18. Carboxylic acids and derivatives
- Syllabus
- 9701–2028–2029
- Section
- 18
- Level
- AS
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Topic 18.1
18.1 Carboxylic acids
Objectives in this topic
Prepare carboxylic acids by oxidation or hydrolysis followed by acidification
Reflux a primary alcohol or aldehyde with acidified dichromate/manganate to obtain a carboxylic acid. Nitriles and esters can be hydrolysed, then an alkaline product is acidified when required.
Choose conditions from the starting functional group. Reflux keeps a primary oxidation product in the flask; hydrolysis breaks C–N or ester bonds and acidification converts the carboxylate into the free acid.
Propan-1-ol → propanoic acid under reflux. CH₃CH₂CN + 2H₂O → CH₃CH₂COOH + NH₃ after acid hydrolysis, with the atom balance checked.
Distillation isolates an aldehyde; reflux drives further oxidation to the acid. Do not leave an alkaline carboxylate when the requested product is the acid.
Carboxylic acids react as weak acids and form esters
Carboxylic acids react with reactive metals to release H₂, with alkalis to form salt and water, and with carbonates to form salt, water and CO₂. With alcohols they esterify in concentrated sulfuric acid.
The same acid functional group supports different observations: gas evolution with metal/carbonate, neutralisation with alkali, and a reversible condensation with alcohol. Write the correct products for the reagent.
2CH₃COOH + 2Na → 2CH₃COONa + H₂; CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂. Ethanoic acid + ethanol ⇌ ethyl ethanoate + water.
A carboxylic acid is weak, not non-reactive; and esterification is not complete neutralisation because it is reversible.
Topic 18.2
18.2 Esters
Objectives in this topic
Esterification condenses an alcohol and carboxylic acid under acid catalysis
An ester forms when an alcohol and carboxylic acid react in a condensation reaction. Concentrated H₂SO₄ catalyses the reversible process and water is eliminated.
Name the acid-derived alkyl and alcohol-derived alkyl parts, then balance the equation. Heating and removal of water can improve the ester yield, but equilibrium remains relevant.
Ethanoic acid + ethanol ⇌ ethyl ethanoate + water. The ester has the linkage CH₃COOCH₂CH₃ and a characteristic fruity smell.
The catalyst is not incorporated into the ester, and esterification is not the same as hydrolysis in reverse conditions.
Ester hydrolysis gives different products in acid and alkali
Dilute acid hydrolysis of an ester is reversible and gives a carboxylic acid plus an alcohol. Dilute alkali hydrolysis gives a carboxylate salt plus an alcohol and is effectively driven to completion.
Heat supplies energy and the reagent determines whether the carboxyl group remains protonated. Acidification after alkaline hydrolysis converts the carboxylate into the free acid.
CH₃COOCH₂CH₃ + H₂O ⇌ CH₃COOH + CH₃CH₂OH in acid; with NaOH, products are CH₃COONa + CH₃CH₂OH, followed by acidification if ethanoic acid is required.
Do not write identical products for acid and alkaline hydrolysis, and do not forget reversibility in the acid route.