Chapter 8 · Chemistry
Why Carboxylic Acids Boil So High: Hydrogen-Bonded Dimers
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Four molecules of about the same mass boil from 0 °C to 118 °C. Carboxylic acids win because two hydrogen bonds join them into dimers that survive even in the vapour; the same dimers double acetic acid's measured molar mass in benzene.
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Why Carboxylic Acids Are Acidic: Resonance and SubstituentsWhy 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.Chemistry
Key points
- Similar mass (≈ 60), very different b.p.: butane ≈ 0 °C, propanal 49 °C, propan-1-ol 97 °C, acetic acid 118 °C.
- The O–H of one acid bonds to the C=O oxygen of another, and back: two hydrogen bonds make a cyclic dimer.
- The association is stronger and more extensive than in alcohols, and the dimers survive even in the vapour.
- In benzene, acetic acid's measured molar mass is about 120, double the true 60 (abnormal molar mass).
- Acids up to C4 are miscible with water; solubility falls as the chain grows.
Same mass, different boiling points
| Compound | Family | M (g/mol) | b.p. |
|---|---|---|---|
| butane | alkane | 58 | ≈ 0 °C |
| propanal | aldehyde | 58 | 49 °C |
| propan-1-ol | alcohol | 60 | 97 °C |
| ethanoic acid | carboxylic acid | 60 | 118 °C |
The dimer
Two acid molecules face each other: each O–H hydrogen bonds to the other molecule's C=O oxygen (O–H···O=C, twice), closing an eight-membered ring. Breaking this association needs more energy, so the boiling point is high; most acids remain dimers even in the vapour phase and in aprotic solvents such as benzene.
Solubility
- Up to four carbons: miscible with water (hydrogen bonds with water).
- Longer chains: the hydrocarbon part grows, solubility falls.
- Benzoic acid: nearly insoluble in cold water.
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