Chapter 8 · Chemistry
Why Carboxylic Acids Are Acidic: Resonance and Substituents
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Why vinegar fizzes with baking soda: the carboxylate ion shares its charge equally between two oxygens (resonance, arrow by arrow), which makes carboxylic acids stronger than phenols and alcohols; electron-withdrawing groups such as chlorine make them stronger still.
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Esterification and the C–OH Swap: Esters, Acid Chlorides, Anhydrides, AmidesFruity esters from an acid and an alcohol, with the acid-catalysed mechanism drawn arrow by arrow, and three more ways to replace the –OH of –COOH: acid chlorides (best with SOCl₂), anhydrides and amides.Chemistry
Key points
- Carboxylic acids liberate CO₂ from NaHCO₃ (a test for –COOH); alcohols and phenol do not.
- RCOOH ⇌ RCOO⁻ + H⁺: the more stable the carboxylate, the stronger the acid.
- In RCOO⁻, resonance shares the charge equally between two oxygens; both C–O bonds are equal.
- Acidity: RCOOH (pKa ≈ 4.8) > phenol (≈ 10) > alcohol (≈ 16).
- Electron-withdrawing groups raise acidity (–I effect): pKa 4.8 → 2.9 → 1.3 → 0.7 for 0 → 3 Cl.
Resonance in the carboxylate ion
R–C(=O)–O⁻ ⟷ R–C(–O⁻)=O
An O⁻ lone pair moves in to make C=O; the old C=O π pair moves onto the other oxygen. The two structures are equivalent, so the charge is shared equally and the ion is stable.
Acid, phenol, alcohol
| Anion | Charge spread over | pKa of the acid |
|---|---|---|
| RCOO⁻ | two equal O atoms | ≈ 4–5 |
| C₆H₅O⁻ | O and ring carbons (less electronegative) | ≈ 10 |
| CH₃CH₂O⁻ | one O (no resonance) | ≈ 16 |
Chlorine makes it stronger
| Acid | pKa |
|---|---|
| CH₃COOH | 4.76 |
| ClCH₂COOH | 2.86 |
| Cl₂CHCOOH | ≈ 1.3 |
| Cl₃CCOOH | ≈ 0.7 |
Each Cl pulls electron density through the σ bonds (–I effect), spreading the carboxylate's charge further. Electron-donating alkyl groups do the opposite.
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