Part 4: Amides and condensation polymers

Recognise amide connectivity, prepare amides from acyl chlorides and draw polyester or polyamide repeat units without losing carbon atoms or end-group information.

A carbonyl bonded directly to nitrogen defines an amide

Ethanamide is CH₃C(=O)NH₂ and N-methylethanamide is CH₃C(=O)NHCH₃. Nitrogen is directly bonded to the carbonyl carbon. CH₃CH₂NH₂ is an amine, and CH₃COOCH₃ is an ester; neither has the C(=O)–N connection.

Amides can be prepared from acyl chlorides using ammonia or amines. An acyl chloride plus excess concentrated ammonia gives a primary amide and ammonium chloride; a primary amine gives an N-substituted amide and an alkylammonium chloride. These products preserve the acyl carbonyl.

An acid plus an amine initially tends to form an ammonium carboxylate salt. Making an amide from that pair generally requires suitable heating/dehydrating conditions, so do not promise rapid amide formation by simply mixing them at room temperature.

Turn a single link into repeated links

A dicarboxylic acid and a diamine can form a polyamide under appropriate condensation conditions, eliminating water. Both monomers have two reactive ends, allowing repeated links. A diacyl chloride with a diamine forms the same amide connectivity while releasing HCl instead.

For hexanedioic acid, HOOC(CH₂)₄COOH, and hexane-1,6-diamine, H₂N(CH₂)₆NH₂, a valid repeat unit is [–C(=O)–(CH₂)₄–C(=O)–NH–(CH₂)₆–NH–]ₙ. Each monomer has six carbons: the acid count includes both carbonyl carbons. The polymer is nylon-6,6.

At each link, the OH of a carboxyl group and H of an amino group are removed as water. Keep one N–H on each primary-amine-derived amide N in the backbone. An O between carbonyl carbon and N would make the wrong linkage. A repeat unit must have bonds continuing through both brackets.

Use the functional group to decide which backbone atom appears

A diacid plus a diol gives an ester link, –C(=O)–O–, while the same diacid plus a diamine gives an amide link, –C(=O)–NH–. This single change affects intermolecular interactions and hydrolysis products. The polymer class is polyester or polyamide; the polymerisation type is condensation.

Worked comparison with butanedioic acid: ethane-1,2-diol gives [–OCH₂CH₂O–C(=O)CH₂CH₂C(=O)–]ₙ, whereas ethane-1,2-diamine gives [–NHCH₂CH₂NH–C(=O)CH₂CH₂C(=O)–]ₙ. The acid-derived –COCH₂CH₂CO– segment remains the same.

Draw the complete unit before abbreviating it. Then expand the join between adjacent repeat units to ensure the boundary also contains the correct ester/amide bond. A correct-looking interior cannot rescue an impossible boundary valency.

An amino acid contains both required groups

An amino acid such as H₂NCH₂COOH can self-condense to give [–NH–CH₂–C(=O)–]ₙ. More generally a 2-amino acid H₂NCH(R)COOH gives [–NH–CH(R)–C(=O)–]ₙ when one monomer species is represented. The side chain R stays attached to the same carbon.

A protein normally contains a sequence of several different amino-acid residues rather than one identical repeat. Draw each side chain in its specified position and leave appropriate amino and carboxyl termini when a finite peptide is requested.

For n amino-acid molecules connected into one linear chain there are n − 1 peptide links and n − 1 water molecules. In the idealised high-polymer repeat notation, end groups are often omitted; use the actual count when the question specifies a short chain.

Track protonation as the polymer is broken down

Heating a polyamide under acidic hydrolysis conditions breaks the amide links and gives carboxylic acids and protonated amines. In alkaline solution, carboxylate salts and unprotonated amines are obtained. Draw the medium-dependent forms, not simply the neutral original monomers for every condition.

Polyamide chains containing N–H can form hydrogen bonds from N–H to carbonyl O on nearby chains, as well as London and permanent dipole interactions. Many polyesters lack N–H/O–H donors within their backbones, so they do not show the same interchain hydrogen bonding. Actual melting behaviour also depends on chain structure and packing; there is no universal numerical ordering for all polymers.

The hydrolysable link provides a route to chemical degradation but does not alone establish a rapid biodegradation rate. Industrial recycling and environmental breakdown depend on accessibility, conditions and reaction speed. A mixed-link polymer must be split at both ester and amide links if complete hydrolysis is requested.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official Edexcel mark allocations.

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