Lecture Notes: Analyzing Acidity in Molecules

Key Concepts

• Acidity Factors: When evaluating acidity, consider the following factors:
  • Inductive effect
  • Hybridization
  • Electronegativity
  • Electron delocalization (resonance)
  • Atomic size

Examples and Analysis

Example 1: NH2 vs. PH2

• Comparison: Nitrogen (N) vs. Phosphorus (P)
• Factor: Atomic size is more relevant because elements are in the same column.
  • Conclusion: Phosphorus is larger, stabilizes the negative charge better, hence PH2 is more acidic.
  • Reason: Atomic size

Example 2: Glycine vs. Acetic Acid

• pKa Values:
  • Acetic Acid: 4.75
  • Glycine (Carboxylic Acid group): 2.2 - 2.3
• Reason for Acidity:
  • Primary: Inductive effect due to nitrogen's electronegativity pulling electrons through sigma bonds.
  • Secondary: Electronegativity

Example 3: Hydrogen on Double Bond vs. Single Bond

• Hybridization:
  • SP2 vs. SP3 carbon
• pKa:
  • Alkene: ~44
  • Alkane: 50-60
• Conclusion: Alkene is more acidic.
• Reason: Hybridization

Example 4: Sulfur vs. Phosphorus

• Comparison: Same row elements
• Factor: Electronegativity
  • Conclusion: Sulfur is more electronegative; hence it is more acidic.
  • Reason: Electronegativity (EN)

Example 5: Alpha Hydrogen in Ketones

• pKa Values:
  • Alpha hydrogen next to a ketone: ~19
  • Alpha hydrogen between two 1,3-ketones: ~9
• Reason for Acidity:
  • Primary: Electron delocalization - negative charge distributed over two oxygen atoms.
  • Secondary: Inductive effect
  • Tertiary: Electronegativity (minor role)

Example 6: Allylic Hydrogen vs. Regular Alkane Hydrogen

• pKa:
  • Regular Alkane: 50-60
  • Allylic Position: 40-50
• Reason for Acidity:
  • Primary: Electron delocalization (resonance stabilization of the conjugate base)
  • Conclusion: Allylic hydrogen is more acidic due to better stabilization of negative charge on two carbon atoms.

Summary

• Acidity Strength: Determined by stability of the conjugate base and influenced by factors such as atomic size, electronegativity, hybridization, inductive effect, and electron delocalization.
• Prioritize Factors: Depending on the context (e.g., atomic size, inductive effect) different factors may play a more significant role in determining acidity.