Chapter+5

Chapter 5 – Chemical Reactions and Mechanisms

I. Introduction

  • Overview of topics: Types of organic reactions, Acid-Base reactions, Reaction mechanisms, Thermodynamics and Kinetics, Reaction coordinate diagram, Catalysis

II. Types of Organic Reactions

  • Proton Transfer

  • Substitution Reaction

  • Addition Reaction

  • Elimination Reaction

  • Rearrangement Reaction

  • Oxidation-Reduction Reaction

III. Bond Making and Breaking

  • Homolytic Cleavage: Free radicals (fishhook arrows)

  • Heterolytic Cleavage: Forms ions (double-headed arrows)

  • Concerted Reactions: Simultaneous bond breaking and making; promoted by light (hν) or heat (Δ)

  • Pericyclic Reactions: A type of concerted reaction

IV. Acid-Base Reactions

  • Bronsted Acid: Donates a proton (H+)

  • Bronsted Base: Accepts a proton

  • Equilibrium: Formation of weaker acid and base favored

Strength of Acids (pKa)

  • Lower pKa → stronger acid

  • Example values: HI = -10 (very strong), CH4 = +50 (very weak)

V. Acid Dissociation in Water

  • Measurable pKa Range: -1.74 to 15.7

  • pKa < -1.74: Completely dissociated

  • pKa > 15.7: Not dissociated

VI. Importance of pKa Values

  • Equilibrium Constant (Keq) Calculation:log Keq = pKa (product acid) - pKa (reactant acid)

    • Example: log Keq = 6 favors forward reaction

VII. Factors Influencing Acid-Base Strength

  • Conjugate Base Stability: Stability indicates stronger acids

  • Element Effect: Acid strength increases as CH4 < NH3 < H2O < HF

    • Conjugate base strength reverses

  • Example of Acid Strength: HF < HCl < HBr < HI; F- > Cl- > Br- > I-

VIII. Additional Factors Affecting Acid Strength

  • Orbital Hybridization: More s character = lower pKa

  • Inductive Effect: Electronegativity stabilizes conjugate anion

IX. Resonance Effect in Acidity

  • Stabilizes conjugate base post-deprotonation

  • Example: Phenol stronger than cyclohexanol

X. Selected pKa Values

  • Key Values: R-CH₂-H: 50, R-CHO: 44

XI. Strength of Bases

  • Amine as a Base: Amphiprotic, protonated amines as weak acids

XII. Factors Influencing Basic Strength

  • Hydrocarbon groups enhance basicity via inductive effects

  • Primary amines stronger than ammonia due to stabilization

XIII. Concepts of Lewis Acids and Bases

  • Lewis Acid: Accepts electron pairs

  • Lewis Base: Donates electron pairs

XIV. Organic Reaction Mechanism

  • Defines stepwise electron movements during reactions

    • Nucleophiles donate, electrophiles accept

XV. Thermodynamics and Kinetics

  • Gibbs free energy influences reaction direction

XVI. Kinetics Overview

  • Rate laws relate to concentration and reaction speed

XVII. Catalysis Overview

  • Enhances rates and mechanisms; catalysts remain unchanged post-reaction

XVIII. Biochemical Reactions

  • Significance under biological conditions, overview of enzyme roles in reactions