Ranking Acids and Bases

Ranking Acids and Bases - Fundamentals

  • Stronger Acid: Has a weaker conjugate base.

  • More Stable Base: Is a weaker base and typically has lower energy electrons (less reactive).

Mnemonic for Ranking Bases (Priority Order)

Most comparisons assume bases have the same charge. If not, Charge is the absolute first consideration.

  1. Charge (CARDIO/AARIO):

    • Negative charge: Higher electron energy, stronger base.

    • Positive charge: Lower electron energy, weaker base.

    • Often a moot point as comparisons usually involve bases with the same charge.

  2. Atom (REO/AARIO): Examines the atom bearing the negative charge or acting as the base.

    • Vertical (Same Group): Size is dominant. Smaller atom = stronger base (due to shorter, stronger bond to H^+), e.g., O^- is stronger base than S^-. This is an exception to the 'lower energy electrons = weaker base' trend; larger atoms have higher energy electrons but are weaker bases.

    • Horizontal (Same Period): Electronegativity is dominant. Less electronegative atom = less stable base = stronger base, e.g., N^- is stronger base than O^-.

    • Basicity increases up and to the left on the periodic table.

  3. Resonance (REO/AARIO):

    • Resonance stabilizes a base by delocalizing the negative charge, making it more stable and thus a weaker base.

    • Quality of resonance structures (atoms sharing the charge) is important, not just the number of structures.

  4. Induction (REO/AARIO):

    • Electron-withdrawing (electronegative) atoms nearby pull electron density away from the base, stabilizing it, making it weaker.

    • Factors enhancing inductive effect:

      • Proximity: Closer electronegative atom(s) exert a greater stabilizing effect.

      • Number: More electronegative atoms create a stronger stabilizing effect.

      • Electronegativity: More electronegative atom(s) exert a stronger stabilizing effect.

  5. Orbitals / Hybridization (REO/AARIO):

    • Relates to the s-character of the orbital holding the lone pair.

    • Higher s-character means electrons are closer to the nucleus, lower energy, more stable, and thus a weaker base.

    • Trend in stability (and decreasing basicity):

      • sp (50% s-character) > sp^2 (33% s-character) > sp^3 (25% s-character)

    • So, basicity: sp^3 hybridized base > sp^2 hybridized base > sp hybridized base.

Ranking Acids

  • To rank acids, evaluate the relative basicity of their conjugate bases using the mnemonic.

  • The stronger acid will come from the weaker (more stable) conjugate base.

Important Considerations / Exceptions

  • Memorize key pK_a values (e.g., carboxylic acids \$4-5, phenols \$10, alcohols \$16, terminal alkynes \$26, amines \$38). These can override simple rules in some cases (e.g., phenol vs. carboxylic acid resonance, alkyne vs. amine).

  • The mnemonic is a general guideline; exceptions exist, often resolved by pK_a knowledge.