CCEA A Level Chemistry
CCEA A Level Chemistry 4.7 Aldehydes and Ketones Exam Questions
Free CCEA A Level Chemistry revision for 4.7 Aldehydes and Ketones: what you need to know, common mistakes and exam-style questions with answers. Covers carbonyl properties, preparation, nucleophilic addition of HCN, racemic products, 2,4-DNPH, Tollens' and Fehling's tests and LiAlH4 reduction.
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About CCEA A Level Chemistry 4.7 Aldehydes and Ketones
Section 4.7 of Unit A2 1 covers carbonyl compounds. You need to name and draw aldehydes and ketones up to six carbons in the main chain, explain their boiling points and solubility, and prepare them by oxidising primary or secondary alcohols. The key mechanism is nucleophilic addition of HCN, using propanone as the CCEA example, and you must explain why addition to an unsymmetrical carbonyl gives a racemic mixture. You also need 2,4-DNPH for identifying carbonyl compounds, how acidified dichromate, Fehling's solution and Tollens' reagent distinguish aldehydes from ketones, and reduction with lithal (LiAlH4). This links to alcohols (2.6), isomerism (4.6) and carboxylic acids (4.8).
What you need to know
- Structure and properties Aldehydes have C=O at the end of the chain (-CHO); ketones have C=O within the chain. They have permanent dipole-dipole attractions, so boiling points lie between those of alkanes and alcohols. Small ones dissolve in water as they can accept hydrogen bonds.
- Preparation Oxidise a primary alcohol with acidified dichromate and distil off the aldehyde as it forms; oxidise a secondary alcohol to a ketone.
- Nucleophilic addition CN- attacks the delta+ carbonyl carbon, the C=O pi bond breaks onto oxygen, and the alkoxide ion is protonated by H+ (from HCN), forming a hydroxynitrile. Propanone gives 2-hydroxy-2-methylpropanenitrile.
- Racemic mixtures The carbonyl group is planar, so CN- can attack from either side with equal probability. With aldehydes (other than methanal) and unsymmetrical ketones, a racemic mixture of the two enantiomers forms.
- Tests 2,4-DNPH gives an orange precipitate with any carbonyl; the purified derivative's melting point identifies the compound. Aldehydes turn acidified dichromate from orange to green, Fehling's from blue to a brick red precipitate (Cu2O), and Tollens' gives a silver mirror. Ketones give no change.
- Reduction LiAlH4 (lithal) in dry ether reduces aldehydes to primary alcohols and ketones to secondary alcohols. Write the reductant as [H].
Common mistakes in 4.7 Aldehydes and Ketones
- Saying the propanone and HCN product is chiral. With propanone the carbon has two CH3 groups, so it is not chiral; racemic mixtures form with aldehydes or unsymmetrical ketones.
- Drawing CN- attacking the oxygen. The nucleophile attacks the delta+ carbon.
- Saying Tollens' reagent turns brick red. Tollens' gives a silver mirror; Fehling's gives a brick red precipitate.
- Saying ketones are oxidised by Fehling's solution. Ketones do not react with Fehling's or Tollens'.
- Writing H2 for reduction with lithal. Use [H], for example CH3CHO + 2[H] gives CH3CH2OH.
Try three 4.7 Aldehydes and Ketones exam-style questions
1. Draw the mechanism for the reaction of HCN with ethanal. (4 marks)
Show the mark scheme answer
Delta+ on carbonyl C; arrow from lone pair on CN- (on carbon) to C; arrow from C=O pi bond to O forming O-; arrow from O- lone pair to H+ (or H of HCN), forming 2-hydroxypropanenitrile.
2. Explain why the product of ethanal and HCN is not optically active. (3 marks)
Show the mark scheme answer
Product has a chiral centre; the carbonyl group is planar so CN- attacks from either side with equal probability; equal amounts of both enantiomers form, a racemic mixture, and the rotations cancel.
3. Describe a test to distinguish propanal from propanone, giving the results. (3 marks)
Show the mark scheme answer
Warm with Tollens' reagent (or Fehling's); propanal gives a silver mirror (or brick red precipitate); propanone shows no change.
4.7 Aldehydes and Ketones FAQs
What does CCEA A Level Chemistry 4.7 Aldehydes and Ketones cover?
It covers naming and properties, preparation, nucleophilic addition of HCN, racemic mixtures, 2,4-DNPH, Tollens' and Fehling's tests and reduction with LiAlH4.
Which unit is 4.7 Aldehydes and Ketones examined in?
It is examined in Unit A2 1: Further Physical and Organic Chemistry, a 2 hour paper with a multiple-choice section and structured questions. Related practical skills can also be assessed in Unit A2 3.
Which tests for carbonyls should I learn?
2,4-DNPH for any carbonyl, then acidified dichromate, Fehling's solution and Tollens' reagent to show an aldehyde, each with the exact colour change.
