Acidity
Carboxylic acids are about a million times more acidic than alcohols. Acetic acid has pKa 4.76; ethanol has pKa 16. That difference is about 11 pKa units, equal to a factor of 10¹¹ in Ka. The single reason is that the carboxylate conjugate base is stabilized by resonance, while the alkoxide conjugate base is not.
The Resonance Picture
When a carboxylic acid loses its proton (H⁺), the conjugate base has TWO equivalent resonance structures:
- Structure 1:
R-C(=O)-O⁻. Negative charge on the oxygen that used to have the H; C=O on the other oxygen. - Structure 2:
R-C(-O⁻)=O. Negative charge on the oxygen that was C=O; the former C-OH becomes a double bond.
These two structures are equivalent (same energy, same connectivity), so the real carboxylate anion is a 50:50 hybrid. The negative charge is distributed equally between the two oxygens, and both C-O bonds are equivalent (bond order 1.5).
Why This Makes Carboxylic Acids Strong Acids
The alkoxide anion from an alcohol (RCH₂-O⁻) has the negative charge on ONE oxygen with no resonance partner. Its energy is higher than a resonance-delocalized anion.
The carboxylate anion (RCOO⁻) spreads the same negative charge over TWO equivalent oxygens. Charge delocalization always reduces energy. The more atoms sharing a charge, the more stable the anion.
Quantifying the effect: carboxylate’s resonance stabilization is about 30-40 kJ/mol, which corresponds to about 10¹¹ in Ka. The observed pKa difference (16 - 4.8 ≈ 11) matches perfectly.
Comparison with Phenols
Phenols (aromatic ring -OH) have pKa ~10. Their conjugate base (phenoxide) delocalizes charge into the aromatic ring (4 contributing structures, hitting C2, C4, and C6 positions). Phenol is about a million times more acidic than ethanol but a million times less acidic than a typical carboxylic acid.
Why the ranking carboxylic acid > phenol > alcohol?
- Carboxylate: 2 equivalent atoms sharing charge; both are electronegative oxygens.
- Phenoxide: 4 atoms sharing charge; but 3 of them are carbons (less electronegative than O); only 1 is oxygen. Net effect: less stabilization than carboxylate despite more resonance partners.
- Alkoxide: 1 atom (oxygen); no resonance.
The quality of the atoms sharing the charge matters as much as the number of atoms.
pKa Values to Memorize
| Substrate | pKa |
|---|---|
| Strong mineral acids (HCl, H₂SO₄) | < 0 |
| Halo-carboxylic acids (e.g., chloroacetic) | 2-3 |
| Typical carboxylic acids | 4-5 |
| Aliphatic amino acid alpha-COOH | ~2 |
| Phenols (unsubstituted) | 10 |
| Ammonium (R-NH₃⁺) | 9-10 |
| Water | 15.7 |
| Alcohols | 16-18 |
| Terminal alkyne | 25 |
| Alpha-H of ester | 25 |
| Alpha-H of ketone | 20 |
| Alkane | ~50 |
Anchor: carboxylic acid pKa ≈ 4-5 is a memorize-now number. It is the most common acid on the MCAT and a biological pKa benchmark.
Behavior at Physiological pH
At pH 7, a carboxylic acid with pKa 4 is about 3 pKa units above its pKa. From Henderson-Hasselbalch: 10^(7-4) = 1000-fold preference for the deprotonated form. So carboxylic acids at physiological pH are essentially 99.9% carboxylate.
This is critical in biochemistry. “Acetic acid” as metabolized by cells is really acetate; “fatty acid” in the bloodstream is fatty acid anion (bound to albumin); “lactic acid” produced in muscle is lactate. The dissociation is essentially complete under biological conditions.