AQA A Level Chemistry
AQA A Level Chemistry 3.3.8 Aldehydes and Ketones Exam Questions
Free AQA A Level Chemistry exam questions for 3.3.8 Aldehydes and Ketones, with a mark scheme for every pack. Covers oxidation of aldehydes, Tollens' and Fehling's tests, reduction with NaBH4, nucleophilic addition mechanisms and hydroxynitrile formation with KCN.
3.3.8 Aldehydes and Ketones exam questions
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About AQA A Level Chemistry 3.3.8 Aldehydes and Ketones
Aldehydes and ketones both contain the carbonyl group, and on the AQA A Level course you learn how their reactions differ. Aldehydes are readily oxidised to carboxylic acids while ketones are not, which is the basis of the Tollens' and Fehling's tests. Both are reduced by NaBH4 to alcohols through nucleophilic addition, and both react with KCN followed by dilute acid to form hydroxynitriles. You must also discuss the hazards of cyanide reagents and why a racemic mixture can form. The topic builds on 3.3.5 Alcohols and links to 3.3.7 Optical Isomerism. Examiners ask for full mechanisms and balanced equations using [O] and [H].
What you need to know
- Oxidation Aldehydes are oxidised to carboxylic acids by acidified K2Cr2O7: RCHO + [O] → RCOOH. Ketones are not oxidised under these conditions.
- Tollens' and Fehling's Tollens' reagent ([Ag(NH3)2]+) gives a silver mirror with aldehydes as Ag+ is reduced to Ag. Fehling's solution (Cu2+) gives a brick-red precipitate of Cu2O. Ketones give no change.
- Reduction with NaBH4 NaBH4 in aqueous solution reduces aldehydes to primary alcohols and ketones to secondary alcohols. Use [H] in equations, e.g. CH3COCH3 + 2[H] → CH3CH(OH)CH3.
- Nucleophilic addition mechanism The nucleophile (H- from NaBH4 or CN- from KCN) attacks the δ+ carbonyl carbon, the C=O π bond breaks with electrons moving onto O, and the negative oxygen is then protonated by water or H+.
- Hydroxynitriles KCN followed by dilute acid adds a CN group and an OH group. Ethanal gives 2-hydroxypropanenitrile. KCN is used instead of HCN because HCN is a very toxic gas and KCN provides more cyanide ions, but KCN is still toxic and must be handled carefully.
Common mistakes in 3.3.8 Aldehydes and Ketones
- Naming hydroxynitriles without counting the nitrile carbon. The CN carbon is carbon 1, so ethanal gives 2-hydroxypropanenitrile.
- Drawing the first curly arrow from the C=O bond rather than from the lone pair on the nucleophile.
- Forgetting the arrow from the C=O π bond to oxygen, or leaving out the protonation step.
- Stating that ketones cannot be reduced. Ketones are reduced by NaBH4 to secondary alcohols; they just resist oxidation.
- Unbalanced [H] equations. Reduction of a carbonyl compound to an alcohol needs 2[H].
Try three 3.3.8 Aldehydes and Ketones exam-style questions
1. Outline the mechanism for the reduction of propanone with NaBH4 and name the product. (4 marks)
Show the mark scheme answer
Curly arrow from the lone pair on the hydride ion (H-) to the δ+ carbon of C=O; curly arrow from C=O bond to O, giving an alkoxide intermediate with O-; curly arrow from a lone pair on O- to H of water (or H+); propan-2-ol.
2. Name the product formed when ethanal reacts with KCN followed by dilute acid, and give one reason why KCN is used rather than HCN. (2 marks)
Show the mark scheme answer
2-hydroxypropanenitrile; HCN is a highly toxic gas (or KCN fully ionises to give a higher concentration of CN- ions).
3. Write an equation for the oxidation of butanal using [O], and describe what is observed when butanal is warmed with Fehling's solution. (2 marks)
Show the mark scheme answer
CH3CH2CH2CHO + [O] → CH3CH2CH2COOH; blue solution forms a brick-red precipitate.
3.3.8 Aldehydes and Ketones FAQs
What does AQA A Level Chemistry 3.3.8 Aldehydes and Ketones cover?
It covers oxidation of aldehydes, Tollens' and Fehling's tests, reduction with NaBH4, nucleophilic addition mechanisms and forming hydroxynitriles with KCN, including hazards and racemic products.
Which paper is 3.3.8 Aldehydes and Ketones examined in?
It is examined in Paper 2, and Paper 3 can also test it.
Which mechanisms do I need for aldehydes and ketones?
You need nucleophilic addition with H- from NaBH4 and with CN- from KCN. Practise both with aldehydes and ketones, and be ready to explain why the cyanide product can be a racemic mixture.
