Carboxylic acids (HT)
Carboxylic acids contain the –COOH (carboxyl) functional group. They have general formula CₙH₂ₙ₊₁COOH and are named with the "-oic acid" ending.
The first four carboxylic acids
| Name | Formula |
|---|---|
| Methanoic acid | HCOOH |
| Ethanoic acid | CH₃COOH |
| Propanoic acid | C₂H₅COOH |
| Butanoic acid | C₃H₇COOH |
Ethanoic acid is the acid in vinegar.
Carboxylic acids are weak acids
In water, only a small fraction of molecules ionise:
CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq)
Note the ⇌ — partial ionisation (see C4.8). This makes carboxylic acids weaker than HCl/H₂SO₄: at the same concentration, carboxylic acids have higher pH (less acidic).
Reactions of carboxylic acids
1. With metal carbonates (test)
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
Bubbles of CO₂ given off; turns limewater cloudy. (This is how vinegar reacts with bicarbonate of soda.)
2. With reactive metals
Mg + 2CH₃COOH → Mg(CH₃COO)₂ + H₂
Slower than with HCl because [H⁺] is lower.
3. With alcohols → esters (info: see C7.7 polymers)
CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O
(Acid catalyst.) Esters smell sweet/fruity — used in flavourings and perfumes.
Salts of carboxylic acids
The –COOH loses a proton to give –COO⁻ (the carboxylate ion). With NaOH:
CH₃COOH + NaOH → CH₃COONa + H₂O
(sodium ethanoate)
Solubility and pH
Short-chain carboxylic acids are very soluble in water. Their pH at 0.1 mol/dm³ is typically 2–3 (vs ~1 for strong acids of the same concentration).
⚠Common mistakes
- Forgetting the ⇌ when writing ionisation — partial only.
- Using "carbon acid" — the correct names are methanoic, ethanoic, etc.
- Saying ethanoic acid doesn't react with metals — it does, just more slowly than HCl.
- Confusing –COOH with –CHO (aldehyde) — the carboxyl group has both =O and –OH on the same carbon.
Links
Builds on C7.5 (oxidation of alcohols), C4.8 (weak acids HT). Sets up C7.7 (condensation polymers).
AI-generated · claude-opus-4-7 · v3-deep-chemistry