Physical Properties
Carboxylic acids have anomalously high boiling points for their molecular weight. Acetic acid (MW 60) boils at 118°C - higher than 1-propanol (MW 60, 97°C) and much higher than propanal (MW 58, 48°C). The reason is a unique feature of carboxylic acids: they form hydrogen-bonded dimers in the liquid state.
The Cyclic Dimer
Two carboxylic acid molecules face each other so that each one’s -COOH can hydrogen-bond to the other’s. The result is a planar, cyclic 8-membered ring with two simultaneous hydrogen bonds:
R-C(=O)-O-H · · · O=C-(O-H) · · · O=C-R
Each molecule donates one H (from its OH) to the other’s C=O, and accepts an H (on its C=O) from the other’s OH. Both donor-acceptor relationships happen simultaneously.
The cyclic dimer is stable enough to persist partially in the gas phase - acetic acid vapor contains significant amounts of dimer at moderate temperatures. In the solid and liquid phases, dimerization dominates.
Boiling Points Comparison
For C2 molecules:
| Compound | MW | Boiling point | IMF |
|---|---|---|---|
| Ethane (CH₃CH₃) | 30 | -89°C | London |
| Ethanol (CH₃CH₂OH) | 46 | 78°C | H-bond (monomer) |
| Acetic acid (CH₃COOH) | 60 | 118°C | H-bond (dimer) |
Even though acetic acid is larger than ethane and similar-sized to ethanol, its dimer makes it behave like a “C4 molecule” (MW 120) for boiling-point purposes.
Water Solubility
Short-chain carboxylic acids (C1-C4) are fully miscible with water in all proportions. They hydrogen-bond strongly with water on both the carbonyl oxygen (H-bond acceptor) and the -OH (both donor and acceptor).
Longer-chain carboxylic acids (C6 and up) become progressively less water-soluble as the hydrophobic chain grows. Fatty acids (C12-C24) are essentially insoluble in water but form micelles and bilayers in aqueous environments - the basis of soap and membrane chemistry.
Melting Points
Due to strong intermolecular H-bonding (even more extensive in solid dimers), carboxylic acids often have relatively high melting points compared to similar-sized alcohols or ketones:
- Formic acid: 8°C.
- Acetic acid: 17°C (hence “glacial” acetic acid, which becomes a crystalline solid below this temperature).
- Benzoic acid: 122°C.
- Stearic acid: 69°C.
Dicarboxylic Acids Have Even Higher Melting Points
Diacids like succinic acid (C4, 188°C) and malonic acid (C3, 136°C) have higher melting points than monoacids because they can form extensive hydrogen-bonded networks with multiple partners. The more -COOH groups per molecule, the more potential H-bonds, and the tighter the solid-state packing.
Solvation in Water
When a short-chain carboxylic acid dissolves in water, it:
- Breaks the cyclic dimer structure.
- Forms new H-bonds with water molecules.
- Partially dissociates (pKa ~4-5) to the carboxylate anion.
At physiological pH (7), carboxylic acids are almost entirely deprotonated. Blood acetic acid is essentially all acetate at pH 7.4. Similarly, amino acid carboxyl groups, lactate, pyruvate, and Krebs cycle intermediates all exist as their carboxylate forms at physiological pH.