
    12. Chapter 12
    13. Chapter 13
    14. Chapter 14
    15. Chapter 15
    16. Chapter 16
    17. Chapter 17
    18. Chapter 18
    19. Chapter 19
    20. Chapter 20
    21. Chapter 21
    22. Chapter 22
    23. Chapter 23
    24. Chapter 24
    25. Chapter 25
    26. Chapter 26
    27. Chapter 27
    28. Chapter 28
    29. Chapter 29
    30. Chapter 30
    31. Chapter 31
  38. Index

11.5 • Characteristics of the SN1 Reaction

Just as the SN2 reaction is strongly influenced by the structure of the substrate, the leaving group, the nucleophile, and the solvent, the SN1 reaction is similarly influenced. Factors that lower ∆G, either by lowering the energy level of the transition state or by raising the energy level of the ground state, favor faster SN1 reactions. Conversely, factors that raise ∆G, either by raising the energy level of the transition state or by lowering the energy level of the reactant, slow down the SN1 reaction.

The Substrate

According to the Hammond postulate (Section 7.10), any factor that stabilizes a high-energy intermediate also stabilizes the transition state leading to that intermediate. Because the rate-limiting step in an SN1 reaction is the spontaneous, unimolecular dissociation of the substrate to yield a carbocation, the reaction is favored whenever a stabilized carbocation intermediate is formed. The more stable the carbocation intermediate, the faster the SN1 reaction.

We saw in Section 7.9 that the stability order of alkyl carbocations is 3° > 2° > 1° > methyl. To this list we should also add the resonance-stabilized allyl and benzyl cations. Just as allylic radicals are unusually stable because the unpaired electron can be delocalized over an extended π orbital system (Section 10.4), so allylic and benzylic carbocations are unusually stable. (The word benzylic means “next to an aromatic ring.”) As Figure 11.13 indicates, an allylic cation has two resonance forms. In one form, the double bond is on the “left”; in the other form it’s on the “right.” A benzylic cation has five resonance forms, all of which contribute to the overall resonance hybrid.

Resonance forms and electrostatic potential maps of allyl carbocation and benzyl carbocation. Two resonance forms of allyl carbocation and five resonance forms of benzyl carbocation.
Figure 11.13 Resonance forms of allylic and benzylic carbocations. The positive charge is delocalized over the π system in both. Electron-poor atoms are indicated by blue arrows.

Because of resonance stabilization, a primary allylic or benzylic carbocation is about as stable as a secondary alkyl carbocation, and a secondary allylic or benzylic carbocation is about as stable as a tertiary alkyl carbocation. This stability order of carbocations is the same as the order of SN1 reactivity for alkyl halides and tosylates.

Carbocation stability in increasing order: methyl, primary, allylic or benzylic (these are equivalent), secondary, and tertiary.

We should also note parenthetically that primary allylic and benzylic substrates are particularly reactive in SN2 reactions as well as in SN1 reactions. Allylic and benzylic C–X bonds are about 50 kJ/mol (12 kcal/mol) weaker than the corresponding saturated bonds and are therefore more easily broken.

C Cl bond strength in kJ per mole for ethyl chloride (338), allyl chloride (289), and benzyl chloride (293).
Problem 11-11

Rank the following substances in order of their expected SN1 reactivity:

Chemical structures of ethyl bromide, 3-bromo-1-butene, vinyl bromide, and 2-bromopropane.
Problem 11-12
3-Bromo-1-butene and 1-bromo-2-butene undergo SN1 reaction at nearly the same rate, even though one is a secondary halide and the other is primary. Explain.

The Leaving Group

We said during the discussion of SN2 reactivity that the best leaving groups are those that are most stable; that is, those that are the conjugate bases of strong acids. An identical reactivity order is found for the SN1 reaction because the leaving group is directly involved in the rate-limiting step. Thus, the SN1 reactivity order is

Leaving group reactivity in increasing order: hydroxyl, chloride, bromide, iodide or T o S O ion (these are equivalent), and water.

Note that in the SN1 reaction, which is often carried out under acidic conditions, neutral water is sometimes the leaving group. This occurs, for example, when an alkyl halide is prepared from a tertiary alcohol by reaction with HBr or HCl (Section 10.5). As shown in Figure 11.14, the alcohol is first protonated and then spontaneously loses H2O to generate a carbocation, which reacts with halide ion to give the alkyl halide. Knowing that an SN1 reaction is involved in the conversion of alcohols to alkyl halides explains why the reaction works well only for tertiary alcohols. Tertiary alcohols react fastest because they give the most stable carbocation intermediates.

Figure 11.14 MECHANISM
The mechanism of the SN1 reaction of a tertiary alcohol with HBr to yield an alkyl halide. Neutral water is the leaving group (step 2).
Mechanism of S N 1 between t-butyl alcohol and H Br. Alcohol is protonated, water leaves to form carbocation, bromide quenches carbocation.

