Study Notes on pKa Values

pKa Values to Know and Apply

1. Key Concepts

  • pKa Values: A measure of the acidity of a compound. The lower the pKa, the stronger the acid.

2. Common pKa Values and Corresponding Structures

  • Hydronium Ion (H₃O⁺) or Protonated Alcohol (ROH₂⁺)

    • Approximate pKa: < 0

  • Strong Acids:

    • Hydrochloric Acid (HCl)

      • Approximate pKa: < -8

    • Hydrobromic Acid (HBr)

      • Approximate pKa: < -8

    • Hydriodic Acid (HI)

      • Approximate pKa: < -8

  • Carboxylic Acid (RCOOH)

    • Approximate pKa: 5

  • Ammonium Ion (NH₄⁺) or Protonated Amines

    • Approximate pKa: 9 - 11

  • Pyridinium Ion

    • Approximate pKa: 5

  • Phenol (C₆H₅OH)

    • Approximate pKa: 10

  • Alcohol (ROH) or Water (H₂O)

    • Approximate pKa: 15

  • Ammonia (NH₃)

    • Approximate pKa: 38

  • Ethane (C₂H₆) or Alkyl Groups (RCH₃)

    • Approximate pKa: 50

  • Ethylene (C₂H₄)

    • Approximate pKa: 44

  • Acetylene (HC≡CH)

    • Approximate pKa: 25

3. General Notation

  • For all species, the formula used is represented as:

    • R indicates an alkyl group, which can vary in structure.

4. Visual Structure Representation

  • The chemical structures include functional groups such as hydroxyl (-OH) and amino (-NH₂) groups.

  • Structural notations are vital for understanding the qualitative and quantitative aspects of acidity as they allow visualization of molecular composition and electronic effects.