Chemistry › Carbonyls and carboxylic acids
Carbonyls and carboxylic acids
Unit C20 · 5 lessons.
The C=O group ties these compounds together. Aldehydes and ketones undergo nucleophilic addition and are told apart by Tollens' and Fehling's tests, carboxylic acids form esters, acyl chlorides and anhydrides acylate, and condensation polymers join monomers by the same ester and amide links. Optical isomerism comes first, because several of these reactions make a chiral centre.
The racemic mixture is where explanations fall short. Cyanide ions attack the planar carbonyl carbon from either face with equal likelihood, so the product of adding HCN to an unsymmetrical aldehyde or ketone is an equal mixture of enantiomers and shows no optical activity, and the explanation needs both the planar group and the equal chance.
On the written papers it is set on AQA Papers 2 and 3, OCR A Papers 2 and 3, Edexcel Papers 2 and 3 and Cambridge Papers 1, 2 and 4.
Specification sections: AQA 3.3.7, 3.3.8, 3.3.9.1, 3.3.9.2, 3.3.12.1, 3.3.12.2 · OCR A 6.2.2, 6.1.2, 6.1.3, 6.2.4, 6.2.3 · Edexcel 17.1 to 17.16, 6.14, 18.8 to 18.15 · CAIE 13.4, 29.4, 16.1, 17.1, 19.2, 18.1, 18.2, 33.1, 29.2, 32.1, 33.2, 33.3, 34.1, 34.3, 35.1, 35.2, 35.3
Notes
Illustrated notes for each topic in this unit, with worked examples and a checklist of objectives.
- Optical isomerism
Chiral centres and how to find them, enantiomers as non-superimposable mirror images drawn with wedges, optical activity and the rotation of plane-polarised light, racemic mixtures and why they are optically inactive, why nucleophilic addition to a carbonyl gives a racemate, optical activity as evidence for SN1 and SN2 with partly racemised products, and single-enantiomer drugs including thalidomide and ibuprofen.
- Aldehydes and ketones: nucleophilic addition and the tests that tell them apart
The carbonyl group and its polarity, oxidation of aldehydes by Tollens' reagent and Fehling's solution with the half-equations, reduction of aldehydes and ketones by NaBH₄ to primary and secondary alcohols, the nucleophilic addition mechanisms for hydride and for cyanide, hydroxynitriles from KCN and acid, the hazard of HCN, racemic products, 2,4-DNP with derivative melting points, the triiodomethane test, and LiAlH₄ in dry ether.
- Carboxylic acids, esters and biodiesel
Carboxylic acids as weak acids and their reactions with carbonates, bases and metals, the relative acidities of acids, phenols, alcohols and chloro-substituted acids, carboxylic acids from nitriles, methanoic acid as a reducing agent, LiAlH₄ reduction to alcohols, esterification with an alcohol and a concentrated sulfuric acid catalyst, naming esters, uses of esters, acid and alkaline hydrolysis of esters, fats and oils as triglycerides, soap by alkaline hydrolysis, and biodiesel by transesterification with methanol.
- Acylation: acyl chlorides and acid anhydrides
Acyl chlorides and acid anhydrides as acylating agents, their reactions with water, alcohols, ammonia and primary amines to give carboxylic acids, esters, amides and N-substituted amides, the observations, the nucleophilic addition–elimination mechanism, making acyl chlorides with SOCl₂, PCl₅ or PCl₃, the ease of hydrolysis of acyl, alkyl and aryl chlorides, hydrolysis and LiAlH₄ reduction of amides and why amides are weak bases, phenyl esters from phenol, and why ethanoic anhydride is used to make aspirin industrially.
- Condensation polymers and the disposal of polymers
Condensation polymerisation compared with addition polymerisation; polyesters, including Terylene (PET) from benzene-1,4-dicarboxylic acid and ethane-1,2-diol and poly(lactic acid); polyamides, including nylon-6,6, nylon-6 and Kevlar; drawing repeating units and finding monomers; hydrogen bonding and other forces between chains and the properties they give; hydrolysis of ester and amide links by acid and alkali; biodegradability compared with polyalkenes; disposal by landfill, incineration and recycling, including chemical recycling of PET; and, for the boards that set them, predicting the type of polymerisation from a monomer and photodegradable polymers.
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