Acyl chlorides, esters and polyestersEdexcel International A Level Chemistry: Revision notes
Section 1
Naming and structure
Acyl chlorides contain –COCl and are named from the acid by changing -oic acid to -oyl chloride: ethanoyl chloride CH₃COCl, propanoyl chloride CH₃CH₂COCl.
Esters contain –COO– and are named alkyl alkanoate: the alkyl group is the part from the alcohol, written first; the -oate part comes from the acid. CH₃COOCH₂CH₃ is ethyl ethanoate (ethanoic acid plus ethanol); CH₃CH₂COOCH₃ is methyl propanoate. Be ready to draw structural, displayed and skeletal formulae of both.
Writing the ester name the wrong way round. CH₃COOC₂H₅ is ethyl ethanoate, not ethanoyl ethanoate.
Section 2
Reactions of acyl chlorides
Acyl chlorides are very reactive: the carbonyl carbon is attached to two electronegative atoms (O and Cl), so it is strongly δ+ and is attacked by nucleophiles. Reactions happen rapidly at room temperature and give off hydrogen chloride (white steamy fumes):
- water: CH₃COCl + H₂O → CH₃COOH + HCl
- alcohols: CH₃COCl + C₂H₅OH → CH₃COOC₂H₅ + HCl
- concentrated ammonia: CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl (amide)
- amines: CH₃COCl + CH₃NH₂ → CH₃CONHCH₃ + HCl (N-substituted amide; the HCl reacts with excess amine to form a salt).
These reactions are not reversible, unlike esterification with a carboxylic acid, so acyl chlorides give better yields.
Water, alcohol, ammonia and amines are all nucleophiles. In each case the group replaces Cl, and HCl is lost.
Section 3
Hydrolysis of esters
Esters are hydrolysed by heating under reflux with water and either an acid or alkali.
Acid hydrolysis (dilute HCl or H₂SO₄ as catalyst) is the reverse of esterification and is reversible:
CH₃COOC₂H₅ + H₂O ⇌ CH₃COOH + C₂H₅OH
Alkaline hydrolysis (aqueous NaOH) goes to completion because the carboxylic acid is converted to its salt, which does not react with the alcohol:
CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH
Adding a dilute strong acid to the mixture afterwards releases the free carboxylic acid.
Section 4
Condensation polymerisation: polyesters
A polyester forms from a monomer with two –COOH groups (a dicarboxylic acid) and a monomer with two –OH groups (a diol). Each time a –COOH reacts with an –OH an ester link forms and a small molecule, water, is lost. This is condensation polymerisation.
Terylene is made from benzene-1,4-dicarboxylic acid and ethane-1,2-diol. The repeat unit is –OC–C₆H₄–CO–O–CH₂CH₂–O–.
In a condensation polymer, two molecules of water are lost per repeat unit. Ester links can be hydrolysed by acid or alkali, which breaks the polyester back into its monomers, unlike the unreactive C–C backbone of addition polymers.
Showing the repeat unit with brackets that omit the ester link. The ester groups must be drawn, with open bonds at both ends.
Section 5
Worked example: polymer mass balance
The repeat unit of Terylene is C₁₀H₈O₄, Mr = 192.0. This equals the diacid (166.0) plus the diol (62.0) minus two waters (36.0).
For 96.0 kg of polymer: amount of repeat units = 96 000 ÷ 192.0 = 500 mol. So 500 mol of diol (31.0 kg) and 500 mol of diacid (83.0 kg) are used and 1000 mol of water (18.0 kg) is lost: 83.0 + 31.0 − 18.0 = 96.0 kg.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Acyl chlorides, esters and polyesters
- Ethanoyl chloride, CH₃COCl, is a colourless liquid that fumes in moist air. It is used in the laboratory to make a range of other organic compounds.Write a balanced equation for the reaction of ethanoyl chloride with concentrated ammonia and name the organic product.2 marks
- Ethyl ethanoate is an ester used as a solvent. A student hydrolyses it by heating under reflux, first with dilute hydrochloric acid and then, in a second experiment, with aqueous sodium hydroxide.Write the equation for the alkaline hydrolysis of ethyl ethanoate and state what must be added to the product mixture to obtain ethanoic acid.2 marks
- Propanoyl chloride, CH₃CH₂COCl, is added separately to ethanol, to methylamine, CH₃NH₂, and to concentrated ammonia.Name the organic product formed in each of the three reactions.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).