OxfordAQA International A Level Chemistry
OxfordAQA International A Level Chemistry 3.3.8 Aldehydes and Ketones Exam Questions
Free OxfordAQA International A Level Chemistry revision for 3.3.8 Aldehydes and Ketones: what you need to know, common mistakes and exam-style questions with answers. Covers oxidation of aldehydes, Tollens' and Fehling's tests, reduction with NaBH4 and nucleophilic addition of cyanide.
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About OxfordAQA International A Level Chemistry 3.3.8 Aldehydes and Ketones
Aldehydes and ketones is an A2 organic topic examined in Unit 4. You need to explain why aldehydes are readily oxidised to carboxylic acids while ketones are not, and use Fehling's solution and Tollens' reagent to tell them apart. You also need the reduction of aldehydes and ketones by NaBH4 in aqueous solution to primary and secondary alcohols, with the mechanism of nucleophilic addition, and the reaction with KCN followed by dilute acid to form hydroxynitriles, explaining why racemic mixtures form. The hazards of using HCN and KCN are also required. It builds on alcohols and links to optical isomerism.
What you need to know
- Oxidation Aldehydes are oxidised to carboxylic acids by acidified potassium dichromate(VI). Ketones are not easily oxidised.
- Tests Tollens' reagent ([Ag(NH3)2]+) gives a silver mirror with aldehydes. Fehling's solution gives a red precipitate of copper(I) oxide with aldehydes. Ketones give no change.
- Reduction NaBH4 in aqueous solution reduces aldehydes to primary alcohols and ketones to secondary alcohols. In equations the reducing agent is written as [H], for example CH3CHO + 2[H] → CH3CH2OH.
- Nucleophilic addition mechanism The nucleophile (H- from NaBH4 or CN-) attacks the delta positive carbon of C=O, and the pi bond electrons move to oxygen. The negative oxygen is then protonated.
- Hydroxynitriles KCN followed by dilute acid gives hydroxynitriles, for example ethanal gives 2-hydroxypropanenitrile. Aldehydes and unsymmetrical ketones give racemic mixtures because the C=O is planar.
- Hazards HCN is a highly toxic gas and KCN is toxic, so these reactions need a fume cupboard and careful handling. KCN is used rather than HCN to reduce the risk.
Common mistakes in 3.3.8 Aldehydes and Ketones
- Using H+ as the nucleophile in the reduction mechanism. The nucleophile is the hydride ion, H-, from NaBH4.
- Starting the curly arrow at the carbon of the carbonyl. The nucleophile's lone pair attacks the carbon, and a second arrow goes from the C=O bond to O.
- Forgetting that symmetrical ketones such as propanone do not give racemic mixtures, because the product has no chiral centre.
- Saying ketones react with Tollens' reagent. Only aldehydes are oxidised by it.
Try three 3.3.8 Aldehydes and Ketones exam-style questions
1. Outline the mechanism for the reaction of propanone with NaBH4. (4 marks)
Show the mark scheme answer
Lone pair on H- attacks delta positive carbon of C=O; curly arrow from C=O pi bond to O; alkoxide intermediate; O- is protonated (lone pair to H of water / H+), forming propan-2-ol.
2. Name the product of the reaction of propanal with KCN followed by dilute acid. (1 mark)
Show the mark scheme answer
2-hydroxybutanenitrile.
3. Describe a chemical test to distinguish butanal from butanone. (2 marks)
Show the mark scheme answer
Warm with Tollens' reagent; butanal gives a silver mirror while butanone shows no change.
3.3.8 Aldehydes and Ketones FAQs
What does OxfordAQA International A Level Chemistry 3.3.8 Aldehydes and Ketones cover?
It covers oxidation of aldehydes, Tollens' and Fehling's tests, reduction with NaBH4 and nucleophilic addition of cyanide.
Which unit is 3.3.8 Aldehydes and Ketones examined in?
It is examined in Unit 4, an International A2 paper covering organic and physical chemistry. The Unit 5 practical and synoptic paper can also assess it.
Why is KCN used instead of HCN?
HCN is a highly toxic gas that is difficult to handle safely. KCN is still toxic but is a solid, so it is easier to control, and dilute acid provides the H+ needed in the second step.
