4.8.5Unit 4. Rates, Equilibria and Further Organic Chemistry
Acid and Alkaline Hydrolysis of Esters — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 4 parts Lesson 49 of 56 Unit quiz
Curriculum Equation Key Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Part 1 of 4Acid Hydrolysis
Hydrolysis breaks the ester linkage using water.
Reagents
- The ester is heated under reflux with dilute hydrochloric acid or dilute sulfuric acid as a catalyst.
Equation
- CH₃COOC₂H₅ + H₂O ⇌ CH₃COOH + C₂H₅OH, giving the carboxylic acid and alcohol.
Reversible
- This is the reverse of esterification, so it reaches equilibrium and conversion is not complete.
Pushing it
- Using a large excess of water moves the position of equilibrium toward the products.
Observation
- During reflux the ester layer gradually disappears and a single homogeneous solution remains, showing the ester has fully reacted.
Part 2 of 4Alkaline Hydrolysis
Using alkali changes the outcome.
1
The ester is heated under reflux with aqueous sodium hydroxide solution.
2
The reaction is CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH, giving a carboxylate salt and an alcohol.
3
The carboxylate cannot react back with the alcohol, so the reaction goes to completion.
4
To obtain the free carboxylic acid, the mixture is acidified afterwards with a strong acid.
Part 3 of 4Soaps and Fats
Soaps and Fats
Triglycerides
- Fats and oils are triesters of propane-1,2,3-triol (glycerol) with three long-chain carboxylic acids.
Saponification
- Heating a fat with sodium hydroxide gives glycerol and the sodium salts of the fatty acids.
Soap
- The sodium salts of the long-chain acids are soaps, with an ionic head and a long hydrocarbon tail.
Cleaning action
- The hydrocarbon tail dissolves in grease and the ionic head dissolves in water, so grease is lifted away.
| Feature | Acid hydrolysis | Alkaline hydrolysis |
|---|---|---|
| Reagent | Dilute H₂SO₄ or HCl | NaOH(aq) |
| Extent | Reversible | Goes to completion |
| Organic products | Acid and alcohol | Carboxylate salt and alcohol |
Part 4 of 4Exam Checklist & Tips
4.8.5 – Key Points for the Exam
- Hydrolysis Reaction with water that splits a molecule.
- Saponification Alkaline hydrolysis of a fat to make soap.
- Triglyceride Triester of propane-1,2,3-triol.
- Carboxylate The anion formed from a carboxylic acid.
Common Mistakes
- Writing the carboxylic acid as the product of alkaline hydrolysis rather than its salt.
- Calling acid hydrolysis irreversible.
- Forgetting to heat under reflux.
- Using the wrong alcohol name when naming products.
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