How do carboxylic acids and their derivatives react?
The acidity and reactions of carboxylic acids, the formation and hydrolysis of esters, acyl chlorides and acid anhydrides as reactive derivatives, and the production of soaps and biodiesel by ester hydrolysis and transesterification.
A CCEA A-Level Chemistry answer on carboxylic acids and their derivatives, covering acidity and reactions, ester formation and hydrolysis, acyl chlorides and acid anhydrides as reactive derivatives, and the production of soaps and biodiesel.
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What this dot point is asking
CCEA wants you to explain carboxylic acid acidity, write their reactions, form and hydrolyse esters, use acyl chlorides and anhydrides as reactive acylating agents, and describe how soaps and biodiesel are made by hydrolysis and transesterification.
Acidity of carboxylic acids
Esters: formation and hydrolysis
Esterification (Fischer): reflux acid and alcohol with concentrated catalyst; an equilibrium.
Acid hydrolysis reverses this (dilute acid, reflux). Base hydrolysis (saponification) is irreversible because the carboxylate salt cannot re-esterify:
Reactive derivatives
Soaps and biodiesel
Boiling a fat (a triester of glycerol) with gives soap (sodium salts of long-chain acids) and glycerol. Reacting the fat with methanol and a base catalyst is transesterification, swapping glycerol for methyl groups to give biodiesel (fatty acid methyl esters) and glycerol.
Examples in context
Soap manufacture (saponification) and biodiesel production are large-scale uses of ester chemistry: a vegetable oil reacted with yields bar soap and glycerol, while the same oil reacted with methanol and a small amount of yields biodiesel and glycerol. Aspirin synthesis, a common CCEA practical, acylates the phenol group of salicylic acid using ethanoic anhydride rather than the acid itself, because the anhydride is a far more reactive acylating agent and gives a higher yield.
Try this
Q1. Write the equation for the reaction of ethanoic acid with sodium carbonate. [1 mark]
- Cue. .
Q2. State why base hydrolysis of an ester is irreversible. [1 mark]
- Cue. The carboxylate salt formed cannot re-esterify, so the reaction goes to completion.
Q3. Name the products of the transesterification of a vegetable oil with methanol. [2 marks]
- Cue. Biodiesel (fatty acid methyl esters) and glycerol.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20194 marksExplain why chloroethanoic acid is a stronger acid than ethanoic acid, and describe how you would prepare ethyl ethanoate from ethanoic acid in the laboratory.Show worked answer →
Chloroethanoic acid is stronger because the electronegative chlorine withdraws electron density along the chain (a negative inductive effect, ). This stabilises the carboxylate anion by spreading the negative charge, so the equilibrium of dissociation lies further to the right and is larger.
Preparation of ethyl ethanoate: reflux ethanoic acid with ethanol and a few drops of concentrated sulfuric acid as catalyst.
The ester is then distilled off and washed with sodium carbonate solution to remove acid. Markers reward (1) the effect of chlorine, (2) stabilisation of the carboxylate anion, (3) reflux with concentrated catalyst, (4) the equilibrium equation.
CCEA 20214 marksAn ester with relative molecular mass is hydrolysed to give ethanoic acid and an alcohol. Determine the molecular formula of the ester and name the alcohol, then write the equation for the acid hydrolysis.Show worked answer →
Ethanoic acid contributes the part with . The ester is , so the alkyl group from the alcohol has mass , which is (ethyl, ).
So the ester is ethyl ethanoate, , molecular formula . The alcohol released on hydrolysis is ethanol.
Acid hydrolysis (reversible) with dilute acid and reflux:
Markers reward (1) subtracting to find the alkyl mass, (2) identifying ethyl and the formula , (3) naming ethanol, (4) the balanced hydrolysis equation.
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Sources & how we know this
- CCEA GCE Chemistry specification — CCEA (2016)