2025 CH3_LecturePPT.pptx

Chapter 3: Acids and Bases

Overview of Organic Chemistry

• Reference: Organic Chemistry Fourth Edition by David Klein.

• The chapter focuses on the principles of acids and bases in organic chemistry, including their definitions, behaviors, and importance in chemical reactions.

3.1 Bronsted-Lowry Acids and Bases

• Brønsted-Lowry Definition:

  • Acids: donate a proton (H+).

  • Bases: accept a proton (H+).

  • Example: Water (H2O) can act both as an acid and a base.

3.1 Conjugate Acids and Bases

• Conjugate Acid: formed when a base accepts a proton.

• Conjugate Base: formed when an acid donates a proton.

• Relationship Example:

  • If HA is an acid, A− is its conjugate base after HA donates a proton.

3.2 Curved Arrows in Reactions

• Illustrates the movement of electrons during acid-base reactions:

  • Single-Step Reactions: The base attacks the acid, indicating two pairs of electrons move simultaneously—one for bond breaking and another for bond formation.

  • Multistep Reactions: Identify proton transfer steps within a series of reactions.

3.3 Quantifying Acidity

Introduction

• Understanding acid and base strength aids in predicting reaction behavior.

  • Quantitative analysis: Using pK a values to compare acid strengths.

  • Qualitative analysis: Analyzing structural stability of acids/bases.

K a

• Acid Dissociation Constant (K a): Measures the strength of an acid in water.

  • K a > 1: strong acid; K a < 1: weak acid.

  • pK a values range from -10 to 50; lower pK a indicates a stronger acid.

Common Compounds Table

• Refer to Table 3.1 for pK a values of several acids.

  • Example: H2SO4 (pK a = -9) is 100 times stronger than HCl (pK a = -7).

3.4 Qualifying Acidity

Introduction

• To assess acid strength without pK a values, evaluate the stability of conjugate bases.

  • Stronger acids have more stable conjugate bases.

Factors Affecting Stability (ARIO)

1. Atom: The atom bearing the charge influences stability.

   • Down a column: size (larger atoms stabilize negative charges better).

   • Across a period: electronegativity (more electronegative atoms stabilize negative charges better).

2. Resonance: Stabilizes lone pairs by delocalization across multiple atoms.

3. Induction: Electron-withdrawing groups stabilize a negative charge by pulling electron density.

4. Orbital: The type of orbital (s vs. p) affects charge stability; s-character indicates closer proximity to nucleus, enhancing stability.

Limitations of ARIO

• ARIO is a guideline; exceptions occur in certain cases requiring direct pK a comparisons.

3.10 Lewis Acids and Bases

• Lewis Definition:

  • Lewis Acid: accepts a pair of electrons.

  • Lewis Base: donates a pair of electrons.

• Some reactions fit the Lewis definition but not the Brønsted-Lowry definition.

Conclusion

• Understanding acid-base equilibria is crucial for predicting the direction of chemical reactions.

• Mastery of pK a values, stability comparisons, and definitions of acids/bases is essential for success in organic chemistry.