Oxidising an aldehyde (from 3.3.8) gives a carboxylic acid, and this page follows the family that grows from it. The unifying idea is the acyl group, R–C(=O)–: swap what is attached to it and you move between the acid, ester, acyl chloride, acid anhydride and amide. One mechanism — nucleophilic addition–elimination — links most of them.
Carboxylic acids
Carboxylic acids contain the –COOH group. They are weak acids — they only partially dissociate in water — but they are acidic enough to show the usual reactions:
- with carbonates and hydrogencarbonates they release CO2 (effervescence) — the test that distinguishes a carboxylic acid from a phenol or alcohol;
- with bases they form carboxylate salts + water.
2CH3COOH + Na2CO3 → 2CH3COONa + H2O + CO2
Warmed with an alcohol and a concentrated sulfuric acid catalyst, a carboxylic acid forms an ester — the reaction is reversible (esterification), so it reaches an equilibrium:
CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O
🧪 Exam-style questions
An alcohol is warmed with ethanoic acid and a few drops of concentrated sulfuric acid, then poured into sodium hydrogencarbonate solution. Suggest a simple way to detect if a reaction occurred, and give a reason for pouring the mixture into sodium hydrogencarbonate solution.
Show answer
A reaction has occurred if there is a sweet / fruity smell (an ester forms). 1 mark
The sodium hydrogencarbonate reacts with / removes (neutralises) the excess acid… 1 mark
…so that the ester smell can be detected clearly. 1 mark
Source: AQA A-Level Chemistry past papers.
Esters & hydrolysis
Esters have the group –COO– and are named alkyl alkanoate — the alkyl part comes from the alcohol, the alkanoate part from the acid. So ethanoic acid + ethanol gives ethyl ethanoate.
Ethyl ethanoate, CH3COOC2H5: ethyl = the C2H5 from ethanol; ethanoate = the CH3COO from ethanoic acid. Write the alcohol-derived group first, the acid-derived group second.
Hydrolysis — the two routes
Hydrolysis splits the ester back apart. Which products you get depends on the conditions:
- Acid hydrolysis (dilute acid, heat under reflux) is the reverse of esterification, so it is reversible and reaches an equilibrium — giving the carboxylic acid + alcohol.
- Base hydrolysis (saponification) (aqueous NaOH, heat under reflux) goes to completion because the carboxylate salt formed does not react back — giving the carboxylate salt + alcohol.
CH3COOC2H5 + NaOH → CH3COONa + C2H5OH
Fats, oils & biodiesel
Fats and oils are triesters (triglycerides) of the triol glycerol (propane-1,2,3-triol) with three long-chain fatty acids. Biodiesel is a mixture of methyl esters of those fatty acids, made by reacting the triglyceride with methanol and a catalyst (transesterification), which releases glycerol as a by-product.
Their pleasant smells and low reactivity make esters useful as solvents, plasticisers, and in perfumes and flavourings.
🧪 Exam-style questions
Which compound is formed when phenyl benzenecarboxylate (C6H5COOC6H5) is hydrolysed under acidic conditions?
Ethyl ethanoate is heated under reflux with aqueous sodium hydroxide. Name this type of reaction, write an equation, and state why it goes to completion rather than reaching an equilibrium.
Show answer
Base hydrolysis / saponification. 1 mark
CH3COOC2H5 + NaOH → CH3COONa + C2H5OH 1 mark
The ethanoate ion (carboxylate salt) formed does not react back with the alcohol, so the reaction is not reversible / goes to completion. 1 mark
Source: AQA A-Level Chemistry past papers.
Acyl chlorides & acid anhydrides
Replace the –OH of a carboxylic acid with –Cl and you get an acyl chloride (e.g. ethanoyl chloride, CH3COCl); join two acid molecules with loss of water and you get an acid anhydride (e.g. ethanoic anhydride, (CH3CO)2O). Both are acylating agents: they hand the acyl group CH3CO– to a nucleophile. Acyl chlorides are the more reactive of the two.
The four nucleophiles — one pattern
Each of the four common nucleophiles attacks the acyl group and displaces the leaving group. The product is set by the nucleophile:
| Nucleophile | Organic product | Equation |
|---|---|---|
| Water | carboxylic acid | CH3COCl + H2O → CH3COOH + HCl |
| Alcohol (e.g. ethanol) | ester | CH3COCl + C2H5OH → CH3COOC2H5 + HCl |
| Ammonia | amide | CH3COCl + 2NH3 → CH3CONH2 + NH4Cl |
| Primary amine (e.g. CH3NH2) | N-substituted amide | CH3COCl + 2CH3NH2 → CH3CONHCH3 + CH3NH3Cl |
One molecule of the nucleophile forms the product; a second mops up the HCl released (as NH4+Cl− or the alkylammonium salt). Balance the equation with 2NH3 / 2 amine, not one.
