Chapter 2: Acids and Bases

Lesson 1: Acid-Base Definitions

• Lewis acid: A substance that accepts electrons.

• Lewis base: A substance that donates electrons.

• Note: Focuses on electron pair donation/acceptance rather than proton transfer (though overlaps with Brønsted-Lowry).

Lesson 2: Conjugate Base-Acid Relationship and pH Scale

• Conjugate relationships:

  • Stronger acid → Weaker conjugate base.

  • Stronger base → Weaker conjugate acid.

• Acid strength metrics:

  • ↑ K_A (acid dissociation constant) = ↓ pK_A = ↑ acid strength.

• Stability and strength:

  • More stable/weaker conjugate base → Stronger acid.

  • More stable/weaker conjugate acid → Stronger base.

• pK_A values for organic compounds: (Refer to standard tables; e.g., carboxylic acids ~4-5, alcohols ~15-18, amines ~38).

• pH scale: Measures [H⁺]; relates to pK_A for determining protonation state.

Lesson 3: Ranking Acids and Bases with CARDIO (Charge)

• CARDIO mnemonic: Charge, Atom, Resonance, Dipole Induction, Orbitals (factors for ranking acidity/basicity).

• Charge factor: If other factors similar:

  • More positively-charged compound → More acidic.

  • More negatively-charged compound → More basic.

• Example: Compare H₃O⁺ (positive, acidic) vs. H₂O (neutral, less acidic).

Lesson 4: Ranking Acids and Bases with CARDIO (Atom)

• Atom factor: If other factors similar, H acidity increases as attached atom:

  • Goes left-to-right across a row (↑ electronegativity pulls electrons, stabilizes negative charge on conjugate base).

  • Goes down a column (↑ atomic size spreads negative charge, stabilizes conjugate base).

• Example: HF > H₂O > NH₃ > CH₄ (electronegativity); HI > HBr > HCl > HF (size).

Lesson 5: Ranking Acids and Bases with CARDIO (Resonance)

• Resonance factor:

  • More stable conjugate base (via resonance delocalization) → Stronger acid.

  • More stable conjugate acid → Stronger base.

• Resonance stabilizes charges by spreading them over multiple atoms.

• Example: Carboxylic acids (resonance in carboxylate ion) more acidic than alcohols (no resonance in alkoxide).

Lesson 6: Ranking Acids and Bases with CARDIO (Dipole Induction)

• Dipole Induction (Inductive effect):

  • Electron-withdrawing groups (EWGs) increase acidity (stabilize negative conjugate base).

  • Electron-donating groups (EDGs) decrease acidity (destabilize negative conjugate base).

• Effects stronger when closer to acidic site; e.g., halogens, nitro groups are EWGs; alkyl groups are EDGs.

• Example: Chloroacetic acid (Cl as EWG) more acidic than acetic acid.

Lesson 7: Ranking Acids and Bases with CARDIO (Orbitals)

• Orbitals factor: If other factors similar, acidity trend:

  • H–sp³ atom < H–sp² atom < H–sp atom (less to more acidic).

• Rationale: ↑ s-character (25% in sp³, 33% in sp², 50% in sp) → More electronegative orbital holds electrons closer, stabilizes conjugate base anion.

• Example: Terminal alkynes (sp, pK_A ~25) more acidic than alkenes (sp², pK_A ~44) or alkanes (sp³, pK_A ~50).

Lesson 8: Acid and Base Review

• Sorting acids/bases by strength:

  • Convert acid to conjugate base.

  • Apply CARDIO in order:

    1. Charge: Positive → acidic; negative → basic.

    2. Atom: More electronegative/larger atom with negative charge → Stronger acid.

    3. Resonance: More resonance stabilization → Stronger acid.

    4. Dipole Induction: EWGs → ↑ acidity; EDGs → ↓ acidity.

    5. Orbitals: ↑ s-character → ↑ acidity (sp > sp² > sp³).

• Use sequentially; stop at first differentiating factor.

Lesson 9: pH and Amino Acids

• Determining amino acid structure/charge:

  • pH < pK_A of functional group → Protonated (e.g., -COOH, -NH₃⁺).

  • pH > pK_A of functional group → Deprotonated (e.g., -COO⁻, -NH₂).

• Isoelectric point (pI): Average of relevant pK_A values; net charge zero.

• Key pK_A for amino acids: Carboxyl ~2, amino ~9-10, side chains vary (e.g., Asp ~4, Lys ~10).

• Example: At pH 7, most amino acids zwitterionic (-COO⁻ and -NH₃⁺).