Lecture on pKa and Acid-Base Chemistry

Jun 14, 2024

Lecture on pKa and Acid-Base Chemistry

Introduction

  • Continuing discussion on pKa and acid-base chemistry.
  • Focus on estimating pKa values using reference molecules and guiding principles.

Estimating pKa Values

Using Reference Molecules

  1. Find Similar Structures: Locate a molecule with the same functional group.
    • Example: Compare ethanol (pKa ~ 16) to isopropyl alcohol (pKa ~16.5).
  2. Approximate pKa: Use similar structures to estimate pKa.
    • Example: Benzoic acid's approximate pKa using acetic acid (pKa ~ 4.75).

Limitations

  • Strategy may fail with molecules influenced by electronegative atoms (e.g., TFA).
  • More advanced guiding principles needed when tables aren’t useful.

Guiding Principles (CARDINAL Rule)

  • C: Charge
  • A: Atoms
  • R: Resonance
  • D: Delocalization
  • I: Inductive Effects

Charge

  • More acidic when proton attached to positively charged atom.
  • Energy Diagram Insight: Higher activation energy indicates lower acidity.

Atoms

  • Trends within Periods: Acidity increases across a period (Left to Right).
    • Example: NH3 (pKa 36) vs. NH4+ (pKa 9.4).
  • Energy Levels: Higher electronegativity means more stable conjugate base.

Periodic Table Trends

  • Within Periods: Electronegativity determines stability of conjugate bases.
    • Example: Methane vs. Ammonia reactions and their conjugate bases.
  • Within Groups: Stability of conjugate bases depends more on size than electronegativity.
    • Example: HCl (pKa -7) vs. HF (pKa 3.2).
    • Size Argument: Larger atoms can stabilize negative charge better.

Resonance and Delocalization

  • Resonance Stabilization: Delocalization of negative charge in conjugate base increases acidity.
    • Example: Acetic acid (resonance) vs. Ethanol (no resonance).

Inductive Effects

  • Electronegative Substituents: Pull electron density and stabilize conjugate base.
    • Example: Fluorinated alcohol vs. simple alcohol.

Positive Charges

  • **Comparing Positively Charged Acids: ** More unstable acid is more acidic.
    • Electronegativity’s Role: More electronegative atoms with positive charges are less stable (e.g., Hydronium vs. Ammonium).

Effective Electronegativity

  • SP vs. SP2 vs. SP3 Hybridization: SP has more s-character, hence more effective electronegativity, stabilizing the conjugate base better.
    • Example: Alkynes are more acidic than alkanes and alkenes.

Applying Multiple Principles

  • Future discussions will involve using all principles together to analyze the acidity of complex molecules.

End of Video Note: The lecture covered examples focusing on one principle at a time; future classes will integrate multiple principles.

Conclusion

  • Guiding principles help predict pKa values without tables.
  • Importance of understanding stability and reactivity at the molecular level.