Acyl chlorides and estersEdexcel A-Level Chemistry: Revision notes
Section 1
The acyl chloride and ester functional groups
An acyl chloride contains –COCl and is named with the suffix -oyl chloride (ethanoyl chloride, CH₃COCl). It is a carboxylic acid with the –OH replaced by –Cl. An ester contains –COO–, named from the alcohol and the acid: alkyl alkanoate. CH₃COOCH₂CH₃ is ethyl ethanoate (the alkyl group comes from ethanol, the ethanoate from ethanoic acid).
The carbonyl carbon in an acyl chloride is bonded to two electronegative atoms (O and Cl), so it is strongly δ+ and is attacked readily by nucleophiles. Acyl chlorides are therefore much more reactive than carboxylic acids.
To name an ester, the alkyl group on the oxygen is named first. CH₃COOCH₂CH₂CH₃ is propyl ethanoate, not ethyl propanoate.
Section 2
Reactions of acyl chlorides
Acyl chlorides react vigorously with water, alcohols, ammonia and amines at room temperature, with no catalyst. In every case HCl is formed (or its salt) and the CH₃CO– group is transferred.
- Water: CH₃COCl + H₂O → CH₃COOH + HCl (steamy fumes of HCl)
- Alcohol: CH₃COCl + CH₃CH₂OH → CH₃COOCH₂CH₃ + HCl (ester)
- Concentrated ammonia: CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl (ethanamide, white smoke)
- Amine: CH₃COCl + CH₃CH₂NH₂ → CH₃CONHCH₂CH₃ + HCl (N-substituted amide; with excess amine the HCl forms a salt)
Making esters this way is better than from a carboxylic acid because the reaction is irreversible and fast, so the yield is higher.
Ammonia is needed in excess because the second molecule removes the HCl as NH₄Cl. Do not write an amine as the product; the C=O is retained in the amide.
Section 3
Hydrolysis of esters
Esters are split by water into a carboxylic acid and an alcohol. Both methods need heating under reflux.
Acid hydrolysis: dilute H₂SO₄ or HCl as catalyst. CH₃COOCH₂CH₃ + H₂O ⇌ CH₃COOH + CH₃CH₂OH. It is the reverse of esterification, so it is reversible and ends in an equilibrium mixture.
Alkaline hydrolysis: aqueous NaOH. CH₃COOCH₂CH₃ + OH⁻ → CH₃COO⁻ + CH₃CH₂OH. This is not reversible, because the carboxylate ion is not attacked by the alcohol, so the ester is hydrolysed completely. The free carboxylic acid is obtained by adding excess dilute strong acid: CH₃COO⁻ + H⁺ → CH₃COOH.
Worked example: propyl ethanoate with dilute acid gives ethanoic acid and propan-1-ol. With NaOH it gives sodium ethanoate and propan-1-ol.
Alkaline hydrolysis does not give the acid directly. It gives the salt; you must acidify to get the carboxylic acid.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Acyl chlorides and esters
- Ethanoyl chloride, CH₃COCl, is a colourless liquid that fumes in moist air. A technician adds it carefully to several different reagents in a fume cupboard.Ethanoyl chloride reacts with ethylamine. Name the organic product and write the equation for the reaction.2 marks
- Ethyl propanoate, CH₃CH₂COOCH₂CH₃, is hydrolysed in two separate experiments. In the first it is heated under reflux with dilute sulfuric acid. In the second it is heated under reflux with aqueous sodium hydroxide.Describe how propanoic acid can be obtained from the mixture formed after alkaline hydrolysis, and write an ionic equation for the reaction involved.2 marks
- A flavour company makes propyl ethanoate, which has a pear-like smell. It can be made from propan-1-ol using either ethanoic acid or ethanoyl chloride. The company also tests samples of the ester by hydrolysing them.Give the conditions for the acid hydrolysis of propyl ethanoate and name the two organic products.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).