The mechanism: nucleophilic addition–elimination
Every one of those reactions goes by the same two-stage mechanism. The carbonyl carbon is Cδ+ (electron-poor), so:
- Addition: the nucleophile’s lone pair attacks Cδ+; the C=O π bond breaks onto the oxygen, giving a tetrahedral intermediate.
- Elimination: the C=O reforms and pushes out the leaving group (Cl− from an acyl chloride, or the carboxylate from an anhydride). A proton is then lost from the nucleophile to give the neutral product.
Acyl chlorides vs anhydrides — and aspirin
Acyl chlorides react violently with water, giving off misty fumes of HCl; the Cl− released also gives a white precipitate with acidified silver nitrate. Acid anhydrides are gentler and easier to control. That is why the industrial synthesis of aspirin uses ethanoic anhydride rather than ethanoyl chloride:
salicylic acid + (CH3CO)2O → aspirin + CH3COOH
Ethanoic anhydride is cheaper, less corrosive and less vulnerable to hydrolysis than the acyl chloride, and its reactions are easier to control. Crucially it does not release corrosive HCl gas — the only by-product is ethanoic acid.
🧪 Exam-style questions
Which reaction involves addition–elimination?
Which compound reacts with warm dilute aqueous sodium hydroxide?
Which compound forms a white precipitate when added to aqueous silver nitrate?
Aspirin is produced by reacting salicylic acid with ethanoic anhydride. Give one reason why ethanoic anhydride is used rather than ethanoyl chloride, and outline how you would test the by-product to show that no acyl chloride was used.
Show answer
Ethanoic anhydride is cheaper / less corrosive / less readily hydrolysed / does not release corrosive HCl gas (any one). 1 mark
Add (acidified) silver nitrate solution to the reaction mixture. 1 mark
An acyl chloride would give a white precipitate (of AgCl); with the anhydride there is no white precipitate, confirming no chloride is present. 1 mark
Source: AQA A-Level Chemistry past papers.
The derivatives map
Pull the whole topic together around the acyl group. Reactivity runs acyl chloride > acid anhydride > ester > carboxylic acid/amide — the better the leaving group, the more reactive the acylating agent. Reading the map is the exam skill: name the reagent that makes each product, and vice versa.
| Add this nucleophile | Get this product | Product type |
|---|---|---|
| Water | CH3COOH | carboxylic acid |
| Alcohol | CH3COOR | ester |
| Ammonia | CH3CONH2 | amide |
| Primary amine | CH3CONHR | N-substituted amide |
- Naming the mechanism substitution — it is nucleophilic addition–elimination (the tetrahedral intermediate forms first).
- Forgetting the second equivalent of NH3 / amine that removes the HCl — the equation must balance.
- Confusing the hydrolysis routes: acid gives the acid + alcohol (reversible); base gives the carboxylate salt + alcohol (one-way).
- For the acyl chloride, missing the observations: violent reaction with water, misty HCl fumes, white precipitate with AgNO3.
Examiner reports repeatedly note that the addition–elimination mechanism is drawn carelessly — the curly arrow must begin at the nucleophile’s lone pair and the elimination arrow must run from the C–Cl bond as the C=O reforms. Precise arrows earn the marks; vague ones do not.
- Carboxylic acids (–COOH) are weak acids: they fizz with carbonates (CO2 — the distinguishing test) and form salts with bases.
- Esterification: carboxylic acid + alcohol ⇌ ester + water (conc H2SO4 catalyst, reversible).
- Ester hydrolysis: acid hydrolysis is reversible (→ acid + alcohol); base hydrolysis (saponification) is one-way (→ carboxylate salt + alcohol). Fats/oils are triesters of glycerol; biodiesel is their methyl esters.
- Acyl chlorides (RCOCl) and acid anhydrides ((RCO)2O) are reactive acylating agents. With a nucleophile they give: + H2O → acid; + alcohol → ester; + NH3 → amide; + amine → N-substituted amide.
- Mechanism: nucleophilic addition–elimination. Aspirin is made from salicylic acid + ethanoic anhydride (cheaper and less corrosive than the acyl chloride).