Carboxylic acids
Carboxylic acids contain the –COOH group. They are weak acids: they only partly ionise in water, RCOOH ⇌ RCOO⁻ + H⁺. Hydrogen bonding gives them high boiling points, and they can form dimers. They are stronger acids than alcohols and phenol. They react with metals, bases and carbonates like other acids.
| Acid | Source or use |
|---|---|
| Methanoic acid (HCOOH) | Ant stings; used to coagulate rubber latex |
| Ethanoic acid (CH₃COOH) | Vinegar, 5% solution; making plastics and acetate fibres |
| Benzoic acid (C₆H₅COOH) | Food preservative |
| Citric acid | Citrus fruit |
| Fatty acids | Fats and oils; soap |
- acid dissociation constant
- −log K_a
Try itpH of a weak acid
Choose the concentration and the pKₐ of a weak acid. Find the pH, assuming only a small fraction ionises.
Esters, acyl chlorides and amides
| Derivative | Made from | Properties |
|---|---|---|
| Ester, RCOOR′ | Carboxylic acid + alcohol, concentrated H₂SO₄ catalyst, heat (reversible) | Sweet, fruity smells; solvents and flavours; fats and oils are esters of glycerol |
| Acyl chloride, RCOCl | Carboxylic acid + SOCl₂ or PCl₅ | Very reactive; fumes in moist air; used to make esters and amides |
| Amide, RCONH₂ | Acyl chloride + ammonia, or heating the ammonium salt | Neutral; found in proteins and nylon |
| Acid anhydride | Two acids joined with loss of water | Reactive; aspirin is made with ethanoic anhydride |
| Hydrolysis of an ester | Conditions | Products |
|---|---|---|
| Acid hydrolysis | Heat with dilute acid (reversible) | Carboxylic acid + alcohol |
| Alkaline hydrolysis (saponification) | Heat with aqueous NaOH (goes to completion) | Sodium carboxylate (soap if the acid is a fatty acid) + alcohol |
Key terms
- Carboxylic acid
- A compound containing the –COOH group.
- Ester
- A compound RCOOR′ made from an acid and an alcohol.
- Saponification
- The alkaline hydrolysis of fats and oils to make soap.
- Acid dissociation constant
- K_a, a measure of the strength of a weak acid.
Summary
- Carboxylic acids are weak acids; K_a and pK_a measure their strength.
- They form esters, acyl chlorides and amides.
- Esters are hydrolysed by acid (reversible) or alkali (complete: saponification).
- Fats and oils are esters of glycerol and fatty acids.