The Nucleophile

The nature of the nucleophile plays a major role in the SN2 reaction but does not affect an SN1 reaction. Because the SN1 reaction occurs through a rate-limiting step in which the added nucleophile has no part, the nucleophile can’t affect the reaction rate. The reaction of 2-methyl-2-propanol with HX, for instance, occurs at the same rate regardless of whether X is Cl, Br, or I. Furthermore, neutral nucleophiles are just as effective as negatively charged ones, so SN1 reactions frequently occur under neutral or acidic conditions.

A reaction between 2-methyl-2-propanol and hydrogen halide (H-X) forms t-butyl halide and water. Text reads same rate for X equals Cl, Br, I.

The Solvent

What about the solvent? Do solvents have the same effect in SN1 reactions that they have in SN2 reactions? The answer is both yes and no. Yes, solvents have a large effect on SN1 reactions, but no, the reasons for the effects on SN1 and SN2 reactions are not the same. Solvent effects in the SN2 reaction are due largely to stabilization or destabilization of the nucleophile reactant, while solvent effects in the SN1 reaction are due largely to stabilization or destabilization of the transition state.

The Hammond postulate says that any factor stabilizing the intermediate carbocation should increase the rate of an SN1 reaction. Solvation of the carbocation—the interaction of the ion with solvent molecules—has such an effect. Solvent molecules orient around the carbocation so that the electron-rich ends of the solvent dipoles face the positive charge (Figure 11.15), thereby lowering the energy of the ion and favoring its formation.

Orbital diagram of solvation of a carbocation (with empty s p 2 orbital) by water. Water molecules are oriented with oxygens toward carbocation.
Figure 11.15 Solvation of a carbocation by water. The electron-rich oxygen atoms of solvent molecules orient around the positively charged carbocation and thereby stabilize it.

The properties of a solvent that contribute to its ability to stabilize ions by solvation are related to the solvent’s polarity. SN1 reactions take place much more rapidly in strongly polar solvents, such as water and methanol, than in less polar solvents, such as ether and chloroform. In the reaction of 2-chloro-2-methylpropane, for example, a rate increase of 100,000 is observed upon going from ethanol (less polar) to water (more polar). The rate increases when going from a hydrocarbon solvent to water are so large they can’t be measured accurately.

2-chloro-2-methylpropane reacts with R-OH giving HCl and a t-butyl ether. Solvent reactivity increasing order: ethanol, 40% water by 60% ethanol, 80% water by 20% ethanol, and water.

It should be emphasized again that both the SN1 and the SN2 reaction show solvent effects, but that they do so for different reasons. SN2 reactions are disfavored in protic solvents because the ground-state energy of the nucleophile is lowered by solvation. SN1 reactions are favored in protic solvents because the transition-state energy leading to carbocation intermediate is lowered by solvation.

A Summary of SN1 Reaction Characteristics

The effects on SN1 reactions of the four variables—substrate, leaving group, nucleophile, and solvent—are summarized in the following statements:

Substrate The best substrates yield the most stable carbocations. As a result, SN1 reactions are best for tertiary, allylic, and benzylic halides.
Leaving group Good leaving groups increase the reaction rate by lowering the energy level of the transition state for carbocation formation.
Nucleophile The nucleophile must be nonbasic to prevent a competitive elimination of HX (Section 11.7), but otherwise does not affect the reaction rate. Neutral nucleophiles work well.
Solvent Polar solvents stabilize the carbocation intermediate by solvation, thereby increasing the reaction rate.

Worked Example 11.2

Predicting the Mechanism of a Nucleophilic Substitution Reaction

Predict whether each of the following substitution reactions is likely to be SN1 or SN2:

1-chloropropylbenzene reacts with sodium acetate in acetic acid and water. Chlorine in the reactant is replaced by O Ac in the product. 3-bromopropylbenzene reacts with sodium methoxide in dimethylformamide. Bromine in the reactant is replaced by methoxy group in the product.

Strategy

Look at the substrate, leaving group, nucleophile, and solvent. Then decide from the summaries at the ends of Section 11.3 and Section 11.5 whether an SN1 or an SN2 reaction is favored. SN1 reactions are favored by tertiary, allylic, or benzylic substrates, by good leaving groups, by nonbasic nucleophiles, and by protic solvents. SN2 reactions are favored by primary substrates, by good leaving groups, by good nucleophiles, and by polar aprotic solvents.

Solution

(a) This is likely to be an SN1 reaction because the substrate is secondary and benzylic, the nucleophile is weakly basic, and the solvent is protic.

(b) This is likely to be an SN2 reaction because the substrate is primary, the nucleophile is a good one, and the solvent is polar aprotic.

Problem 11-13
Predict whether each of the following substitution reactions is likely to be SN1 or SN2:
(a)
A secondary alcohol reacts with HCl and methanol. Hydroxyl in the reactant is replaced by chlorine in the product.
(b)
3-bromo-2-methylpropene reacts with sodium methanethiolate and acetonitrile. Bromine in the reactant is replaced by S C H 3 group in the product.
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