Edexcel International A Level Chemistry
Edexcel International A Level Chemistry Carbonyl Compounds Exam Questions
Free Edexcel International A Level Chemistry exam questions for Carbonyl Compounds, with a mark scheme for every pack. Covers aldehydes and ketones, Fehling's and Tollens' tests, 2,4-DNPH, LiAlH4 reduction, nucleophilic addition of HCN and the iodoform reaction.
Carbonyl Compounds exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About Edexcel International A Level Chemistry Carbonyl Compounds
Carbonyl compounds in Unit 4 of Edexcel International A Level Chemistry covers aldehydes and ketones and the reactions of the C=O group. You learn how they are made by oxidising alcohols, why they have higher boiling temperatures than alkanes but lower than alcohols, and how to distinguish them with chemical tests. A key mechanism is nucleophilic addition of hydrogen cyanide, which also links to Chirality because the planar carbonyl group can be attacked from either side. Examiners frequently test reagents and observations for the tests, the nucleophilic addition mechanism and the iodoform reaction for identifying methyl ketones.
What you need to know
- Structure and properties The C=O bond is polar, so carbonyl compounds have permanent dipole-dipole forces. They cannot hydrogen bond with each other but small ones dissolve in water by hydrogen bonding with water molecules.
- 2,4-DNPH test 2,4-dinitrophenylhydrazine gives an orange precipitate with both aldehydes and ketones. Melting temperatures of the purified derivative can identify the compound.
- Distinguishing aldehydes Aldehydes are oxidised to carboxylic acids. Fehling's or Benedict's solution changes from blue to a red precipitate; Tollens' reagent gives a silver mirror; acidified dichromate turns from orange to green. Ketones give no change.
- Reduction Lithium tetrahydridoaluminate(III), LiAlH4, in dry ether reduces aldehydes to primary alcohols and ketones to secondary alcohols.
- Nucleophilic addition of HCN Using KCN with a little acid, CN- attacks the delta positive carbon, the pi bond electrons move onto oxygen, then the alkoxide intermediate is protonated to form a hydroxynitrile. Attack from either side of the planar C=O gives a racemic mixture.
- Iodoform reaction Iodine in alkali gives a yellow precipitate of triiodomethane (CHI3) with compounds containing CH3CO (methyl ketones and ethanal) or the CH3CH(OH) group.
Common mistakes in Carbonyl Compounds
- Stating that ketones give a silver mirror or red precipitate. Only aldehydes are oxidised by Tollens' and Fehling's.
- Starting the nucleophilic addition arrow from the carbon of CN- without showing the lone pair, or from the negative charge on N. The lone pair is on carbon.
- Forgetting the second step of the mechanism, where H+ (or HCN) protonates the O- intermediate.
- Describing the 2,4-DNPH precipitate as yellow and confusing it with the iodoform precipitate. Describe it as orange (or yellow-orange) and use it only to show a C=O group.
- Using LiAlH4 in water. It reacts violently with water, so dry ether is used.
Try three Carbonyl Compounds exam-style questions
1. Describe a test to distinguish propanal from propanone. Give the reagent and the result for each. (3 marks)
Show the mark scheme answer
Add Tollens' reagent and warm (or Fehling's solution); propanal gives a silver mirror (or red precipitate); propanone shows no change (stays colourless or blue).
2. Draw the mechanism for the reaction of ethanal with HCN in the presence of KCN. Name the product. (4 marks)
Show the mark scheme answer
Dipole on C=O; arrow from lone pair on carbon of CN- to delta positive carbon; arrow from C=O bond to O; intermediate O- with arrow from lone pair to H of HCN (or H+); product 2-hydroxypropanenitrile.
3. Which of these gives a yellow precipitate with iodine in alkali: propanone, propanal, butan-2-ol? Explain. (2 marks)
Show the mark scheme answer
Propanone and butan-2-ol; propanone contains CH3CO and butan-2-ol contains CH3CH(OH), while propanal contains neither.
Carbonyl Compounds FAQs
What does Edexcel International A Level Chemistry Carbonyl compounds cover?
It covers aldehydes and ketones, their physical properties, tests with 2,4-DNPH, Fehling's, Tollens' and iodine in alkali, reduction with LiAlH4 and nucleophilic addition of HCN.
Which unit is Carbonyl compounds examined in?
Carbonyl compounds is part of Unit 4, and these reactions are used again in Unit 5 organic synthesis.
Which tests do I need to know for carbonyls?
Learn 2,4-DNPH for any carbonyl, Fehling's or Benedict's and Tollens' to identify aldehydes, acidified dichromate for oxidation, and iodine in alkali for methyl carbonyls.
