How do carboxylic acids behave as acids and how are esters and acyl chlorides made?
Carboxylic acids as weak acids that react with carbonates. Esterification of carboxylic acids with alcohols and the uses and hydrolysis of esters. Acylation by acyl chlorides and acid anhydrides reacting with water, alcohols, ammonia and amines. The industrial advantages of using acid anhydrides.
A focused answer to the AQA A-Level Chemistry 3.3.9 and 3.3.10 specification points on carboxylic acids and their derivatives. Covers acidity, esterification and ester hydrolysis, acylation reactions of acyl chlorides and anhydrides, and why anhydrides are preferred industrially.
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What this dot point is asking
AQA wants you to explain why carboxylic acids are weak acids that react with carbonates, write esterification equations and describe ester hydrolysis and uses, and describe acylation by acyl chlorides and acid anhydrides with water, alcohols, ammonia and amines, including why anhydrides are industrially preferred.
Carboxylic acids as weak acids
Carboxylic acids only partially dissociate in water, so they are weak acids:
They are weak because the carboxylate ion left after losing a proton is stabilised by delocalisation of the negative charge over both oxygen atoms, but this stabilisation is not enough to make ionisation complete. They react with carbonates and hydrogencarbonates to give a salt, water and carbon dioxide (fizzing), which distinguishes them from phenols and alcohols (phenols are too weakly acidic to react with carbonates, and alcohols are neutral).
Esterification
A carboxylic acid reacts with an alcohol, with a concentrated sulfuric acid catalyst, to form an ester and water (reversible).
Esters are used as solvents, plasticisers, perfumes and flavourings because many are sweet-smelling and volatile.
Hydrolysis of esters:
- Acid hydrolysis (reflux with dilute acid) is reversible, giving the carboxylic acid and alcohol.
- Base (alkaline) hydrolysis (reflux with ) is irreversible, giving the carboxylate salt and alcohol (saponification produces soap from fats).
Acylation
Acyl chlorides () and acid anhydrides () are reactive acylating agents. They react with nucleophiles, replacing the leaving group with the nucleophile.
| Nucleophile | Product | Also formed |
|---|---|---|
| Water | Carboxylic acid | or carboxylic acid |
| Alcohol | Ester | or carboxylic acid |
| Ammonia | Primary amide | or carboxylic acid |
| Primary amine | N-substituted amide | or carboxylic acid |
The reactivity of acylating agents reflects how good the leaving group is: the chloride ion of an acyl chloride leaves readily, making acyl chlorides the most reactive of the derivatives, which is why they react vigorously even with cold water. Acid anhydrides are less reactive but still useful, and they form the same products as the corresponding acyl chloride (an acid with water, an ester with an alcohol, an amide with ammonia or an amine), differing only in the by-product, a carboxylic acid rather than hydrogen chloride. This combination of adequate reactivity with a less hazardous by-product is exactly why anhydrides are chosen for large-scale manufacture such as aspirin, where releasing corrosive HCl gas would be a serious safety and corrosion problem.
Try this
Q1. What is observed when a carboxylic acid reacts with sodium carbonate? [1 mark]
- Cue. Effervescence (carbon dioxide released).
Q2. Give two reasons anhydrides are preferred over acyl chlorides industrially. [2 marks]
- Cue. Cheaper, less corrosive, react less violently with water, no toxic HCl by-product (any two).
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20184 marksEthanol reacts with ethanoic acid in the presence of a concentrated sulfuric acid catalyst. Write an equation for the reaction, name the organic product, state the role of the sulfuric acid, and explain why the yield of ester is limited.Show worked answer →
Equation: . The organic product is ethyl ethanoate.
Role of sulfuric acid: it acts as a catalyst (it also helps by removing water, shifting the equilibrium towards the ester).
Limited yield: the reaction is reversible and reaches equilibrium, so not all the reactants are converted; some ester and water react back to the acid and alcohol.
Markers reward the balanced reversible equation, the name ethyl ethanoate, sulfuric acid as a catalyst, and the equilibrium reason for the limited yield.
AQA 20203 marksExplain why an acid anhydride is preferred to an acyl chloride for the industrial manufacture of aspirin, giving two reasons, and state the by-product formed when the anhydride reacts.Show worked answer →
Two reasons (any two): the anhydride is cheaper than the acyl chloride; it is less corrosive and reacts less violently with water and moisture, so it is safer and easier to handle; it does not release toxic, corrosive hydrogen chloride gas.
By-product: the anhydride reaction produces a carboxylic acid (ethanoic acid) rather than HCl.
Markers reward two valid advantages and identifying ethanoic acid (the carboxylic acid) as the by-product instead of HCl.
Related dot points
- Alcohols as products of fermentation and hydration of alkenes. Classification as primary, secondary and tertiary. Oxidation of alcohols with acidified potassium dichromate(VI) to aldehydes, carboxylic acids and ketones. Elimination to form alkenes. The biofuel debate.
A focused answer to the AQA A-Level Chemistry 3.3.5 specification points on alcohols. Covers fermentation and hydration routes, primary, secondary and tertiary classification, oxidation with acidified dichromate, dehydration to alkenes, and the ethanol-as-biofuel debate.
- Aldehydes and ketones as carbonyl compounds. Oxidation of aldehydes to carboxylic acids and the use of Tollens' and Fehling's reagents to distinguish them from ketones. Reduction with NaBH4 to alcohols. Nucleophilic addition of HCN to form hydroxynitriles and the production of a racemic mixture.
A focused answer to the AQA A-Level Chemistry 3.3.8 specification points on carbonyl compounds. Covers oxidation of aldehydes, distinguishing tests, reduction with NaBH4, and the nucleophilic addition of HCN with its mechanism and racemic outcome.
- Tests for alkenes, alcohols, aldehydes and carboxylic acids. Use of bromine water, acidified potassium dichromate(VI), Fehling's and Tollens' reagents, and sodium carbonate. Determination of empirical and molecular formulae from combustion or composition data. The principle of mass spectrometry and infrared spectroscopy for structure determination.
A focused answer to the AQA A-Level Chemistry 3.3.6 specification points on organic analysis. Covers chemical tests for the main functional groups, identification of products, and the use of mass spectrometry and infrared spectroscopy to determine structure.
- Amines as bases and nucleophiles. Preparation of aliphatic amines by reaction of halogenoalkanes with ammonia and by reduction of nitriles. Preparation of aromatic amines by reduction of nitro compounds. The relative base strength of ammonia, primary aliphatic and aromatic amines. Amines as nucleophiles in further substitution.
A focused answer to the AQA A-Level Chemistry 3.3.12 specification points on amines. Covers preparation of aliphatic and aromatic amines, their behaviour as bases, the order of base strength, and their reactions as nucleophiles.
- Addition polymers from alkenes. Condensation polymers, including polyesters and polyamides, from two monomers or one monomer with two functional groups. Identifying the repeating unit and the monomers. Hydrolysis of condensation polymers. Biodegradability and disposal of polymers.
A focused answer to the AQA A-Level Chemistry 3.3.13 specification points on polymers. Covers addition polymerisation of alkenes, condensation polyesters and polyamides, identifying repeat units and monomers, hydrolysis of condensation polymers, and the disposal and biodegradability of plastics.
Sources & how we know this
- AQA A-level Chemistry (7405) specification — AQA (2015)