Description and Properties
High-Yield Summary
- Carboxyl group = carbonyl + hydroxyl on the same carbon, always terminal — one of the most oxidized functional groups, second only to CO₂.
- IUPAC: -e → -oic acid, carboxyl carbon is always C1 (highest naming priority of any functional group). Cyclic: cycloalkane + "carboxylic acid" suffix. Salts: -ic acid → -ate. Dicarboxylic acids: -dioic acid.
- Strongly polar (C=O + O-H) → extensive hydrogen bonding → forms dimers in solution → unusually high boiling points vs. similarly sized molecules.
- Acidic because the carboxylate anion is resonance-stabilized (negative charge delocalized equally over both O atoms). Electron-withdrawing substituents increase acidity; electron-donating substituents decrease it.
- Dicarboxylic acids: 1st deprotonation easier than a comparable monoacid (two electron-withdrawing carboxyls stabilize each other); 2nd deprotonation harder (electrostatic repulsion between two negative charges).
Key Terms & Common Names
- Formic acid
- Common name for methanoic acid; found in ant venom.
- Acetic acid
- Common name for ethanoic acid; main component of vinegar.
- Propionic acid
- Common name for propanoic acid.
- Oxalic acid
- Common name for ethanedioic acid (dicarboxylic); binds calcium, forms kidney stones.
- Malonic acid
- Common name for propanedioic acid (dicarboxylic).
- Succinic acid
- Common name for butanedioic acid (dicarboxylic); central to the Krebs cycle.
- β-dicarboxylic acid
- Dicarboxylic acid with carboxyl groups separated by one carbon; that carbon's α-hydrogens are dramatically acidic (stabilized by resonance + inductive withdrawal from both carbonyls).
Dicarboxylic Acid: 1st vs. 2nd Deprotonation
| Deprotonation | Relative acidity vs. a comparable monoacid |
|---|---|
| 1st (removing first proton) | More acidic — both carboxyls' electronegative O's stabilize the resulting single negative charge |
| 2nd (removing second proton) | Less acidic — electrostatic repulsion between two negative charges destabilizes the dianion |
Common MCAT Trap
- Carboxylic acid always outranks other groups in IUPAC naming — carbon 1 is always the carboxyl carbon, even if a ketone or other group is also present (named as a substituent, e.g. "oxo-").
- Don't confuse the two deprotonation trends in a dicarboxylic acid: 1st is MORE acidic than a monoacid, 2nd is LESS acidic — opposite directions, easy to mix up.
- Electron-withdrawing substituents increase acidity (stabilize the anion); electron-donating substituents decrease it (destabilize the anion) — the same logic used throughout organic acid-base chemistry.
Quick Recall
Why do carboxylic acids have unusually high boiling points for their size?
Why is the carboxylate anion more stable than a simple alkoxide, making carboxylic acids more acidic than alcohols?
In a dicarboxylic acid, why is the second deprotonation harder than the first?