Lesson 24 of 34
Choose between acid-base reaction and changing the acyl group
Salt formation leaves the carbon skeleton intact; reduction and substitution transform the functional group.
A-Level 9476 (2026-2027)
I can describe four carboxylic-acid transformations.Syllabus
Syllabus 9476, 11.8(b). Describe four carboxylic-acid transformations.
Choose between acid-base reaction and changing the acyl group
Salt formation leaves the carbon skeleton intact; reduction and substitution transform the functional group.
| Transformation | Reagents/conditions | Representative equation |
|---|---|---|
| Salt with reactive metal | Suitable metal such as sodium or magnesium. | 2CH3COOH + Mg → (CH3COO)2Mg + H2. |
| Salt with alkali | Aqueous hydroxide. | CH3COOH + OH- → CH3COO- + H2O. |
| Salt with carbonate | Aqueous carbonate; effervescence. | 2CH3COOH + CO32- → 2CH3COO- + CO2 + H2O. |
| Ester | Alcohol, concentrated H2SO4 catalyst, heat. | CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O. |
| Acyl chloride | PCl5. | CH3COOH + PCl5 → CH3COCl + POCl3 + HCl. |
| Primary alcohol | LiAlH4 in dry ether, followed by aqueous work-up. | CH3COOH + 4[H] → CH3CH2OH + H2O. |
Carboxylic acids release CO2 with hydrogencarbonate or carbonate. Phenol does not, under the usual test conditions. Benzoic acid forms the same kinds of products as ethanoic acid. In esterification, the part from the acid keeps its C=O, and the alcohol provides the group joined through oxygen.
Esterification is reversible. Using one reactant in excess, or removing a product, raises the equilibrium yield. Concentrated sulfuric acid is the catalyst. Using more catalyst does not change the equilibrium constant at a fixed temperature. Heat under reflux. Then choose separation, washing, drying and distillation steps that suit how soluble and volatile the product is.
Check your understanding
Which test tells ethanoic acid apart from phenol?
Show the answer
Add sodium carbonate: only the acid fizzes, giving CO2
Carboxylic acids are strong enough to release CO2 from carbonate or hydrogencarbonate. Phenol is too weak an acid to do this under the usual conditions.