Amides, polyamides and addition polymersEdexcel International A Level Chemistry: Revision notes
Section 1
Making amides from acyl chlorides
Acyl chlorides react vigorously with ammonia and amines by nucleophilic addition–elimination to form amides:
CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl (ethanamide)
CH₃COCl + CH₃NH₂ → CH₃CONHCH₃ + HCl (N-methylethanamide)
With ammonia, two moles are needed because the second takes the HCl as NH₄Cl. The amide link is –CONH–. Amides are named from the acid with -amide, and groups on the nitrogen are shown with N-.
Section 2
Condensation polymerisation: polyamides
In condensation polymerisation monomers join and a small molecule (H₂O or HCl) is lost each time. A polyamide forms from a monomer with two –NH₂ groups and a monomer with two –COOH (or –COCl) groups.
Nylon-6,6 comes from hexane-1,6-diamine, H₂N(CH₂)₆NH₂, and hexanedioic acid, HOOC(CH₂)₄COOH (with H₂O lost), or hexanedioyl dichloride, ClOC(CH₂)₄COCl (with HCl lost). Its repeat unit is:
–NH(CH₂)₆NHCO(CH₂)₄CO–
Proteins are natural polyamides made from amino acids, which have an –NH₂ and a –COOH group on the same molecule, so one monomer type is enough.
To draw the repeat unit, keep the bonds at each end open and include one amide link only.
Section 3
Properties of polyamides
Polyamide chains have polar N–H and C=O groups. Hydrogen bonds form between the N–H of one chain and the C=O of a neighbouring chain, in addition to London forces. These are stronger than the London forces alone in a polymer like poly(ethene), so polyamides have high melting temperatures and high tensile strength, and can be drawn into strong fibres.
Section 4
Addition polymers: poly(propenamide) and poly(ethenol)
In addition polymerisation the C=C bond of an alkene monomer opens and many molecules join with no small molecule lost. The backbone is a carbon–carbon chain.
Poly(propenamide) is made from propenamide, CH₂=CHCONH₂. Repeat unit: –CH₂–CH(CONH₂)–. The amide groups are side groups, so this is not a polyamide.
Poly(ethenol) has repeat unit –CH₂–CH(OH)–. Ethenol is unstable, so the polymer is not made directly from its monomer.
A polymer with –CONH₂ side groups is not a polyamide. A polyamide has –CONH– in the backbone.
Section 5
Hydrogen bonding and the solubility of poly(ethenol)
Poly(ethenol) has many –OH groups, which form hydrogen bonds with water, so it dissolves in water. The energy released on forming these bonds compensates for breaking hydrogen bonds between chains and between water molecules. This makes it useful for soluble laundry bags and the film around liquid-detergent capsules (liquitabs), which dissolve in the wash without leaving a plastic bag to remove.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Amides, polyamides and addition polymers
- A technician makes ethanamide, CH₃CONH₂, in a fume cupboard by reacting an acyl chloride with a nitrogen-containing compound.Write an equation for the reaction of ethanoyl chloride with ammonia, and explain why two moles of ammonia are needed for each mole of ethanoyl chloride.2 marks
- Nylon-6,6 is made by condensation polymerisation. In a demonstration, a solution of hexanedioyl dichloride, ClOC(CH₂)₄COCl, in an organic solvent is carefully covered with an aqueous solution of hexane-1,6-diamine, H₂N(CH₂)₆NH₂. A film of nylon forms where the two liquids meet and is pulled out as a continuous rope.Explain why nylon-6,6 fibres have a high tensile strength.2 marks
- A chemical company makes two water-soluble addition polymers: poly(ethenol), used for dissolvable laundry bags and for the film around liquid detergent capsules (liquitabs), and poly(propenamide), made from propenamide, CH₂=CHCONH₂. Ethenol, CH₂=CHOH, is unstable, so poly(ethenol) is not made directly from it.Give the repeat unit of poly(ethenol) and explain, in terms of intermolecular forces, why it dissolves in water.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).