Acids and Bases
High-Yield Summary
- Brønsted-Lowry: acid = proton (H⁺) donor, base = proton acceptor. Donating a proton forms a conjugate base; accepting one forms a conjugate acid.
- Lewis: acid = electron pair acceptor, base = electron pair donor — broader, includes reactions without proton transfer. Every Brønsted-Lowry reaction is also a Lewis reaction.
- Ka = [H₃O⁺][A⁻]/[HA]; larger Ka = stronger acid. pKa = −log(Ka); lower pKa = stronger acid.
- Stronger acids have more stable (weaker) conjugate bases — the conjugate base of a strong acid is weak, and vice versa.
- Resonance stabilization of the conjugate base explains the biggest acidity jumps: carboxylate ions and enolates are far more stable than a non-resonance-stabilized alkoxide.
Acid Dissociation Constant
Ka = [H₃O⁺][A⁻] / [HA]; pKa = −log(Ka)
- Ka = Acid dissociation constant — larger value = stronger acid
- pKa = −log(Ka) — lower value = stronger acid
Functional Group Acidity Ladder (weakest → strongest)
| Group | Approx. pKa |
|---|---|
| Alkanes | ≈ 50 |
| Alkenes | ≈ 43 |
| H₂ | ≈ 42 |
| Amines (e.g. NH₃) | ≈ 35 |
| Terminal alkynes ≈ Esters | ≈ 25 |
| Aldehydes / Ketones | ≈ 17–20 / ≈ 20–24 |
| Alcohols ≈ Water | ≈ 17 / ≈ 16 |
| Carboxylic acids | ≈ 4 |
| Hydronium ion (H₃O⁺) | ≈ −1.7 |
Key Terms
- Conjugate base / conjugate acid
- What remains after an acid donates a proton (conjugate base) or a base accepts one (conjugate acid).
- Coordinate covalent bond
- Bond formed when a Lewis acid and Lewis base react; both bonding electrons originate from the Lewis base.
- Carboxylate ion (R-COO⁻)
- Conjugate base of a carboxylic acid; negative charge delocalized equally across both oxygens via resonance, making it highly stable.
- Enolate
- Conjugate base of an aldehyde/ketone; delocalizes negative charge between carbon and oxygen, explaining their elevated acidity vs. alkanes.
Common MCAT Trap
- Lower pKa = stronger acid — easy to flip under time pressure; anchor to carboxylic acids (pKa ≈ 4, strong) vs. alkanes (pKa ≈ 50, essentially non-acidic).
- A strong acid's conjugate base is weak/stable, not strong — don't assume the conjugate base of a strong acid must be reactive.
- Carboxylic acids beat alcohols in acidity purely from conjugate-base resonance stabilization, not from any difference in the O-H bond itself.
Quick Recall
What are the Brønsted-Lowry and Lewis definitions of an acid?
Why are carboxylic acids (pKa ≈ 4) so much more acidic than alcohols (pKa ≈ 17)?
If Ka increases, what happens to pKa and acid strength?
Rank alkanes, alcohols, and carboxylic acids from weakest to strongest acid.